Prominent Role for Leaf Calcium as a Yield and Quality Determinant in Upland Cotton (Gossypium hirsutum L.) Varieties Grown Under Irrigated Mediterranean Conditions

Seven cotton (Gossypium hirsutum L.) accessions were tested over 2 years under irrigated Mediterranean conditions on a loamy soil with nitrogen (N) as the only nutrient input. The study aimed to identify the critical nutritional and physiological factors determining seedcotton yield and fibre quality. A suite of leaf physiological traits [chlorophyll content (assessed by SPAD), carbon isotope discrimination (Δ), ¹⁵N natural abundance (δ¹⁵N), leaf water potential, N and C concentrations, C/N ratio, K, Na, Ca and Mg concentrations, their sum and ratios] was assessed, and their interrelationships then analysed. It was found that physiological indices such as SPAD, Δ and δ¹⁵N failed to discern genotypes for yield and did not relate with fibre quality traits. At the same time, leaf Ca concentration was the trait that showed the strongest correlation with both seedcotton (SY) and lint yield (LY). An increase of K/Na ratio up to 5.74 was beneficial for SY but higher ratios impacted yield adversely. In this line, exclusion of K in favour of Ca (lower K/Ca ratios) increased both SY and LY. The above results could be explained by Ca²⁺ control over activity of tonoplast and plasma membrane cation channels, resulting in redistribution of K⁺ between cell compartments. It is suggested that Ca²⁺‐rich plants are more efficient in sequestering higher K⁺ quantities in leaf vacuoles, at the expense of cytosolic K⁺. Under K⁺‐limiting conditions, such redistribution may trigger programmed cell death and enhance leaf senescence. This would remobilize and translocate nutrients (e.g. N) and organic substances to sinks (seedcotton), contributing to higher yields reported in the present work.