The aim of this study was to investigate carbohydrate metabolism in rice seedlings subjected to salt-alkaline stress. Two relatively salt-alkaline tolerant (Changbai 9) and sensitive (Jinongda 138) rice cultivars, grown hydroponically, were subjected to salt-alkaline stress via 50 mM of salt-alkaline solution. The carbohydrate content and the activities of metabolism-related enzymes in the leaves and roots were investigated. The results showed that the contents of sucrose, fructose, and glucose in the leaves and roots increased under salt-alkaline stress. Starch content increased in the leaves but decreased in the roots under salt-alkaline stress. The activities of sucrose-phosphate synthase, sucrose synthase, amylase, and ADP-glucose pyrophosphorylase increased whereas the activities of neutral invertase and acid invertase decreased in the leaves under salt-alkaline stress. The activities of sucrose-phosphate synthase, sucrose synthase, amylase, neutral invertase, and acid invertase increased in the roots under salt-alkaline stress. In conclusion, salt-alkaline stress caused the accumulation of photosynthetic assimilates in the leaves and decreased assimilation export to the roots.

Context In recent years, direct-seeded rice (DSR) cultivation methods have been widely adopted in Asia. Aims The objective of this study was to evaluate the grain yield, nitrogen use efficiency and associated plant traits of DSR in northeastern China. Methods A 2-year field experiment was conducted using two varieties under three cultivation methods (1) conventional transplanted rice (CTR); (2) wet direct-seeded rice (WDSR); and (3) dry direct-seeded rice (DDSR). Key results The grain yield of CTR and DSR ranged from 10.16 to 9.39 t ha−1 and from 8.85 to 6.90 t ha−1, respectively, accross varieties and years. The wide difference was attributed to the differences in spikelet number per panicle and biomass. Compared with the CTR, the DSR decreased the spikelet number per panicle, which resulted in lower spikelet number per m2. Meanwhile, a significant positive correlation was observed between spikelet number per m2 and nitrogen uptake. The nitrogen uptake of CTR and DSR ranged from 151 to 153 kg ha−1 and from 96 to 132 kg ha−1, respectively. The root biomass, root length and zeatin + zeatin riboside contents in roots were higher in CTR than DSR, which contributed to the higher nitrogen uptake. Conclusions These results suggest that DSR reduced nitrogen uptake, thereby resulting in decreased number of spikelets per panicle, and hence lower rice grain yield. Implications These findings have highlighted the deficiencies in direct seeded rice system, and further research is required to overcome these deficiencies.

Lodging is an important factor that limits rice yield and the large‐scale promotion of direct‐seeded rice (DSR). The objective of this study was to clarify the relationship between the characteristics of basal internodes and lodging and the physiological mechanism underlying this process in DSR. A field study was conducted in Changchun, Jilin Province, China, using a japonica rice variety Jiyujing with two direct seeding cultivation methods, including dry DSR (DDSR), wet DSR (WDSR) and conventional‐transplanted rice (CTSR) as a control in 2019 and 2020. Lodging‐related physical parameters, morphological characteristics and carbohydrate components of basal internodes were investigated at heading stage (HS) and 30 days after heading stage (HS30). The results showed that WDSR increased lodging index (LI) and lodging rate compared with CTSR and DDSR. LI increased rapidly from HS to HS30, primarily because of the significant reduction in the breaking strength (M). Correlation analysis revealed that the M of N4 internode was significantly positively correlated with culm plumpness and structural carbohydrate proportions at HS30. Culm plumpness decreased significantly, due to a decrease in nonstructural carbohydrate (NSC) content then primarily the decrease in starch proportions and content. Compared with CTSR and DDSR, WDSR decreased culm wall thickness, dry weight per centimetre of the culm and leaf sheaths, and the proportions and contents of cellulose, lignin and starch of internode, resulting in the decrease in internode breaking strength. Thus, it was concluded that the DSR reduced internode strength by reducing internode plumpness and carbohydrate content, thus decreasing lodging resistance.

Direct-seeded rice has been widely adopted in Asia in recent years. However, its low and unstable yields severely limit the increase in rice production and directly affect food security. The aim of this research was to investigate the effects of different concentrations of paclobutrazol seed soaking on the biomass production and yield formation of direct-seeded rice and to find the optimal concentrations to provide a theoretical basis for the high-yielding cultivation of direct-seeded rice. Two rice varieties, Jiyujing and Jijing305, were used in this experiment, and the seeds were soaked in paclobutrazol at concentrations of 0, 50, 100, 150, and 200 mg L−1. The appropriate concentration of paclobutrazol seed soaking increased the yield of direct-seeded rice; this was attributed to the increase in the spikelet per unit area and the seed setting rate. However, the too-high paclobutrazol concentration was not conducive to a yield increase. Regression analysis showed that the appropriate concentration of paclobutrazol seed soaking was 100–105 mg L−1. The appropriate concentration of paclobutrazol for seed soaking ensured the seedling emergence rate; improved the seedling quality, stem tillers, and productive tillers; increased the efficient leaf area, chlorophyll content, and net photosynthetic rate, resulting in an increase in dry matter accumulation; coordinated the source-sink relationship and dry matter distribution; and promoted the material translocation from the “source” to the “sink”. In conclusion, using the appropriate concentration of paclobutrazol for seed soaking is an effective means of obtaining high yields of direct-seeded rice.