Root Morpho-Physiological Characteristics and Yield Formation of Rice Under Combined Salinity-Drought Stress

Because of inadequate freshwater resources and poor irrigation facilities, salinity and drought often co-occur for rice production in saline lands. The root is the primary and most vulnerable organ for detecting and perceiving salinity and drought stresses in soil. Still, little information is available on the root morpho-physiological characteristics and grain yield of rice when subjected to the combined salinity-drought stress. The present study was conducted under two salinity levels (NS, non-salinity treatment; S, salinity treatment) and three drought levels imposed from jointing to heading (ND, non-drought treatment; MD, moderate drought treatment; SD, severe drought treatment). Salinity and drought treatments shortened the duration from heading to maturity by 5-9 days and total growth duration by 3-6 days. Grain yield was reduced (p < 0.01 or p < 0.05) under salinity and drought, and the reduction was more significant under their combined stress. The aggravated yield loss under the combined salinity-drought was attributed to lower yield components relative to salinity and drought alone. The combined stress caused greater decreases in root and shoot biomass and root/shoot ratio at heading and maturity, although there was an increase in harvest index. Individual salinity and drought, and especially their combined stress, reduced root length and root volume at heading and maturity and increased reduction rates of root length and root volume after heading. Root-bleeding rate and root oxidative activity after heading were decreased under salinity and drought, and the decreases were greater under the combined salinity-drought stress; similar trends were detected for flag leaf photosynthetic rate and zeatin (Z) and zeatin riboside (ZR) contents in the root-bleeding sap and flag leaf. Our results suggested a greater yield penalty of rice when subjected to the combined stress of salinity and drought. Individual salinity, drought, and especially their combined stresses deteriorated root morphology and physiology, which shortened growth duration, accelerated plant senescence, weakened leaf photosynthesis and biomass accumulation, and led to poor grain yield.