Effects of planting density and mode on summer maize chloroplastD1 protein and photo-system II

The effects of different planting densities and modes on grain yield and physiological parameters of two summer maize varieties ("Zhengdan 958" and "Xundan 20") were studied under high yield conditions in North Henan Province. With 67 500~90 000 plants per hm^-2, D1 protein content, optimal photochemical efficiency of PSII (Fv/Fm), photochemical quenching coefficient (qP), photosynthetic rate (Pn) and activities of foliar superoxide dismutase (SOD) decreased. However, peroxidase (POD) activity, non-photochemical quenching (NPQ) and malondialdehyde (MDA) content increased. At the same time, bald needle length increased and ear weight decreased. Yield increased to a peak then decreased with increasing plant density. When plant density exceeded 82 500 plants per hm^-2, Fv/Fm, qP, Pn and SOD activity decreased dramatically. These parameters were lower at densities of 90 000 plants per hm^-2 than at 82 500 plants per hm^-2; dropping by an average from 1.2%~21.7%. NPQ, POD activity, and MDA content increased with increasing plant density. These parameters were also higher at densities of 90 000 plants per hm^-2 than at 82 500 plants per hm^-2, increasing by an average 3.2%~15.0%. Fv/Fm, qP, Pn and SOD activity in wide-narrow row planting mode were higher than in equidistant row planting mode, while NPQ, POD activity and MDA content exhibited the polar opposite. When the density exceeded 75 000 plants per hm^-2, Fv/Fm, qP, Pn and SOD activity in "Zhengdan 958" were higher than those in "Xundan 20", but NPQ, POD activity and MDA content exhibited the polar opposite. D1 protein content, Fv/Fm, qP, NPQ, Pn, activities of SOD and POD, MDA content, bald needle length, ear weight and yield were significantly influenced by variety, plant density and planting mode. The interactions of the three factors were distinctively significant. Yield was highest at a density of 82 500 plants per hm^-2 in the wide-narrow row planting mode for the two summer maize varieties.