Maize (Zea mays L.) grain yield is closely associated with kernel number at harvest. This yield component is a function of the physiological condition of the crop at a critical period bracketing silking. The objectives of this study were to analyze for that critical period by assessing (i) the relationship between final kernel number and plant or crop growth rate, (ii) the partitioning of dry matter to reproductive structures, and (iii) the effect of duration of this period on kernel set. Kernel number (per plant and per unit area) and growth rates of crops, plants, and ears were measured in 12 field experiments conducted at Balcarce, Argentina, from 1989 to 1996 with hybrid Dekalb 636. Experimental sources of variation included shading, plant density, sowing date, and night temperature. Shading and night temperature treatments were established during a 31‐d period bracketing silking. In one of the density experiments, plant growth rate was estimated for individual plants by means of a non‐destructive methodology based on correlations between dry matter and morphometric measurements. The relationship between kernel number per plant and plant growth rate was curvilinear with threshold values of approximately 1 g d−1 for kernel set and 6 g d−1 for prolificacy. At high and at low plant growth rates during the period bracketing silking, Dekalb 636 set a low number of kernels per unit of plant growth rate. This effect was explained by limited partitioning of biomass to reproductive structures during that period. When duration of the critical period for kernel set was considered by expressing plant growth rate in thermal units, a better estimation of kernel number per plant was obtained.

The response of grain yield to narrow rows can be analyzed in Westgate et al., 1997).terms of the effect on the amount of radiation intercepted by the crops. The objective of this work was to study the effect of row spacing There are times during the crop cycle that are most on grain yield and radiation interception (RI) during the critical period critical for yield determination. These times comprise for grain set in three crop species. Ten experiments were conducted the period bracketing flowering in maize (Kiniry and with maize (Zea mays L.), sunflower (Helianthus annuus L.), or soy- Ritchie, 1985; Fischer and Palmer, 1984) and sunflower bean [Glycine max (L.) Merr.] under irrigation or under dryland con- (Chimenti and Hall, 1992, Connor and Sadras, 1992; ditions without severe drought during flowering and grain filling. The Cantagallo et al., 1997) and extend to more advanced treatments consisted of two row distances combined with other factors reproductive stages in soybean (Shaw and Laing, 1966; such as plant density, cultivar, defoliation, etc. Grain yield responses Board and Tan, 1995; Egli, 1997). Higher crop growth to decrease distance between rows were inversely proportional to RI rates during these periods would allow more grains to achieved with the wide-row control treatment during the critical pebe set and thus higher grain yields (Andrade et al., riod for grain number determination (r 2 ϭ 0.62, 0.54, and 0.86 for maize, soybean, and sunflower, respectively). Moreover, when row 1999). Crop growth rate is directly related to the amount spacing was reduced, grain yield increases and RI increases during of radiation intercepted by the crop (Gardner et al., the critical periods for grain set were significantly and directly corre-1985). Therefore, the response of grain yield to narrow lated in the three crop species (r 2 ϭ 0.71, 0.64, and 0.94 for maize, rows can be analyzed in terms of the effect on the soybean, and sunflower, respectively). For the conditions of these amount of RI at the critical periods for kernel set. In

Variations in N availability affect growth and development of maize (Zea mays L.) and may lead to changes in crop physiological conditions at flowering and in kernel set. The objectives of this study were (i) to establish the effect of N availability on crop development, crop radiation interception, radiation use efficiency, and dry matter partitioning; and (ii) to study the relationship between kernel number and crop growth at flowering and between kernel number and crop N accumulation at flowering. Three experiments with a commercial hybrid (DK636) were carried out under field conditions at the INTA Baicarce Experimental Station, Argentina, without water limitations. The treatments consisted of different radiation levels, obtained by shading, combined with different levels of N availability obtained by the addition of N fertilizer or organic matter to inmobilize N. Nitrogen deficiencies delayed both vegetative and reproductive phenological development, slightly reduced leaf emergence rate, and strongly diminished leaf expansion rate and leaf area duration. Nitrogen deficiencies reduced radiation interception as much as radiation use efficiency and their effects on the ear dry matter/total dry matter ratio at harvest were associated with crop growth rate reductions at flowering. Dry matter partitioning to reproductive sinks at flowering and the ear dry matter/total dry matter ratio at harvest were reduced by N shortages. Significant relationships between kernel number and N accumulation rate or crop growth rate at flowering were fitted by linear + plateau functions with thresholds above which kernel number and grain yield did not increase.

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