In maize (Zea mays L.), the later‐fertilized ovaries often abort, thereby reducing kernel set. We examined whether altering the time interval between pollination of florets within an ear or between ears could affect final kernel number per plant. Synchrony of pollination was varied by natural‐ and hand‐pollination of four hybrids, contrasting in prolificacy (ears plant−1). Plants were grown in the field at low (2.5 and 3 plants m−2) and high (7.5 and 9 plants m−2) plant populations, without water or nutrient stress. Increasing plant population generally delayed silk appearance, but most silks were exposed within 5 d after silking (DAS). Synchronous pollination of all exposed silks on apical and sub‐apical ears 5 DAS improved kernel number (KN) per plant and the floret fertility index (FFI = number of kernels/number of pollinated silks), relative to open‐pollinated plants. At low plant populations, the KN plant−1 increase resulted primarily from a large increase (39–535%, depending upon the hybrid) in kernels on sub‐apical ears. At high plant populations, only apical ears set kernels. Synchronous pollination increased KN in these ears 8 to 31%, depending on the hybrid. Thus, timing of pollination had a large impact on kernel set, and the disadvantage associated with an ontogenetic delay in silk emergence could be partially overcome by synchronous pollination. Because delayed pollination of early‐silking ovaries allowed a greater number of the late‐silking ones to set kernels, factors other than assimilate availability per fertile floret likely are involved in controlling kernel set.

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Shoot dry weight of maize (Zea mays L.) depends on the amount of photosynthetically active radiation intercepted by the crop (IPAR). The present work was conducted to analyze the variation in shoot dry weight production and its partitioning to reproductive sinks when seasonal changes of temperature and solar radiation occur during the growing cycle of the crop. Four commercial hybrids were grown at 8 plants m−2 on four sowing dates (20 Aug., 20 Sept., 20 Oct., and 20 Nov.) at Rojas (34° 08' S, 60° 59' W), Argentina, on a silty clay loam soil (Typic Argiudoll) during 1990‐1991 and 1991‐1992, with no water or nutrient restrictions. Shoot dry weight at physiological maturity was associated with the amount of IPAR, with radiation use efficiency before silking (4.14 g MJ−1) higher than after silking (2.45 g MJ−1). Grain yield was correlated with shoot dry weight at physiological maturity, resulting in a stable (0.46 ± 0.02) harvest index. Shoot dry weight at silking showed a significant relationship with final grain number (r2 = 0.52, n = 32) as well as with grain yield (r2 = 0.55, n = 32). Ear dry weight at silking was associated with grain yield particularly for prolific hybrids (r2 = 0.64, n = 16). Provided postsilking conditions do not limit assimilate supply to the grains, shoot dry weight at silking could be considered a good grain yield predictor. In temperate regions, maize potential productivity seems to be more limited by the amount of solar radiation available around silking (determinant of grain set) than during grain filling (determinant of grain weight). Early and intermediate sowings tend to best utilize solar radiation for grain production.

period. An early study (Reddy and Daynard, 1983) showed that the number of endosperm cells was es-In maize (Zea mays L.), the negative effects of increased stand tablished during the lag phase, and that this number densities on final kernel weight (KW) are attributed to reductions in was related to kernel sink potential (Jones et al., 1985). the effective grain-filling period, and not in kernel growth rate. This suggests that competition for assimilates among kernels only occurs atOn the other hand, in an in vitro study, Hanft et al. the last stages of grain filling. To test this hypothesis, two commercial (1986) observed that assimilate supply during the grainhybrids of different KW were grown at two stand densities (3 and 9 filling period affected final KW, regardless of endoplants m Ϫ2 ) during 1998 to 1999 and 1999 to 2000. Pollination treatsperm cell number. Similar results were obtained with ments were performed in order to modify kernel number per plant maize cropped at contrasting sowing dates (Cirilo and (KNP) and to obtain a range of source-sink ratios. Pollination treat-Andrade, 1996) or with reductions in KNP at flowering ments altered KNP, and negative relationships were established bein a related species like sorghum (Sorghum bicolor (L.) tween KW and KNP, with no differences between years. On the basis Moench; Kiniry, 1988), where significant differences in of regression analysis of the response of KW to changes in KNP, KW KW could not be attributed to the number of endoincreased between 0.09 to 0.28 mg kernel Ϫ1 per unit decrease in KNP, sperm cells. depending on stand density and genotype. The theoretical potential KW was independent of preanthesis plant population effects, which When maize source-sink ratio was altered only during affected ear growth significantly (P Ͻ0.01). Kernel weight was closely the effective grain-filling period (i.e., after the lag related to variations in kernel growth rate during the effective grainphase), KW varied in relation to the level of assimilate filling period (r 2 ϭ 0.84; P Ͻ0.001), and not to modifications in the (C1417DSE), Argentina. Received 11 Dec. 2000. *Corresponding author (borras@agro.uba.ar). Abbreviations: DAS, d after silking; KNP, kernel number per plant; KW, kernel weight; TT, thermal time.

Maize (Zea mays L.) kernel set can be improved through synchronous pollination within and between ears. Reductions in kernel set could be expected because of asynchronous pollination between early‐ and late‐appearing silks. We analyzed the effect of (i) selective ear heating around the time of silking, and (ii) different time gaps between early‐ and late‐pollinated silks in an attempt to modify kernel set. Tip ear heating was expected to minimize the advantage of early silking ovaries. Lateral heating and pollination gaps were expected to exaggerate this advantage. Three pollination gaps (2, 4, and 6 d) were tested for two plant populations (3 and 9 plants m−2). Ear temperature in the heated zone averaged 4.5°C above air temperature. Temperature in the nonheated side closely followed air temperature. Treatments promoted greater differences in maize kernel number (KN) per ear (73% variation) than in the number of silks exposed 5 d after silking (6% variation). Lateral ear heating reduced KN per ear in comparison with the nonheated control, but tip ear heating did not modify KN per ear. At 9 plants m−2, synchronous pollination resulted in ∼15% increase in KN per plant. Pollination gaps of 2 and 4 d reduced KN per plant drastically (up to 51%), but the reduction was smaller for the 6‐d gap. This study (i) gives evidence of the negative impact of delayed pollination timing among silks on kernel set, which was not related to reduced silk receptivity, and (ii) defines the time gap for maximum interference of early‐ on late‐pollinated ovaries, a period shorter than 4 d.