Physiological and growth responses of selected sweet potato (Ipomoea batatas (L.) Lam.) cultivars to water stress

Drought is one of the most serious environmental problems affecting sweet potato (Ipomoea batatas (L.) Lam.) growth and yield in many tropical countries including Highland Papua of Eastern Indonesia, where it is grown as a staple crop under rainfed conditions. Some genotypes survive during drought, while many others fail. This condition regularly devastates sweet potato growth and threatens the lives of highland Papuan people. On the other hand, research into the effects of drought and on the growth, yield, and water relations of sweet potato remains limited. A series of pot and field trials therefore was conducted to identify the drought tolerance of sweet potato cultivars available in Australia. The influence of soil water regimes on the physiology, growth, and yield of sweet potato was studied to determine the critical soil water levels that sweet potato could tolerate and produce acceptable yields. The influence of nutrient supply (N and K) on water use efficiency in sweet potato was also studied. A grafting trial was carried out to examine the nature of the physiological signal between shoot and root in relation to transpiration efficiency and yield responses in sweet potato. The cultivars Lole and Hawaii showed more strongly developed drought resistant characters than all of the other cultivars. They have better water use efficiency, maintain higher plant water status under drought stress by delaying wilting, and lower percentage decrease in leaf water potential indicating their greater tolerance to water stress at the stage of vegetative growth. Field trials were conducted to determine tuber yields from the same 15 sweet potato cultivars under well-watered conditions. The Lole and Hawaii cultivars produced low tuber yields, whereas the Beerwah Gold and Wanmun cultivars produced the greatest yields when not water stressed. The Lole cultivar representing the vegetative tolerant and Wanmun cv representing the susceptible genotypes were grown in a glasshouse to observe the degree to which sweet potato cultivars could withstand water stress conditions and still produce good tuber yields. The Wanmun cultivar, which grew and produced good yields under well-watered conditions, was strongly affected by water stress. On the other hand, the Lole cultivar showed more drought tolerance, indicated by greater plant water status, including leaf water potential and relative water contents. At maturity, tuber sucrose contents increased while the starch contents decreased. Under drought conditions, cv Lole tubers had higher sucrose and lower starch contents than cv Wanmun. Tuber yields were greater in cv Wanmun when not stressed; Lole on the other hand produced marketable tuber size when water was restricted to ≥ 40% of soil field capacity. The overall results suggested greater drought tolerance of the Lole cultivar. Results of a study of the influence of nitrogen and potassium on water stress and productivity showed that a greater nitrogen supply resulted in greater shoot dry weight, leaf weight, and leaf area. Increasing the soil nitrogen content beyond 100 kg per ha reduced tuber yields, due to greater top growth and inefficient carbon translocation for tuber development. Low soil nitrogen contents (20 kg of N/ha), on the other hand, lowered the biomass production, reducing tuber formation and development. Potassium had a significant effect in increasing tuber yields when nitrogen supply was optimal. Although cv Wanmun consistently produced greater tuber yields than cv Lole under well-watered conditions, the Lole cultivar supplied with 100 kg of N/ha and 160 kg of K/ha produced greater tuber yields under water stress. The lower transpiration under higher soil potassium contents suggest that potassium plays an important role in improving water use efficiency. Grafting was conducted with the aim of producing plants that embody cv Lole's tolerance to drought and cv Wanmun's high tuber yields. The results showed that growth and physiological aspects of the Lole and Wanmun cultivars and their grafted combinations decreased with water stress. Wanmun scions when grafted onto Lole rootstocks grew very poorly, however, Lole scions grafted onto Wanmun rootstocks grew well and produced higher tuber yields than the parent plants, especially under water stressed conditions. Therefore, cv Wanmun and cv Lole are recommended to be independently propagated under good rainfall and drought condition, respectively, and combinations of drought tolerant cv Lole scions grafted onto good storage root capacity of cv Wanmun rootstocks improved tuber yields under water stress condition. Productivity of sweet potato, as measured by tuber production, was reduced by soil water stress. Under well-watered conditions, cv Wanmun (drought sensitive) produced high tuber yields. Nitrogen and potassium at optimal application levels increased yields of both drought sensitive and tolerant cultivars under both well watered and water stressed conditions. Under drought conditions, cv Lole (drought tolerant) produced good tuber yields, and Lole scions grafted onto Wanmun rootstocks produced even better tuber yields. The interactions between genotype and environmental constraints, including drought, require further study to produce sweet potato cultivars with high tuber yield potentials that are well suited to different local conditions