Root growth dynamics and yield responses of rice (Oryza sativa L.) under drought-Flood abrupt alternating conditions

Drought and flash flooding are two common water stresses that negatively affect the rice growth and development in South China. Unfortunately, in some cases, rice plants have to experience these two stresses within a short time. A study on root plasticity responses and their relationships with grain yield (GY) in rice under drought-flood abrupt alternating (DFAA) water stresses at jointing-booting stage was conducted in a specially designed experimental pool. The two-year outdoor pot experiment was composed of six DFAA treatments with combinations of different droughts and floods. Three additional treatments, namely, the drought-followed-by-no-flood (DNF), no-drought-followed-by-flood (NDF), and control treatment, were prepared for each of the DFAA treatment. Results showed that responses of root and yield related traits in rice to DFAA were not the additive effect of drought alone and flooding alone and there existed interaction (positive/negative compensation) between them. DFAA increased bleeding intensity (BI) significantly at the end of flooding, but the increases were less than those of DNF. Compared to DNF and NDF, DFAA showed the highest reduction in total root length, root and shoot dry weight, root mean diameter, root surface area (RSA), and root volume at the end of flooding. However, specific root length and RSA/root volume ratio increased. Root/shoot ratio in DFAA and DNF varied from 2016 to 2017, while it was enhanced in NDF in both years. During recovery period, differences in root traits among treatments still existed. The preceding drought weakened the effects of subsequent flooding on GY in rice. The yield losses in DNF, DFAA, and NDF (reduced by 13.0%, 24.9%, and 33.5% on average, respectively) were mainly attributed to a smaller total number of spikelets and thinner filled spikelets. The percentage of filled grains (FG) depended on the degrees of drought and flooding. Rewatering after drought tended to increase FG, while flooding after drought tended to decrease it. Yield-related traits in DFAA were significantly associated with most of root traits at the end of flooding, while these correlations were not significant in DNF and NDF.