Effects of water and nitrogen interaction on photosynthetic fluorescence and physiological characteristics of pomegranate seedlings

Abstract: 【Objective】This experiment aimed to investigate the interaction of water and nitrogen on photosynthetic fluorescence and physiological characteristics of the soft seed pomegranate (Punica gra-natum L.) seedlings under water shortage in Shihezi, Xinjiang in order to offer a clue for saving water and determine the optimal combination of water and nitrogen. Tunisia soft seed pomegranate seedlings were used as the test materials. The effects of different water and nitrogen treatments on the photosyn-thetic characteristics and chlorophyll fluorescence of the leaves of the seedlings were investigated, and the osmotic regulation ability of the seedlings and their effects on nitrogen metabolism were also stud-ied.【Methods】The pot water control method was used to set three irrigation levels (W1: 35%-45% of the field water capacity, W2: 55%-65% of the field water capacity, W3: 75%-85% of the field water ca-pacity) and four nitrogen application levels (N0: 0 g·kg^-1 soil, N1: 0.1 g·kg^-1 soil, N2: 0.2 g·kg^-1 soil,N3: 0.3 g·kg^-1 soil), respectively. The chlorophyll content, photosynthesis and chlorophyll fluorescence characteristics of the seedlings were measured on the 40th day, and osmotic regulation substance con-tent and key enzyme activities of nitrogen metabolism were measured on the 10th, 20th, 30th and 40th days after water-nitrogen interaction treatments.【Results】In terms of the photosynthetic characteristics and chlorophyll fluorescence, the chlorophyll content, net photosynthetic rate (P_n), stomatal conduc-tance (G_s), transpiration rate (T_r), photochemical quantum efficiency (Fv/Fm), photochemical quenching coefficient (qP), photosynthetic electron transfer rate (ETR) and effective quantum yield [Y(Ⅱ)] of pho-tochemical energy conversion in the leaves of the soft seed pomegranate seedlings achieved the optimal results under the W2 irrigation condition. Under various irrigation conditions, the increase of the nitro-gen application rate could significantly increase the chlorophyll content and net photosynthetic rate, sto-matal conductance and transpiration rate values, while intercellular CO2concentration (Ci) was negative-ly correlated with the net photosynthetic rate change under W1 irrigation condition, but positively corre-lated with the net photosynthetic rate change under the W2 and W3 irrigation condition. The variation trend of the non-photochemical quenching coefficient (NPQ) was opposite to that of the photochemical quantum efficiency. In terms of osmotic regulation and nitrogen metabolism, the contents of soluble pro-tein and soluble sugar and proline in the leaves of the seedlings decreased with the decrease of the irri-gation amount. The nitrate reductase (NR), glutamine synthetase (G_S), glutamic oxaloacetate transami-nase (GOT) and glutamic pyruvate transaminase (GPT) achieved the highest activities under the W2 irri-gation condition. Except that the glutamine synthetase activity treated with N2 (0.2 g·kg^-1 soil) reached the optimal results, the contents of soluble protein, soluble sugar and proline and the activities of nitrate reductase, glutamic oxaloacetate transaminase and glutamic pyruvate transaminase increased with the increase of nitrogen application level under the W3 irrigation condition (the upper and lower limits of ir-rigation were 75% to 85% of the field water capacity). Under the condition of the W2 (the upper and lower limits of irrigation were 55%-65% of the field water capacity), the soluble protein content and the activities of key enzymes of the nitrogen metabolism reached the highest levels in the N2 treatment, and the contents of soluble sugar and proline increased with the increase of nitrogen application rate. Under the W1 irrigation (the upper and lower limits of irrigation were 35%-45% of the field water capacity), the contents of soluble protein and soluble sugar decreased with the increase of nitrogen application rate, while the proline content remained an opposite alteration. The activity of nitrogen metabolism en-zymes was the highest in the N1 treatment, followed by the N2, N3 and N0 treatments.【Conclusion】The photosynthetic fluorescence characteristics of the soft seeds pomegranate seedlings could be inhibit-ed by drought to a certain extent. The drought stress could be effectively alleviated by the osmotic regu-lation ability, key enzyme activity, and nitrogen metabolism through the appropriate increase of nitro-gen application. In addition, W2N2 seems to be the best combination of water and nitrogen in this study. The results could provide a theoretical basis for raising the soft seed pomegranate seedlings in Shihezi, Xinjiang to a certain extent.