Mixed-nitrogen nutrition-mediated enhancement of drought tolerance of rice seedlings associated with photosynthesis, hormone balance and carbohydrate partitioning

Abstract

To investigate whether mixed-N (NO3 − + NH4 +) nutrition can enhance rice growth under water-deficit condition, a hydroponic experiment in which rice plants were supplied with different N forms (NO3 −, NH4 + and mixed-N) was conducted, and the intrinsic mechanisms involved in photosynthesis, root-shoot carbon partitioning, and hormone signalling were investigated. Water stress was found to decrease rice biomass, leaf area, chlorophyll and Rubisco contents. However, mixed-N nutrition substantially alleviated these inhibitions compared with NO3 − nutrition alone. Mixed-N nutrition also maintained a higher electron transport rate, actual photochemical efficiency of PSII, and non-photochemical quenching, causing higher photosynthesis and photochemical efficiency. Water stress up-regulated leaf sucrose-phosphate synthase (SPS), but down-regulated acid invertase (InvA). However, leaf InvA and root sucrose synthase in the cleavage direction (SSc) in NO3 − nutrition was higher than that in mixed-N nutrition. Water stress decreased indole acetic acid (IAA) content in leaves and cytokinins content in roots, but their contents in mixed-N nutrition were higher than those in NO3 − nutrition. In mixed-N nutrition, the up-regulation of SPS and IAA in leaves and the reduction of sucrose metabolism (SSc and InvA) in roots jointly resulted in the accumulation of sucrose in leaves and the inhibition of its transportation to roots, finally reducing the root:shoot ratio (R/S). The reduced R/S provides more photosynthates for shoots and increases the utilisation efficiency, thereby strengthening the water-deficit tolerance of plants. We concluded that the strengthened water-deficit tolerance in mixed-N-supplied rice was closely associated with higher accumulation of dry matter mainly via improvement of photosynthesis and photochemical efficiency, hormone balance, and coupling with root-shoot carbon partitioning.