© 2003 Plant Management Network.
Tassel Morphology as an Indicator of Potential Pollen Production in Maize
Agustin E. Fonseca, and Mark E. Westgate, Department of Agronomy, Iowa State University, 1563 Agronomy Hall, Ames 50011; and Lahcen Grass, and David L. Dornbos, Jr., Syngenta Seeds Inc., Washington, IA 52353
Corresponding author: Mark E. Westgate. email@example.com
Fonseca, A. E., Westgate, M. E., Grass, L., and Dornbos, D. L., Jr. 2003. Tassel morphology as an indicator of potential pollen production in maize. Online. Crop Management doi:10.1094/CM-2003-0804-01-RS.
Adequate pollen production is an essential prerequisite for achieving high yields in commercial corn (Zea mays L.) production and for insuring high levels of genetic purity in the production of hybrid seed. Documenting the timing and intensity of pollen shed are fundamental to these goals, but methods to describe patterns of pollen release from maize tassels are limited and laborious. Our objective was to explore characteristics of tassel morphology that could be used as simple and indirect measures of pollen production per plant under field conditions. The progress of tassel development was documented using a nine-stage scale based on easily-quantified morphological characteristics. Genetic variation among hybrids and inbreds as well as environmental variation across planting densities and years was correlated with levels of pollen production. This analysis revealed that a change in tassel dry weight during pollen shed was not an accurate measure of pollen production per tassel. Likewise, no single morphological characterization captured all the genetic and environmental variation in pollen production per tassel. But a combination of morphological traits incorporated into a Tassel Area Index (TAI) accounted for up to 89% of the variation in pollen production among hybrids in response to population density, and 64% of the variation in pollen production among inbred heterotic groups. Because data collection is simple, quick, and non-destructive, the Tassel Area Index approach is well-suited for distinguishing genetic variation in pollen production and relative responses to treatments under field conditions. The accuracy of the technique could be increased, if necessary, by incorporating additional information about flower density or pollen production per anther. But this would entail a much greater investment of time and resources.
Pollination in maize (Zea mays L.) can occur only if pollen shed by the tassel is captured by the stigmas (silks) on the ear. Since the introduction of hybrid seed production, much effort has been directed towards managing this process to ensure maximum kernel set and high levels of genetic purity. Current practices include crop rotations, high-purity parent seed, mechanical and hand detasseling of the female parent, temporal or spatial isolation from corn in nearby fields, and use of male border rows around the field (3). These strategies are laborious and not always successful. Managing pollen dispersion also is an important consideration in seed production, particularly for managing genetically-modified (GMO) materials, due to the potential for flow of transgenes into landraces, wild relatives of maize, and non-GMO commercial hybrids (12). Managing for maximum kernel set and high levels of genetic purity requires greater knowledge of the factors that affect pollen production and dispersion coupled with simple methods to quantify these processes.
Environmental conditions can affect pollen availability by modifying the synchrony between pollen shedding and silk emergence, by affecting how long pollen remains viable, or by changing the amount of pollen produced per tassel (2,9). Distinguishing between these possibilities has important implications for germplasm selection as well as designing seed production practices to ensure genetic purity. Attempts to describe the quantitative patterns of pollen release from maize tassels are limited, generally because collecting and counting pollen are laborious. Flottum et al. (5) determined the intensity of pollen shed by collecting samples on spinning rods and counting adhering pollen visually. Sadras et al. (14) collected the pollen shed daily within tassel bags, and counted the pollen with a light microscope. Bassetti and Westgate (1) monitored pollen shed intensity with passive pollen traps, and counted the pollen grains by computer-aided video imaging. More recently, Fonseca et al. (6) used passive traps to collect pollen in the field and capitalized on its capacity to fluoresce to quantify pollen shed intensity. Although simple and accurate, this latter technique can be time consuming.
There is considerable information on tassel development and pollen shed, but it has not been used to predict pollen production per tassel. Pollen shed from an individual tassel usually begins after the tassel is fully expanded, beginning on the central rachis close to the apex and continuing in apical and basal directions (11). Branches start to shed one or two days after the beginning of the central rachis and follow the same pattern. An individual tassel may shed pollen for 2 to 10 days, depending on genotype and environmental conditions. Daily pollen release will depend to some extent on moisture and temperature conditions, but it will generally last about 4 to 5 hours, starting one hour after sunrise (5; Westgate unpublished data). Corn pollen is 70 to 100 µm in diameter and spherical in shape (10,17; Fonseca unpublished data) and among the largest particles that are commonly airborne (13). The dry weight of individual pollen grains has been estimated at 250 ng (8). Total pollen production per plant can vary considerably. Reported values of 20 to 42.2 million grains for old cultivars (9,14) contrasts with more recent values of 9.6 to 11.3 million grains (15) and 2.2 to 3.3 million grains (6) observed for modern hybrids. This decrease in pollen production apparently reflects the smaller tassel size of today’s maize hybrids (4,7). This trend is also true for male inbreds, and the higher incidence of outcrosses in modern hybrid seed production may be associated with reduced pollen production of modern male parents. The average pollen yield for male inbreds ranges from 200,000 to more than 3 million pollen grains per tassel (Grass, unpublished data). The general relationship between tassel size and pollen production suggests that a quantitative estimate of pollen production can be obtained from tassel morphological characteristics. To our knowledge, however, no satisfactory method has been established to provide this estimate.
Westgate et al. (16) developed an indirect method of predicting pollen shed in the field from male flowering dynamics. They calculated a "population index" for daily pollen shed considering the progression of pollen shedding in a population of tassels, from the initiation of shed through maximum shedding to the end of shedding. This index, coupled with an estimate of average pollen production per tassel, predicted pollen shed dynamics very accurately (16). While this technique provides a link between tassel development and pollen shed, it requires an accurate estimate of pollen production per tassel.
Therefore, our objective was to explore characteristics of tassel morphology that could be used as indirect measures of pollen production under field conditions. If successful, this approach would have immediate utility as a rapid, simple, and inexpensive way to characterize inbred "maleness," provide a basis for inbred selection, define optimum male/female planting patterns, and estimate pollen shed in models to predict kernel set and pollen dispersal.
Experiments to Identify Predictive Morphology
A series of related experiments was conducted to identify simple measures of tassel morphology and development that might be suitable for predicting pollen production per tassel. The logic of these experiments progressed from an evaluation of the "dry weight difference" method, to the development of a morphometric measure of pollen production, and an evaluation of this approach for predicting pollen production from commercial inbred lines.
Experiment 1. An inbred (CM105), a grain hybrid (Pioneer 3893) and a sweet corn hybrid (‘Delectable’) were planted in the field at Swan Lake, Minnesota, on 13 May 1997 in 76-cm rows in four-row plots (3.05 × 12 m). Three population densities were tested: 2.5, 7.5, and 12.5 plants per m2. The statistical design was a split-plot, with population as main plots and genotypes as subplots, replicated four times. Plots were fertilized with 150 kg N per ha as anhydrous ammonia prior to planting.
The progress of tassel development was documented using a nine-stage scale, based on easily-identifiable morphological characteristics. Key elements of the scale are shown in Fig. 1. Tassels are at Stage 1 when the tip of the main branch is visible. At Stage 2, tassel is expanding rapidly and the peduncle at the base of the tassel is visible. Stage 3 indicates when the tassel is fully expanded and has entirely emerged from the whorl. Stage 4 is the beginning of pollen shed. The first anthers have begun to shed pollen, which generally occurs in the center of the main tassel branch. Stages 5, 6, and 7 document the progress of pollen shedding on the lateral branches: Stage 5, anthers on one branch shedding; Stage 6, half of the branches are shedding pollen; Stage 7, all branches are involved in pollen shedding. At Stage 8, pollen shed from all anthers is complete. No new anthers are exposed. Stage 9 occurs 7 days after pollen shed is complete and indicates when most of the anthers and other floral structures have been shed from the tassel. Stages 4 through 8 are of particular interest in this study since they bracket the period in which pollen shed occurs. Presumably, the change in weight during this time reflects pollen liberated from the tassel. We defined Tassel Weight Loss (TWL) as the difference in tassel weight between Stage 4 and Stage 8.
Five plants per plot were tagged and observed daily to determine the progress of tassel development. Five additional tassels per plot were removed and weighed at each of the Stages 1 through 8. The amount of pollen produced per tassel was quantified by covering 5 tassels per plot with tassel bags at Stage 3, i.e., immediately before they started to shed pollen. Bags were sealed at the base with expanding foam to prevent pollen loss, and left in place until pollen shed was complete. After pollen shed, bagged tassels were excised from the plant and taken to the laboratory for dissection. Bags were carefully removed, anthers and debris removed by hand, and all the shed pollen was collected, dried at 60°C, and weighed. Main stem length, number of branches, main stem weight, and branch weight also were recorded after drying at 60°C.
Experiment 2. Two hybrids, Dekalb 611 and Holdens LH198 × LH185, were planted in the field at the Bruner Research Farm near Ames, Iowa, on 8 May 2001 in 76-cm rows in sixteen-row plots (6.10 × 15 m) at three population densities (1, 8 and 18 plants per m2). The statistical design was a split-plot with population as main plots and genotypes as subplots, replicated three times. Plots were fertilized with 168 kg N per ha as anhydrous ammonia prior to planting.
Five representative tassels per plot were selected for sampling when 40% of the population had begun to shed pollen. Pollen was collected daily in clear bags (Pantek, Montesson, France) designed to exclude moisture but allow gas exchange around the tassel. Prior to pollen shed and continuing for 4 to 7 days, bags were placed over the tassels, wrapped tightly around the peduncle to prevent pollen loss, and locked in place with a clip. Bags were replaced at about 1900 hours each day and carefully removed to avoid losing pollen or damaging the tassel. Collected bags were slowly compressed to exclude air and carried to the lab for pollen collection. Bags were allowed to air-dry prior to processing. Pollen was harvested from the bags by washing with approximately 30 ml of Isotone II solution (Coulter Corporation, Florida, USA). The solution containing pollen was filtered through a stainless steel mesh to remove anthers and large debris, brought to 60 ml, and stored at room temperature.
The number of pollen grains per ml in triplicate 0.5-ml aliquots were quantified using a Coulter Multisizer II (Coulter Electronics Limited, Luton, Beds, England), which was calibrated to detect particles between 60 and 100 µm in diameter. Pollen grains per tassel were calculated from the total sample volume used to wash the pollen collection bags. The remaining solution was dried and weighed to estimate individual pollen grain weight. The weight added by solutes in the Isotone II solute was insignificant. In contrast to Experiment (Exp.) 1, tassel morphology was measured non-destructively on the same plants from which pollen was collected and quantified. Main stem length, effective main stem length, main stem diameter, number of branches, total branch length, and effective branch length were recorded when tassels reached maximum pollen shed (Stage 6) . These measurements were taken prior to replacing the clear collection bags. Effective main stem length and effective branch length were defined as the length in which anthers were exposed. Main stem length was measured from the insertion of the first branch on the stem to the tip of the main tassel branch (Fig. 1). Total branch length was calculated as the sum of all branch lengths. Main stem diameter represented the minimum diameter measured immediately below the insertion of the first branch.
Tassel Area Index (TAI) was calculated to integrate these parameters as
When pollen shed was complete (Stage 8), tassels were cut just below the insertion of the first branch and transported to the lab. Main stem length, main stem diameter, number of branches, and total branch length were measured immediately to minimize changes due to desiccation. Total dry weight was recorded after tassels were dried at 60°C for 24 h.
Experiment 3. Hybrids Dekalb 611 and Holdens LH198 × LH185 were planted in the field at the Bruner Research farm near Ames, Iowa, on 6 May 2002 in 76-cm rows in 20-row plots (15.2 × 15 m) at three population densities (1, 4, and 8 plants per m2) and replicated three times. Crop management was as described for Exp. 2. Pollen production per tassel was quantified as in Exp. 2, except that the collection bags were replaced every third day instead of on a daily basis. Main stem length, total branch length, and main stem diameter were recorded at Stage 6 and TAI was calculated from Stage 6 morphology data.
Experiment 4. Pollen production and tassel morphology of 22 inbreds from Syngenta Seeds Inc. were characterized in 2002. The inbred lines were planted in a randomized complete block design, replicated three times at Washington, IA on 23 May 2002 in 4-row plots (6.1 × 3 m) at a population of 8.6 plants per m2. Pollen production per tassel was quantified as in Exp. 2 using clear tassel bags. Main stem length, number of branches, total branch length, and main stem diameter were measured at maximum pollen shed (Stage 6), and TAI was calculated from these values. Additionally, tassel weights at the beginning (Stage 4) and end of pollen shed (Stage 8) were recorded on 5 representative tassels in each plant population. All observations were recorded on plants in the two center rows of each plot.
Change in Tassel Weight as Measure of Pollen Production
Tassel morphology changes dramatically as it emerges from the whorl, sheds pollen, and senesces. Thus, attempts to relate tassel morphology with pollen production must consider this phenology and document the stage at which tassels are sampled. A simple visual scale was developed for that purpose (Fig. 1). Data from the three genotypes evaluated in Exp. 1 showed that tassel weight was increasing as tassels emerged from the whorl, reached their maximum as they began to shed pollen, decreased as pollen was shed, and continued to do so after pollen shed was complete (Fig. 2). The fact that tassel weight continued to decrease after pollen shed was completed indicates that not all of the weight loss could be attributed to pollen shed. Other floral structures such as anthers, palea, lemma, and glumes also are shed from the tassel and can account for a significant portion of the weight loss. While this general pattern is typical of all hybrids and most inbreds we have measured, there may be exceptions. In an unrelated study, tassels of one inbred continued to increase in weight during the early stages of pollen shed (Syngenta, unpublished data). This observation underscores the difficulty in relating changes in tassel weight directly to pollen shed.
In Exp. 1, Total Weight Loss values for plants grown at the commercial density of 7.5 plants per m2 were 0.5, 1.18, and 6.26 g for the inbred, grain hybrid, and sweet corn hybrid, respectively (Fig. 3). The accuracy of this approach for estimating pollen shed was tested by comparing TWL values with the weight of pollen collected from individual tassels of these three genotypes grown at three population densities. On average, the inbred, grain hybrid, and sweet corn hybrid grown at 7.5 plants per m2 produced 0.44, 0.99, and 2.54 g of pollen. Pollen shed accounted for 89% of the weight loss in the inbred, 84% in the hybrid, and 41% in the sweet corn hybrid (Fig. 3). These results clearly demonstrated that TWL was not an accurate way of comparing genotype maleness, even when weight loss was measured only during pollen shed (Stage 4 to 8). For this particular inbred (CM105), we overestimated pollen production per tassel by about 11%, or 200,000 pollen grains per tassel (assuming an average weight of 250 ng per grain). This overestimation might not be significant in terms of seed production for a male that produces 3 million pollen grains per tassel, but it is certainly a concern for inbreds considered "poor males" because they produce much less pollen. Similarly, TWL overestimated pollen shed by 16% for the grain hybrid and 59% for the sweet corn hybrid, corresponding to 633,600 and almost 6,000,000 pollen grains per tassel, respectively. The error in estimating pollen production increased with the size of the tassel, which probably reflects a greater proportion of anthers and other floral structures contributing to the weight loss during pollen shed.
Comparing these three genotypes at extreme population densities produced similar results (Fig. 3). The sweet corn hybrid showed similar TWL values of 6.16 and 6.26 g when grown at 2.5 and 7.5 plants per m2, but pollen production decreased from 3.74 to 2.54 g (about 15,000,000 to 10,160,000 pollen grains). The fact that contrasting amounts of pollen production were obtained with a similar TWL is additional evidence that the "weight loss" approach is imprecise at best. The overestimation of pollen production using TWL increased as plant population density increased. This could be due to an increase in the number of sterile anthers per tassel and/or a decrease in the number of pollen grains per anther with increasing plant population densities. No data were collected to distinguish between these possibilities.
Analysis of 22 inbred lines used in commercial seed production led to the same conclusion (Fig. 4). In this case, however, the actual number of pollen grains shed per tassel was measured with a flow-cytometer. The expected change in tassel weight due to pollen loss was calculated assuming 250 ng per pollen grain and compared to the measured TWL. In every case, TWL overestimated measured pollen production. And more importantly, there was large variation in TWL that could not be accounted for by variation in pollen weight loss.
In this analysis, tassels were sampled at well-defined stages of development to ensure TWL was measured from the beginning to the end of pollen shed. It is likely that even greater uncertainty would have resulted if TWL were measured using tassels collected some time after pollen shed has ended since genotypes obviously differ in weight loss during pollen shed and after pollen shed is complete (Stage 8 to 9, Fig. 2).
The inaccuracy of the TWL method for estimating pollen production prompted us to search for a more accurate measurement system. We attempted to relate total pollen shed (by weight) with individual components of tassel morphology. The genotypes from Exp. 1 were examined for main stem length, number of branches, main stem weight, branch weight and total tassel weight (Table 1). These morphological characteristics failed to explain more than 56% of the variation in pollen production per tassel. Furthermore, the correlations between these characteristics and pollen production per tassel were not consistent across genotypes. Leaving tassel bags in place throughout pollen shed might have affected pollen production by modifying air temperature and moisture content around the tassel. Presumably, the effect of an altered environment around the tassel during pollen shed would depend on tassel size, which varied dramatically by genotype in Exp. 1, and by population density (Fig. 3). The numbers of grains per tassel estimated for the inbred (1,300,000 to 1,760,000) and the hybrid (2,440,000 to 4,920,000), however, were similar to values obtained using clear bags (Exp. 2 and 3) and for other genotypes using passive pollen traps (6,16). We do not have prior experience with sweet corn hybrids, but our estimates of pollen production per tassel of 8,200,000 to 14,960,000 pollen grains are consistent with their much larger size.
Table 1. Linear correlations between pollen production and various tassel morphological characteristics for three contrasting genotypes grown at 2.5, 7.5, and 12.5 plants per m2. Morphological characteristics were measured on five representative plants per plot at the end of pollen shed. Pollen production is the average per tassel collected on the same plants. N = 12 plots.
* Indicates significance at P = 0.10
** Indicates significance at P = 0.05
ns, nonsignificant at the 0.10 level of probability
Direct comparison between genotypes, environments or individual plants using the TWL approach assumes that weight per pollen grain is uniform and remains fairly constant. To examine this issue, we measured the average weight of pollen grains shed by the two hybrids examined in Exp. 2, in which pollen production per tassel varied from 28,000 to 1,900,000 grains per day. Figure 5 shows pollen grain weight typically varied between 250 and 350 ng with no obvious differences between hybrids or among population density treatments. Greater variability in weight per grain at low levels of pollen production probably reflects the greater measurement error for these small pollen samples. While this analysis confirms that the average weight of pollen was fairly stable across a wide range of population densities for these two hybrids, measurement errors alone could mask the relationship between tassel morphology and pollen production, if the amount of pollen production were determined only by a change in tassel weight.
Tassel Morphological Characteristics Related to Pollen Production
In Exp. 2, we related the number of pollen grains shed per tassel to the morphological characteristics of the same tassel. We minimized the effects on the tassel environment by using clear bags that allowed for gas exchange during pollen collection. In this case, we adopted a simplified development scale to define when tassel morphology was measured relative to the beginning, maximum and end of pollen shed. These stages correspond to Stages 4, 6, and 8 in Fig. 1. Of the morphological variables examined, main stem length and main stem diameter at Stage 6 were closely correlated with pollen production, explaining 62 to 80% of the variation in pollen production per tassel (Fig. 6). All morphological parameters were more closely correlated with pollen production when measured during intense pollen shed (Stage 6), rather than after pollen shed was complete (Stage 8). Number of branches, effective main stem length, and effective branch length accounted for less than 2%, 49% and 46% of the variation in pollen production among treatments (data not shown). For this reason, we did not evaluate these characteristics in Exp. 3 and 4.
Reasoning that the number of flowers per tassel determined total pollen production, and that this number should be distributed over the total tassel surface area, we combined main stem length, total branch length, and main stem diameter to calculate a tassel area index (TAI) for each plant. This index explained 89% of the variation in pollen production per tassel in 2001 (Fig. 6). This index assumes a lower density of flowers on the lateral branches and that they contribute half as much pollen per unit area relative to the main stem. More detailed information about the density of fertile flowers per cm of tassel main stem and branches or the amount of viable pollen produced per flower could be used to refine this relationship. The additional resources required to obtain this detailed information, however, would be considerable and negate the primary benefit of estimating pollen production by individual tassels from simple measures of their morphology and development in the field.
We tested whether main stem length, total branch length, main stem diameter, or TAI would vary similarly with pollen production for the same hybrids grown in a different environment (Exp. 3). We also examined the possibility of using these parameters to distinguish the capacity of inbred lines to produce pollen (Exp. 4). Considering the data from all three experiments together, the relationships between the morphological indices and pollen production were positive, but they explained a smaller portion of the variability in pollen production than observed in 2001 (Fig. 7). Main stem length was the most consistent and robust parameter, explaining 67% of the variation in pollen production across years and genotypes. There was considerably more variation in the TAI values for the inbreds, which generally produced 0.5 to 3 million pollen grains per tassel. Also, the extremely high plant density (18 plants per m2) was not included in the second environment, which decreased the range of tassel size, and therefore the capacity to resolve the relationship between pollen production and tassel morphology for the two hybrids. These results indicated that the TAI approach could provide a useful indication of the relative response of pollen production to growth conditions that cause significant variation in tassel size. It was not sufficiently sensitive, however, to resolve variation in pollen production among individual plants or genotypes with similar tassel morphologies grown at the same population density. As suggested earlier, additional information on floral or anther density per unit length of tassel could be required to increase the resolving power of this technique.
To determine which tassel characteristics provided the most consistent measure of pollen production, we calculated the Coefficient of Variability (CV) for each morphological characteristic and compared them to the CV for pollen production for each of the three treatment groups. Figure 8 shows that main stem length, total branch length, and main stem diameter (except for the inbreds) varied much less than did pollen production in each group of plants. As such, these tassel parameters were relatively stable in response to genetic and environmental treatments that cause pollen production to vary considerably (Fig. 6 and 7). The CV for TAI, however, was of similar magnitude to that of pollen production per tassel for each treatment group. Therefore, this tassel characteristic captures the inherent variability in pollen production associated with relatively large changes in tassel morphology. Refinements may be needed to account for the seasonal variation and plant-to-plant variability observed in this study.
Tassel Area Index for Inbred Lines
We observed earlier that pollen production and tassel morphological characteristics varied considerably among the inbred lines (Fig. 7). Therefore, it was of interest to determine whether TAI or other characteristics could be used to categorize inbred males as poor, fair, or good pollen shedders according to common practice in the hybrid seed industry. Figure 9a shows levels of pollen production for each of 22 inbred lines organized by heterotic group. Variation in pollen production per tassel was as great within groups (notably D and E) as it was between groups (compare A and F).
In general, tassel morphological characteristics were poorly correlated with pollen production per inbred line when lines were considered independently (r2 = -0.01 to 0.31, Table 2). The variation in TAI among inbreds, for example, shows little correspondence to observed levels of pollen production (Fig. 9b). No single morphological characteristic was sufficient to properly identify the inbreds destined to produce < 0.5 million grains (poor), 0.5 to 2 million grains (fair), or > 2 million grains (good).
Table 2. Mean values for pollen production per tassel and tassel morphological characteristics for 22 inbred lines from six heterotic groups. Regressions with pollen production are calculated for the 22 inbred lines as a group, and for mean values of each heterotic group. TAI = tassel area index, TWL = tassel weight loss.
* Indicates significance at P = 0.10
** Indicates significance at P = 0.05
ns, nonsignificant at the 0.10 level of probability
The relationships between tassel characteristics and pollen production improved considerably, however, when values were averaged by heterotic group (Table 2). The number of tassel branches explained 80% of the variation in pollen production between groups. This was the only tassel parameter, however, to provide a better estimate of pollen production per tassel than simply measuring TWL during pollen shed (r2 =0.57).
The poor relationship between the TAI of inbred tassels and the amount of pollen they produce indicates that critical information about other important tassel traits is still missing from the TAI equation. Total number of flowers, flower density, amount of pollen per anther, and loss of floral structures will impact this relationship. It is quite conceivable that the reduction in tassel size that has accompanied selection for high grain yield (4) has had a disproportionate impact on one or more of these morphological characteristics on the male inbred parents. Detailed measurements of inbred tassel morphology are needed to resolve this possibility.
Using the Tassel Area Index as an indicator of pollen production per plant emerged as an interesting alternative to tassel weight loss due primarily to its speed and simplicity. It is suitable for describing genotype response to increasing population density or for documenting large genetic differences among inbred lines, such as between heterotic groups. Seed companies in particular might find this approach useful for characterizing male inbred response to population density or across a range of environmental conditions. The technique as currently defined, however, is not sufficiently accurate to distinguish "maleness" of inbreds within a heterotic group. A better characterization of flower density and pollen production per anther could improve estimates of pollen production based on simple morphological measures. But these additional measurements would reduce the speed and simplicity of characterization, which makes the TAI approach more attractive than other techniques. Current studies are aimed at refining this measurement technique to account for genetic variation in floral density and structure.
1. Bassetti, P., and Westgate, M. E. 1994. Floral asynchrony and kernel set in maize quantified by image analysis. Agron. J. 86:699-703.
2. Bolaños, J., and Edmeades, G. O. 1993. Eight cycles of selection for drought tolerance in lowland tropical maize. II. Response in reproductive behavior. Field Crops Res. 31:253-268.
3. Burris, J. S., and Lauer, M. J. 2001. Adventitious pollen intrusion into hybrid maize production fields. Technique report for the Association of Official Seed Certifying Agencies.
4. Duvick, D. N. 1997. What is Yield? In Developing drought and low N-tolerant maize. G.O. Edmeades, M. Banziger, H. R. Mickelson, and C.B. Peña-Valdivia, eds. Proc. of a symposium, March 25-29, 1996, CIMMYT, El Batan, Mexico D.F., CIMMYT.
5. Flottum, P. K., Robacker, D. C., and Erickson, E. H. 1984. A quantitative sampling method for airborne sweet corn pollen under field conditions. Crop Sci. 24:375-377.
6. Fonseca, A. E., Westgate, M. E., and Doyle, R. T. 2002. Application of fluorescence microscopy and image analysis for quantifying dynamics of maize pollen shed. Crop Sci. 42:2201-2206.
7. Galinat, W. C. 1992. Evolution of corn. Adv. Agron. 47:203-231.
8. Goss, J. A. 1968. Development, physiology, and biochemistry of corn and wheat pollen. Bot. Rev. 34:333-358.
9. Hall, A. J., Villela, F., Trapani, N., and Chimenti, C. 1982. The effect of water stress and genotype on the dynamics of pollen-shedding and silking in maize. Field Crops Res. 5:349-363.
10. Jones, M. D., and Newell, L. C. 1948. Size, variability and identification of grass pollen. J. Am. Soc. Agron. 40:136-143.
11. Kiesselbach, T. A. 1999. The Structure and Reproduction of Corn. 50th Anniversary Edition. Cold Spring Harbor Laboratory Press. Cold Spring Harbor, New York.
12. Luna, S., Figueroa, V. J., Baltazar, M. B., Gomez, M. R., Townsend, L. R., and Schoper, J. B. 2001. Maize pollen longevity and distance isolation requirements for effective pollen control. Crop. Sci. 41:1551-1557.
13. Raynor, G. S., Eugene, C. O., and Janet, V. H. 1972. Dispersion and deposition of corn pollen from experimental sources. Agron. J. 64:420-427.
14. Sadras, V. O., Hall, A. J., and Schlichter, T. M. 1985. Kernel set of the uppermost ear in maize: II. A simulation model of effects of water stress. Maydica 30:49-66.
15. Uribelarrea, M., Carcova, J., Otegui, M. E., and Westgate, M. E. 2002. Pollen production, pollination dynamics, and kernel set in maize. Crop Sci. 42:1910-1918.
16. Westgate, M. E., Lizaso, J., and Batchelor, W. 2003. Quantitative relationships between pollen shed density and grain yield in maize (Zea mays L.). Crop Sci., in press.
17. Wodehouse, R. P. 1935. Pollen Grains: Their Structure, Identification, and Significance in Science and Medicine. MacGraw-Hill Book Co. Inc., New York.