Effects of Different Planting Densities on Photosynthesis in Maize Determined via Prompt Fluorescence, Delayed Fluorescence and P700 Signals

The mutual shading among individual field-grown maize plants resulting from high planting density inevitably reduces leaf photosynthesis, while regulating the photosynthetic transport chain has a strong impact on photosynthesis. However, the effect of high planting density on the photosynthetic electron transport chain in maize currently remains unclear. In this study, we simultaneously measured prompt chlorophyll a fluorescence (PF), modulated 820 nm reflection (MR) and delayed chlorophyll a fluorescence (DF) in order to investigate the effect of high planting density on the photosynthetic electron transport chain in two maize hybrids widely grown in China. PF transients demonstrated a gradual reduction in their signal amplitude with increasing planting density. In addition, high planting density induced positive J-step and G-bands of the PF transients, reduced the values of PF parameters PI(ABS), RC/CS(O), TR(O)/ABS, ET(O)/TR(O) and RE(O)/ET(O), and enhanced ABS/RC and N. MR kinetics showed an increase of their lowest point with increasing high planting density, and thus the values of MR parameters V(PSI) and V(PSII-PSI) were reduced. The shapes of DF induction and decay curves were changed by high planting density. In addition, high planting density reduced the values of DF parameters I(1), I(2), L(1) and L(2), and enhanced I(2)/I(1). These results suggested that high planting density caused harm on multiple components of maize photosynthetic electron transport chain, including an inactivation of PSII RCs, a blocked electron transfer between Q(A) and Q(B), a reduction in PSI oxidation and re-reduction activities, and an impaired PSI acceptor side. Moreover, a comparison between PSII and PSI activities demonstrated the greater effect of plant density on the former.