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：在巴西半干旱区域，高盐度水体频发，已成为制约当地农业生产的关键限制性因素。灌溉水盐管理策略是减轻盐胁迫对植物产生有害影响的可行技术途径。本研究旨在评估咸水灌溉与氮肥配施模式下，‘Sugar Baby’西瓜植株的气体交换特性与产量表现。试验采用随机区组设计，设置6×2因子试验方案，共5次生物学重复：其中6种咸水灌溉策略分别在作物不同物候期施加盐胁迫，对照处理为全生育期采用低盐灌溉，其余5组分别在营养生长期、营养/开花期、开花期、坐果期和果实成熟期施加盐胁迫；同时设置2个氮肥施用量水平，即50%和100%，分别对应每千克土壤施氮50 mg和100 mg。本研究设置两个灌溉水盐度梯度，低盐组与高盐组的电导率（electrical conductivity）分别为0.8 dS·m⁻¹和3.2 dS·m⁻¹。在营养/开花期与果实成熟期施加3.2 dS·m⁻¹的盐胁迫，会降低植株的气孔开度、蒸腾速率与CO₂同化速率。施氮50%水平下，植株可获得更高的CO₂同化速率与鲜果质量。‘Sugar Baby’西瓜在营养生长与开花期对盐胁迫更为敏感，该时期的盐胁迫会导致果实尺寸显著减小。