The semi-arid region of Northeastern Brazil has water limitations in terms of both quantity and quality, with salt stress as a limiting factor for increasing yield in most crops. In this context, the present study aimed to evaluate cell damage, gas exchange, and growth of custard apple under salt stress and potassium fertilization. The research was carried out at the Experimental Farm of CCTA/UFCG, in São Domingos-PB, Brazil. A randomized block design was arranged in a 2 × 5 factorial scheme, with two levels of electrical conductivity of irrigation water (ECw; 1.3 and 4.0 dS m-1) and five potassium doses (10, 15, 20, 25, and 30 g of K2O per plant per year). Water salinity of 4.0 dS m-1 negatively affected the stem diameter and number of leaves in custard apple at 179 and 210 days after transplanting (DAT). The highest relative growth in stem diameter in the period of 179-245 DAT was obtained in plants irrigated with 4.0 dS m-1 water and fertilized with 20 g of K2O per plant. Potassium doses of up to 30 g of K2O resulted in a higher percentage of cell damage and relative water content in custard apple leaf tissue. Water saturation deficit decreased with the increase in K2O doses in plants irrigated with water of 1.3 dS m-1. Irrigation with 1.3 dS m-1 water and estimated K2O doses ranging from 16 to 22 g per plant resulted in an increase in stomatal conductance, transpiration, CO2 assimilation rate, and instantaneous carboxylation efficiency in custard apple plants at 210 DAT.

HIGHLIGHTS Irrigation water salinity alters gas exchange and biosynthesis of photosynthetic pigments in po-megranate. The reduction in CO2 assimilation in pomegranate plants under salt stress is related to non-stomatal factors. Increasing nitrogen doses increase electrolyte leakage in pomegranate seedlings.

The salt stress caused by irrigation water with high concentration of salts stands out as one of the main limiting factors in agricultural production in the semiarid region of Northeastern Brazil. In this context, the objective of this study was to evaluate the photosynthetic pigments, the photochemical efficiency, and the growth of custard-apple irrigated with saline water and potassium doses. The research was carried out under field conditions in a randomized block design in a 2 × 5 factorial scheme, corresponding to two values of electrical conductivity of irrigation water - ECw (1.3 and 4.0 dS m-1) and five potassium doses (50, 75, 100, 125 and 150% of the recommendation). The dose referring to 100% corresponded to the application of 20 g of K2O per plant per year. ECw of 4.0 dS m-1 reduced the synthesis of chlorophyll a, total chlorophyll, and carotenoids in custard-apple, at 245 days after transplanting. Fertilization doses of 50 to 150% of the recommendation inhibited the synthesis of chlorophyll b and the absolute and relative growth rates in stem diameter of custard-apple plants irrigated with water of highest electrical conductivity. Reduction in the quantum efficiency of photosystem II in custard-apple cultivated under ECw of 4.0 dS m-1 is related to photoinhibitory damage to photosystem II. Potassium fertilization did not alleviate the stress caused by water salinity on the growth of custard-apple, during 151-245 days after transplantation.

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