Besides the paucity of information on the effects of drought stress on photosynthesis and yield in sweet potato [Ipomoea batatas (L.) Lam.], available reports are also contradictory. The aim of this study was to shed light on the effects of long-term restricted water supply on shoot development, photosynthesis and storage root yield in field-grown sweet potato. Experiments were conducted under a rainout shelter where effects of restricted water supply were assessed in two varieties (Resisto and A15). Large decreases in stomatal conductance occurred in both varieties after 5 weeks of treatment. However, continued measurements revealed a large varietal difference in persistence of this response and effects on CO(2) assimilation. Although restricted water supply decreased leaf relative water content similarly in both varieties, the negative effects on stomatal conductance disappeared with time in A15 (indicating high drought acclimation capacity) but not in Resisto, thus leading to inhibition of CO(2) assimilation in Resisto. Chlorophyll a fluorescence measurements, and the relationship between stomatal conductance, intercellular CO(2) concentration and CO(2) assimilation rate, indicated that drought stress inhibited photosynthesis primarily through stomatal closure. Although yield loss was considerably larger in Resisto, it was also reduced by up to 60% in A15, even though photosynthesis, expressed on a leaf area basis, was not inhibited in this variety. In A15 yield loss appears to be closely associated with decreased aboveground biomass accumulation, whereas in Resisto, combined effects on biomass accumulation and photosynthesis per unit leaf area are indicated, suggesting that research aimed at improving drought tolerance in sweet potato should consider both these factors.

It was assumed that the genetic manipulation of the proline (Pro) level would also affect the (homo)glutathione content as both compounds have a common precursor, glutamate. To test this hypothesis, the levels of Pro, reduced and oxidized (homo)glutathione [(h)GSH and (h)GSSG] and other antioxidants were compared under simultaneous drought and heat stress conditions and in a control treatment in a time course experiment on wild‐type soybean (Glycine max cv. Ibis) and on transgenic plants containing the cDNA coding for l‐Δ1‐pyrroline‐5‐carboxylate reductase (EC 1.5.1.2), the last enzyme involved in Pro synthesis, in the sense and antisense directions. At the end of the recovery period, the highest H2O2 and lipid hydroperoxide concentrations were observed in the antisense transformants, which exhibited the greatest injury, while the lowest H2O2 content was detected in the sense transformants, which exhibited the lowest injury percentage. During stress treatment, the highest Pro and ascorbate (AA) levels were detected in the sense transformants, while the highest GSH and hGSH contents, AA/dehydroascorbate (DHA) and (h)GSH/(h)GSSG ratios and ascorbate peroxidase (APX) activity were found in the antisense transformants. The greatest APX (EC 1.11.1.11) activity was observed in the first part of the stress treatment in the antisense transformants, and the greatest glutathione reductase (EC 1.6.4.2) activity was observed in the second part of the treatment in the same genotype. The present experiments indicate that the manipulation of Pro synthesis affects not only the (h)GSH concentrations, but also the levels of other antioxidants.

The antioxidant contents (beta-carotene, chlorogenic acid, and vitamin C) as well as the antioxidant capacity (ORAC, FRAP, and ABTS) of 4 sweetpotato varieties were measured in this study. The sweetpotato varieties were cultivated under different water regimes and also subjected to thermal processing. The results show that the 2 orange-fleshed varieties (Resisto and W-119) contain significant more beta-carotene, chlorogenic acid, and vitamin C than the 2 cream-fleshed varieties (Bosbok and Ndou). Thermal processing decreased the carotenoid and vitamin C content of all the varieties but increased the chlorogenic acid content and antioxidant capacity. Drought stress appears to increase the beta-carotene, vitamin C, and chlorogenic acid contents as well as the antioxidant capacity of some of the sweetpotato varieties, especially W-119.

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Sweet potato is generally considered as a drought tolerant crop, however, due to large genotypic differences, appropriate genotypes adapted to drought conditions must be identified. The aim of the study was to investigate the suitability of canopy cover, stem length and stomatal conductance to differentiate performance of sweet potato genotypes subjected to drought stress. Seven sweet potato cultivars and six elite lines were subjected to three water treatments, namely, a control (100%) treatment which received the full complement of plant available water (PAW) once soil water had depleted to 70% PAW, while the other two treatments received 60% (mild stress) and 30% (severe stress) of the calculated water that the control treatment received in rainout shelters. Pearson correlations were performed on measured parameters yield, leaf area index (LAI), stem length and stomatal conductance (g s ). Genotypic differences were observed for LAI values and indicated that the cultivar Purple Sunset was better adapted to soil moisture stress at the mild stress and severe stress conditions than the other cultivars included in the trials. The cultivars Purple Sunset and Blesbok indicated a better ability to adapt to moisture stress, with regards to stem length, by outperforming the other genotypes in Trials 1 and 3. Drought had a severe effect on the yield of all the genotypes, especially at severe stress. Resisto, Bophelo and 199062.1 produced the highest yield at the mild stress, with significant correlations between yield and the parameters LAI, stem length and stomatal conductance. This indicated that above-ground growth had a direct influence on plant growth below-ground. The correlations also indicated that LAI and stomatal conductance played a more important role than stem length in achieving yield, and, thus, could be useful screening methods for drought tolerance of sweet potatato.

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Sweet potato is an important world staple with the potential to address hunger and malnutrition. The Agricultural Research Council of South Africa has led sweet potato breeding efforts in the country since 1952 and released several important cultivars. More detailed quality assessments are necessary in addition to general breeding criteria. The present study investigated sweet potato cultivars and elite clones for (1) their nutritional composition; (2) biochemical reaction to drought stress; (3) correlate biochemical reaction to root yield for possible identification of screening methods for drought tolerance in sweet potato. Orange-fleshed cultivars, in particular Bophelo, had superior contents of Fe, Zn, Mg, Ca, Mn, and dietary fiber. Cream-fleshed cultivars, particularly Ndou, were higher in starch and carbohydrate. When sweet potato cultivars were subjected to drought stress, significant changes were noted in most antioxidant enzymes, chlorophyll and proline concentrations, and 13C discrimination. All of these showed significant correlations with root yield. For screening of drought tolerance, the determination of nitrate reductase, proline, and chlorophyll concentrations at 60 days after planting are recommended. Ndou was less affected by drought stress. Bophelo and Ndou, which are gaining popularity on the informal market, have superior nutritional value and are good cultivar choices for food security and addressing malnutrition.