Disentangling the decrease in R:FR and light intensity on the plant density effect on crop biomass

Abstract

Background and Aim: The plant density effect on plant biomass is due to a decrease in R:FR and PAR availability per plant. These processes are entangled, as the decrease in R:FR contributes to the decrease in PAR per plant with increasing plant density. This is due to its influence on plant architecture, through a positive effect on light absorption, and net photosynthesis. This research aimed to disentangle R:FR and the decrease in PAR per plant to the plant density effect on crop biomass. Methods: An experiment was performed with three plant densities in a climate-room, with RB+FR LEDs. Thus, plant density affected both, R:FR and PAR availability per plant. This was compared to plants grown at the same three densities, but under RB light, without FR. Here, plant density affected only PAR per plant. Results: R:FR decreased with increasing plant density, which was highly correlated with LAI. The amount of absorbed light per plant decreased as R:FR decreased, with the associated negative effect on plant biomass. Without FR light, plant density did not affect the amount of absorbed light per plant, hence neither the biomass per plant. Leaves grown without far-red light had lower, but not statistically significant, net photosynthetic rates. Conclusions: R:FR significantly contributes to the plant density effect on crop biomass due its significant effect on PAR availability per plant. Plant density effects only by a decrease in PAR per plant were not evident at the assessed densities.

Keywords R:FR, Decrease in PAR per plant, Plant density, Plant and crop biomass, Light absorption, Net photosynthesis. Abbreviations R:FR: Red to far-red ratio; FR: Far-red light; PAR: Photosynthetically active radiation; RB: Red and blue light. LAI: Leaf area index.