Genotypic variation for drought stress response traits in soybean. I. Variation in soybean and wild Glycine spp. for epidermal conductance, osmotic potential, and relative water content

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Studies were undertaken to assess genotypic variation in soybean and related wild species for traits with putative effects on leaf turgor maintenance in droughted plants. Traits of interest were (i) epidermal conductance (ge) which influences the rate of water loss from stressed leaves after stomatal closure; (ii) osmotic adjustment (OA) as indicated by tissue osmotic potential (π), which potentially affects the capacity to withdraw water at low soil water potential; and (iii) relative water content (RWC) at incipient leaf death (critical relative water content, RWCC), which is a measure of the dehydration tolerance of leaf tissue. The germplasm comprised a diverse set of 58 soybean genotypes, 2 genotypes of the annual wild species G. soja and 9 genotypes representing 6 perennial wild Glycine spp. indigenous/endemic to Australia. Seedling plants were grown in soil-filled beds in the glasshouse and exposed to terminal water deficit stress from the second trifoliolate leaflet stage (21 days after sowing). Measurements were made on well watered plants, moderately stressed plants, and at incipient plant death, in 2 separate studies. In both studies, there were significant genotypic differences in all 3 traits in the stressed plants. However, across the 3 sample times, ge decreased and the absolute magnitude of π increased, indicating that the expression of these traits changed as the plants acclimated to the stress. RWC was therefore used as a covariate to adjust the genotypic values of π and ge in order to facilitate comparison at a consistent plant water status of 70% RWC. There was statistically significant genotypic variation for the adjusted values, ge70 and π70, in both studies, and genotypic correlations between the 2 studies were significant (P < 0.05) and positive for all 3 traits: ge70 (r = 0.48), π70 (r = 0.50), and RWCC (r = 0.53). Among the soybean genotypes, there was at least a 2-fold range in ge70, a 0.7 MPa range in π70, and a 12 percentage point range in RWCC. Some of the perennial wild genotypes exhibited lower values of ge and RWCC and greater OA than soybean and G. soja, consistent with adaptation to drier environments. While the repeatability of measurement between experiments was variable among genotypes, the studies confirmed the existence of genotypic differences for ge, OA, and RWCC in cultivated soybean, with a wider range among the wild germplasm.