Elucidating the Process of Decline in NERICA Upland Rice Production Caused by Water Shortage to Identify Effective Water Use Functions to Sustain Production

Abstract

The superior productivity under drought conditions in New Rice for Africa (NERICA) upland rice is expected to overcome low yields in sub-Saharan rainfed regions of Africa. However, the core processes and contributing functions of the productivity of this rice under drought are not fully understood. Biomass production (BP) is one component of grain production (GP) (GP = BP × HI, where HI is harvest index) and BP is indicated by the water use efficiency coefficient (k) × transpiration per vapour water deficit of air (T/VWD). Our objective was to determine which of k, T/VWD, and HI strongly contributed to the maintenance of GP during drought conditions in the reproductive stages, thereby identifying a key function in the water use process that maintains GP in NERICA upland rice under drought conditions. First, the k and T/VWD values in four NERICA upland cultivars and three Oryza sativa cultivars with contrasting traits for drought resistance were compared in a 4 L pot held under three different field capacities for 14 days. k was approximately constant under different soil moisture contents and mainly T/VWD changed BP. Second, the responses of T/VWD to soil drying in these seven cultivars were compared in 15 L pots for 10 days. The ratios of T/VWD in desiccated soil to watered control plants (T/T0) in all cultivars similarly decreased with a decrease in the fraction of transpirable soil water (FTSW). Third, the FTSW values were compared for two NERICA upland cultivars and one drought sensitive O. sativa cultivar selected from these seven cultivars in 31 L pots with depths of 1 m irrigated at four different soil depths. The FTSW values weighed by root distribution in NERICA upland cultivars watered deep in their soils were higher than those in the O. sativa cultivars, resulting in higher BP, GP, and HI values. These results indicate that the process by which drought reduced grain production in NERICA upland rice was as follows: the decreased FSW caused by reductions in water supply suppressed biomass production by reducing the transpiration level and moreover, the reduced harvest index due to sterility. Reductions of biomass production and harvest index decreased grain production. Hence, greater FTSW due to more developed roots could be a key elemental function for maintaining rice productivity due to keeping transpiration and harvest index.