Progress on Physiological Mechanisms of Rice Spikelet Degeneration at Different Panicle Positions Caused by Abiotic Stress

Abstract

Rice yield is heavily reliant on the number of spikelets per panicle, a factor determined by the processes of spikelet differentiation and degeneration. In rice cultivars with large panicles, spikelet degeneration negates the advantage of large panicle and constrains the potential for rice yield. Environmental stress-induced metabolic disorders in plants aggravate spikelet degeneration, with the sensitive period for this process commencing approximately 15‒20 d before panicle heading. A notable positional variation is observed within the panicle, with a higher spikelet degeneration ratio occurring at the basal compared with the upper position. An imbalance of carbon and nitrogen metabolism is the primary physiological cause for the aggravation spikelet degeneration under abiotic stress. Reduced carbon and nitrogen metabolism leads to disordered energy metabolism and disrupted respiratory electron transport, which accelerates the apoptosis of young spikelets through an excessive accumulation of reactive oxygen species. Sucrose serves as the main carrier for providing carbohydrates essential for spikelet development, and its utilization demonstrates apical dominance favoring spikelet formation. However, under abiotic stress, the inhibition of sucrose decomposition, rather than the blockage of sucrose transport, contributes to aggravated spikelet degeneration at the basal position of panicles. Brassinolide and auxin have a significant relationship with spikelet formation, potentially mediating apical dominance. Brassinolide can effectively enhance the accumulation and utilization of sucrose, thereby alleviating spikelet degeneration. At present, the mechanisms underlying rice spikelet degeneration have not been fully revealed, and the joint effects of hormones, carbohydrates, and carbon and nitrogen metabolism on this process require further investigation. To reduce the spikelet degeneration, the strategic application of water and fertilizer to establish a stable rice population can enhance the rice plants’ resilience to abiotic stress. An effective approach to reducing spikelet degeneration is to increase the dry matter occupancy of each spikelet during the panicle initiation period.