Foliar Magnesium Supplementation as a Strategy to Mitigate Salt Stress in Guava (Psidium guajava L.) Cultivars: Physiological and Growth Responses

The guava tree (Psidium guajava L.) is a tropical fruit tree of worldwide importance; however, the salinity of irrigation water severely limits its development in semi-arid regions. However, magnesium (Mg) can mitigate this stress by promoting plant photosynthetic activity. The objective was to evaluate the effect of foliar Mg in mitigating saline stress on photosynthesis and the growth of guava cultivar seedlings. The experiment was conducted in a randomized complete block design, in a 2 × 2 × 3 factorial scheme, with two guava cultivars (Kumagai and Paluma), two irrigation water salinity levels (a low-salinity control—0.5 dS m−1, and salt stress—2.5 dS m−1), and three doses of foliar Mg (0, 1, and 2 mL L−1), and six replications. A salinity of 2.5 dS m−1 reduced growth and gas exchange in both cultivars, with a reduction of approximately 30% in total dry mass, and 16% in CO2 assimilation rate. Supplementation with 1 mL L−1 of Mg attenuated the effects of stress, stimulating chlorophyll synthesis and gas exchange, reducing approximately leaf temperature in 3.5%, and vapor pressure deficit (VPD) in 12%. The Paluma cultivar was more responsive to Mg under salinity, with improved CO2 assimilation rate, stomatal control, and water use efficiency. Kumagai showed greater growth in height and diameter with 1 mL L−1 under stress. Foliar application of magnesium (1 mL L−1) is a promising strategy to produce guava seedlings under saline stress.