Harnessing genetic variability for root and shoot morpho-physiological traits contributing to drought stress tolerance in African spider plant gynandropsis gynandra (L) briq accession..

Abstract

African spider plant (Gynandropsis gyanandra (L.) Briq.) is a promising leafy vegetable, which is an important source of zinc, iron, calcium, magnesium, and vitamins. It forms one of the strategic crops that are being used to improve the resilience of local communities to drought. These crops, often referred to as underutilized, forgotten, neglected or orphan crops are adapted to the local environment and have potential to address climate change induced abiotic stresses such as drought. Drought is major biotic stress threatening the sustainability of agriculture and food security, especially in SSA and leads to shortened cropping seasons with significant yield reductions. This study was conducted to i) assess the variation of morpho-physiological and biochemical traits of Gynandropsis gynandra in response to drought stress, ii) estimate the heritability and variance components for morphological and physiological traits in Gynandropsis gynandra under drought stress and iii) assess drought indices together with root traits to identify tolerant African spider plant (Gynandropsis gynandra (L.) Briq.) genotypes. In the first experiment, 18 African spider plant accessions were evaluated in the tunnels at the University of KwaZulu-Natal, Pietermaritzburg Campus, South Africa under drought-stressed and non-stressed conditions using a split-split plot design with four replications across three water regimes: severe drought (30% field capacity), intermediate drought (50% field capacity), and well-watered (100% field capacity). The following morpho-physiological and biochemical traits were recorded; days to 50% flowering (Fl), plant height (Ph), leaf length (Ll), leaf width (Lw), stem diameter (Sd), chlorophyll content (Spad), relative water content (Rwc), net photosynthesis (Photo), transpiration rate (Trans), stomatal conductance (Cond), proline content (Pro), number of leaves per plant (Nl) and leaf yield (Ly). Data were subjected to the following statistical analysis: analysis of variance (ANOVA), Pearson's correlation coefficient, principal component analysis (PCA), and cluster analysis. Proline content rose significantly under stress but was inversely associated with agronomic traits under both optimal and waterlimited conditions. Two accessions with high leaf yield under drought stress and beneficial adaptive traits were identified for further use in breeding for drought tolerance in the crop species. The second experiment investigated variance components, heritability and path coefficient analysis of yield and yield-related traits in the 18 spider plant accessions. A randomized complete block design (RCBD) experiment was carried out with four replications across three water regimes: severe drought (30% field capacity), intermediate drought (50% field capacity),

and well-watered (100% field capacity). The genotypic coefficient of variation (GCV) ranged from 0.10 % (relative water content) to 46.86 % (leaf yield) under well-watered conditions, 0.11% (relative water content) to 54.42 % (leaf yield) under mild stress conditions, and from 0.00% (stomatal conductance) to 74.22% (leaf yield) under severe stress. The phenotypic coefficient of variation (PCV) ranged from 0.10% (relative water content) to 46.91% (leaf yield) under well-watered conditions, 0.11 from 0.10 % (relative water content) to 46.86 % (leaf yield) for mild stress and 0.00% (stomatal conductance) to 74.34% (leaf yield) severe stress. Under drought stress conditions, the experiment showed a high to moderate heritability estimate along with a high to moderate genetic advance for the number of leaves, leaf width, plant height, and stem diameter, indicating that these attributes are governed by additive gene action. The number of leaves per plant, plant height, days to 50% flowering, relative water content, net photosynthesis and leaf length were identified as target traits that could be used to improve spider plant leaf yield under drought-stressed conditions. In the third experiment, the same African spider plant accessions were evaluated to compare changes in root traits under intense drought stress conditions and identify drought-tolerance indices that can be utilized in selecting African spider plant genotypes. The following six drought tolerance indices were evaluated: stress susceptibility index (SSI), yield index (YI), stress tolerance index (STI), geometric mean of productivity (GMP), stress tolerance (TOL), and mean productivity (MP). Six root traits including: total root length (TRL), total root volume (TRV), root dry weight (RDW), root: shoot ratio (RR), total root area (TRA), and average root diameter (ARD) were recorded as well as leaf yield (Ly). The experiments were laid out in a randomized complete block design with three replications and two water regimes treatments namely non-stressed and drought stressed conditions. The ANOVA showed that genotypes varied in leaf yield, but non-significant values were observed for root traits. Five drought tolerance indices namely TOL, GMP, STI, MP, and YI, were discovered to be effective in selecting stress-tolerant genotypes. Root: shoot ratio was identified as a useful trait in the selection of tolerant genotypes. Overall, the study managed to capture and identify genotypes that can be used for future breeding programs for drought stress. Lastly, root traits together with stress indices that can be utilized in selecting drought-tolerant African spider plant genotypes were identified.