Nitrogen (N) deficiency critically impairs leaf photosynthetic capacity and triggers premature senescence. However, the physiological metabolism underlying N deficiency-induced leaf senescence and its relationship with the varying mineral nutrients and reactive oxygen (ROS) concentration in leaf tissues are not well understood. In this paper, the premature senescence of flag leaves (psf) mutant and its wild type (WT) were employed to clarify the senescent-associated changes in the contents of several mineral elements, including potassium (K), manganese (Mn), magnesium (Mg), and iron (Fe) in leaf and root tissues under different N regimes. The roles of K and Mn in regulating ROS generation during N deficiency-induced senescence were further verified by exogenous K and Mn treatments. Results showed that N deficiency accelerated leaf senescence and led to a significant increase in ROS and Mn content in both leaf and root tissues, concurrently with N deficiency-induced declines in K, Mg and Fe contents in senescing leaves. In contrast, sufficient N supply delayed leaf senescence and reduced ROS and Mn accumulation in leaf tissues. The changes in ROS and Mn under varying N conditions exhibited an inverse relationship with the variations in K, Mg, and Fe. K deficiency exacerbated N starvation-induced leaf senescence and promoted ROS accumulation by suppressing antioxidant enzyme activity. Conversely, exogenous K incubation at higher concentration inhibited excessive ROS accumulation and retarded leaf senescence under N deficiency. Furthermore, the elevated Mn accumulation under N-deficiency impaired ROS scavenging capacity of antioxidant enzymes, leading to oxidative stress and excessive ROS. Increased Mn levels in leaves and roots further aggravated N deficiency-induced leaf senescence by triggering ROS burst. N deficiency upregulated the expression of key K⁺ transporter genes (OsHAK1 and OsHAK5) and stimulated K⁺ efflux from leaf tissues. The elevated accumulation of Mn coupled with the loss of K in senescing leaves acts as an important regulatory mechanism driving N deficiency-induced leaf senescence.

Based on known cDNAs of rice starch synthase isoforms, we constructed dsRNA interference vectors for starch synthase I (SSI) to produce transgenic plants containing starch with a moderately high amylose content. We investigated the effect of SSI suppression on grain quality traits, starch biosynthesis, and amylopectin chain distribution in rice plants exposed to two different temperature regimes. The activities and transcripts of BEs, DBEs, and other SS isoforms were further investigated to clarify the effect of SSI suppression on these key enzymes and their specific isoforms under different temperature treatments. Suppression of SSI by RNAi altered grain starch component and amylopectin chain distribution, but it exerted only a slight effect on total starch content (%) and accumulation amount (mg kernel⁻¹) and on starch granule morphology and particle size distribution. Under normal temperature (NT), insignificant differences in kernel weight, chalky kernel proportion, chalky degree, and starch granule morphology between SSI-RNAi line and its wild type (WT) were observed. However, amylose content (AC) level and granule-bound starch synthase (GBSS) activity in rice endosperms were markedly increased by SSI-RNAi suppression. The chalky kernel proportion and chalky degree of SSI-RNAi lines were significantly higher than those of WT under high temperature (HT) exposure at filling stage. Inhibition of SSI by RNAi affected amylopectin chain distribution and raised starch gelatinization temperature (GT) in two ways: directly from the SSI deficiency itself and indirectly by reducing BEIIb amounts in an SSI-deficient background. The deficiency of SSI expression led to an alteration in the susceptibility of grain chalkiness occurrence and starch gelatinization temperature to HT exposure, owing to a pleiotropic effect of SSI deficiency on the expression of other genes associated with starch biosynthesis.