Biochemical, physiological and agronomic response of various sweet potato cultivars/varieties to drought stress in rainout shelters and field conditions.
Drought is and will always be an issue in the cultivation of plants. Some plants have the ability to withstand a drought conditions to a certain degree while others, with other useful attributes, fail dismally. The value of testing genotypes for the ability to tolerate drought cannot be underestimated and will enhance the progress in the selection of drought tolerant genotypes. Thus, the objective of this study was to investigate the physiological, biochemical and agronomical reaction of sweet potato plants to drought and the procedures which could be used to test for sweet potato drought tolerance in the field. This was made possible through the creation of an environment at ARC-Roodeplaat in which sweet potato plants could be subjected to drought stress conditions. Thirty five sweet potato genotypes were planted in three trials in rainout shelters and open fields to analyze their physiological, biochemical and agronomical responses to drought stress. The majority of the genotypes were selected breeding lines with some cultivars from America, Peru and South Africa. These genotypes were chosen due to their range of traits for incorporation in crosses in the sweet potato breeding programme of the Agricultural Research Council (ARC). Drought stress conditions on the plants were induced through selective irrigation practices. In Trial 1 control plants were cultivated at field capacity while drought stressed plants received 60% and 30% of the amount of water of the control, respectively. In Trial 2, genotypes were planted in the field and under rainout shelters respectively. The field plantings acted as the control and received normal rain and irrigation while the rainout shelter planting received irrigation corresponding to 30% of field capacity. The plants in Trial 3 were subjected to control and drought conditions with the drought stressed plants receiving 30% of the water of the control. Leaf harvesting and phenotypical measurements were conducted twice during the trial period i.e. 60 and 120 days after planting. The drought stress impacted the growth of the sweet potato plants significantly. Canopy cover and stem length were severely influenced by the drought stress and resulted in huge declines of the respective values in all trails. Canopy cover values declined by more than twice compared to the control while stem length values were reduced by up to 10 times compared to the control. Antioxidant systems with particular reference to ascorbate peroxidase (AP), super oxide dismutase (SOD) and glutathione reductase (GR) reacted to the stress imposed and increased significantly. It was observed that values of the respective antioxidant enzyme systems increased sharply in the latter part of the trial and that the increase was also more intense at severe stress. The analysis of the antioxidant system made it possible to distinguish between the genotypes regarding their reaction to the stress. Results for carbon discrimination experiments in all the trials indicated that a significant decline in values took place as the drought stress increased. The decline appeared to be slightly more pronounced as the stress progressed. Also, as in the case of the antioxidant systems, it was possible to distinguish between genotypes even in the control treatments. The plants responded to the drought stress to the effect that a similar trend, (compared to the antioxidants), was observed with regards to stomatal conductance although genotypical differentiation was not possible in any of the stress conditions. It was demonstrated in the trials that the relative water content (RWC) values in the leaves of plants subjected to drought stress declined significantly between water treatments. Drought stress in the three trials had a severe impact on the nitrate reductase (NR) activity in the leaves of the plants. The decline in values were substantial but no significant differences could be detected between the genotypes except for the breeding line 2005-1-16 and cultivars Purple Sunset, Beauregard and Zapallo. Slight non-significant differences were observed between the genotypes at mild stress conditions but the severe stress conditions proved too harsh. Significant increases in the proline content of the sweet potato plants subjected to drought stress resulted in differentiation between the genotypes in Trial 1 and Trial 2, especially during the latter stages of the trials and at severe stress. Large reductions, up to 97%, of root yield were detected in the three trials. It appeared that the severe stress treatment proved too harsh to accomplish significant differences between the genotypes in all the trials. In Trial 1 the genotype Resisto differed significantly from the other genotypes and seemed to tolerate the drought the best in the mild stress conditions. Water use efficiency (WUE) values did allow for discrimination between the genotypes in Trial 1. A large decline in WUE values were observed in Trial 2 in general, although a few breeding lines 2005-7-4, 2006-4-4 and ix 2006-7-7 were prominent with high WUE values and could be recommended for use in a breeding programme. In Trial 3 the cultivar Bophelo and 199062.1 also exhibited higher WUE values which correlate well with yield data obtained from the same Trial. This could also prove valuable in the selection process. Due to the fact that multiple traits make a valuable contribution to the decision-making process in the selection for possible screening methods, statistical correlation was undertaken to establish possible relationships between traits. Good correlation was found between yield, stomatal conductance and WUE in Trial 1. This confirmed the assumption that a drop in stomatal conductance will result in lower root yield. Proline correlated also very well with the antioxidant enzyme levels of GR and AP which indicates that while the antioxidant enzymes play a role in combatting oxidants proline aid in possible prevention of moisture loss and stabilization of cell membrane structures. In Trial 2 good correlation was observed between yield, LAI, NR and CCI and to a lesser extent carbon-13 discrimination. This confirmed the belief that a decrease in LAI and CCI should have a negative effect on the yield due to less canopy cover and less chlorophyll for the capture of sunlight for photosynthesis. Results from Trial 3 also indicated good relationships between proline, GR and AP, as well as good relationships between yield, WUE, carbon discrimination and stomatal conductance (gs). It can hereby be concluded that the reaction of sweet potatoes to drought stress revealed results that can be of help for use in the future to successfully establish a protocol whereby successful selection of genotypes can be made in a biochemical, physiological and agronomical way. The study also provided proof that some of the approaches and procedures used in these trials can be successfully implemented in the drought screening of sweet potato.