Supplemental irrigation and modified plant density improved photosynthesis, grain yield and water productivity of winter wheat under ridge-furrow mulching

In the semi-arid and semi-humid areas, ridge-furrow planting with film mulch on the ridge (R) has been found to significantly enhance crop yields and water productivity, but the interaction effects of supplemental irrigation and plant density on the growth, photosynthesis, grain yield and water productivity (WP) of winter wheat under ridge-furrow mulching are still poorly understood. A two-season experiment (during October 2020 - June 2021 and October 2021 - June 2022) was undertaken on winter wheat in a semi-humid but drought-prone area in northwest China, with two cultivation patterns (R, with ridge width of 40 cm and furrow width of 60 cm; F: traditional flat planting), two supplemental irrigation amounts (I0: rainfed; I60: 60 mm of supplemental irrigation equally applied at the overwintering and regreening stages) and three plant densities (D1:240 × 104 plants ha−1; D2: 360 × 104 plants ha−1; D3: 480 × 104 plants ha−1; with row spacing of 20 cm). The results showed that cultivation pattern, supplemental irrigation and plant density had significant effects on soil water content, leaf area index (LAI), aboveground biomass, relative chlorophyll content (SPAD value), photosynthesis, chlorophyll fluorescence parameters and grain yield of winter wheat. In addition, the interaction effects of cultivation pattern, supplemental irrigation and plant density on SPAD value, fluorescence parameters, grain yield, evapotranspiration and WP were significant. Compared to F and I0, R and I60 significantly increased LAI (4.8 % and 4.6 %), aboveground biomass (9.3 % and 8.1 %), SPAD value (5.0 % and 6.4 %), net photosynthetic rate (19.8 % and 11.7 %), maximal PSII quantum yield (Fv/Fm) (5.2 % and 2.4 %), energy transformation potential activities of PSII (Fv/Fo) (4.1 % and 1.5 %), effective PSII quantum yield (ΦPSII) (12.2 % and 9.5 %), grain yield (10.4 % and 13.5 %) and WP (11.8 % and 7.7 %), while they reduced the relative energy excess at the PSII level. Increasing plant density generally improved LAI, aboveground biomass, but it reduced SPAD value, net photosynthetic rate and fluorescence parameters. Grain yield and WP reached their maximum values at D2 under R and at D3 under F, which was because R had a higher plant number (by 25 %) in each row at D3 compared to F due to the decreased row number resulting from the existence of 40-cm-wide ridge under R. The greatest grain yield (7383.5 kg ha−1 and 8577.4 kg ha−1) and WP (20.03 kg ha−1 mm−1 and 21.77 kg ha−1 mm−1) were obtained under RI60D2 during the two growing seasons. These results suggest that ridge-furrow mulching combined with supplemental irrigation and modified plant density of 360 × 104 plants ha−1 can produce high grain yield and WP for winter wheat in the semi-humid but drought-prone areas of northwest China.