Exogenous Kinetin Promotes the Nonenzymatic Antioxidant System and Photosynthetic Activity of Coffee (Coffea arabica L.) Plants Under Cold Stress Conditions

Coffee plants are seasonally exposed to low chilling temperatures in many coffee-producing regions. In this study, we investigated the ameliorative effects of kinetin—a cytokinin elicitor compound on the nonenzymatic antioxidants and the photosynthetic physiology of young coffee plants subjected to cold stress conditions. Although net CO(2) assimilation rates were not significantly affected amongst the treatments, the subjection of coffee plants to cold stress conditions caused low gas exchanges and photosynthetic efficiency, which was accompanied by membrane disintegration and the breakdown of chlorophyll pigments. Kinetin treatment, on the other hand, maintained a higher intercellular-to-ambient CO(2) concentration ratio with concomitant improvement in stomatal conductance and mesophyll efficiency. Moreover, the leaves of kinetin-treated plants maintained slightly higher photochemical quenching (qP) and open photosystem II centers (qL), which was accompanied by higher electron transfer rates (ETRs) compared to their non-treated counterparts under cold stress conditions. The exogenous foliar application of kinetin also stimulated the metabolism of caffeine, trigonelline, 5-caffeoylquinic acid, mangiferin, anthocyanins and total phenolic content. The contents of these nonenzymatic antioxidants were highest under cold stress conditions in kinetin-treated plants than during optimal conditions. Our results further indicated that the exogenous application of kinetin increased the total radical scavenging capacity of coffee plants. Therefore, the exogenous application of kinetin has the potential to reinforce antioxidant capacity, as well as modulate the decline in photosynthetic productivity resulting in improved tolerance under cold stress conditions.