Salinity affects physiology, growth, and nutrient uptake of papaya in a calcareous soil

Sea level rise is increasing saltwater intrusion into soil and irrigation water in low-lying coastal agricultural regions, posing threat to crop performance and productivity. However, effects of increasing salinity on papaya ( Carica papaya L.) grown in calcareous soils remain poorly understood. This study assessed physiological growth, and nutrient uptake responses of two commercial papaya cultivars (‘Exp 15′ and ‘Red Lady’) grown in an oolitic limestone soil (Krome very gravelly loam) to increasing salinity of the irrigation water. Increasing the electrical conductivity (EC) of the irrigation water to 3, 6, or 9 dS m⁻¹ resulted in declines in physiological parameters. At the highest salinity level (9 dS m⁻¹), net CO₂ assimilation ( A ) decreased by 81.4% in ‘Exp 15′ and 78.4% in ‘Red Lady’ relative to the control (no salt added to the irrigation water). Similar reductions were observed in stomatal conductance ( g s ) (95.5% and 93.5%), transpiration ( Tr ) (98.1% and 96.9%), the ratio of variable to maximum chlorophyll fluorescence ( Fv/Fm ) (21.6% and 20.5%), and the leaf chlorophyll index ( LCI ) (39.7% and 33.3%) in ‘Exp 15′ and ‘Red Lady’, respectively. Leaf water potential ( Ψw) also declined with increasing EC, by 44.6% in ‘Exp 15′ and 73.5% in ‘Red Lady’, whereas osmotic potential ( Ψo ) and relative water content ( RWC ) remained unchanged, suggesting limited osmotic adjustment. Increased salinity altered leaf and root nutrient concentrations with most elements increasing as EC increased. High salinity also reduced leaf area and plant biomass accumulation. The normalized difference vegetative index (NDVI), determined from single multispectral images of all plants in the study, detected salinity stress only at the highest salinity level (9 dS m⁻¹) for ‘Exp 15′, but not for ‘Red Lady’. Overall, increased salinity of the irrigation water impaired physiology and growth of papaya in Krome very gravelly loam soil, apparently through stomatal limitation of carbon assimilation resulting in growth suppression.