As a major oat-producing region in China, Inner Mongolia is located in an arid to semi-arid area where water availability significantly limits stable and high oat yields. This study investigates the physiological mechanisms of yield formation under different reduced irrigation regimes, aiming to provide theoretical basis and practical guidance for developing water-saving and high-yielding cultivation practices for oats in this region.

A fixed-site field experiment was conducted in Ulanqab City, Inner Mongolia, during 2022-2023. Two oat varieties with contrasting grain numbers per spike-Bayou 1 and Dingyou 8-were used. Under rain-out shelter conditions, three irrigation treatments were applied: conventional irrigation (CK), reduced irrigation at the tillering stage (RIt), and reduced irrigation at the jointing stage (RIj). After the irrigation treatments, the shelters were removed at the booting stage. Spikelet development, changes in endogenous hormones in various plant parts, and source-sink-flow characteristics in relation to grain yield were systematically analyzed to clarify the physiological responses of oats to reduced irrigation.

Oat grain yield was significantly affected by year, variety, and reduced irrigation regime. Compared with conventional irrigation, reduced irrigation at tillering decreased the two-year average yield of both varieties by 7.8%-8.9%, whereas reduced irrigation at jointing reduced yield by 17.9%-20.0%. Reduced irrigation at jointing markedly decreased the number of fertile spikelets in the central portion of the panicle while increasing the number of sterile spikelets in the basal portion, resulting in a two-year average reduction in panicle grain number and fertility of 28.2% and 11.3% in Bayou 1, and 25.8% and 15.6% in Dingyou 8. Mantel test and random forest analysis indicated that grain yield was closely associated with endogenous hormones in different organs, with reduced irrigation at jointing decreasing the two-year average (IAA+GA₃+ZR) /ABA ratios in the panicle, leaves, and roots of both oat varieties by 16.4%-32.2%, 22.2%-54.4%, and 21.9%-50.6%, respectively, while reduced irrigation at the tillering stage increased the corresponding ratios by 42.2%-54.8%, 10.3%-55.5%, and 18.4%-94.8%, respectively. Furthermore, reduced irrigation at jointing disrupted source-sink relationships, suppressed photosynthetic capacity, limited assimilate availability, and impeded vascular bundle development, thereby restricting dry matter accumulation in various organs and ultimately reducing grain yield. In contrast, reduced irrigation at tillering had minimal effects on these physiological processes.

Water deficit during the jointing stage disrupts the balance of endogenous hormones and the source-sink relationship in different oat organs, thereby affecting the development of spikelets, reducing the number of grains per panicle and the seed-setting rate, and ultimately leading to a significant decline in grain yield. Therefore, in the arid regions of Inner Mongolia, to mitigate the negative effects of water stress on oat grain yield, implementing moderate water control during the tillering stage combined with irrigation at the jointing stage, along with the selection of varieties with a higher number of grains per panicle, can effectively reduce both water wastage and yield loss.