Influence of early nitrogen application on physiological properties and ultrastructures of functional leaves of potatoes under drought stress

The changes in physiological properties and leaf ultrastructures caused by early nitrogen application for potatoes under drought stress were explored. Potato variety Chuanyu117 was used to conduct a controlled experiment in a sand medium. Nutrient solution treatments (0.84, 1.26, 1.68 and 2.10) were administered, followed by severe drought, moderate drought, and normal (control) water treatments (45, 60, and 75% of soil saturation moisture capacity) in the seedling stage of potatoes. Early additional nitrogen application maintained chlorophyll content in potato leaves under drought stress and increased activities of protective enzymes in the plants. Severe and moderate drought stress treatments resulted in the smallest reductions in chlorophyll content compared with the control and largest increases in POD and CAT activities. Under the same water conditions, potato with high nitrogen treatment (1.68 g N L-1) had significantly higher net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and intercellular CO2 concentration (Ci) than other nitrogen treatment levels; the highest levels of these photosynthetic parameters were obtained under the normal water condition. As drought stress increased, the degree of damage to chloroplasts and mitochondria in potato leaves gradually increased and the number of starch grains increased; however, with increases in the nitrogen application level, the degree of damage to chloroplasts and mitochondria gradually decreased. Early additional nitrogen application had mitigative effects on the damage to cellular structures of potato leaves under drought stress and could improve gas exchange and increase absorption and utilization of light energy in potato leaves. Further, the adaptive capacity of photosynthetic structures in potato leaves increased, leading to greater drought resistance in potato. However, with increases in the nitrogen levels, drought resistance of potato showed an initially increasing trend that decreased subsequently, suggesting that over-application of nitrogen reduced drought resistance in potato.   Key words: Early nitrogen application, drought stress, physiological properties, photosynthetic parameters, ultrastructure.