Physio-biochemical and morphological screening of F4 vegetable-type soybean progeny lines under water-limited stress conditions

Vegetable-type soybean ( Glycine max L.) has emerged as a high-protein crop of increasing importance in South Africa; however, its production is constrained by drought associated with semi-arid conditions and erratic rainfall. Yield reductions of 25–85% under water-limited stress conditions underscore the need to improve drought tolerance. This study evaluated the morphological, physiological, and biochemical responses of 12 F₄ vegetable-type soybean progeny lines derived from the cross AGS354 × UVE17 under water-limited stress (WS) conditions. The parental genotypes were initially selected based on their stress tolerance indices, which were determined during the preceding drought experiment. The parents and 12 F₄ progeny lines (2.1/1, 2.1/2, 2.11/3, 2.12/2, 2.3/1, 2.3/2, 2.3/3, 2.4/3, 2.5/1, 2.6/1, 2.6/2, and 2.6/3) were grown under controlled glasshouse conditions and subjected to WS conditions (30% soil water-holding capacity) in a randomised complete block design with two replications per treatment. Chlorophyll fluorescence parameters, total chlorophyll content (Chl T ), electrolyte leakage, and selected morphological traits were evaluated. In general, WS significantly affected both the physiological and morphological traits. Chlorophyll fluorescence parameters, particularly PI ABS and PI Total , and ẟ Ro differed significantly ( p  ≤ 0.01) between the control and WS treatments. Principal component analysis indicated strong associations between photosynthetic traits and lines 2.3/1 and 2.6/3, whereas lines 2.11/3, 2.3/2, 2.6/1, 2.6/2, 2.1/2, and 2.4/3 were associated with yield-related traits, including total seeds per plant, shoot length, number of pods per branch, and seed mass per plant. Based on the combined physiological, biochemical, and morphological responses observed in this study, we identified lines 2.4/3, 2.11/3, 2.1/2, and 2.6/1 as promising progeny lines for further advancement in the vegetable-type soybean breeding programme. These findings support the integration of physiological and biochemical traits into early-generation drought screening in vegetable-type soybean breeding programmes, although field validation is still required.