| Yunnan Province is the largest coffee production area in China. However, the Arabica coffee(Coffea arabica L.) grown in this region has long been facing the problem of seasonal drought, which seriously affects its quality and yield. Studying the effect of biochar on the drought resistance of Arabica coffee has practical significance for improving the drought resistance of coffee and coping with drought in the coffee production area of Yunnan. To clarify the effects of biochar application on the morphology, stomatal regulation, abscisic acid(ABA) hormone levels and expression of ABA signaling pathway-related genes of Arabica coffee seedlings under moderate drought stress, four treatments were set up: T1, normal water supply, soil moisture content of 55% to 60% of the maximum water holding capacity, no biochar application; T2, moderate drought, soil moisture content of 30% to 35% of the maximum water holding capacity, no biochar application; T3, moderate drought, same water treatment as T2, application of 5% of the dry soil weight of biochar; T4, same water treatment as T1, application of 5% of the dry soil weight of biochar. The experimental period was set as D1 (drought for 1 day), D15 (drought for 15 days), D30 (drought for 30 days), D60 (drought for 60 days), and R30 (re-watering for 30 days after 60 days of drought, maintaining normal water supply for the seedlings after re-watering). The results showed that compared with T1, the stomatal conductance (Gs) of T2 and T3 decreased significantly by 75% and 50% at D30, and the net photosynthetic rate (Pn) of T2 and T3 decreased significantly by 62.4% and 48.4% at D60; at D15, the stomatal length decreased by 15.4% and 3.7% respectively, significantly reducing the stomatal width, and the stomatal density increased by 40% and 37.8% respectively, and the stomatal apparatus became smaller; the relative content of ABA in T4 was significantly higher than that in T1.
Under moderate drought stress, the ABA content in the leaves of Arabica coffee showed a trend of first increasing and then decreasing, and then increasing again after re-watering. Moreover, moderate drought could induce the expression of most genes in the CaNCED family. The relative expression levels of CaNCED family genes and the relative content of ABA in T3 treatment were significantly higher than those in T2 treatment; the expression levels of ABA receptors CaPYL4 and CaPYL8 were the highest at D30. Moderate drought stress promoted the expression of CaPP2C50, CaPP2C16-likeX2, CaPP2C16-like and CaPP2C24 in the leaves, and most genes in the CaSnRK2 family were involved in expression. Among them, the relative expression level of CaSAPK7 in T3 was significantly higher than that in T2 at D30, D60 and R30. Moderate drought induced the expression of most genes in the ABF family, and the period with the most genes involved in expression was at D30. It indicates that adding biochar under normal irrigation conditions can promote the accumulation of ABA in the leaves of Arabica coffee; adding biochar under moderate drought stress can maintain a higher level of ABA in the leaves of Arabica coffee, and has a better inducing effect on the transduction of the ABA signaling pathway, thereby regulating stomata to improve the drought resistance of Arabica coffee; CaNCED6 and CaNCED8 may be potential genes involved in ABA synthesis in Arabica coffee, which can provide theoretical reference for the subsequent breeding of coffee varieties with stress resistance. |