Variation in Drought Tolerance Attributes Among Tepary Bean (Phaseolus acutifolius) Germplasm

Abstract

Tepary bean (Phaseolus acutifolius A. Gray) is an important food legume which originated from South America. In South Africa, it is cultivated by smallholder growers mainly in the drought prone Sekhukhune District of Limpopo Province. Currently, there are no significant breeding efforts aimed at cultivar development of this crop and it remains under-utilized despite the potential of the crop. Therefore, this study evaluated drought tolerance and growth attributes of the tepary bean emphasising on the leaf proline content that are associated with drought tolerance directly or indirectly. The study also determined the drought tolerance and growth relationships as well as identified potentially superior genotypes of tepary bean. The germplasm was evaluated before and after the soil moisture stress treatment which was imposed on the trial by withholding water for 21 days. A 6 x 7 rectangular lattice design replicated three times was used for evaluating 42 genotypes. The results showed that prior to soil moisture stress, there were significant (P<0.05) differences among the 42 genotypes for all the six phenotypic parameters that were measured. The highest (1.05 μmol/g dry weight) and lowest (0.32 μmol/g dry weight) leaf proline content (LPC) were observed for genotypes ‘Ac-35’ and ‘Ac-9’, respectively. The trial mean for proline was 0.69 μmol/g dry weight. The genotype ‘Ac-42’ attained the highest (27.85) leaf chlorophyll content (LCC) which was 48.94% higher than the check genotype (‘Ac-34’). The genotype ‘Ac-33’ achieved almost two-fold higher relative water content (RWC) (84.72%) than genotype ‘Ac-11’ which recorded the lowest (43.12%) RWC. The highest (68.70 mmol m-2s-1) stomatal conductance (SC) was three-fold more than for the check genotype (19.90 mmol m-2 s-1). At least four genotypes (‘Ac-6’, ‘Ac-7’, ‘Ac-22’ and ‘Ac-28’) attained significantly (P < 0.05) greater stem height (SH) than the trial mean (28.63 cm). After the soil moisture stress treatment, the results revealed that the LPC ranged from 1.26 to 0.36 μmol/g dry weight that were observed for genotype ‘Ac-35’ and ‘Ac-9’, respectively. The LPC showed a positive but not significant (P > 0.05) correlation with each of the other remaining attributes both before and after the moisture stress treatment. Similarly, after the soil moisture stress, the LCC maintained a highly significant (P < 0.01) positive correlation with the RWC but a negative correlation with the SH. In both soil moisture conditions, there was no discernible correlation between the SD and the SH. In general, the soil moisture stress lead to a variable increment in the LPC among the genotypes. An independent samples t-test which was used to determine the significance of the change in LPC showed that there was a highly significant (P < 0.00019) difference between the measurements of this amino acid before and after soil moisture stress. The results also showed a reduction in LCC during the soil moisture stress period but there was no clear pattern of the influence of the soil moisture stress on both the SC and RWC. The principal component analysis showed that before the soil moisture stress, the first two principal components accounted for 45.49% of the total variation and three traits (SC, LPC and SH) were highly associated with PC1. In addition, SC contributed the most variation for this component. However, PC2 was highly associated with LPC and RWC. In contrast, PC3 was dominated by SH. The results also showed that after the soil moisture stress, the first two principal components accounted for >50.0% of the total variation. The LPC and SH were highly associated with PC2 but PC3 was dominated by both LCC and SD. In the biplot analysis four genotypes (‘Ac-2’, ‘Ac-19’, ‘Ac-30’ and ‘Ac-41’) were clustered around the origin prior to the moisture stress treatment while five genotypes (‘Ac-3’, ‘Ac-9’, ‘Ac-11’, ‘Ac-28’ and ‘Ac-35’) were distinct and positioned far away from the origin. The genotypes in the right top quadrant (including ‘Ac-4’, ‘Ac-6’, ‘Ac-7’ and ‘Ac-28’) were associated and characterized by high leaf proline, high degree of stomatal opening and tall shoots. The tallest shoots were associated with the genotypes that were grouped in the left top quadrant while the remainder of the genotypes were characterized by thick stems and grouped in the left bottom quadrant. The tepary bean genotypes were grouped into three main clusters with the majority of the genotypes (64.28%) grouped in cluster III. Cluster I consisted of only seven genotypes including ‘Ac-40’ (which was associated with high LCC) as well as ‘Ac-2’, ‘Ac-35’, and ‘Ac-37’ (which were characterized by both LPC and RWC). The check (genotype ‘Ac-34’) was grouped in cluster III in a sub-cluster with genotype ‘Ac-20’. This study discusses the implications of the observed variability among the tepary bean genotypes for these phenotypic attributes and growth parameters. There will be merit in validating these results on a field basis together with grain yield evaluation and genotyping over multiple locations and seasons to determine elite germplasm that breeders and growers can utilize.