Coupling effect of wetting and drying alternative irrigation and nitrogen application rate on photosynthetic characteristics of rice leaves

ObjectivesSoil water potential and fertilizer nutrients are the two main factors affecting rice growth and development. Elucidation of their coupling effects on grain yield of rice (Oryza sativa L.) has enormous significance for high yielding and high efficiency in production. This study observed photosynthetic characteristics of japonica rice under different irrigation and nitrogen supply levels to help understanding the mechanism of suitable irrigation and nitrogen fertilization coupling in yield increase of rice.

MethodsA field experiment was conducted using a mid-season japonica rice cultivar Xindao 20 with three different nitrogen levels, including N0, MN (240 kg/hm²) and HN (360 kg/hm²), and three irrigation regimes, including submerged irrigation (0 kPa), alternate wetting and moderate drying (–20 kPa), and alternate wetting and severe drying (–40 kPa) in 2014 and 2015. The effects of different water and nitrogen managements on rice yield, chlorophyll and nitrogen content in leaves, leaf area index, net photosynthetic rates, photosynthetic nitrogen utilization efficiencies(PNUE), potential activity and maximum photochemical efficiencies of PSⅡ at main growth stages were analyzed.

ResultsThere was a significant interaction between the irrigation regimes and the nitrogen applications, with a similar result in two years. The grain yield was the highest in the treatment of MN coupling with mild water stress due to improved seed filling rate and grain weight. The grain yield increased with alternate wetting and moderate drying, and decreased under the condition of severe drying significantly. Compared with the MN, the grain yield under the condition of HN treatment was decreased, but the difference was not significant, while water stress was severe, the yield of HN was decreased significantly, the result showed that heavy nitrogen fertilizer aggravated the soil drought, and reduced the effect of fertilizer regulation role. In the same nitrogen level, the chlorophyll contents, nitrogen contents, leaf area index, net photosynthetic rates, PNUE, potential activity and maximum photochemical efficiencies of PSⅡ at main growth stages were higher in the treatment of alternate wetting and moderate drying compared with the treatment of submerged irrigation, meanwhile LAI was also increased after the heading stage. The treatment of mild water stress and MN enhanced rice yield, being the best water-nitrogen coupling management model in this study. The opposite result was observed under the condition of alternate wetting and severe drying. In the same irrigation regime, the net photosynthetic rates, potential activity and maximum photochemical efficiencies of PSⅡ at main growth stages were higher under the condition of MN treatment, when compared with no nitrogen applied, meanwhile the chlorophyll contents and leaf area index were enhanced significantly. The opposite results were observed under the condition of HN treatment, which indicated that heavy nitrogen application decreased photosynthetic characteristic of leaves. The grain yield was positively correlated with leaf area index, net photosynthetic rate, PNUE, potential activity and maximum photochemical efficiency of PSⅡ at main growth stages. Positive effects were observed at nitrogen effect in photosynthetic characteristic at main growth stages, both water stress effect and interaction effect were positive under the condition of alternate wetting and moderate drying, while negative effect was observed under the condition of alternate wetting and severe drying.

ConclusionsThe net photosynthetic rate, photosynthetic nitrogen utilization efficiency, potential activity and maximum photochemical efficiency of PSⅡ at main growth stages were higher in treatment of moderate drying and MN, meanwhile the chlorophyll content and leaf area index were also increased after the heading stage. These results suggested that a photosynthetic characteristic, increasing photosynthetic capacity through the appropriate regulation of water coupling with nitrogen application, would be much more beneficial to increasing grain yield of rice.