Abstract
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 PIABS, RC/CSO, TRO/ABS, ETO/TRO and REO/ETO, 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 VPSI and VPSII-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 I1, I2, L1 and L2, and enhanced I2/I1. 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 QA and QB, 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.
Highlights
The analysis of variance (ANOVA) results demonstrated the significant effect of plant density on yield in both years (Table 1)
The mutual shading among individual plants resulting from high planting density inevitably reduced maize ear leaf photosynthesis
Improving maize planting density to achieve high yield works only when the yield loss caused by decreased photosynthesis did not exceed the gain of increased planting density
Summary
Maize is the most productive crop in the world and an important food and feed crop. Improving planting density is a key strategy used to achieve a high yield in maize [1,2]. Maize is a high-stalk crop with long and wide leaves, high planting density inevitably causes mutual shading and the subsequent depression of photosynthesis in leaves around the ear. The photosynthetic performance of leaves around the ear is crucial for the determination of maize yield. Improving our understanding of the effects of mutual shading caused by high planting density on maize ear-leaf photosynthesis can aid in the advancement of plant density strategies for the development of dense-plantingresistant maize varieties
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