JXB Advance Access originally published online on August 8, 2003
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Journal of Experimental Botany, Vol. 54, No. 391, pp. 2307-2312,
October 1, 2003
© 2003 Oxford University Press
Rate of dehydration of corn (Zea mays L.) pollen in the air
Received 27 May 2003; Accepted 23 June 2003
Department of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station, PO Box 1106, New Haven, CT 06504, USA
* Fax: +1 203 974 8502. E-mail: Donald.Aylor{at}po.state.ct.us
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Abstract |
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The water content of corn (Zea mays L.) pollen directly affects its dispersal in the atmosphere through its effect on settling speed and viability. Therefore, the rate of water loss from pollen after being shed from the anther is an important component of a model to predict effective pollen transport distances in the atmosphere. The rate of water loss from corn pollen in air was determined using two methods: (1) by direct weighing of samples containing
5x104 grains, and (2) by microscopic measurement of the change in size of individual grains. The conductance of the pollen wall to water loss was derived from the time rate of change of pollen mass or pollen grain size. The two methods gave average conductance values of 0.026 and 0.027 cm s–1, respectively. In other experiments, the water potential, 
, of corn pollen was determined at various values of relative water content (dry weight basis), 
, either by using a thermocouple psychrometer or by allowing samples of pollen to come to vapour equilibrium with various saturated salt solutions. Non-linear regression analysis of the data yielded (MPa)= –3.218–1.35 (r2=0.94; for –298
–1 MPa). This result was incorporated into a model differential equation for the rate of water loss from pollen. The model agreed well (r2 0.98) with the observed time-course of the decrease of water content of pollen grains exposed to a range of temperature and humidity conditions. Key words: Longevity, settling speed, survival, viability, water content, water potential.
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Introduction |
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The aerial transport of genetically modified (GM) Zea mays pollen has been viewed by some sectors of the public as a potential risk of genetic contamination in managed non-GM organic and conventional production fields and as a potential hazard to non-target species (Losey et al., 1999). Strategies for regulating off-site gene flow to related wild and domesticated plants, estimating the potential off-target effects of pollen containing insect-toxic proteins, and the impact of off-site movement of GM pollen on the potential marketability of non-GM corn all require improved methods for predicting the risk of pollen moving off-site. A quantitative model of aerial dispersal of pollen can be a useful tool for helping to predict this potential risk by identifying and integrating critical biophysical parameters that affect pollen movement.
The relative water content of corn pollen plays an important role both in the viability and in the flight dynamics of pollen in the atmosphere. Corn pollen is known to be sensitive to dehydration (Jones and Newell, 1948; Barnabas, 1985; Buitink et al., 1996; Luna et al., 2001). Depending mainly on the vapour pressure deficit of the air, the water status of corn pollen can change from being fully hydrated to being nearly dehydrated in from 1–4 h. Several physical changes take place during drying: for example, the shape of corn pollen changes from a prolate spheroid to a crinkled, prismatic solid, its specific gravity increases by about 16%, and its settling speed decreases by about 34% (Aylor, 2002). These physical changes can have a large direct effect on potential transport distances. Furthermore, the viability of maize pollen is related importantly to its water content and to the drying conditions (vapour pressure deficit) of the atmosphere (Buitink et al., 1996; Luna et al., 2001). Thus, the relative water content of corn pollen, through its effects on settling speed and on survival, can play a major role in determining ‘effective’ aerial transport distances of corn pollen (Aylor, 1999). In this paper, data are presented that relate the relative water content of corn pollen to its water potential and to its change in size and shape. These relationships are used to derive a model for the dynamics of water loss from pollen under a variety of temperatures and relative humidity conditions. This is one important component of an aerobiological model to predict effective pollen transport distances, which is needed, for example, to assess gene flow from GM cornfields or for assessing purity in seed production fields.
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Materials and methods |
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Source of pollen
Fresh corn pollen was collected as needed from a commercially available hybrid corn variety (37M81, Pioneer Hi-Bred International, Des Moines, Iowa, USA), which was planted in the greenhouse every 3–4 d to maintain a ready supply of fresh pollen. The plants were grown in 4.0 l plastic pots in a 1:2:1 by vol. mixture of sand, Promix BXR, and PerliteR. Fresh pollen was collected by gently tapping the main stem of the plant while holding a large, flat tray covered with aluminium foil below the tassels (Kiesselbach, 1949). Pollen was used in the experiments described below starting about 5 min after its collection from the anthers.
Pollen water potential and relative water content
Throughout this paper, the relative water content of corn pollen is expressed as a fraction of dry weight, namely, =mw/md, where mw is the mass of water in a sample of corn pollen and md is the mass of the same sample of oven-dry pollen. The value of mw at a given time was determined by subtracting md from the pollen mass at that time. For the corn pollen used in these studies, ranged from 1.35 for hydrated corn pollen, fresh from the anther, to zero for oven-dry pollen. A relationship between water potential, (MPa), and relative water content, (dimensionless), for corn pollen was obtained using the two different methods described below.
Moisture equilibrium above saturated salt solutions: In the first method, samples of freshly collected pollen were initially weighed in aluminium weighing dishes within about 5 min of collection from the tassel. During the brief time between collection and the first weighing, the pollen samples were kept inside closed glass Petri dishes, which, in turn, were kept inside a plastic bag containing a moist paper towel. After the initial weighing, the samples were placed above saturated solutions (slurries) of various salts at room temperature (22–25 °C) inside small, closed glass chambers. The salts used and the nominal relative humidity at 23 °C for each of them were (Dhingra and Sinclair, 1985; List, 1966; Wexler and Hasegawa, 1954): LiCl (11%), MgCl2 (33%), Mg(NO3)2 (54%), NaCl (75%), KCl (85%), and KNO3 (94%). Temperature and relative humidity of the air at the location of the samples (<0.5 m) were recorded at 10–15 min intervals during the experiments.
Periodically, the pollen sample dishes were removed briefly (5 s) from the chambers, weighed, and returned to the chambers. When the change in weight of the samples was less than 0.5%, the sample dishes were sealed with ParafilmR and allowed to equilibrate for 12–14 h (usually overnight). The pollen samples were then reweighed to obtain the equilibrium mass, me, and then they were oven-dried at 53 °C until their weight changed by less than 0.05% to obtain the mass, md, of the dry pollen sample. Individual determinations were made using samples that contained 104–105 pollen grains. The equilibrium water content of pollen was expressed as a fraction of dry weight, i.e. e=me/md.
Moisture equilibrium using a thermocouple psychrometer: The second method was to determine the water potential of samples of pollen using a thermocouple psychrometer (comprised of a Model HR-33T Dew point microvolt meter and a Model C-52 sample chamber, Wescor, Inc., Logan, Utah, USA). Fresh samples were placed in the sample holders, preweighed, and then placed in the sample chamber where the water potential was determined. The samples were then dried in the oven and reweighed to determine the dry weight, wd (g), from which the value of was determined. This procedure was done for samples having a range of initial water contents.
Pollen germination and relative water content
Samples of corn pollen were weighed at regular intervals and their dry weight was determined, as described above. Periodically, subsamples of pollen were taken and placed in liquid germination medium (LGM), and allowed to germinate. A pollen grain was counted as germinated if the length of the germ tube was 
one diameter of the pollen grain (i.e. approximately 100 µm long). The LGM followed the recipe given by Walden (1994) except the amount of sucrose used was 222 g l–1.
Measured drying rates
Change in weight of samples containing many (104) grains: Corn pollen was distributed in a thin layer (averaging 1–2 grains thick) on the bottom of an aluminium weighing dish. Standard weighing dishes were cut down to reduce their height and weight. A small amount (50–100 mg) of fresh pollen was placed in the dish and distributed over the bottom by gently moving the dish from side to side (fresh pollen has a ready tendency to roll on this surface). The dish containing the pollen was immediately weighed and placed in small glass chambers (2 cmx9 cm diameter) with various saturated salts to maintain various RH values, already described above. RH values for the various determinations ranged from about 11–94%. Periodically, the samples were removed briefly from the chamber, weighed, and immediately returned to the chamber; this process took about 5–7 s each time.
Change in size of individual grains: The diameters of individual corn pollen grains were measured using a microscope at a magnification of 100x, at various times during their exposure to air streams having a range of temperatures and humidity. This was accomplished by placing the pollen grains inside a small, flow-through chamber, which was fixed to the stage of a compound microscope. The chamber (5.5 cm long x 2.0 cm wide x 0.8 cm high) was constructed using brass for the sides, copper tubing for the flow inlet and outlet, and glass for the bottom and top. The top was a 2.5x5.0 cm glass cover slip and the bottom was a 5.0x8.0 cm glass microscope slide. Freshly collected pollen grains were placed on the slide (chamber bottom) and the top of the chamber was fixed to the slide using a thin coating (on the edge of the chamber) of vacuum grease (Dow Corning). The rate of flow through the chamber (180 cm3 min–1) gave an average air flow speed in the cross-section of 2 cm s–1, resulting in an exchange of the air in the chamber about once every 3 s.
The inlet and outlet of the chamber were connected to a flow system using tygon tubing. The flow system comprised a supply of pressure-regulated, dry air, a humidifying chamber, and a splitter valve. A portion of the air flow was moistened by passing it through the humidity chamber and then mixed with the dry air stream to achieve various values of humidity in the air stream passing through the chamber. The dew point of the stream of air exiting the chamber was measured using a thermoelectric (chilled mirror) dew point hygrometer (Model 880-C1, EG&G, Waltham, Massachusetts, USA). The temperature of the air stream was measured just downstream of the dew point hygrometer using a thermocouple. Temperature and relative humidity of the laboratory air were monitored using a probe (Model CS-500, and 21X data logger, Campbell Scientific, Inc., Logan, Utah, USA) and, occasionally, by using a force-ventilated, wet- and dry-bulb psychrometer.
For each determination, the flow conditions in the chamber were first equilibrated, and then the bottom slide of the chamber was replaced by a fresh, clean slide onto which a small amount of fresh pollen was added. Pollen densities on the surface were 300–420 grains cm–2, which gave separation distances between the grains of 4–10 diameters. The change in size and shape of eight pollen grains were observed for each determination, and the experiment was repeated 16 times (128 grains) for vapour pressure deficits (VPD) ranging from 0.4–2.4 kPa.
The principal diameters of the grains were measured microscopically at frequent intervals and their dimensions and shapes were recorded. The shape of a fully hydrated pollen grain was approximated by a prolate spheroid (Aylor, 2002) with major and minor principal diameters, L1 and L2. The ratio L1/L2 for the corn pollen tested here was between 1.1 for freshly collected pollen and 1.2 for partially dehydrated pollen. For this small degree of eccentricity, it is reasonable to replace L1 and L2 with a volume-equivalent diameter, De=(L1L22)1/3.
Calculated drying rates
In the model described below it is assumed that the driving force for loss of water from a pollen grain is the vapour pressure difference between an evaporating surface within the pollen wall and the ambient air outside the grain. Pollen grains have two distinct wall layers. The outer layer (exine) is composed mainly of a complex biopolymer, sporopollenin, and the inner layer (intine) is composed mainly of pectin and cellulose (Lewis et al., 1983). Corn pollen has a single pore (aperture) through which the pollen germ tube emerges and grows. Prior to the start of the germination process, this pore is mainly covered (closed) with wall material (operculum). Detailed knowledge of the combined hydraulic and diffusive pathway for water movement through the plasma membrane and wall layers of the pollen grain is not presently available. Therefore, the conductance of the plasma membrane, the inner and outer wall layers, and the aperture area have been conceptually combined into a single conductance, which is referred to from now on as the conductance of the pollen wall.
In this model, the instantaneous rate of water loss (kg s–1) from a pollen grain can be expressed as (Bird et al., 1960; Monteith and Unsworth, 1990):
where dmw/dt (kg s–1) is the change in mass of the water in the pollen grain occurring in the small time interval dt, Ap (m2) is the surface area of the pollen grain, gv (cm s–1) is the overall conductance for water loss of the pollen wall and of the surrounding boundary layer of air, and Cp and Ca (g cm–3) are the concentrations of water vapour at the evaporating surface of the pollen grain and in the surrounding air (outside the boundary layer), respectively. The thermodynamic gas law gives a relationship between concentration and vapour pressure, i.e.:
Expressing mw in terms of (=mw/md) and substituting for C in equation 1 yields:
where Mw (0.018 kg mol–1) is the molecular weight of water, R (8.3143 J mol–1 °K–1) is the universal gas constant, T (°K) is temperature, and pp and pa (Pa=J m–3) are the partial pressures of water vapour at the evaporating surface of the pollen grain and in the outside air, respectively, and RH is the relative humidity in the surrounding air. In writing equation 3, it has been assumed that the temperature of the pollen grain is at the temperature of the air. This is a reasonable assumption because the resistance to water loss of the pollen integument is relatively large, so that the rate of latent heat loss from the grain due to evaporation is small compared to the rate of sensible heat gain by the pollen grain (to be justified ex post facto, see Discussion). In addition, the effect of direct absorption of short- and long-wave radiation has been neglected.
In order to solve equation 3, a relationship for the vapour pressure of water both inside the pollen grain, pvp, and in the ambient air, pva, is required. For these, take:
where pv,sat is the saturation vapour pressure of water at temperature, T, and where hr() is the ‘effective’ relative humidity inside the wall of the pollen grain. An expression for hr can be written as (Campbell and Norman, 1998):
where (MPa) is the net water potential at the evaporating surface and Vw (1.8x10–5 m3 mol–1) is the molar volume of water. In the second equality in equation 5, the following functional form has been substituted for (Campbell and Norman, 1998):
= –a–b(6)
The coefficients a and b in equation 6 were determined by non-linear regression using the data obtained by the two methods for determining water potential described above. With the assumption that Tp=Ta (justified because gw<<gbl, see next section), then:
Equation 7 is non-linear and was solved numerically using a fourth order Runge–Kutta scheme, subject to the initial condition (t=0)=0. Equation 7 indicates that pollen loses water only as long as hr >RH/100. Note that the equilibrium water content of a pollen grain at a given temperature and relative humidity is given by equations 5 and 6.
Conductance of the wall: Over a limited range of near full hydration, it is reasonable to assume that gv, Ap, and hr are all approximately constant (i.e. do not depend on ). In this case, equation 7 indicates that decreases approximately linearly with time for a limited range of water content, near full hydration of the pollen grain. Estimated values of gv were obtained from equation 7 by expressing the change in mass of water in the grain in finite difference form, i.e. as md 
/t, where the change in mass was obtained either from the change in mass of a small sample of pollen (method 1) or from the change in size of individual grains (method 2), as described above.
The conductance, gv (cm s–1), in equation 7 is the combined conductance for water loss of the wall, gw, and the surrounding boundary layer of air, gbl, and is given by (Monteith and Unsworth, 1990):
The boundary layer conductance for a sphere can be expressed as:
gbl=ShD/d(9)
where Sh is the Sherwood number, which is given in terms of the Reynolds number (Re) and the Schmidt number (Sc) (Bird et al., 1960; Monteith and Unsworth, 1990) as:
SH=2+0.6Re1/2Sc1/3(10)
In the pure diffusion limit (Re=0, which is the worst-case scenario for water loss), equation 10 gives that Sh=2, so that gv=2x0.245/0.01=49 cm s–1. For a corn pollen grain settling in still air at its terminal velocity (Aylor, 2002), Re 1.6, Sh 2.6, and the conductance of the boundary layer is even larger. Therefore, in the present case gw<<gbl, so it is justified to take gv=gw.
To solve equation 7 for a wide range of water contents, it is necessary to account for a decrease in the size of a pollen grain as it loses water and to express the surface area of a pollen grain in terms of its water content, i.e. Ap(). Initially, a dehydrating pollen grain acts like a fully inflated balloon, in that its volume and surface area decrease in a commensurate way with a loss of water. However, below some value , a pollen grain begins to act more like a deflated basketball (i.e. having a ‘stiff’ wall) and the surface begins to change its confirmation (indent and crinkle), rather than actually decrease in surface area (Fig. 1). To help account for this, it was assumed that the ‘effective’ area decreased with as:
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Ap(
)=0.5
De2()[1+/0](11)
where De is the volume-equivalent diameter of a pollen grain (defined above), and is a function of time through , and where 0 is the relative water content of a fully hydrated pollen grain. Equation 11 was arrived at by a heuristic argument, and is intended to compensate for the effective decrease in surface area as the grain crinkles and flattens disc-like onto the experimental surface. Over the course of dehydration, equation 11 gives that AP of a pollen grain decreases in value from the surface area of the volume-equivalent sphere to one-half that value as approaches 0.
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Results |
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Water potential and water content
The results from both methods were combined in a single graph to determine the water potential,
(MPa), as a function of relative water content, (Fig. 2). These data were fitted using non-linear regression to obtain:
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= –3.218–1.35; r2=0.94 (12)
Pollen germination and relative water content
Germination remained high for values of >0.3, but decreased rapidly for lower values of (Fig. 3). The data were described well by a cumulative Weibull distribution given by G=65 (1–exp(–(–0.05)/0.2887)1.915) determined by non-linear regression (n=112, r2=0.89). This equation gives G/Gmax >0.9 for >0.5, and G/Gmax=0.5, 0.25, and 0 at =0.3, 0.2, and 0.05, respectively.
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Conductance values
Values for the wall conductance, gw, of corn pollen were determined by direct weighing and by measuring changes in size of individual grains. The mean value of gw determined by direct weighing was 0.0258±0.0012 (sem) cm s–1 (0.0245 median, range 0.0171–0.0445, n=27), while the mean value of gw determined from size changes of individual grains was 0.0269±0.0024 (sem) cm s–1 (0.0215 median, range 0.0015–0.1338, n=128). The two means were not significantly different by a t-test (P=0.59) or by a Mann–Whitney U test (P=0.08). Values of conductance determined by either method were not significantly correlated [P=0.36 (weighing) and P=0.85 (sizing individuals)] with vapour pressure deficit values over the experimental range from 0.4–2.4 kPa.
Loss of water versus time
The change in relative water content of pollen exposed to a range of conditions was described well by the model embodied in equations 5, 7, 11, and 12 (Fig. 4). The wall conductance, gw, was set equal to 0.0263 cm s–1 (the average of the values determined by the two methods) for all calculations. The model curves level out as they approach the equilibrium values of water content that are obtained by taking the inverse of equation 5 and using the relationship for given by equation 12. Equilibrium water contents (at 23.5 °C) for RH=20, 33, 54, and 75% were about 4.4, 5.8, 8.9, and 15.7%, respectively.
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Discussion |
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The rate of water loss from fresh, whole pollen grains was determined. This was described by a phenomenological first-order model that assumes that the rate of water flow from a pollen grain is determined by the vapour pressure difference between an evaporating surface somewhere within the wall (the exact location is not known) and an overall conductance through the plasma membrane and wall layers. The physical details of the hydraulic and diffusive pathways for water movement through the plasma membrane and wall layers of the pollen grain are not presently known. Part of the pathway for water transport may be via the liquid phase and part may be via the vapour phase. No distinction is made here in this regard, however, it is reasonable to assume that the liquid–vapour interface lies somewhere within the wall, rather than at the outside surface, because the measured conductance values of the pollen grain (0.026–0.027 cm s–1) are much smaller than the conductance of boundary layer of air surrounding the grain. In addition, judging by the rapid rate at which pollen grains rehydrate when they come into contact with liquid water, it does not appear that the hydraulic conductivity of the pollen wall is limiting water loss from the grain (assuming that liquid water moves just as readily in either direction across the wall). It was not possible, with the experiments presented here, to separate the wall conductance into its component parts in the different layers.
The conductance of the pollen wall found here (2.6x10–4 m s–1) is one to two orders of magnitude greater than the conductance values typical of leaf cuticles (Riederer and Schreiber, 2001). This suggests that the biopolymer covering pollen grains is very different from cuticles with respect to their ability to act as barriers for water diffusion across the cell boundaries. Recent experiments comparing the rate of water loss from isolated pine pollen exines and from intact pine pollen suggest that the exine may not be the main factor in limiting water loss (Bohne et al., 2001). In the future it may be possible to build a more complete physical model that describes the wall conductance in terms of its component parts.
The rate of loss of water vapour from corn pollen is controlled by an effective conductance of the wall (including the aperture area), the temperature and humidity of the surrounding air, and the surface area of the pollen grain (equation 7). The conductance of the boundary layer of air outside the pollen grain for water vapour is much greater than gw and can be safely neglected. The determination of the conductance of the pollen wall, gw (cm s–1), was made for 1.0 < 1.4. For this range of water contents, the effect of water potential, , on reducing water vapour pressure inside the wall is <4% (Fig. 2). Furthermore, for these values of , changes in the surface area of a pollen grain can be described by congruent changes in a spheroid as the pollen grain loses mass with a commensurate loss in volume.
It is noteworthy that the two methods of measuring the conductance for water loss from corn pollen gave essentially the same mean values (0.026 and 0.027 cm s–1). The range of values was greater for the determinations made on individual pollen grains (0.0015–0.1338) compared to (0.0171–0.0445) for the weight determinations. There are at least two reasons for this difference. Firstly, a population of 5x104 grains is expected to average out the effect of any outliers, which obviously can have a large impact for determinations made on individual grains. Secondly, the size determinations made using a microscope can have an undetermined error for those grains that change their dimensions disproportionately in the vertical direction (i.e. into the plane). All things considered, there was remarkable agreement between the two methods.
It was assumed that gw was not a function of , and that changes in the product gwAp were ascribed entirely to the surface area term. This may not be true for the entire range of for dehydrating pollen. In the present experiments, it was not possible to separate the two effects. Nevertheless, it is perfectly legitimate to combine the two parameters into a single parameter 
=gwAp and use equation 11; then the predictive power of the model is unabated. The form of and the coefficient of 0.5 in equation 11 were not determined from the data, but rather, were arrived at by a heuristic argument to account for the effective loss in exposed surface area as a pollen grain deflates ‘pillow-like’ onto the substrate. It bears emphasis that this relationship was used throughout, and was not adjusted for specific cases.
As the pollen grains dry to values of <0.5, decreases rapidly and approaches –250 MPa at =0.04. For normal temperatures and relative humidity encountered in temperate climates, will tend to limit pollen water contents above 9%. However, germination has already dropped to a low level for this value of . Values of 4.5% outdoors are expected only under hot, dry conditions.
To apply the present model to free-drifting pollen, it is only necessary to replace the Sherwood number Sh=2 with the full equation 10. This will have little effect on predicted evaporation rates because gbl remains <<gw. Pollen grains may temporarily come to rest on a corn leaf before being entrained, with the potential to be carried onto the silks. In this case, it is the relative humidity immediately next to the leaf surface, not the ambient RH, that should be used in equation 7. The former is normally expected to be higher than ambient (Campbell and Norman, 1998), and water loss from the pollen is expected to be slower. In principle, it is not difficult to account for this effect using the model presented here.
The model presented gives a good estimate for the time-course of dehydration over a wide range of environmental conditions encountered in the atmosphere. For example, if Tair=23 °C and RH=50%, then it can be expected that >0.5 for 60 min and >0.2 for 120 min. An important application of the model (Fig. 4) is to predict the length of time it would take for to reach some critical value, crit, under a given set of environmental conditions where, for example, crit might be related to pollen viability (Fig. 3). This would allow pollen survival times to be estimated as a function of time after being shed from the anther and thus would allow ‘effective’ dispersal distances to be estimated.
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Acknowledgements |
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I thank P Thiel for excellent technical assistance. This material is based upon work supported in part by Hatch Funds and by the Cooperative State Research, Education, and Extension Service, US Department of Agriculture, under Agreement No. 2001-52106-11528.
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References |
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Aylor DE. 1999. Biophysical scaling and the passive dispersal of fungus spores: Relationship to integrated pest management strategies. Agricultural and Forest Meteorology 97, 275–292.[CrossRef]
Aylor DE. 2002. Settling speed of corn (Zea mays) pollen. Journal of Aerosol Science 33, 1599–1605.
Barnabas B. 1985. Effect of water loss on germination ability of maize (Zea mays L.) pollen. Annals of Botany 55, 201–204.
Bird RB, Stewart WE, Lightfoot EN. 1960. Transport phenomena. New York: John Wiley and Sons.
Bohne G, Woehlecke H, Richter E, Wegener AK, Lerche D, Ehwald R. 2001. The barrier function of pollen exines for water, low molecular-weight solutes and polymers. Proceedings of the 9th international cell wall meeting, Toulouse, France.
Buitink J, Walters-Vertucci C, Hoekstra FA, Leprince O. 1996. Calorimetric properties of dehydrating pollen: analysis of a desiccation-tolerant and intolerant species. Plant Physiology 111, 235–242.[Abstract]
Campbell GS, Norman JM. 1998. An introduction to environmental biophysics. New York: Springer-Verlag.
Dhingra OD, Sinclair JB. 1985. Basic plant pathology methods. Boca Raton, FL: CRC Press.
Jones MD, Newell LC. 1948. Longevity of pollen and stigmas of grasses: buffalo-grass, Buchloe dactyloides (Nutt.) Englem. and corn, Zea mays L. Agronomy Journal 40, 195–204.
Kiesselbach TA. 1949. The structure and reproduction of corn. Lincoln, NE: University of Nebraska Press.
Lewis WH, Vinay P, Zenger VE. 1983. Airborne and allergenic pollen of North America. Baltimore, MD: Johns Hopkins University Press.
List RJ. 1966. Smithsonian meteorological tables, 6th edn. Smithsonian Miscellaneous Collections, Vol. 114, Publication 4014. Washington, DC: Smithsonian Institution.
Losey JE, Rayor LS, Carter ME. 1999. Transgenic pollen harms monarch larvae. Nature 399, 214.[Medline]
Luna VS, Figueroa MJ, Baltazar MB, Gomez LR, Townsend R, Schoper JB. 2001. Maize pollen longevity and distance isolation requirements for effective pollen control. Crop Science 41, 1551–1557.
Monteith JJ, Unsworth MH. 1990. Principles of environmental physics. London: Edward Arnold.
Riederer M, Schreiber L. 2001. Protecting against water loss: analysis of the barrier properties of plant cuticles. Journal of Experimental Botany 52, 1023–2032.
Walden DB. 1994. In vitro pollen germination. In: Freeling M, Walbot V, eds. The maize handbook. New York: Springer-Verlag, 733–734.
Wexler A, Hasegawa S. 1954. Relative humidity-temperature relationships of some saturated salt solutions in the temperature range 0–50 °C. Journal of Research of the National Bureau of Standards 53, 19–25.
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