Translocation of photoassimilates in melon vines and fruits under salinity using 13C isotope

Both yield and quality of Cantaloupe melon in semi-arid regions are threatened by high levels of salinity in irrigation waters. Our study aimed to characterize the translocation of photoassimilates in melon plants cultivated under salinity condition, using carbon 13 ((CO2)-C-13) isotopic enrichment. Plants of the melon hybrid 'Zielo' were cultivated and drip-irrigated with waters of electrical conductivity (ECw) of 0.5 and 3.5 dS m(-1). Leaves were evaluated for concentrations of Na, Cl, and sucrose and their CO2 assimilation rate. Fruits were evaluated for their weight, yield, sucrose, and soluble solids. Salinity stress affected both assimilation and accumulation of sugars, decreased fruit number and size. The natural (before enrichment) and enriched C-13 in leaves from three stem regions and fruits were evaluated in the two final weeks of the crop cycle. The isotopic enrichment was applied to leaves of the basal and apical regions of the primary stem and apical leaves of the secondary stem. Although Na and Cl accumulated in all leaves, accumulation was the highest in basal leaves, causing a significant reduction of net assimilation of CO2 in these leaves. Although salinity significantly reduced fruit numbers and weight, it did not affect fruit soluble solids. Under saline conditions, leaves had a greater isotopic preference for the fixation of C-13 than C-12. Two weeks before fruit harvest, regardless of salinity, fruits were the preferential sinks for carbon originated from leaves from the basal and apical parts of the stem. During fruit harvest week, basal and apical leaves of plants irrigated with water of ECw =0.5 dS m(-1) had little or no C-13 contribution to the fruit, whereas leaves (mainly basal) of plants irrigated with ECw =3.5 dS m(-1) continued to send photoassimilates to the fruit. Thus, saline stress reduced fruit size and number significantly and delayed carbon translocation to fruits, delaying fruit maturation. Results suggest that under saline stress, eliminating fertigation two weeks before fruit harvest to reduce fertilizer costs may not be beneficial to melon producers. However, fertigation can be stopped in the last week before harvest when using low-salinity water. Further studies are needed to determine if extending the crop cycle under salinity may compensate for delayed fruit maturation and increase fruit sugars.