Effects of root characteristics on panicle formation in japonica rice under low temperature water stress at the reproductive stage

Cold stress during the reproductive stage causes spikelet sterility and limits rice productivity in high-altitude and high-latitude regions. In higher plants, phytohormones regulate root and panicle development under environmental stress. Maintaining high root physiological activity during the reproductive stage may be a strategy for high yields in crops under abiotic stress. However, little is known about the root physiological activity and panicle size response mechanisms associated with rice cold tolerance at the reproductive stage. In this study, we investigated the effects of low root zone water temperature (Tw) during reproductive growth on root physiological characteristics and root phytohormones associated with panicle development in japonica rice from 2018 to 2019. The rice plants were subjected to 17 °C low Tw for 5, 10, and 15 days during reproductive growth. The results showed that, compared with the control, rice pollen grain development was hindered, and the percentage of fertile pollen decreased under low Tw; the number of surviving total panicle branches decreased significantly; the number of degraded branches increased significantly; the range of change increased gradually with the extension of low Tw stress time; and the impact of low Tw stress on the secondary branches and spikelets was greater than on the primary branches and spikelets. The abundance of zeatin (ZR) and auxin (IAA) in the roots, the root physiological activity, and the number of surviving spikelets and branches had a significant positive correlation in variety DN428. DN428 exhibited strong cold tolerance and possessed a high level of ZR and IAA in the roots, which allowed it to maintain root antioxidant enzyme activity, root vitality, and the root absorption surface area at a relatively stable level, resulting in the roots not accumulating large amounts of soluble protein, soluble sugar, malondialdehyde, and proline. This facilitated the accumulation and transfer of photosynthetic materials above the ground and provided a stable material foundation for spikelet and branch formation.