Intra-specific competition in maize: Contribution of extreme plant hierarchies to grain yield, grain yield components and kernel composition

Maize (Zea mays L.) cropping conditions that promote high intra-specific competition pressure generate an increased plant-to-plant variability within the stand, and the appearance of individuals with different ability to capture scarce resources (i.e. dominant and dominated plants). The objectives of this paper were to analyze (i) stand density effects on plant biomass at physiological maturity (R-6), grain yield per plant (GYP), GYP components (KNP: kernel number per plant; KW: kernel weight), and kernel composition (starch, oil and protein contents per kernel) of the mean plant of the stand (i.e. considering all individuals) and of the dominant (D) and dominated (d) individuals; and (ii) the contribution of these extreme plant hierarchies to GYP, GYP components and kernel composition of the mean plant of a stand. Four maize hybrids of contrasting KW (small and large KW) were cropped at a wide range of stand densities (3-15 pl m(-2))during 1999/2000 and 2001/2002 in Argentina. The mean value of measured variables declined as plant density increased from 3 to 15 pi m-2, and plant-to-plant variability (CV: coefficient of variation) of the same variables increased with enhanced crowding. The magnitude of the reduction in mean plant values differed among variables: plant biomass at R-6, GYP and KNP underwent a larger reduction (ca. 66%) than KW (ca. 14-19%) or kernel contents (ca. 22% for oil and protein contents, and 13% for starch content). Similarly, the increase in CVs was larger for plant biomass at R6 (from ca. 13 to 40%) and GYP (from ca. 30 to 58 and 15 to 38% for small and large KW hybrids, respectively) than for KW (ca. from 7 to 20%). Only a slight increase in CVs of oil (6-17%) and protein (9-12%) concentrations of large KW hybrids was recorded. The CV of KNP followed a trend similar to that for GYP. Differences between plant categories increased when mean GYP and KNP of all individuals of the stand were smaller than 157 g pl(-1) and 649 kernel pl(-1), respectively. Below these thresholds, the d/D ratio dropped from 0.76 to 0.30 (small KW hybrids) or to 0.40 (large KW hybrids) for GYP (r(2) = 0.76, P < 0.001), and from 0.75 to 0.38 (small KW hybrids) or to 0.46 (large KW hybrids) for KNP (r(2) = 0.59, P < 0.001). In contrast, the d/D ratio for KW varied always from 1 to 0.80 in response to decreased mean KW (r(2) = 0.39, P < 0.01). The concentration of kernel contents did not differ between plant types. Results indicate that grain yield of maize crops grown at high stand densities is composed by plants bearing very different kernel numbers, with slight differences in kernel size, and similar starch, oil and protein concentration. (c) 2005 Elsevier B.V. All rights reserved.