Pre-breeding of okra (abelmoschus esculentus [L.] moench) for drought tolerance.

Abstract

Okra (Abelmoschus esculentus [L.] Moench; 2n = 2x = 130) is an important vegetable and oil crop. It is extensively grown in tropical and subtropical regions with limited and erratic rainfall conditions. A lack of improved cultivars with drought tolerance hinders the production of okra in sub-Saharan Africa (SSA). Considerable phenotypic and genotypic variation present in okra genetic resources from SSA useful for cultivar design with enhanced fresh pod and oil yields and drought tolerance. However, the genetic diversity in SSA’s okra germplasm collection is yet to be explored for breeding targeting economic and horticultural traits. There has been limited progress in the breeding of okra for drought tolerance. Therefore, the specific objectives of this study were i) to determine the response of selected okra genotypes to drought stress using fresh fruit yield and yield-related traits to identify and select candidate genotypes for drought tolerance breeding, ii) to determine genetic diversity present among okra accessions using simple sequence repeats (SSR) and complementary phenotypic markers and to select genetically divergent and superior parental accessions for pre-breeding, iii) to assess the levels of drought tolerance in preliminarily selected okra accessions based on leaf gas exchange and chlorophyll fluorescence to determine best-performing genotypes for drought-tolerance breeding and iv) to determine the combining ability and heterosis of selected okra accessions for yield and yield-related traits to identify superior parents and progenies for breeding.

The first part of the study involved 26 okra genotypes that were evaluated in glasshouse and field environments under drought-stressed (DS) and non-stressed (NS) conditions using a 13 × 2 alpha lattice design with two replications. The findings revealed significant (P < 0.05) genotype x testing environment x water condition interaction effects for most traits, allowing for the selection of okra genotypes suited for drier conditions. Yield per plant (YPP) positively and significantly correlated with fresh pod length (FPL) (r = 0.66; P ≤ 0.001), dry pod weight (DPW) (r = 0.80; P ≤ 0.001) and number of pods per plant (NPP) (r = 0.58; P ≤ 0.001) under DS condition in the field environment. The study identified genotypes with high yield and other desirable phenotypic attributes, which are useful genetic resources for future crosses and the selection of promising progenies based on combining abilities analyses and heritability under water-limited environments.

In the second study, 26 preliminarily selected okra accessions were assessed using nine highly polymorphic SSR markers and phenotyped under DS and NS environmental conditions using a 13 × 2 alpha lattice design with two replications. The SSR markers revealed a mean heterozygosity value of 0.54, indicating moderate genetic diversity among the tested okra accessions. Cluster analysis based on phenotypic and SSR markers differentiated the accessions into three distinct genetic groups. Pod yield per plant (PYPP) was positively and significantly correlated with fresh pod length (FPL) (r = 0.81), above-ground biomass (ABG) (r = 0.69), and harvest index (HI) (r = 0.67) under DS conditions, and FPL (r = 0.83) and AGB (r = 0.60) under NS conditions. Genetically complementary accessions such as LS04, LS05, LS06, LS07, LS08, LS10, LS11, LS15, LS18, LS23, LS24, and LS26 were identified for their high yield potential and related yield-improving traits under DS conditions. The identified accessions were recommended as parents for hybridization and selection programs to improve the yield potential of okra under drought-stressed environments.

In the third part of the study, 26 genetically diverse okra accessions were screened for physiological traits response under NS and DS conditions in a controlled glasshouse environment using a 13 × 2 alpha lattice design and three replications in two growing seasons. Statistical analyses revealed a significant genotype × water condition interaction effect for transpiration rate (T), net CO2 assimilation (A), intrinsic water use efficiency (WUEi), instantaneous water use efficiency (WUEins), minimum fluorescence (Fo′), maximum fluorescence (Fm′), maximum quantum efficiency of photosystem II photochemistry (Fv′/Fm′), the effective quantum efficiency of PSII photochemistry (ɸPSII), photochemical quenching (qP), nonphotochemical quenching (qN) and relative measure of electron transport to oxygen molecules (ETR/A). The results suggested variable drought tolerance of the studied okra accessions for selection. Seven principal components (PCs) contributing to 82% of the total variation for assessed physiological traits were identified under DS conditions. Leaf gas exchange parameters, T, A and WUEi, and chlorophyll fluorescence parameters such as the ɸPSII, Fv′/Fm′, qP, qN, ETR and ETR/A had high loading scores and correlated with WUEi, the ɸPSII, qP and ETR under DS conditions. The study identified drought-tolerant accessions, namely LS05, LS06, LS07 and LS08 based on high A, T, Fm′, Fv′/Fm′ and ETR, and LS10, LS11, LS18 and LS23 based on high AES, Ci, Ci/Ca, WUEi, WUEins, ɸPSII and AES. The selected genotypes are high yielding (≥5 g pods/plant) under drought stress conditions. The data presented will complement phenotypic data and guide breeding for water-limited agroecologies.

Eight selected okra genotypes were crossed in the fourth part of the study to generate new genetic combinations and breeding populations. The parents were selected based on their high yield potential and tolerance to drought stress. The genotypes were sourced from the Agricultural Research Council-Vegetable, Industrial and Medicinal Plants (ARC-VIMP), South Africa, assembled from diverse regions of origin. The selected eight parents were crossed using an 8 × 8 half diallel mating design during the 2021 cropping season. The parents were planted under field conditions at the ARC-VIMP research station during the 2021/2022 growing season. Subsequently, 28 new generations were developed. The crosses and eight parents were field evaluated using a 12 × 3 lattice design with three replications. The genotypes were evaluated under NS and DS conditions at two locations, namely the ARC – Loskop and ARC – Brits sites. Significant (P<0.01) effects of genotype, environment, and genotype × environment interaction was recorded for fresh pod yield and component traits. General combining ability (GCA) and specific combining ability (SCA) effects were significant (P<0.05) for most traits, indicating the role of additive and non-additive gene action underlying the inheritance of the assessed traits. The GCA × environment and SCA × environment interaction effects were significant for days to flowering (DTF), number of leaves per plant (NOL), fresh pod length (FPL), number of fresh pods per plant (NFPP) and pod yield per plant (PYP). Parental genotypes LS09, LS10 and L24 showed positive GCA effects for PYP under DS conditions and were selected to be valuable germplasm for variety design to widen genetic variability for drought tolerance and yield-related traits. Crosses LS01 × LS17, LS01 × LS18, LS09 × LS10, LS09 × LS18, LS09 × LS24, LS15 × LS18, LS15 × LS21, LS15 × LS24 and LS17 × LS21 expressed positive SCA effects for PYP under DS condition and are recommended for genetic advancement, production, and commercialization in water-scarce environments of South Africa. Overall, the study discerned considerable genetic diversity among the evaluated okra genotypes. Further, the study selected parental lines and new families with good product profiles, drought tolerance and combining ability for genetic advancement and variety design for water-limited environments in South Africa and similar agroecologies.