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CERES-Maize simulates kernel set as a sourcelimited process based on average plant growth rate during the lag phase after flowering. Yet the number of kernels formed by maize (Zea mays L.) also depends on timely interaction between male and female flowers, which can limit formation of reproductive sinks under some conditions. Failure to account for sink-limited kernel set may contribute to simulation error observed under conditions that affect dynamics of pollen shed or silking, but do not alter crop growth rate. We developed algorithms for a Flowering Model to simulate sink-limited kernel set from flowering dynamics. This model was calibrated against kernel production in hybrid seed production fields and then linked to CERES-Maize. The Modified CERES-Maize was calibrated against two years of field data involving three hybrids, eight population densities, and seven N levels. Integrating the capacity to simulate sinklimited kernel set with source-limited kernel set increased simulation accuracy dramatically, relative to original CERES-Maize. For 13 commercial fields tested, Modified CERES-Maize decreased simulation error for kernels per plant from 17.1 to 2.3%, improved P between measured and simulated values from 0.77 to 0.87, and decreased simulation error indicators mean error, root mean square error, and mean square deviation by 85, 40, and 64%, respectively. Modified CERES-Maize accounts for a much greater range of variability in the biological processes controlling kernel set.

Production of hybrid corn (Zea mays L) depends on cross-pollination between male and female inbred parents. As such, reproductive isolation of seed fields is required to ensure genetic purity of the hybrid progeny. Customer demand for improved genetic purity prompted the seed industry to examine the level of genetic purity resulting from current isolation practices. A 3-yr study was conducted to monitor purity of hybrid seed produced in 315 fields from 24 seed companies in North America. Each field was near a commercial corn field shedding pollen synchronously with the seed parent: a worse case scenario. Seed samples were collected at five locations along a 200-m transect established perpendicular to the nearest potential adventitious pollen source, and 100 seed from each location were subjected to isozyme analysis to determine percent out-crossing. Isolation distance, seed field size, field block size, number of border rows, adventitious field size, male: female row ratio, male population, and male pollen class were analyzed as continuous predictors and as variable class predictors of out-crossing at the field margins and field midpoints. Year had a significant (p [less than or equal to] 0.05) impact on observed out-crossing. Isolation distance, border rows, and male pollen class were significant predictors of out-crossing at the field margin but not at the field midpoint. A significant interaction between isolation distance and border rows was observed at both the field margin and midpoint. There also was an interaction between male: female ratio and male pollen class on out-crossing at the field margin. These results indicate that current practices used to isolate hybrid seed fields often achieve the goal of producing [greater than or equal to]99% genetically pure seed, but much higher levels of out-crossing can and do occur. Because out-crossing generally was greater and more variable at the field margins than at the field midpoint, adjustments to field management that focus on minimizing out-crossing at the field margins should lead to consistently high levels of genetic purity from hybrid seed fields.

Accurate prediction of kernel number per plant is critical for yield simulation in maize (Zea mays L.). Model predictions of kernel number generally are based on empirical relationships between final kernel number and carbohydrate supply or plant growth at silking, without regard to flowering dynamics. The objective of this study was to develop a model for predicting kernel set from seasonal dynamics of pollen shed and silk exsertion. Hybrids were planted in isolated plots at four ratios of male fertile (MF) and male sterile (MS) plants. Dynamics of pollen shed in each plot was described by a Gauss curve and by a population index ([P.sub.ind]) derived from the percentage of plants at beginning, maximum, and ending pollen shed. Percentage of plants at silking followed a sigmoid curve, whereas the number of silks exposed daily per ear followed a monomolecular model. Pollen shed and silk exsertion curves were translated to potential kernel set using a published relationship. Excellent agreement was observed between predicted and measured number of florets per hectare available for pollination. The model overpredicted kernel number per hectare at high pollen shed densities. Predicted kernels per hectare were within 5% of actual kernel numbers at low pollen shed densities when delayed development of subapical ears and asynchronous pollination within ears were considered. These results indicate kernel set in maize can be predicted from simple measures of the flowering process. They confirm that kernel set is limited primarily by factors other than pollen amount when all plants in the population are MF., THE DEVELOPMENTAL PROCESSES that determine kernel number per plant in maize can be divided into three consecutive stages for simulation purposes. In the first stage, the reproductive structures are initiated [...]

Maize (Zea mays L.) pollen production has been found not to limit kernel set, but there is scarce information on pollen production of modern hybrids and the effect that breeding for reduced tassel size has had on this trait. Our objectives were to study genotypic differences in pollen production and flowering dynamics, and to estimate pollen availability per exposed silk. Four F1 hybrids were grown at different plant densities (between 2.5 and 12.5 plants [m.sup.-2]) in two distinct environments (cool Midwest USA and temperate Argentine Pampa). Pollen availability was also modified by delayed plantings and detasseling treatments. We measured the dates of anthesis and silking of individual plants, the anthesis-silking interval (ASI), the number of exposed silks per apical ear, the number of pollen grains per square meter (PGM), and kernel number per ear. The number of pollen grains produced per tassel (PGT) and per exposed silk were estimated. Increased plant density promoted an increase in ASI, matched by an enhanced interplant variability in this parameter, and a reduction in PGT. The latter was reduced from 10.3 x [10.sup.6] or 11 x [10.sup.6] at 2.5 plants [m.sup.-2] to about 3 x [10.sup.6] at 12.5 plants [m.sup.-2] (USA), and from 9.7 x [10.sup.6] or 113 x [10.sup.6] at 3 plants [m.sup.-2] to 4 x [10.sup.6] or 3.6 x [10.sup.6] at 9 plants [m.sup.-2] (Argentina). We estimated two thresholds beyond which kernel set could be affected: (i) 227 pollen grains [cm.sup.-2] [d.sup.-1] at the end of pollen shedding and (ii) two pollen grains per exposed silk. Some cropping conditions were closer to the second threshold than others., A SHORT anthesis-silking interval (ASI = anthesis date minus silking date) is a key trait for obtaining high grain yield in maize (Bolanos and Edmeades, 1993). An increase in this [...]

Plant water deficits during flowering cause maize (Zea mays L.) kernels to abort. Lack of current and reserve photosynthate account for much of the kernel loss, but partitioning to ovaries at low ovary water potential ([[Psi].sub.w]) may also be limited by lack of assimilate demand. To test this possibility, we measured the water status, carbohydrate content, and growth of ovaries on plants grown in the field in pots containing 22 kg of soil under one of three light environments [Control, 75-cm rows, 43 055 plants [ha.sup.-1]; Shade, same as Control except under shade cloth (55% light interception) from the sixth leaf stage until physiological maturity; or Isolated, 122-cm rows, 6727 plants [ha.sup.-1]] and exposed to a water deficit during pollination. Water was withheld at silk emergence and plants were hand-pollinated 4 d later when silk [[Psi].sub.w] was [approximately equal to] - 1.1 MPa, leaf [[Psi].sub.w] was [approximately equal to] - 1.8 MPa, and photosynthesis was completely inhibited. The water deficit decreased kernel set, which was correlated with the inhibition of ovary dry matter accumulation. The concentration of sucrose and glucose increased in ovaries of water-deficient plants, and ovary turgor remained at or above control levels. Thus, inhibition of ovary growth at low [[Psi].sub.w] was not related to a loss of turgor, nor was it caused by a depletion of ovary sugars. Sugar accumulation at low [[Psi].sub.w] suggested that metabolism may have been impaired. Coupled with a low level of reserves, failure to utilize available sugars at low [[Psi].sub.w] would severely inhibit assimilate flux to the ear and render kernel set highly vulnerable to water deficits during pollination.

Water deficits during pollination increase the frequency of zygotic abortion in maize (Zea mays L.). Recent work suggests that reproductive failure occurs because assimilate supply in water-deficient plants is not sufficient to maintain growth of all newly formed zygotes. We tested this possibility by altering the vegetative source to reproductive sink ratio in the inbred line A188 prior to imposing a water deficit at pollination. Relative assimilate supply was modified by growing plants in 20-L pots containing 22 kg of amended soil in three environments: (i) the field, (ii) the field under 55% shade, or (iii) a growth chamber. Sink size was modified genetically using a chromosome interchange (reciprocal translocation) mutant (T3-7c) that is near-isogenic to the wild type (A188WT) but sets only 50% of the fertilized ovaries. Six treatment-genotype combinations produced a seven-fold range in photosynthesis per kernel and a four-fold range in reserve sugars per kernel at silk emergence. Water was withheld on the first day silks emerged and plants were hand pollinated 4 to 7 d later when leaf photosynthesis was

Water deficits during anthesis cause maize (Zea mays L.) kernels to abort soon after fertilization. Inhibition of photosynthesis accounts for much of the kernel loss, but lack of assimilate reserves might also limit kernel development. We tested whether varying the level of C and N reserves prior to anthesis affected kernel set of plants pollinated at low water potential ([[Psi].sub.w]). Plants were grown in sand/soil media in pots in the field under one of three light environments: Control, 75-cm rows, 4.3 plants [m.sup.-2]; Shade, same as Control except under 55% shade cloth from V6 until maturity; or Isolated, 122-cm rows, 0.7 plants [m.sup.2]. These treatments produced a 3.3-fold range in total extractable carbohydrate (TEC) and a 1.7-fold range in total Kjeldahl nitrogen (TKN) per plant at silk emergence. Water was withheld at silk emergence and plants were pollinated by hand with excess pollen 4 d later when silk [[Psi].sub.w] was [is approximately equal to] -1.1 MPa, leaf [[Psi].sub.w] was [is approximately equal to] -1.8 MPa, and photosynthesis was completely inhibited. Plants were rewatered 2 d after pollination and remained well watered until physiological maturity. The brief water deficit decreased kernels per ear by 45% in the Controls, 72% in the Shade treatment, and 49% in the Isolated treatment. Lower kernel set in the Shade treatment was associated with low levels of TEC and TKN in the vegetative stalk, leaves, and reproductive stalk, compared with Control plants. However, accumulation of TEC and TKN in excess of Control values in the Isolated plants did not prevent kernel loss. Kernel set in all three treatments was more closely related to the rate of photosynthesis prior to imposing the water deficit than to the level of reserves accumulated prior to flowering. These results suggest that selection for high levels of reserves at pollination will not improve kernel set at low [[Psi].sub.w] in maize.