Gas exchanges and production of watermelon plant under salinity management and nitrogen fertilization

ABSTRACT In the Brazilian semi-arid region, the occurrence of water with high salt concentrations is common, making it a limiting factor for the agricultural production. The water-use salinity management strategies are an alternative capable of minimizing the deleterious effects of the stress on plants. This study aimed to evaluate the gas exchanges and production of ‘Sugar Baby’ watermelon plants under strategies of irrigation with saline water and nitrogen fertilization. The experiment was conducted in a randomized block design, arranged in a 6 x 2 factorial scheme, with five replicates, corresponding to six irrigation strategies with saline water applied at different phenological stages of the crop (control - irrigation with low-salinity water throughout the entire crop cycle, and salt stress at the vegetative stage, vegetative/flowering stage, flowering, fruiting and fruit maturation) and two nitrogen rates (50 % and 100 %, equivalent to 50 mg and 100 mg of N kg-1 of soil). Two levels of water salinity were studied: one with a low and the other with a high level of electrical conductivity (0.8 dS m-1 and 3.2 dS m-1, respectively). The salinity of 3.2 dS m-1 in the vegetative/flowering and fruit maturation phases decreases the stomatal opening, transpiration and CO2 assimilation rate. The level with 50 % of N provides a higher assimilation rate of CO2 and fresh fruit mass. The watermelon plant expresses a greater sensitivity to saline stress in the vegetative and flowering phases, a situation that results in a decreased fruit size.

摘要（ABSTRACT）：巴西半干旱地区高盐度水体频发，成为农业生产的限制性因素。盐分胁迫会对植物产生有害影响，而水分利用盐分管理策略可有效减轻此类危害。本研究旨在评估不同咸水灌溉策略与氮肥配施下，‘糖宝宝’（Sugar Baby）西瓜植株的气体交换特性与产量表现。本试验采用随机区组设计（randomized block design），设置6×2析因试验方案（factorial scheme），共5次重复。其中6个咸水灌溉策略分别针对作物不同物候期（phenological stages）设置，具体为：全生育期灌溉低盐度水（对照组）、营养生长期盐胁迫、营养/开花期盐胁迫、开花期盐胁迫、坐果期盐胁迫以及果实成熟期盐胁迫；2个氮肥施用量水平分别为50%和100%，对应每千克土壤施氮50 mg与100 mg。本试验设置两个水质盐度梯度：低电导率（electrical conductivity）水平（0.8 dS m⁻¹）与高电导率水平（3.2 dS m⁻¹）。研究发现，在营养/开花期及果实成熟期采用3.2 dS m⁻¹的咸水灌溉，会降低植株气孔开度、蒸腾速率与CO₂同化速率；施氮50%水平可提升CO₂同化速率与果实鲜重。西瓜植株对盐分胁迫的敏感性在营养生长期与开花期最高，此阶段的盐胁迫会导致果实尺寸减小。