Ye Wang

Nitrogen (N) availability significantly influences plant metabolism and productivity. The aim of this study was to assess the effects of low N stress and subsequent N supplementation on key enzymes of nitrogen metabolism, nitrogen metabolism-related substances, and chlorophyll a fluorescence kinetic parameters in rice genotypes with different nitrogen utilization efficiencies. We used the Jijing 88 (low-N tolerant) and Xinong 999 (low-N sensitive) as test materials. During the seedling, tillering, and booting stages, the 1/2N and 1/4N treatments were restored to the 1N treatment level. Nine treatments were used in this experiment: CK (1N), A1 (1/2N), A2 (1/2N restored to 1N during the seedling stage), A3 (1/2N restored to 1N during the tillering stage), A4 (1/2N restored to 1N during the booting stage), B1 (1/4N), B2 (1/4N restored to 1N during the seedling stage), B3 (1/4N restored to 1N during the tillering stage), and B4 (1/4N restored to 1N during the booting stage). Key physiological responses, nitrogen compounds, enzymes activities, and chlorophyll a fluorescence kinetics were analyzed. Under low nitrogen conditions, the growth and nitrogen assimilation of rice were inhibited. Compared to XN 999, JJ 88 maintains higher levels of dry matter, nitrate reductase activity (NR), glutamine synthetase activity (GS), glutamate oxaloacetate transaminase activity (GOT), glutamate pyruvate transaminase activity (GPT), as well as nitrate (NO₃⁻) and ammonium (NH₄⁺) nitrogen contents. After N supplementation during the early growth stage, both JJ 88 and XN 999 exhibit recovery capabilities. However, in the late growth stage, JJ 88 demonstrates superior recovery capabilities. In addition to enhancing nitrogen metabolism levels, there is also an increase in the content of osmotic regulation substances such as soluble sugars, free amino acids, and proline, along with responses in chlorophyll fluorescence kinetic parameters. This was primarily manifested in the enhancement of performance index (PI_ABS, PI_total), and quantum yield (φ_EO, φ_RO, ψ_EO), which maintain photosynthetic performance and electron transport efficiency. The research findings indicated that reducing N supply during the early growth stage and restoring N levels in the later stage are beneficial for the recovery of low-nitrogen-tolerant rice varieties. Therefore, in the context of sustainable agricultural production, the breeding of low-nitrogen-tolerant rice varieties and the optimization of N fertilizer management are crucial.

Rice is a globally important food crop, and phosphorus is an essential nutrient element for rice growth. In many of China’s arable lands, there is a deficiency in available phosphorus content. Therefore, screening and breeding rice germplasm resources that are tolerant to low phosphorus can enhance the growth capability of rice in low-phosphorus soils. This study set up treatments with two phosphorus concentrations: H₂PO₄⁻ at 0.18 mmol/L, referred to as normal phosphorus (NP), and H₂PO₄⁻ at 0.009 mmol/L, referred to as low phosphorus (LP). Using hydroponic methods, 156 different genotype rice germplasms were treated for 35 days, after which the morpho-physiological traits of the rice seedling shoots, root morphology, and material content were measured. An analysis of the coefficient of variation (CV) for low phosphorus tolerance coefficients across different rice germplasm resources revealed that 16 indicators had CVs greater than 10%, which can be used as criteria for screening rice varieties with low phosphorus tolerance at the seedling stage. The relevant indicators and low-phosphorus resistance characteristics of different rice varieties were comprehensively evaluated using principal component analysis, correlation analysis, membership function, and cluster analysis methods. The results indicate that the principal component analysis transformed 23 indicators into 5 comprehensive indicators, with a cumulative contribution rate of 86.947%. The D value was evaluated in a comprehensive evaluation of low-phosphorus resistance, and 156 rice germplasm resources were divided into four types by cluster analysis. A scatter plot was created using the comprehensive phosphorus efficiency values of different rice germplasms under normal phosphorus and low phosphorus conditions. Through further verification, the germplasms with strong low-phosphorus tolerance finally selected through comprehensive screening were Y3-14, Y3-35, Y3-21, Jinnongda 705, Changjing 625, and Jinnongda 873. The germplasms with poor low-phosphorus tolerance were Jijing 338, Jingu 981, Tong 35, Y3-31, and Longdao 20.

Nitrogen is a key element in promoting crop growth and development and improving photosynthesis. This study aimed to study the response of two rice genotypes to the restoration of N supply after varying periods of N deficiency. We used the low-nitrogen-tolerant rice Jijing 88 (JJ 88) and the nitrogen-sensitive rice variety Xinong 999 (XN 999) as test materials. The results of this study indicated that, compared to XN 999, JJ 88 has a higher content of the photosynthetic pigments. Photosynthesis in JJ 88 has strong adaptability under low-nitrogen conditions. Upon an increase in the nitrogen supply level, the maximum regeneration rate of ribulose biphosphate (RuBP, J_max) and the maximum carboxylation rate of RuBP (V_cmax) in JJ 88 showed a relatively large increase. The chlorophyll fluorescence parameters, including the effective quantum yield of photosystem II (Φ_PSII), the efficiency of excitation capture by open PSII centers (Fv′/Fm′), photochemical fluorescence quenching (qP), and the electron transfer rate (ETR) decreased slightly, while the non-photochemical fluorescence quenching (NPQ) increased slightly. Under low-nitrogen conditions, low-nitrogen-tolerant rice varieties maintain reasonable growth during the seedling stage. With an increase in the nitrogen supply level, the dry matter accumulation, photosynthetic pigment content, photosynthesis, and electron transfer ability of plants improve, but not to normal nitrogen supply levels. However, compared with XN 999, JJ 88 has a more proactive recovery ability. The research results provide valuable guidance for the breeding of nitrogen-efficient rice varieties and nitrogen fertilizer management.