Characteristics of gas exchange in sweet potato leaf under differentsoil water content and fertilization

To understand the effect of soil water and fertilization on gas exchange in two cultivars of sweet potato leaf (“Qinshu 4” and “619”), photosynthesis rate (Pn), water use efficiency (WUE), transpiration rate (Tr), stomatal conductance (Gs), stomatal limitation (Ls) and intercellular CO₂ (Ci) were measured in a pot experiment. Results show that soil water has a significant influence on Pn, Tr, WUE, Ls, Gs and Ci of sweet potato. Fertilization has marked effect on these indexes as well, except for Gs in “Qinshu4” cultivar. With the decrease of soil water, a reducing tendency is found for Pn, Tr of sweet potato. Under light soil drought, Gs and Ls slightly decrease or increase but the change in Ci is unapparent. Under severe soil drought, however, both Gs and Ls significantly drop, while Ci increases significantly. WUE of “Qinshu 4” decreases with decreasing soil water content while light soil drought significantly enhances WUE in “619”, relative to CK or severe soil drought. Change in Ls and Ci with decreasing soil water content indicates that gas change in sweet potato leaf is mainly limited by stomatal factors under slight soil drought, and non-stomatal factors under severe soil drought. Sweet potato Pn is enhanced with increasing fertilization. Relative to CK, Tr slightly drops in “Qinshu 4” or significantly increases in “619” at moderate fertilization. However, Pn in the two varieties significantly increases at high fertilization. WUE apparently increases at moderate fertilization though high fertilization does not lead to any positive effect. The effect of fertilization on Gs is positive for “619” and insignificant for “Qinshu 4”. With improvement in soil nutrient, Ls initially shows an increasing tendency but eventually decreases. For Ci, leaf gas exchange is mainly limited by stomatal factors under moderate fertilization and non-stomatal factors under high fertilization. A significant interaction exists between soil water content and fertilization for all six indices. This suggests that Pn, Tr and WUE of sweet potato can be improved by selecting appropriate fertilization and soil water content. However, the degree of response to fertilization is different for the two varieties.