Leaf water relations of vegetable amaranth (Amaranthus spp.) in response to soil drying

Amaranth is a promising vegetable crop species grown under semi-arid conditions. However, until now the water relations characteristics and the possible mechanisms that are responsible for its adaptation to drought stress have not been investigated in this species. This study was initiated to determine the effects of soil drying on leaf water relations and osmotic adjustment (OA) in four genotypes of vegetable amaranth, viz. 'Hin Choi', 'Co.2', 'WS80-192' and 'RRC 1027' with contrasting morphological characteristics. The plants were grown in a temperature-regulated greenhouse in pots with loamy sand. Midday leaf water potential (LWP), leaf osmotic potential (LOP), and relative water content (RWC) were followed in well-watered and drought-stressed plants during soil drying until all the transpirable soil water was used. The relative capacitance (C-r) of amaranth leaves was about 0.14 MPa-1 for Hin Choi, Co.2 and WS80-192, but only 0.07 MPa-1 for RRC 1027. The ratio between turgid weight and dry weight (TW/DW) of amaranth leaves ranged from 8 to 12 in well-watered plants and decreased significantly by drought stress to 6-9. OA was observed in all the four genotypes with the highest in RRC 1027 (1.24 MPa) and lowest in Hin Choi (1.08 MPa) in severely stressed plants. The degree of OA positively correlated to the relative reduction of TW/DW. This result suggested that changes in TW/DW might provide a simple screening method for OA in vegetable amaranth. Except for Hin Choi, the relative yield reduction in others negatively correlated with OA. Conclusively, the leaf water relations of vegetable amaranth were characterised by a high capacity of OA that may sustain turgor maintenance; hence dry matter production during drought stress. (C) 2002 Elsevier Science B.V. All rights reserved.