Drought is the single most important factor limiting soya bean (Glycine max L.
Merr.) yields in the field. The following study was therefore undertaken to
identify phenotypic markers for enhanced drought tolerance in nodulated soya
beans. Leaf and nodule parameters were compared in three genotypes: Prima
2000, glyphosate-resistant A5409RG and Jackson, which had similar shoot biomass
and photosynthesis rates at the third trifoliate leaf stage under waterreplete
conditions. When water was withheld at the third trifoliate leaf stage,
photosynthesis, nodule numbers, nodule biomass and symbiotic nitrogen fixation
(SNF) were greatly decreased. Significant cultivar–drought interactions
were observed with respect to photosynthesis, which also showed a strong positive
correlation with nodule SNF, particularly under drought conditions. Prima
leaves had high water-use efficiencies, and they also maintained high photosynthetic
electron transport efficiencies under long-term drought. Moreover, Prima
had the highest shoot biomass under both water-replete and drought conditions.
A-5409RG was the most drought-sensitive genotype showing early closure
of stomata and rapid inhibition of photosynthesis in response to drought.
In addition to classifying the genotypes in relation to drought tolerance, the
results demonstrate that the ability to sustain shoot biomass under nitrogen
limitation is an important parameter, which can be easily applied in germplasm
screening for drought tolerance in soya bean.