Leaf Expansion, Stomatal Conductance, and Transpiration of Vegetable Amaranth (Amaranthus sp.) in Response to Soil Drying

This study was designed to quantify the responses of leaf expansion, stomatal conductance, and transpiration of four genotypes of vegetable amaranth Amaranthus tricolor L. (Hin Choi), A . tricolor L. (Co. 2), A . blitum L. (WS80-192), and A . cruentus L. (RRC 1027) to soil drying. Two greenhouse experiments were conducted during 1999 and 2000. Soil water status was expressed as the fraction of transpirable soil water (FTSW). Leaf expansion rates, stomatal conductances, and transpiration rates of the stressed plants were determined relative to those of nonstressed plants, and expressed as relative leaf expansion (RLE), relative stomatal conductance (RSC), and relative transpiration (RT), respectively. The rate of soil water extraction differed among genotypes, with RRC 1027 depleting soil water fastest and Hin Choi slowest. Whereas in 1999 all genotypes were equally efficient in soil water use, RRC 1027 extracted a greater volume of transpirable soil water than the other genotypes in 2000. The responses of RLE, RSC, and RT to FTSW were well described by linear-plateau models which allowed calculation of soil-water thresholds for leaf expansion (C L ), stomatal conductance (C S ), and transpiration (C T ). Values for C L were higher than for C S and C T . C L was similar for the four genotypes in each year, whereas, C S and C T differed among genotypes. C S and C T was lowest for Hin Choi and highest for WS80-192. Differences of C L , C S , and C T between the two experiments might have been due to the different soils used in the experiments and the different evaporative demands during the drought cycles. Under drought stress, the reduction of transpiration of vegetable amaranth was due mainly to reduction of stomatal conductance, not to reduction of leaf expansion. The relative reduction of dry weight caused by drought stress was positively correlated with C S or C T across the four genotypes. Variation in C S and C T among amaranth genotypes revealed different responses to drought stress, which could make them suitable for different drought situations.