Physiological and Growth Responses of Two Sweetpotato Cultivars at Early Season Soil Moisture Deficit

Even though soil moisture deficit is one of the important abiotic stresses causing reductions in growth and yield of several crops including sweetpotato [Ipomoea batatas L. (Lam)], limited quantitative information is available that can be used for irrigation management during the growing season. To quantify soil moisture stress effects on sweetpotato, an experiment was conducted by varying soil moisture levels, 0.256, 0.216, 0.164, 0.107, and 0.058 m³·m^-3 soil, (representing 100%, 80%, 60%, 40%, and 20% of field capacity) from transplanting to 55 days on two commonly grown cultivars, Beauregard and Evangeline, for plants grown in pots under greenhouse conditions. Soil moisture treatments were imposed prior to planting and maintained throughout the experiment at the respective treatment levels through sensor-based programmed irrigation. Six pots were harvested at weekly intervals starting 15 days after transplanting and growth and developmental rates were determined from the destructive harvests. Gas exchange and other physiological measurements were recorded during the last two weeks of the experiment. Numbers of storage and non-storage roots were recorded based on their visual and anatomical features through microscopic observation of roots at each harvest. Although vine length was greater in 'Evangeline' than 'Beauregard' across all soil moisture levels, total leaf area, leaf number, and total biomass were not different between the cultivars and declined quadratically with decreased soil moisture. The rate of photosynthesis decreased in both cultivars with increasing soil moisture deficit and the magnitude of reduction was significantly different between cultivars at soil moisture levels below 0.164 m³·m^-3 soil. Less time was required for 'Beauregard' to initiate storage roots than 'Evangeline', across all soil moisture levels. Storage root initiation of both cultivars showed a quadratic decline with decreasing soil moisture content. Soil moisture optima for storage root initiation were 0.168 and 0.199 m³·m^-3 soil for 'Beauregard' and 'Evangeline', respectively. Even though more total and storage roots were produced in 'Evangeline' than 'Beauregard' across soil moisture levels, storage root biomass was not different between the cultivars and the response to soil moisture stress was similar, which declined with declining soil moisture levels. The functional algorithms developed from these data will be useful in predicting sweetpotato growth and development under varying soil moisture levels.