Seed number as a function of growth. A comparative study in soybean, sunflower, and maize

Seed number, the main yield component of cereals and oil-seed species, strongly depends on the physiological status of the crop during a critical period for seed set. Using a comparative approach including three species with contrasting reproductive strategies, we investigated the relationship between seed number per plant (SNP) and plant growth rate during the critical period for seed set (PGR(C)). Indeterminate soybean [Glycine mar (L.) Merr.], sunflower (Helianthus annuus L.), and maize (Zea mays L.) crops were grown under a wide range of plant densities to generate contrasting availability of resources per plant. Growth of individual plants was estimated by a novel, nondestructive method based on relationships between actual shoot dry matter and morphometric variables, including stem diameter, plant height, and dimensions of reproductive structures. Seed number per plant ranged from 0 to 890 in soybean, 0 to 4096 in sunflower, and 0 to 1348 in maize and PGR(C) (g d(-1)) from 0.01 to 4.3 in soybean, 0.3 to 17.6 in sunflower, and 0.4 to 12.3 in maize. Our study showed that (i) the relationship between SNP and PGR(C) was linear in soybean, reflecting the reproductive plasticity of this species, and curvilinear in sunflower and maize, reflecting morphogenetic restrictions to generate reproductive sinks under favorable growing conditions; (ii) the PGR(C) threshold below which no seed was set varied among species, being negligible in soybean, close to 0.35 g d(-1) in sunflower, and I g d(-1) in maize. Quantitative relationships between seed number and plant growth rate during the critical period of seed set could be useful for crop modeling.