The effect of nitrogen supply on potato yield, tuber size and pathogen resistance in Solanum tuberosum exposed to Phytophthora infestans

Little is known about the effects of nitrogen supply on plant productivity and defensive response to pathogen exposure. Here, tuber yield and resistance to the fungal pathogen Phytophthora infestans were investigated in potato plants supplied with four different nitrogen levels (N1: 45 kg-hm-2, N2: 90 kg-hm-2, N3: 135 kg-hm-2, and N4: 180 kg-hm-2). The N2 level of nitrogen promoted the highest tuber yield and largest tubers under pathogen-free conditions. However, N3 promoted the strongest pathogen resistance and highest potato production in response to P. infestans exposure. Also, the highest level of chlorophyll accumulation was observed with N3 relative to the other treatment groups under pathogen-infected but not pathogen-free conditions. Further study showed that, in response to pathogen infection, the N3 level of nitrogen induced the highest activity levels for the defensive enzymes polyphenol oxidase and phenylalanine ammonia-lyase (but not chitinase and β-1,3-glucanase) and the most abundant accumulation of phenolic and flavonoid compounds. Our data demonstrated that modest increases in the nitrogen supply provided potato plants with optimum growth and defensive capability in response to P. infestans exposure.   Key words: Nitrogen, tuber yield, pathogen resistance, Phytophthora infestans, Solanum tuberosum.