Effects of planting density on photosynthetic characteristics of flag leaf and senescence of leaf and root under wide-width sowing condition.

To determine the optimal planting density under wide-width sowing condition, we investigated the effects of different planting densities on photosynthetic characteristics of flag leaves, senescence characteristics of flag lea-ves and roots, grain yield, and water use efficiency under four planting density levels, 90×10 ⁴ plants·hm ^-2 (D ₁ ), 180×10 ⁴ plants·hm ^-2 (D ₂ ), 270×10 ⁴ plants·hm ^-2 (D ₃ ) and 360×10 ⁴ plants·hm ^-2 (D ₄ ), in field condition set in Yanzhou, Shandong during the growing season of 2018-2019 and 2019-2020. The results showed that compared with D ₁ and D ₄ treatments, D ₂ treatment significantly improved photosynthetic characteristics of wheat flag leaves during grain filling, increased the activity of superoxide dismutase (SOD) and soluble protein content, reduced the malondialdehyde (MDA) content, and delayed the senescence of flag leaves and roots. Compared with other treatments, D ₂ treatment significantly increased root length, root surface area and root volume in 0-40 cm soil layer. Compared with D ₁ , D ₃ and D ₄ treatments, the grain yield of D ₂ treatment was increased by 11.8%, 2.5%, 6.4% in 2018-2019 and 22.7%, 5.7%, 17.1% in 2019-2020, respectively. In addition, water use efficiency was increased by 9.2%, 8.8%, 14.2% in 2018-2019 and 21.1%, 6.2%, 21.5% in 2019-2020, respectively. The planting density at 180×10 ⁴ plants·hm ^-2 improved photosynthetic characteristics of flag leaves and root morphology during filling stage, delayed plant senescence, increased grain number per spike and grain weight. Consequently, the highest grain yield and water use efficiency were obtained under D ₂ treatment, which was the optimal treatment under the experimental wide-width sowing condition.