Tolerância ao déficit hídrico de mudas de Coffea arabica L. suplementadas com magnésio foliar

Abstract

Coffea arabica L. is one of the most commercially cultivated perennial species, but water deficit is the most limiting abiotic stress to it, negatively affecting its photosynthetic activity and yield. Thus, magnesium (Mg) being the enzymatic cofactor acting in the photosynthetic process and in the translocation of sugars, the hypothesis that coffee trees supplemented with Mg on the leaves would present drought tolerance due to the maintenance of sugar production and translocation, greater accumulation of carbohydrates in the system root, in addition to better recovery of water status and photosynthetic apparatus when rehydrated, was tested. Thus, aimed to evaluate whether Mg foliar application in coffee trees under water deficit would stimulate the maintenance of photosynthetic activity and the accumulation of carbohydrates in the root system. Coffee seedlings (cultivar Mundo Novo) were submitted to two leaf Mg supplements (MgSO4 . 7H2O) (Control, Mg0% and Mg2%) and two water conditions [wellwatered (WW) and water deficit (WD)]. Assessments were performed at three times defined based on the intensity of stress [50 and 30% of the field capacity (FC)] and rehydration (REC). It was evaluated: leaf water status; biometric; gas exchange; photochemical activity; carbohydrates in old leaf (OL), new (NL) and root (R); foliar nutrient content and accumulation, in addition to nutrient use efficiency (NUE) and the stress tolerance index (STI), through root dry mass (RDM) and total dry mass (TDM). In 50% FC there was maintenance of the number of leaves (NL), CO2 assimilation (A), and transpiration (E), as well as higher STIRDM and STITDM, in WD + Mg2% plants. There was a significant reduction in leaf water potential at dawn (Ψwpd), relative water content (RWC), total soluble sugars in root (TSSR), starch in old leaf (StarchOL) and sucrose in root (SUCR) in WD + Mg2% plants. In 30% FC, there was a significant difference between the treatments of Mg leaf, showing lower STIRDM and STITDM. The highest values of gas exchange were observed in plants WW + Mg2%, but they accumulated less starch in the analyzed tissues. However, plants with WD + Mg2% presented the highest values of TSSR, SUCR and intrinsic water use efficiency (WUEi), in relation to plants WD + Mg0%. In REC, no significant difference was observed between treatments with leaf Mg for leaf water potential, RWC and instantaneous carboxylation efficiency (A/Ci), with recovery of water status. After REC, WD + Mg2% plants had higher WUEi, ratio between apparent electron transport and CO2 assimilation (ETR/A), photochemical fluorescence extinction coefficient (qP), RDM, root/shoot ratio (RDM/SDM), TSSOL, SUCOL, StarchR, TSSR and SUCR, calcium (Ca), potassium (K) and Mg. Greater content and accumulation of Mg in leaves were found in plants that received this treatment. Plants WW + Mg2% had higher EUN, for N, K and Ca. Therefore, coffee seedlings supplemented with foliar Mg were more tolerant to moderate water deficit, through the maintenance of photosynthetic activity, of the greater translocation of sugars and of the greater accumulation of starch in the root. Under good water availability, coffee seedlings show greater efficiency in the use of nutrients when supplemented with foliar Mg.