Pollen is an important vector of gene flow in maize (Zea mays L.). Experiments were conducted to investigate the duration of pollen viability and the effectiveness of isolation distance for controlling gene flow. Pollen longevity was tested by collecting pollen at dehiscence and exposing it in a thin layer in the open air and sunshine for prescribed time periods before assessing pollen viability by measuring seed set after pollination and scoring visual appearance. Isolation distance efficacy was evaluated by growing 12.8‐m2 plot of maize at various distances from a 4000‐m2 pollen source. The pollinator contained either a genetic leaf or seed marker that allowed pollen flow to be measured. Pollen maintained viability for 1 to 2 h after dehiscence depending on atmospheric water potential. The theoretical, maximum distance viable pollen could move was 32 km, assuming pollen was transported linearly at the maximum average afternoon windspeeds for our location, viability was maintained for 2 h, and pollen settling rate was ignored. Cross pollinations occurred at a maximum distance of 200 m from the source planting, and only a limited number of cross pollinations occurred at the shortest distance (100 m). No cross pollinations occurred at 300 m from the source planting. The results are consistent with conclusions that maize pollen is desiccation intolerant and has a high settling rate. The results indicate isolation distance can be a useful tool for controlling gene flow via pollination in research scale plantings.
In maize (Zea mays L.) large decreases in kernel number result when low water potentials (',,) and high temperatures occur during pollination. To gain insight into the basis for the decreased seed set, silk, pollen, and ear-leaf 'I.',,, the capability for silk osmotic adjustment, and pollen appearance were measured to determine their relationship to seed set. A multiple-eared or prolific (high carbohydrate availability to the pistillate inflorescence) hybrid (B73 x FR25), a heat sensitive hybrid (WF9 x A632), and a commercial hybrid (B73 x Mol7) were studied. A crosspollination experiment, with pollination limited by pollen amount, was conducted to determine the impact on seed set of water and heat stressing the tassel and water stressing the ear. At low I,,, silk I,, and seed set were decreased whereas pollen I,,, appearance, and viability were unaffected. High temperature resulted in a 2 megapascal decrease in pollen I,,, visually damaged pollen being shed, decreased pollen viability, and, in two of the hybrids, substantially decreased pollen shed. Prolificacy did not result in increased silk solute accumulation but did result in superior seed production by the pistillate inflorescence at low ',,. The magnitude of the decrease in silk solute potential was small (0.2 megapascal) and similar for all genotypes. One hybrid maintained a relatively high silk turgor but this hybrid also decreased the most in seed production when the pistillate inflorescence was water deficient. These results indicated an adverse effect of high temperature on pollen development, a positive relationship between seed production and silk water status, and no advantage to high silk turgor after silk emergence in maintaining seed production. Additionally, there was no evidence of variation in silk solute regulation capability among hybrids which varied in prolificacy, a trait important in drought tolerance, but the seed production of the pistillate inflorescence of the prolific hybrid was least affected by water deficit.In maize the number of ovules that are fertilized and which develop into grains (seed set) decrease rapidly when drought or high temperature conditions occur during flowering (4). The failure of silk elongation such that pollen shed is complete before silk emergence is often cited as being responsible for the decreased kernel numbers (9,12). However, recent evidence indicates that pollen viability and the capability of the pistillate flower to produce seed after being pollinated may be factors which determine seed production when drought or high temper-
Gene flow between maize [Zea mays (L.)] and its wild relatives does occur, but at very low frequencies. Experiments were undertaken in Tapachula, Nayarit, Mexico to investigate gene flow between a hybrid maize, landraces of maize and teosinte (Z. mays ssp. mexicana, races Chalco and Central Plateau). Hybridization, flowering synchrony, pollen size and longevity, silk elongation rates, silk and trichome lengths and tassel diameter and morphology were measured. Hybrid and open-pollinated maize ears produced a mean of 8 and 11 seeds per ear, respectively, when hand-pollinated with teosinte pollen, which is approximately 1-2% of the ovules normally produced on a hybrid maize ear. Teosinte ears produced a mean of 0.2-0.3 seeds per ear when pollinated with maize pollen, which is more than one-fold fewer seeds than produced on a maize ear pollinated with teosinte pollen. The pollination rate on a per plant basis was similar in the context of a maize plant with 400-500 seeds and a teosinte plant with 30-40 inflorescences and 9-12 fruitcases per inflorescence. A number of other factors also influenced gene-flow direction: (1) between 90% and 95% of the fruitcases produced on teosinte that was fertilized by maize pollen were sterile; (2) teosinte collections were made in an area where incompatibility systems that limit fertilization are present; (3) silk longevity was much shorter for teosinte than for maize (approx. 4 days vs. approx. 11 days); (4) teosinte produced more pollen on a per plant basis than the landraces and commercial hybrid maize; (5) teosinte frequently produced lateral branches with silks close to a terminal tassel producing pollen. Collectively these factors tend to favor crossing in the direction of teosinte to maize. Our results support the hypothesis that gene flow and the subsequent introgression of maize genes into teosinte populations most probably results from crosses where teosinte first pollinates maize. The resultant hybrids then backcross with teosinte to introgress the maize genes into the teosinte genome. This approach would slow introgression and may help explain why teosinte continues to co-exist as a separate entity even though it normally grows in the vicinity of much larger populations of maize.
Pollen viability and ear receptivity may be important determinants of maize (Zea mays L.) seed set under drought and high temperature conditions. Little quantitative information is available concerning the influence of water and heat stress on pollen viability, as determined with seed‐set‐limiting pollen amounts, or ear receptivity. The purpose of this investigation was to determine the effect of a water deficit on ear receptivity, of heat and water stress on pollen viability using a seed‐set‐limiting amount of pollen in pollination, and the genotypic variability for these effects. During a 2‐y period, field‐grown potted maize plants, grown in a soil mix containing a Typic Argiudoll, were well‐watered or water‐stressed during flowering. Tassels included heat‐stressed, water‐stressed, and well‐watered. A second experiment, conducted on a Typic Haplaquoll, involved mixing heat‐stressed and nonstressed pollen with field‐produced R‐nj pollen and using kernel ratios to determine viability. The heat‐sensitive, commercial, and prolific hybrids decreased in kernel number by 31,15, and 9% respectively when the female plants were water stressed. Kernel number did not decrease when pollen from a water‐stressed plant was used. Heat stressing the tassel resulted in kernel number decreases of 87, 53, and 72% for the heat‐sensitive, commercial, and prolific hybrids, respectively. Pollen viability and amount shed limited seed set when the heat‐sensitive and prolific hybrids' tassels were heat stressed. Pollen viability limited seed set when the commercial hybrid's tassel was heat stressed. The pollen mixing experiment also indicated a large effect of heat stress on pollen viability. Thus, water and heat stress had large effects on ear receptivity and pollen viability, respectively, but pollen viability was unaffected by a water deficit.
High temperatures during maize (Zea mays L.) pollination limit kernel number. Knowledge is limited on the effect of heat stress on both pollen viability and tassel shedding characteristics or of possible heat tolerance sources and testers. The purpose of this research was to determine the tassel response to heat stress, as measured by in vitro pollen viability and shedding characteristics, and the combining ability for these traits using a diallel of four heat‐tolerant and three heat‐intolerant inbreds. Tassels from field‐grown plants were excised at the beginning of anthesis, immediately placed in sufficient water to cover the base, and transferred to growth chambers. The growth chambers were maintained at light period temperatures of 29 and 38°C and dark period temperatures of 24 and 32°C, respectively. In vitro germination, anther emergence, and tassel color were determined after 24 h in the growth chamber. Due to bursting, low levels of germination were obtained, but the percentage of nongerminated pollen increased greatly at the high temperature (65% mean increase). The increase in nongerminated pollen varied among the genotypes, and all except B73 × Mo17 and N28 × Mo17 had statistically significant increases at the high temperature. Mo17 had the most desirable general combining ability (GCA) effect at the high temperature for nongerminated pollen. Specific combining ability effects were significant for nongerminated pollen and accounted for 51% of the genotype and genotype × temperature sum of squares. Anther emergence of the central tassel spike decreased differentially among the hybrids at the high temperature. Mo17 had the most desirable GCA effect for anther emergence. Anther emergence and pollen viability did not always respond the same to high temperature. Results from this study indicate pollen shed and viability must be considered in tassel heat tolerance and hybrids involving Mo17 had the most heat tolerant tassels.
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