Vegetative plasticity and floral branching compensate low plant density in modern spring rapeseed

Abstract

Sunlight is a crucial environmental factor for photosynthesis. Plant density affects both quality and quantity at light penetration into the canopy. The effects of plant density on the expression of photomorphogenic traits in cultivated spring rapeseed (Brassica napus L.), and their consequences on seed production per unit area are unknown. The aims of this work were to: i) analyze the magnitude of shade avoidance responses to plant densities in five spring rapeseed genotypes, ii) describe the dynamics of floral branching in response to genotype and plant density, and iii) study yield and its components and seed quality in field-cultivated plants with contrasting plant densities (15 and 240 pl m−2). Rosette diameter was the main attribute of vegetative plasticity modified by plant density, without significant changes in petiole length. Plant density changed the relationship between intercepted solar radiation (ISR) and red/far red ratio (R/FR) during crop development, but at flowering all densities reached ISR = 95% and R/FR ratio <0.1. From flowering to maturity, the lower the plant density, the lower the R/FR, associated to increased silique area by a promotion of floral branching. Growth dynamics of floral branches at first, second and third orders were strongly affected by plant density and genotype. Seed yield per plant could be described by a negative power-law function as a function of plant density at harvest, exhibiting a high reproductive plasticity capable of compensating grain yield per unit area. Fruiting efficiency per plant was around 100–160 seeds g−1, regardless of plant density. Seed oil and protein content were not responsive to plant density. We concluded that plant density could be reduced without significant penalty for seed production per area, because short-cycle spring rapeseed genotypes expressed a strong vegetative and reproductive plasticity at individual level when plants were grow under well- watered and fertilized conditions.