Effects of phytochromes B on growth variability and competitive capacity of maize plants in a canopy

Abstract

Inter-plant competition is a key trait of maize (Zea mays, L.) crops growth and grain yield. Spatial and temporal availability of resources like water, nutrients and radiation have been proved to impact on this trait. It is suspected that light-quality signals operate on plants´ growth variability of irrigated and fertilized maize crops. Since photoreceptors phytochromes B1 and B2 are involved in light-signals-mediated detecting neighbors, it has been speculated that phytochromes B i) affect plants growth variability from early stages of maize cycle, ii) are involved in the increasing inter-plant variability by increasing plant density, and iii) confer a higher competitive capacity of plants within a canopy. It is also unclear if shade avoidance responses are detrimental or beneficial for plants growth and grain yield in maize crops. To test these hypothesis, plants of maize inbred line France 2 wild type (WT) and the isogenic mutants lacking either phyB1 or phyB2 (phyB1 and phyB2), were cultivated in the field during two seasons in monocultures (WT, phyB1, phyB2; hypothesis i and ii) and polycultures (WT/phyB1, WT/phyB2, phyB1/phyB2 and WT/phyB1/phyB2; hypothesis iii) at contrasting plant densities (low and high) irrigated and fertilized. Plant biomass of ten tagged plants per plot were nondestructively estimated from seedling emergence to 15 days after flowering, and from individual samples at physiological maturity and coefficient of variation (CV) of plant biomass was calculated as a proxy of plants growth variability. Plant leaf area, stem length, and plant growth rate of tagged plans were measured around female flowering, i.e. silking (PGRs) and kernel number per plant (KNP) and grain yield were quantified at physiological maturity. At the lower density, no differences in plants growth variability were detected among genotypes. By contrast, at the higher density CV of WT was higher than those of mutant lines only when canopies were fully developed (i.e. after flowering). Taller plants with larger leaf area characterized WT phenotype at all densities and polycultures, which were generally reflected on higher PGRs. These reactions of WT plants allowed them to acquire more competitive ability and to set more KNP and grain yield than phyB1 plants. phyB2 may have offset differences in KNP with WT by greater grain weights. Hence, in maize crops, phytochromes B are key photoreceptors mediating the response of plants growth variability to crowding stress, without any detrimental effect on grain yield because the ability of plants to forage for light, sustains PGRs and kernel setting.