Tolerância de ziziphus joazeiro mart. ao déficit hídrico induzido em casa de vegetação e à seca sazonal em comunidades naturais

Abstract

Ziziphus joazeiro Martius is an arboreal species of great economic, medicinal, agro-industrial, and phytochemical potential. It occurs in the phytogeographic domains of the Cerrado, Atlantic Forest, and it is predominantly distributed in semiarid environments as Caatinga, where it faces periods of temporary drought during the year. Thus, to identify which are the physiological and biochemical responses of Z. joazeiro during periods of drought is crucial to identify which mechanisms are used by this species when it is subjected to water deficiency in different environments. The aim of this study was to investigate the effects of water deficit in Z. joazeiro, with the following objectives: i) evaluate the efficiency of the photosynthetic apparatus and antioxidant system of Z. joazeiro in response to different types of soil, associated with water deficit and subsequent rehydration under greenhouse conditions; ii) identify and evaluate the physiological mechanisms used by Z. joazeiro to tolerate periods of seasonal droughts in its natural habitat; and iii) evaluate the physiological responses, the efficiency of the photosynthetic apparatus and antioxidant system in natural communities of Z. joazeiro in two environmental conditions: humid tropical and semiarid in the dry and rainy seasons, in the Northeastern Brazil. Experiment I was carried out in a greenhouse. The treatments consisted of: I) loam-clay soil, II) loam-sandy soil and III) sandy soil, versus 1) plants always irrigated according to the field capacity of each soil and 2) plants with suspended irrigation until the net photosynthetic rate of these plants reached values close to zero. Subsequently, plants where rehydrated until the recovery of the net photosynthetic rate. In this study, Z. joazeiro was able to adjust its physiological and biochemical characteristics during the period of water deficiency and rehydration. The physiological and biochemical mechanisms used under water deficit are associated with increased activity of antioxidant enzymes such as ascorbate peroxidase and catalase. They reduce lipid peroxidation, prevent the degradation of photosynthetic pigments and alleviate the photoinhibition of photosystem II. The high content of malondialdehyde and hydrogen peroxide are indicators of oxidative stress in Z. joazeiro plants under water stress. Sandier soils are favorable to the photosynthetic performance of Ziziphus joazeiro under water deficit. Experiment II was carried out using 20 adult matrices of Z. joazeiro, located in a humid tropical forest environment. During the period of study, Z. joazeiro was able to use compensatory mechanisms such as the reduction of stomatal conductance in order to tolerate temporary drought. Although, it maintained a good photochemical capacity and CO2 assimilation, which allows the plant to survive in environments with long periods of low precipitation rates. In experiment III, 20 mother trees of Z. joazeiro were randomly selected in their natural environment in the humid and semiarid tropical regions. From the results obtained, we suggest that the stomatal closure mechanism and the maintenance of leaf water potential can be considered adaptive strategies used by Z. joazeiro aiming to save water during the periods when the soil has low water availability. In addition, an efficient photosynthetic apparatus and the ability to increase the water use efficiency are functional strategies that are likely to prevent photoinhibition in Z. joazeiro during periods of seasonal drought in semiarid regions. An increase in the antioxidant activity of the enzyme ascorbate peroxidase, associated with the enzyme catalase is an efficient defense system to control the reactive oxygen species content and mitigate the oxidative stress in the natural communities of Z. joazeiro growing in tropical humid and semiarid regions.

Keywords: Water deficit. Antioxidant enzymes. ROS. Photosynthesis. Tropical forest. Chlorophyll a fluorescence. Seasonal dry. Semiarid. Rhamnaceae. Gas exchange.