Effects of single and combined exogenous application of abscisic acid and melatonin on cotton carbohydrate metabolism and yield under drought stress

Drought is a major abiotic factor that adversely affects plant growth and crop productivity. Abscisic acid (ABA) and melatonin (MT) are two commonly used hormones, whose exogenous application has been shown to improve crop drought tolerance under water deficiency, when used separately; however, studies on the effects of their combined application on crops are scarce. To amend this, pot studies were conducted to study the effects of exogenous ABA and MT application (single or combined) on cotton leaf photosynthetic carbon metabolism and yield under drought. Results revealed that single ABA or MT application significantly improved plant water status under drought by different pathways, which curtailed drought-induced yield losses. ABA+MT application effects on plant water status were similar to those of MT, however, ABA+MT application had synergistic effects on alleviating drought-induced yield losses. ABA spraying had no effects on the net photosynthetic rate (Pn) of drought-stressed leaves, but MT application improved Pn by increasing stomatal conductance and CO2 fixation, and ABA+MT was more effective than MT in alleviating the Pn. Furthermore, ABA application reduced leaf sucrose synthesis by decreasing sucrose synthase (SuSy) activity, and MT and ABA+MT application also inhibited sucrose synthesis by reducing SuSy and sucrose phosphate synthase activities. Extra ABA down-regulated acid invertase activity to inhibit the hydrolysis of sucrose to hexose, but MT spraying decreased both acid and alkaline invertases activities of drought-stressed leaves, and ABA+MT had similar effects as MT. Although ABA or MT down-regulated GhSWEET5, GhSWEET20 and GhSWEET55 expression, thus repressing sucrose transport to the cell wall, their application up-regulated GhSUT3A/D and GhSUT4 expression in drought-stressed leaves, improving in that way sucrose transport to the sieve-companion cell complex and ultimately benefiting yield formation. Similarly, ABA+MT application had additive effects on GhSUT3A/D and GhSUT4 expression under drought. Consequently, ABA or MT application decreased sucrose content in drought-stressed leaves, while ABA+MT application showed a synergistic effect. Hexose, as the substrate for starch biosynthesis, content decreased in ABA- or MT-treated leaves under drought, however, starch accumulation was not affected by ABA application, and MT spraying increased starch content of drought-stressed leaves, by increasing ADP-glucose pyrophosphorylase activity. Nevertheless, ABA+MT application was more effective in increasing starch synthesis under drought than either single hormone application. In conclusion, the effects of ABA+MT on photosynthetic carbon metabolism of cotton leaves were more similar to those of MT under drought, but there were significant synergistic effects in mitigating the negative effects of drought on Pn, sucrose accumulation, sucrose transport, starch synthesis as well as final yield.