It was proposed to evaluate various hydric schemes on the morphophysiological behavior and performance of watermelon plants at field conditions in the semiarid region of Paraíba state (Brazil). The experiment was carried in the State University of Paraíba. The cultivar Crimson Sweet was used, and plants were spaced 2.2 m x 0.6 m; the experiment was designed in randomized blocks with four treatments [40, 70, 100 and 130% of the reference evapotranspiration value (mm day-1)] and eight replications. The maximum expressions of growth were verified for the irrigation level of 204 mm, with leaf area of 1.831 m², leaf area index of 1.35 m² m-2 and growth rate of 0.04 m² day-1. The stomatal resistance was greater during the afternoon, regardless of irrigation level. The initial, maximum and changing fluorescences and the quantum efficiency of photosystem II reduced abruptly for the highest level of irrigation. The maximum fruit yield (52,400 kg ha-1) was obtained for the irrigation level of 266 mm -1 cycle-1; the water use efficiency verified was 19.69 kg m-3. During the irrigation management for the watermelon plant, it is possible to adopt values between 70-100% for estimating the ETo due to calculate the culture irrigation level.
Citrullus lanatus Thumb. Mansf; transpiration; irrigation; photosystem II
