Climate-change-induced temperature fluctuations pose significant threats to global rice production, particularly through their impact on photosynthetic efficiency. The differential mechanisms by which low and high temperatures affect leaf photosynthetic processes in rice remain poorly understood. Here, we investigate the effects of temperature stress (15 °C, 30 °C, 45 °C) on rice photosynthetic performance across a gradient of nitrogen supply levels: low nitrogen (LN), medium nitrogen (MN), and high nitrogen (HN). The low temperature exhibited stronger negative impacts on photosynthesis than the high temperature, primarily through increased mesophyll limitation and disrupted cellular CO₂ diffusion, while the high temperature showed less pronounced effects, particularly under HN and MN conditions. While photosynthetic nitrogen use efficiency (PNUE) decreased with increasing nitrogen under the optimal temperature, moderate nitrogen supply maintained optimal PNUE under temperature stress, suggesting that a balanced nitrogen level is crucial for maximizing both photosynthetic capacity and nitrogen use efficiency. Plants with adequate nitrogen maintained higher intrinsic water use efficiency (_iWUE) under both temperature extremes through improved coordination between CO₂ uptake and water loss. Our findings reveal distinct mechanisms underlying low- and high-temperature stress effects on photosynthesis and highlight the importance of optimizing nitrogen management for enhancing crop resilience to temperature extremes under climate change.

Research indicates that, owing to the enhanced grain-filling rate of tetraploid rice, its yield has notably improved compared to previous levels. Studies conducted on diploid rice have revealed that optimal planting density and fertilization rates play crucial roles in regulating rice yield. In this study, we investigated the effects of different nitrogen application and planting density treatments on the growth, development, yield, and nitrogen utilization in tetraploid (represented by T7, an indica–japonica conventional allotetraploid rice) and diploid rice (Fengliangyou-4, represented by FLY4, a two-line super hybrid rice used as a reference variety for the approval of super rice with a good grain yield performance). The results indicated that the highest grain-filling rate of T7 could reach 77.8% under field experimental conditions due to advancements in tetraploid rice breeding. This is a significant improvement compared with the rate seen in previous research. Under the same conditions, T7 exhibited a significantly lower grain yield than FLY4, which could be attributed to its lower grain-filling rate, spikelets per panicle, panicle number m⁻², and harvest index score. Nitrogen application and planting density displayed little effect on the grain yield of both genotypes. A higher planting density significantly enhanced the leaf area index and biomass accumulation, but decreased the harvest index score. Compared with T7, FLY4 exhibited a significantly higher nitrogen use efficiency (NUE_g), which was mainly due to the higher nitrogen content in the straw. Increasing nitrogen application significantly decreased NUE_g due to its minimal effect on grain yield combined with its significant enhancement of nitrogen uptake. Our results suggest that the yield and grain-filling rate of T7 have been improved compared with those of previously tested polyploid rice, but are still lower than those of FLY4, and the yield of tetraploid rice can be further improved by enhancing the grain-filling rate, panicle number m⁻², and spikelets per panicle via genotype improvement.

Heat stress exerts detrimental effects on the survival, growth, and productivity of Gentiana rhodantha Franch. ex Hemsl., a medicinal herb endemic to Southwestern China. The application of exogenous protective substances has been widely recognized as an effective and economical strategy for alleviating heat damage in crops. However, most previous studies have focused on conventional agents such as phytohormones, osmolytes, or mineral nutrients, while the potential of emerging nanomaterials and microbial-derived compatible solutes remains largely unexplored. In the present study, we investigated the effects of foliar application of two promising exogenous candidates, (i) carbon quantum dots (CQD), a newly emerged carbon-based nanomaterial with unique photochemical and electron-transfer properties, and (ii) ectoine (ECT), a microbial-derived compatible solute renowned for its exceptional protein and membrane-stabilizing capacity under abiotic stress, applied individually or in combination (CQD + ECT) on the agronomic and physiological traits of two G. rhodantha genotypes (Kaili and Dafang) under normal temperature (NT, 25/20 °C) and high temperature (HT, 35/30 °C) conditions, with H₂O application as the control. The results showed that HT inhibited plant growth, reduced canopy traits, altered root architecture, and disrupted antioxidant enzyme activities and osmolyte accumulation, with pronounced genotypic differences. The KL genotype exhibited greater inherent heat tolerance than the DF genotype, as reflected by relatively less injury to canopy traits and root architecture, as well as more favorable physiological adjustments under HT. Exogenous application of CQD and ECT, particularly their combination, generally mitigated heat-induced injury. Under HT, the combined CQD + ECT treatment preserved canopy structure and root morphology, enhanced SPAD values and leaf nitrogen concentrations, and modulated antioxidant enzyme activities and osmolyte levels, with the most pronounced effects observed in the KL genotype. The combined application of CQD and ECT consistently exhibited superior protective effects across multiple parameters, suggesting that it represents a novel and promising strategy for alleviating heat injury in G. rhodantha cultivation. These findings provide new insights into the application of nanomaterial and compatible solute-based approaches for improving heat tolerance in medicinal plants.