A Report on the Dispersal of Maize Pollen
Dr Jean Emberlin
Beverley Adams-Groom BSc and Julie Tidmarsh BSc
National Pollen Research Unit
Worcester WR2 6 AJ
commissioned by SOIL
Based on evidence available from publications and internet
1 Summary 2 Introduction 3 The features of Maize pollen and pollination 4 Evidence of maize pollen dispersal from monitoring surveys 5 Estimates of pollen transport by the wind in the vicinity of the crop and rates of cross pollination 6 Potential long range dispersal 7 The transfer of maize pollen by Bees and other insects 8 Discussion and conclusions 9 References 10 Appendices a List of searches for material. b Web sites of interest
The report reviews evidence from published sources and also from communications from named authorities about maize pollen dispersal. Background material is given on the characteristics of maize pollen including morphology and duration of viability, together with quantities produced and the salient features of pollination.
This information is used with data from empirical studies, dispersion theory models and particle deposition theory to give estimates of deposition rates and concentrations of pollen remaining airborne downwind from a source. However it is not possible to provide accurate assessments for practical use as most empirical work has been done within a narrow range of weather conditions and many studies suffer from the constraints of monitoring only close to the source and of errors in sampling. For example, evidence from previous work can be used as a basis for generalised estimates of percentages of pollen concentrations remaining airborne downwind in low to moderate wind speeds compared with concentrations at 1m from the source. These are approximately 2% at 60m, 1.1% at 200m and between 0.75 % and 0.5 % at 500m. The implications of these figures for potential cross pollination are considered but it is emphasised. that they should be used as rough guidelines only. In addition it should be noted that dispersal gradients would be altered by factors such as climatic conditions and local topography.
Transport on the airflow over longer distances is likely to occur under a range of weather situations including uplift and horizontal movement in convection cells, and uplift and transport in frontal storms. As the maize pollen grains remain viable for about 24 hours in normal weather conditions pollination could occur at sites remote from the source ( e.g. 180 km).
Dispersal away from the vicinity of the crop also takes place by carriage on bees. Evidence is cited that maize pollen is collected by bees in notable amounts. In this way the pollen is transported several miles from the crop plot in suitable weather conditions.
In any assessment of pollen flow from maize plots consideration should be given to the limitations of evidence from empirical studies and from results based on theoretical models. Acknowledgement must be given to the potential movement of maize pollen by bees and the possibilities of long range transport under certain weather conditions.
This report examines the evidence for patterns of pollen dispersal from maize and considers the potential distances for effective pollination between different plants. It utilises a range of available information including empirical data from specific monitoring experiments, dispersion theory models, particle deposition theory, evidence of maize pollen transport by insects and aerobiological evidence from studies of long range transport of pollen.
The report discusses the evidence for patterns of maize pollen dispersal by wind in the vicinity of the crop and also examines the probability of longer range dispersal either by carriage on vectors or by long range transport. The probability of cross pollination events occurring between GM maize and ordinary maize is assessed for various separation distances.
3. FEATURES OF MAIZE POLLEN AND POLLINATION
Maize (Zea mays) is a highly variable, naturally cross-pollinated, markedly heterogeneous, complex species, in which all forms hybridise freely (Purseglove, 1972). It is generally pollinated by wind and gravity (anemophilous) and is also visited by bees (e.g. Percival 1950) (see section 6).
3.1 The maize pollen grain; morphology and duration of viability.
Maize pollen is amongst the largest of that of the grass (gramineae) family with dimensions in the region of 90 to 125 x 85 microns (Erdtman, 1952, Smith, 1990). The grains of pollen are usually mono-porate, more or less spheroidal to ovoid and with the aperture as a rule slightly protruding, crassimarginate and operculate (Erdtman, 1952). The grain has a volume of about 700 x 10-9 cm3 and a weight of about 247 x 10-9 g (Miller 1985; Goss 1968).
Published data for the length of time that maize pollen remains viable under natural conditions differs from about 24 hours through to several days. In artificially warm conditions or exceptionally hot weather this time could be reduced to a few hours. Conversely in cool conditions it could be extended to 9 days. Purseglove (1972) mentions maize pollen vitality as being 'about 24 hours, but is killed more rapidly in very hot dry weather.' Jones and Newell (1948) refer to Pfundt (1910) who reported that Zea mays pollen lasts for one day and they also quote the following: 'The maximum retention of the fertilising power of corn pollen is reported by Knowlton (1922) to be 70 to 80 hours at a temperature of 5o to 10o C and a humidity of 50 to 80%'. However, McCluer (1942) reports that when corn pollen was kept dry it retained its viability for several days. The research of Jones & Newell (1948) in 1944 and 1945, used two methods for determining the longevity of maize pollen viability. Firstly, they refrigerated the pollen in the tassel and found that the pollen would set seed with decreasing success over 9 days. Secondly, they used pollen that had been stored in a beaker in a refrigerator and that was viable, again with decreasing success, over 8 days. In both cases the first two days were very successful. Jones & Newell (1948) also found that maize pollen stored in pollinating bags in direct sunlight under a maximum temperature of 96oF was effective in fertilising silks for only 3 hours, while pollen stored in the shade of the plants under a maximum temperature of 86oF remained viable for 30 hours. They suggest that cool temperature and high relative humidity appear to be important factors in extending the life of the pollen.
The maize plant is monoecious and diclinous, with male and female flowers borne separately on the same plant. The male flower is called a 'tassel' and the female flower is called the 'ear', while the style is usually called a 'silk', being long and thread-like. Maize is protandrous with pollen being shed before the silks are receptive, but as there is some overlap, up to five per cent self-pollination can occur. The tassel spreads fully before anthesis begins. Opening of the flower begins near the middle of the central spike and passes upwards and downwards, followed by the lateral branches, and ends with the tips and bases of the lower branches.
The upper flower of each pair of pair of spikelets usually opens before the lower, so that anthesis may take place in two waves. A tassel sheds pollen from 2-14 days, more usually 5-8 days, with maximum shedding about the third day (Purseglove 1972). A corn plant sheds its pollen on several successive days with dehiscence occurring in the mornings between 630 and 1100 hours, although it may be delayed by two hours in cold, cloudy weather ( Miller 1985).
The oldest spikelets are at the base of the cob and their silks begin to elongate before the upper spikelets and are the first to protrude from the husks, 2-3 days after pollen has begun to shed on the plant. Under favourable conditions, all the silks emerge during a period of 3-5 days and are receptive on emergence and can remain so for 14 days. The silks dry up on pollination, but grow abnormally long and remain turgid if not pollinated. The pollen grains are caught on the moist sticky stigmatic hairs and germinate immediately. Fertilisation of the ovule occurs 12-28 hours after pollination, during which time the pollen tube may have traversed a distance of 25 cm down the longest silks. Although there is usually an overlap of pollen shedding and silk emergence on the same plant, under normal field conditions at least 95% of the ovules are fertilised by pollen from other plants. Drought and unfavourable weather may result in silks emerging after most of the pollen has been shed (Purseglove 1972).
3.3 Pollen Quantities.
Maize pollen is produced in enormous quantities. Each of the numerous florets within a tassel contains three anthers, each of which produces in the order of 2,000 (Miller 1985 ) or 2,500 grains (Kiesselbach 1949) to 7,500 (Goss, 1968) pollen grains, depending on the plant, the variety, and the conditions of growth. This means that a spikelet produces c.15,000 grains, and a tassel between 2-5 million grains. Estimates for the numbers of pollen grains produced by an average-sized plant range from 14 million (Miller 1985), to about 50 million (Miller 1985), to fertilise approximately 1000 kernels per plant (Evans, 1975) so that there are 20,000 to 30,000 pollen grains for each silk (Purseglove, 1972). This high ratio of pollen grains per female flower available for fertilisation is typical of plants fertilised by wind pollination or anemophilous plants such as corn and pine.
Nowakowski and Morse (1982) found that "many sweet corn varieties produce over 150 pounds of pollen per acre". They collected sweet corn pollen from the plants using paper cones and found that even more pollen was collected by taking plants into the still conditions of a laboratory the day before pollen shedding started. Although a field of maize may release pollen over a period of up to 13 days each plant will be active for less than this. An average of 3.5 g of pollen was produced per plant in total in their experiments. Maize is typically cultivated at 20,000 plants per acre giving a pollen output of approximately 154 pounds/ 70 Kg per acre.
Table 1 : Summary of salient features of maize pollen
Grain size c.90 m (with approx. range 85- 125)
Deposition coefficient Cap . 0.20
Terminal velocity Vs 20 cm s-1
Vg ( velocity of deposition ) 40 cm s-1
Duration of viability 3 hours to 8 days depending on conditions
4. EVIDENCE OF MAIZE POLLEN DISPERSAL FROM MONITORING SURVEYS
Numerous trials have been conducted on maize pollen dispersal because of its economic and archaeological importance. Many of these studies were done in the 1940s through to the 1970s over short downwind distances ( c.50 to 500m ) to investigate exclusion distances, or within crop stands to investigate cross pollination ( see 4.3 ). Most of the studies have examined the deposition gradients of pollen with distance from the crop and some have sampled air borne pollen at low level. There has been little consideration of vertical profiles of maize pollen concentrations or of dispersal potential other than that in the immediate vicinity of the crop in the predominant downwind direction. All of the studies suffer from the disadvantage that the work was carried out in a limited range of weather conditions. Some work has also been constrained by problems with sampling efficiencies of monitoring equipment.
The data from some of these trials has been used by various workers in dispersion models to produce generalised patterns of dispersion and to predict downwind rates of deposition and airborne concentrations of pollen. The results of this type of work are often quoted without reference to the constraints of the models and the complexity of particulate dispersion patterns. Any numerical estimates for the gradients and amounts of pollen remaining airborne can only be approximations and must be viewed with caution. Similarly the implications for pollination of concentrations of pollen airborne at distance from the source must be assessed in the context of pollen viability and other factors such as competition from pollen produced by the receptor stand.
It is important to appreciate that the results of limited experimental data from one area may not represent the prevailing situation in another location and climate. Similarly the data derived from theoretical models should be used only as guidelines with acknowledgement of the assumptions made. In order to provide realistic estimates of pollen transport distances in particular cases local variables need be considered including the prevailing climate and topography. In the following sections the literature relevant to making estimates of maize pollen transport from plots is considered both for pure stands and for plots with mixed planting of GM and ordinary maize.
4.2 Dispersal within stands
Available evidence indicates that total deposition within the source plot is greater than that outside (Raynor et al. 1972) but there will always be some dispersal beyond the borders of the plot. In situations where GM maize is planted within plots such as in rows bordered by ordinary maize it is useful to consider the relative contributions that the plants make to the concentration of pollen emanating from the edge of the crop. Evidence for this comes from studies of dispersal within the crop stands. Paterniani and Stort (1974) conducted trials using one central plant with dominant yellow endosperm colour in each of 4 plots of white endosperm maize ranging in size from 15 x 20 m to 40 x 40 m. The plants were harvested individually and the seeds of each type counted. The mean percentages of pollination by the central plant in relation to increased distances were calculated. The findings indicate a panmitic population in which male gametes are considerably mixed within the stand. At distances between 10 to 20 m the mean percentage pollination by the central plant is 0.01% decreasing to 0.005% at 30m. This rate remains fairly constant with increasing distance to the edge of the field. The greatest amount of pollination occurs in proximal plants. The results indicate that 50% of the kernels of any individual plant result from pollen of plants within a radius of about 12m. This involves about 800 to 2500 plants. The remaining 50 % must obviously come from pollen further afield. The authors state that "What is surprising is that after some distance the amount of pollination even though small at about a few kernels per 100,000 seeds remains reasonably constant up to the borders of the fields which is about 170 plants from the central one". Clearly pollen from the central plant could travel to the edges of the plot despite the filtering and impact effects of the maize crop.
If GM maize is grown within a stand of ordinary maize the maximum contribution that GM plants could make to the pollen flow from the mixed stand would be in the ratio of the plants grown. This could be considerably less depending on the planting scheme but would not decrease to zero even if there were only a few GM plants and these were in the centre of the plot.
4.3 Dispersal downwind from stands
To some extent studies have produced contradictory results but there is general agreement that the typical downwind dispersal pattern of maize pollen by the airflow in low to moderate wind speeds results in a relatively steep deposition gradient. This would be expected as the dispersion of corn pollen is influenced by its large size and rapid settling rate. Maize pollen is one of the largest normally dispersed by the wind from a low level source and compares in size to the largest particles commonly airborne ( Raynor et al. 1972).
Published results of measurements of the deposition gradients differ considerably. One point of comparison is the measure of the half distance that is the distance over which the concentration decreases by half as the distance from the source increases by a constant increment (the half distance). This figure provides indications of decay rates but has limitations and should not be extrapolated to distances outside the observed range. Some studies report short half distances. For instance the work of Bateman (1947) gives a figure of 3.77m. He describes experiments showing that contamination from cross pollination of different maize cultivators may drop by 99 per cent over a distance of 12 to 15m. Similarly the work of Hodgson (1949) indicates a steep decay deposition curve with a half distance of 8.25 m. In comparison Jones and Newell (1948) report data showing a half distance of 47.47m indicating a potential for cross pollination over several hundred metres.
Estimates of half distances have been applied in dispersal models. These make numerous assumptions about the features of dispersal but they have proved useful in some cases for describing natural and experimental gradients generally within a few hundred metres of the source under certain weather conditions. For instance Fitt et al. (1987) used data published on pollen dispersal from various crops to develop power law and exponential models, achieving a high percentage of explanation of variance (r2). The greatest r2 maize was for data from Hodgson (1949) with a half distance of 8.25m. This indicates concentrations at 50 m being less than 1% of that at 1m from the crop edge. However using data from Jones and Newell (1948) in the same model but with a half distance of 47.47m in this case, the concentration at 500 m is estimated to be 0.05 % of that at 1m from the crop. McCartney (1994) also uses the maize pollen I/2 distance of 47m in a negative exponential relationship with similar results.
All models have limitations as they are essentially descriptive, not interpretative and should not be extrapolated outside the observed range. The exponential model applies to particle dispersal when the decrease in concentrations with distance away from the source is predominantly from deposition. For maize pollen there will always be some dilution and diffusion within the airflow that will distort deposition rates so that the model will not be entirely accurate. The processes involved in particle dispersal are very complex making it impossible to describe gradients in a model using a single parameter. For instance deposition rates do not indicate ambient concentrations of the fractions remaining. Factors such as frictional turbulence and thermal convection which can cause steep deposition gradients can transport large numbers of pollen grains to great heights where they can disperse over long distances (see section 6).
Some workers have used bi-axial gaussian dispersion models using estimates of standard deviations of dispersion gradients taken from Pasquill (1971). Gaussian models are not appropriate in the case of maize pollen because they assume that the concentration of the particles is normal about a central axis. Dispersion in the vertical axis is unlikely to be even for maize pollen due to the weight of the pollen grains. For example Raynor et al. (1972) found that vertical dispersion patterns were more flattened in the case of maize pollen than with Timothy grass or ragweed. Their results demonstrated that the rate of settling opposes the rate of upward dispersion so that the height of pollen plume does not increase continuously with downwind distance. Similarly dispersion in the cross wind axis is unlikely to be even because the grains may not take up the motion of eddies.
One of the most detailed studies on maize pollen dispersal was that conducted by Raynor, Ogden and Hayes (1972). Dispersion and deposition of corn pollen emitted from plants in two 18m diameter plots were studied in 39 tests. Concentrations were measured by wind impaction samplers mounted at four heights from 0.5 to 4.6m and at five distances from the source on 20 degree radii extending to 54.9m downwind in 1963 and 59.5m in 1964. Deposition was measured by greased microscope slides on the ground. The study indicated at 60m from the source the total amount of pollen remaining airborne was about 5% of that at 1m from the source and that the deposition per unit area at this distance downwind was only 0.2% of that near the source. In the trials by Raynor et al. attenuation of airborne maize pollen grains between the source and 60m was 50:1 whereas attenuation of deposition was 2500:1. This much heavier deposition near to the source corresponds to the grains that never became effectively airborne. This point is relevant when considering data based on deposition readings. Taking concentrations at 1m as 100% the mean results of 15 samples indicated concentrations at 54.9m to be 1.3% (SD 3.30) and at 59.5m to be 1.1% ( SD 2.1).
In contrast the results from Jones and Newell (1948) show approx. 1% concentrations remaining at 427m and those from Jones and Brooks (1950) approximately 0.75% at 503 m. It is likely that these shallower slopes of the deposition curves result from greater source areas and the greater wind speed occurring during their work. Faster winds would result in dispersal over larger distances but they would also cause more depletion by impaction. The range of weather conditions during the trials by Raynor et al ( 1972) were small and did not vary enough to permit documentation of the effects of wind speed.
In most of the data on maize pollen dispersal from empirical studies the loss in air concentrations compared with deposition are uniformly high indicating an over estimate of deposition or an underestimate of air concentrations. Monteith (1975) proposes that the peak concentrations are not adequately estimated. He suggests that the poor retention efficiency of samplers for corn pollen may be enough to cause errors in computing concentrations. The wide range of estimates for deposition rates coupled with the narrow range of weather conditions explored and the possible errors involved in monitoring bring in to question the validity of the results.
More reliable information on potential spread of pollen comes from observations of outcrossing. For example Jones and Brooks (1950) researched the effectiveness of distance and border rows in preventing outcrossing in corn. In one of three years the outcrossing exceeded the maximum mixture permitted by international standards at distances of 300m isolation . At a distance from the contaminating field of 400m the mean percent outcrossing was 0.42 and at 500 m the mean percent outcrossing was 0.32. Other cited levels of cross breeding between maize at various distances include Jones and Newell (1948) 7.2% at 250m, Jones and Brooks (1950) 2.47% at 200m and Salamov (1940) 0.21% at 805m.
5. ESTIMATES OF POLLEN TRANSPORT BY THE WIND IN THE VICINITY OF THE CROP AND RATES OF CROSS POLLINATION
The publications described in section 4 have produced statistics that have lead many workers to report that an isolation distance of 200 to 400 metres is considered satisfactory ( Airy 1955). However most studies have been done in low to moderate wind speeds so insufficient evidence is available from empirical studies to allow the prediction of the features of maize pollen transport in a variety of weather conditions. In gusty weather, deposition gradients near to crops may be steeper but that of the distance travelled above crops may be enhanced ( McCartney 1990 b ). In general higher wind speeds result in longer travel distances for pollen but the increased turbulence leads to greater impaction.
Taking the evidence of published trials and use of data in dispersion models described previously, estimates can be made of the transport of maize pollen in low to moderate wind speeds . However little confidence is attached to these due to the reasons outlined in section four. In the following section the pollination due to pollen from one maize plot ( the donor plot) is considered for plants in another plot (the receptor plot ). Based on consideration of the limited evidence available the generalised estimate figures for maize pollen concentrations from the donor plot at downwind distances under dry weather conditions with low to moderate wind speed are as follows: ( expressed as percentages of concentrations at 1m from the source, Qd ):
Table 2 . Estimates of percentages of pollen concentrations at various distances downwind compared with those at 1m from the source ( based on evidence presented in section 4).
60m from the crop edge would be approximately 2%
200 m downwind from the source would be approximately 1.1%
500m it would be approximately 0.75 % to 0.5 %
These relatively small percentages could still result in considerable concentrations in the receptor plot due to the large amounts of pollen released from maize. If it assumed that there was no competing pollen being released from within the receptor plot these amounts could result in high rates of cross pollination. For example if the figures quoted in 3.1 are considered, a conservative estimate of pollen production per plant is in the order of 25 million grains. This could result in the following approximate amounts of pollen per plant in the donor plot being available for pollination in the receptor plot over the duration of pollen release. Obviously the amounts would not be consistent for all plants in the plot as there would be some deposition and filtering within the stand. Plants on the edge of the plot could contribute most of their pollen to the airflow leaving the crop.
Table 3. Possible numbers of pollen grains remaining air borne from an individual plant at various distance downwind (based on estimates of 25 million grains produced per plant).
500,000 at 60 m
275,000 at 200m
187,500 to 125000 at 500m
However in order to arrive at a realistic estimate of the amount of cross pollination likely to occur with these percentages and amounts the following aspects need to be considered:
1. Synchronisation of maturation of the flowers ( both male and female parts).
2. Relative concentration strengths of the pollen produced by the donor plot and the receptor plot at the point of pollination. There may be an overlap in pollen production periods in the two stands so there may be competition for pollination between pollen from the two sources.
3. The amount of self or cross sterility in the variety.
4. Density of the stands.
If the donor crop releasing pollen was a mixed GM / ordinary maize plot the source strength ( Qd) could be described in terms of the ratio of the GM/ ordinary pollen (Qgm/Qn) e.g. 1:10, as this would give the maximum contribution of the GM pollen. The corresponding percentage of the concentration remaining at various downwind distances could be calculated e.g. if 10% of the stand was GM maize the concentration of GM pollen remaining at 60m downwind would be in the order of 0.2% of the total pollen concentration at 1m from the source.
If it is assumed that there is an overlap in pollen release between the two plots of maize there will be competition for pollination. The donor plot source strength (Qd) will need to be considered in relation to receptor plot source strength (Qr)
If flowering is synchronous and both stands produce equal amounts of pollen then the relative concentrations of pollen from the donor plot and the receptor plot can be considered e.g. at 60m this will be Qd x 2%: Qr giving a qualification to the probability of pollination. For instance, taking the figures from Table 2, the Qd component in the prevailing pollen concentration at the edge of a receptor plot at 60m would be 1.9%, at 200m would be 1.08% and 500m would be 0.74 to 0.49%.
At these concentration ratios the rates of cross pollination would be in the order of:
at 60 m 1 kernel per 53
at 200 m 1 kernel per 93
at 500 m 1 kernel per 135 to 204
If GM maize is grown within the donor plot the corresponding figures for the contributions that its pollen makes to the amounts at the receptor plot can be calculated from the ratios of GM to ordinary maize in the donor plot.
These estimates should be considered as rough guidelines only as they would be altered by factors such as climatic conditions which effect the transport of pollen, and the numbers of bees and other insects around, which would be likely to increase the amounts of pollen transfer. Also the generalised predictions based on empirical evidence and theoretical models would need to be modified in relation to local topography and climate, and the sizes and lay-outs of the plots. The estimates given in this section are for conditions of low to moderate wind speeds. In higher wind speeds with gusty conditions the maize pollen dispersal will be different. Some grains are likely to travel further downwind but impaction rates will also increase. More empirical work needs to be done to investigate the details of dispersal under these and other weather patterns.
6. POTENTIAL LONG RANGE DISPERSAL
In certain weather conditions particles, including pollen grains, can travel long distances on the airflow. Long-distance dispersal of maize pollen needs to be considered within the constraint of its viability time ( in the region of 24 hours under normal weather conditions, Purseglove, 1972). The following section indicates that maize pollen can travel for long distances within its viability period, given the right conditions for doing so.
Maize pollen is relatively large and heavy (section 3). Empirical work on its dispersal has indicated relatively steep declines in concentrations with distance away from the source and limited upward spread of the plume (see section 4 ). However these studies have investigated dispersal only to heights of c. 4.6 m above ground in the downwind direction and to distances of c. 500m maximum downwind. The studies have been conducted in a narrow range of weather conditions. No work has been done specifically on the movement of maize pollen in convection currents, or on movement aloft in turbulent conditions or during the passage of weather fronts. Research done on the dispersal of other pollen types has demonstrated that long range transport does take place including pollen from low level sources.
Vertical transport of pollens takes place by several mechanisms. On warm days with low wind speeds convective currents driven by the heating of the ground by the sun lead to mixing through the boundary layer. This activity has a marked diurnal influence with particles being dispersed laterally through convection cells during the day, and descending when the convection subsides ( Oke 1978). As long as there is a positive convective air movement the net result will be to keep pollen in the atmosphere. Settling out of pollen usually occurs during cooling in the evening when the majority of grains will return to the surface.
Most anemophilous pollen will be liberated during day time in dry, warm weather. Days like this usually have thermals rising turbulently that will have a positive effect in bringing pollen grains up into the higher strata. The upper limit for convective ascent is marked by the thermal inversion, often shown by the presence of cumulus clouds. Convective cells are typically 1 -3 km in diameter, reaching some 1-2,000 m in height and last about 20 to 30 minutes each, during which time they can move downwind. The individual cells may form composite cells 5-10 km across and last for several hours. Upward velocity of cell tops reaches 0.5-1.5m/sec and horizontal expansion of 0.5-1.0 m/sec ( Hardy and Ottersten 1968 ). Some pollen grains will have reached the inversion layer when the bubble collapses. It may then be transported horizontally considerable distances depending on weather conditions. During the evening and night time convection will cease and the pollen will tend to fall towards the ground but this may be impeded by low level inversions. The usual length of time available for pollen to travel as it is kept aloft by convection is a maximum of one day. This would be equivalent to a distance of about 50 - 180 km, although it is well known that much longer transports do occasionally take place (Faegri & Iversen, 1989) when suitable meteorological conditions occur.
On days with less solar heating and higher wind speeds, pollen can be dispersed vertically by turbulence generated either by instability in the lapse rate or by rough surfaces such as uneven topography. Biological particles introduced into the boundary layer have been observed in detectable concentrations to distances of several hundred kilometres downwind ( e.g. Hjelmroos 1991). Penetrative transport to great heights can also take place through updraughts generated by deep intense convective storms. In such storms large masses of air, originally lying near the surface, are transported in a few tens of minutes to heights typically of the order of 8 to 12 km. At such heights, in the middle latitudes, winds are often very strong, in the range 25 to 50 m per second, so that pollen can travel great distances in a matter of hours (Mandrioli et al. 1984).
Hirst and Hurst (1967) sampled air for pollen and spores over the north seas. Their results include a case in which a pollen cloud generated over Britain could later be found as a pollen concentration cloud over the North Sea. Pollen released during one day was found the following day 300-400 km off the coast. Transport took place over the sea where dispersal conditions could be different from those over land depending on the weather. For example pollen transport over land could be enhanced by increased convection but conversely the concentrations could be depleted by more deposition due to turbulence. Tyldesley (1973) found appreciable quantities of arboreal pollen (up to 30 per m3) in the air in the treeless Shetlands, 250-380 km away from the nearest forests, in connection with favourable meteorological conditions, i.e. cyclonic storms.
A frontal storm can lift air masses several kilometres up in the air in a very short time and thus place pollen grains far above the day and night cycle (Faegri & Iversen, 1989). Once pollen has arrived in the upper atmosphere it can travel for many hundreds of kilometres on the airflow until finally being deposited or it may be captured by water drops and return to the surface in precipitation (Mandrioli, 1984). In general long range transport occurs most efficiently in dry conditions with limited mixing depth and moderate to high wind speeds.
Pollens can also be re-suspended from surfaces in gusts of wind and re-deposited. For example, Erdtman (1938) described finding Zea pollen in the air in Sweden during midwinter (Faegri & Iversen, 1989). If re-suspension took place within the time of pollen viability it could extend the effective transport distance.
It is reasonable to assume that maize pollen can remain viable and capable of effective fertilisation for at least 24 hours in most weather conditions prevailing in the UK (2. 1). This means that with mean horizontal wind speeds of 2 m/s, that can occur on summer days with convection currents that could keep the pollen grains aloft (section 7), they could travel I km in 4.16 minutes, 7.2 km in an hour (potentially 172.8 km in a day). In wind speeds of 10 m/s some pollen grains would travel greater downwind distances before deposition than in slower wind speeds. Winds of 10 m/s would give rise to turbulent conditions in the boundary layer keeping some pollen airborne for longer than in non turbulent air flows. If the pollen remained airborne it could travel 36 km in an hour and nearly 864 km in 24 hours.
7. THE TRANSFER OF MAIZE POLLEN BY BEES AND OTHER INSECTS
Zea mays is generally regarded as being anemophilous (successful pollination relies on wind dispersal of pollen) but maize pollen is also collected by bees and may be transported by flies. Maize pollen is not uncommon in honey but would not be the major pollen type. There is often 90% of one main type and a 10% mix of many different species, which does contain some wind-pollinated types such as grass and maize ( Hodges 1984). Bees may pick up maize pollen whilst out foraging for a better source in perhaps a hedgerow (personal communication, Ms Sarah Brookes, Manager of the Bee Unit, Luddington, Warwickshire for 13 years, and Mr Paul Wilkins The National Bee Unit, Central Science Lab, National Bee Unit, Sand Hutton, York. YO41 1LZ ).
Evidence that maize pollen is collected by bees is available from experimental results and monitoring (Percival 1947, 1955, Nowakowski and Morse 1982, Vaissiere and Vinson 1994). Nowakowski and Morse from the Department of Entomology, Cornell University, New York, conducted research on honey-bee behaviour in sweet-corn fields to help minimise effects of insecticide spraying on foraging bees. They state that,"Although corn is wind pollinated it produces such copious amounts of pollen that it is highly attractive to bees, both honey bees and a wide variety of solitary bees."
Vaissiere and Vinson (1994) report the use of maize pollen, amongst others, in laboratory conditions to test the effectiveness of honey bee pollen collection. They note that maize pollen was readily collected by nearly all of the bees in the experiment, however, as the bees had been starved of pollen for a week this may not give a true representation of natural behaviour. Although reference is made to bees collecting maize pollen in the open this activity would be governed by a number of factors such as distance from hive and availability of more nutritious food sources. Percival (1947) notes that "any plant offering a fair amount of pollen per flower-form will be worked for pollen by the honey-bee, provided that (a) it grows within 1/4 mile of the hive, and (b) it attains a reasonable density."
Miller (1985) describes how bees may be used for collecting maize pollen. Once the bees have collected the pollen in their normal manner, the pollen is then collected from the bees and used. However, it has been noticed that some biochemical alterations can occur in pollen collected by bees.
Percival conducted work in the 1950s to investigate why honey bees collected some types of pollen more than others and Zea mays was included in the 86 species she studied (1955). She discovered that Z. mays presented pollen between 0700 hrs and 1800 hrs but that approximately 40% of the daily amount was produced between 1000-1100 hrs. She also found that in Z. mays all the anthers in one flower dehisce simultaneously.
Percival's conclusions were strengthened by research by Maurizio (1951) who showed that bee colonies had a need for pollen in early spring but that nectar was more desirable in June/July. Maurizio had also discovered that some pollen types were extremely important to bees. Z.mays was amongst the pollens that were found to "stimulate the development of brood food glands, ovaries and the fat body and also prolong length of life". This means that bees could be actively choosing certain pollen types not simply being attracted to quantity and proximity. Percival managed to calculate the amount of pollen produced per 'flower-form' for maize and found it to be 494 mg in total, 16 mg per day. This is a considerable amount of pollen, as might be expected for a wind-pollinated plant. Its attraction to bees would seem to be mostly due to its biological value because it flowers at a time when there is no shortage of other pollen around and maize does not produce nectar. Z. mays has a peak of pollen production at a time of day which is usually warm enough for insect activity.
Problems encountered in trying to estimate the distance of insect-carried pollen travel are discussed in a paper by Richards and Ibrahim (1978). Work on Primula veris found that pollen travel increased in fine, warm weather and attributed this to increased pollinator activity. This could be due to foraging time extending into evening hours on warm, dry days. Estimates on the distance bees will travel to find pollen or nectar differ. Early estimates tended to give relatively short distances e.g. Carter (1946) considers an economical flight for a honey-bee to be about 1 mile (at speeds of approximately 12-15 m.p.h.), Hooper gives a distance of about 1.5 miles (1976). Morse ( 1972) agrees with this for the majority of pollen collection but says that a bee might travel up to 8 or 9 miles if necessary. More recently it has been widely accepted that bees will regularly travel about 3 miles from the hive but will not go this far if a good source of pollen is available closer ( pers. com. Ms Sarah Brookes, and Mr Paul Wilkins).
In addition to transport by bees it is likely that occasional random encounters with other insects could lead to the transfer of maize pollen to neighbouring fields. Research into this topic has found evidence to support this hypothesis. Work carried out on Plantago lanceolata by Stelleman (1978) showed that despite being considered anemophilous, the flowers were frequently visited by certain flies. This in itself was not proof of pollination but trials with marked pollen, reported in this paper, showed that it was being transported from one inflorescence to another. The belief that some species use both wind and insect pollination is widely accepted (e.g. Proctor 1978) . The extent to which one method or the other is used cannot be taken for granted and must be discovered by observation and experiment.
8. DISCUSSION AND CONCLUSIONS
8.1 Empirical work and estimates of pollen dispersal downwind from plots.
Most empirical work on maize pollen dispersal has been done under a limited range of weather conditions and for relatively short distances downwind. In some cases the use of the results is further constrained by errors in sampling and in others the data have been applied to inappropriate dispersion models. The published estimates of deposition gradients give a wide range of readings but it is reasonable to assume from the features of the pollen grains that the deposition gradients would be steep in low to moderate wind speeds ( section 4.3). Estimates of airborne concentrations remaining at distances from the source are more problematic due to lack of reliable data. However it is reasonable to assume that concentrations of pollen remaining airborne downwind from the source would be more attenuated in high wind speeds.
Evidence from patterns of pollen dispersal within crop stands shows that pollen from central plants contributes to the pollen stream leaving the crop edge.
Considering the lack of empirical evidence it is not realistic to give exact predictions of maize pollen dispersal. Estimates have been given in section 4.3 based on available information. However these are qualified with a statement of low confidence because of the poor quality of the information base.
The information available indicates that in low to moderate wind speeds the percentages of pollen concentrations remaining airborne compared with those at 1m from the source would be approximately 2% at 60m, 1.1% at 200 m and approximately 0.75 % to 0.5 % at 500m. The implications of these amounts for pollination are discussed in section 4.3.
8.2 Movement by Bees and other insects.
The spread of pollen by the wind would be enhanced by transport by bees and other insects. Research has shown that Bees regularly visit maize flowers and transport maize pollen. Bees could move maize pollen several miles from the crop each day in suitable weather. Evidence shows that significant amounts of pollen are involved but it is not possible to quantify this component from published data found in searches so far.
8.3 Long range transport
Substantial evidence exists for long range transport of considerable numbers of pollen grains especially under the influences of movement in convective cells or in frontal storms. It is probable that notable amounts of maize pollen are transported by these mechanisms for several kilometres and that decreasing amounts are carried further with exceptional events giving rise to transport of hundreds of kilometres. Maize pollen remains viable under normal conditions for approximately 24 hours giving potential for pollination by grains that had travelled many hundreds of kilometres on the airflow.
8.4 Assessments of pollen flow from Maize.
Evidence currently available from empirical studies is insufficient to allow accurate assessments to be made of maize pollen dispersal under the different weather conditions prevailing in the UK during the maize pollen season. In any assessment of pollen flow from maize plots consideration should be given to the limitations of evidence from empirical studies and from the results obtained using theoretical models. In addition acknowledgement must be given to the potential movement of maize pollen by bees and the possibilities of long range transport under certain weather conditions. Overall it is clear that maize pollen spreads far beyond the 200m metres cited in several reports as being an acceptable separation distance to prevent cross pollination.
9. REFERENCE LIST
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Dispersal in Agricultural Habitats (Eds. Bunce and Howard) pp 133-158.
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Percival MS. 1950. Pollen presentation and pollen collection. New Phyto. 49 : 40-63.
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Additional relevant publications not referred to in text.
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Pollen and Spores, Form and Function. Academic Press, London.
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Erdtman, G. 1935. Pollen Statistics. pp 110 – 125. In: Wodehouse (1935). Pollen Grains. Mc Graw - Hill (N.Y.)
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Exploration and Conservation. Blackwell Scientific, Oxford.
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Kozumplik, and Christie, .1972. Dissemination of orchard-grass pollen. Can.j.plant Sci. 52: 997-1002.
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McCartney, H.A., and Bainbridge, A. 1984. Deposition gradients close to a point source. Phytopathol. Z. 109: 219-236.
McCartney, H.A. 1990. Dispersal mechanisms through the air. In: Dispersal in Agricultural Habitiats (eds R G H Bunce and D C Howarth ) pp133-158. Belhaven Press, London.
Proctor M and Yeo P. 1973. The Pollination of Flowers. Collins
Raynor GS, Hayes JV and Ogden EC. 1970. Experimental data on dispersal and deposition of Timothy grass and Corn pollen from known sources. Brookhaven National Laboratory Report. BNL, (New York) 50266 (T/595).
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Tauber, H. 1965. Differential Pollen Dispersion and the interterpretation of pollen diagrams. Dan.geol.unders. 2 rk 89. 69 pp
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