Tolerância de cultivares de cana-de-açúcar ao déficit hídrico: relações hídricas,trocas gasosas, fluorescência da clorofila e metabolismo antioxidante

Abstract

The sugarcane is a crop of great economic importance due to its use as a source of raw material for the food industry and energy, Brazil is the world's largest producer. However, the occurrence of seasonal droughts has been one of the main factors limiting sugarcane production and longevity. Thus, this study proposed to investigate the effects of drought in sugarcane, with the following objectives: i) to discriminate different sugarcane cultivars in drought tolerance by means of physiological, biometric and biochemical evaluations; ii) to understand the role of osmoprotectants on the photosynthetic machinery of contrasting sugarcane cultivars in drought tolerance; e iii) to evaluate the exogenous effects of osmoprotetor glycine betaine on the photosynthetic apparatus of the sugarcane cultivar more tolerant to drought when submitted to water deficit. For this, three experiments were sequentially performed. The experiment I was perfomed in pots 170 L disposed the open sky. The treatments consisted of six cultivars of sugarcane and three treatments of water. The cultivars were: RB72910, RB99382, RB72454, RB92579, RB855536 and RB931011, and the water treatments were: control - 80 to 100% water available (AW); moderate water stress - 40 a 60% (AW) and severe water stress - 0 a 20% (AW). The cultivar RB92579 was more drought tolerant, with less changes in physiological and biometric characteristics, higher enzymatic activity, besides low degradation of photosynthetic pigments, lipid peroxidation and biomass reduction. Other way, the RB72454 was the more sensitive, with higher reduction of gas exchanges, of biomass production, of photochemical efficiency, besides low enzymatic activity, and increased degradation of pigments and lipid peroxidation. The experiment II was perfomed in greenhouse. The cultivars contrasting to drought, RB92579 and RB72454 were submitted to drought stress, with to suspension of irrigation. When the plants reached maximum drought stress, were rehydrated. The cultivar RB92579 had less reduction of electron transport rate and of effective efficiency PSII. Furthermore, increased the proline concentration and glycine betaine under drought stress, maintaining this concentration in rehydration even higher than the control. While the RB72454 had higher reduction of electron transport rate and of effective efficiency PSII with increased of quenching non fotochemistry, besides little and any increased in the concentration of proline and glycine betaine, respectively, under drought stress. The experiment III was perfomed in greenhouse. The cultivar more drought tolerant, RB92579, was submitted to foliar application of glycine betaine (without, with one, and two applications). Subsequently, the plants were submitted to drought stress, with the suspension of irrigation, and when reached maximum drought stress, were rehydrated. The foliar application of glycine betaine helped in the maintenance of gas exchanges, of photochemical efficiency PSII, of water condition in tissues, and of carboxylation efficiency. Furthermore, increased the water use efficiency, and decreased the excess of energy and the photoinhibition. Glycine betaine attenuate the effects caused by drought stress in the physiology of sugarcane, also helping in the recovery.