Inherent physiological mechanism of nitrate nitrogen in suppressing Fusarium oxysporum infection and increasing growth of cucumber plants

Objectives Continuous cropping obstacles seriously impact the development of facility agriculture. Nitrogen forms affect the occurrence of Fusarium wilt of cucumber, the inherent physiological mechanism is studied in the paper.

Methods A hydroponic experiment was conducted using cucumber cultivar of Jinchun 2 and Fusarium oxysporum f. sp. cucumerinum (FOC) as materials. Four nutrient solution treatments included ammonium-N supply and without or with FOC inoculation in the grown plants, nitrate-N supply and without or with FOC inoculation in the grown plants. The plant height, root length, biomass, disease index, chlorophyll content, photosynthetic characteristics, leaf temperature were measured at 8 days after inoculation. The ultrastructure of mesophyll cells were observed, and plant total N, soluble protein and soluble sugar content were analyzed.

Results The nitrate-N nutrition significantly suppressed the disease index of cucumber Fusarium wilt as compared to ammonium-N nutrition. Plant growth and biomass production were markedly increased in nitrate nutrition. Without FOC inoculation, the plants supplied with ammonium-N had higher net photosynthesis rate, stomatal conductance, transpiration rate, carboxylation efficiency and apparent quantum yield as compared to those in plants supplied with nitrate-N. With ammonium-N nutrition, the infection of FOC resulted in damage to the chloroplast structure, which significantly reduced the photosynthetic rate, stomatal conductance, transpiration rate, intercellular CO₂ concentration, carboxylation efficiency and apparent quantum efficiency. However, the photosynthetic characteristics of nitrate supplied plants were not affected by FOC infection. Under non-inoculation of FOC, the leaf temperature and water use efficiency (WUE) of ammonium nutrition plants were significantly lower than those of nitrate nutrition plants. FOC infections markedly increased the leaf temperature and WUE of ammonium nutrition plants, while had no effects on nitrate nutrition plants. Leaf temperature was significantly and negatively correlated with transpiration rate, and positively correlated with WUE. The contents of total N, soluble protein and soluble sugar in the root of ammonium supplied plants were significantly higher than those of nitrate supplied plants, thus inducing the pathogen infection in ammonium nutrition plants. After FOC infection, the soluble protein content in the root of ammonium supplied plants was significantly increased, while the soluble sugar content was decreased. FOC infection had no significant effect on the soluble protein and soluble sugar content of nitrate supplied plants.

Conclusions Nitrate nutrition can effectively suppress the disease index of cucumber Fusarium wilt, maintain the integrity of the chloroplast structure, keep the normal photosynthesis and growth of cucumber plants, and reduce the transport of carbohydrates to the root, thereby inhibit the infection of pathogens and the occurrence of diseases. In the facility cultivation of cucumber, we should appropriately increase the application of nitrate fertilizer and decreased the input of ammonium fertilizer to suppress the occurrence of soil-borne wilt.