Modulação da rede de trocas gasosas em um cultivar de cana-de-açúcar tolerante a seca: o papel do fósforo

Abstract

Drought is one of the main abiotic stresses causing significant reduction of crop yield in most of agricultural areas in tropical and subtropical regions. Particularly, water deficit is the major environmental factor limiting sugarcane yield. The objective of this study was to identify what kind of changes in the photosynthetic network would be linked to the homeostasis modulation of a drought tolerant sugarcane cultivar subjected to severe water deficit. Moreover, we tested the potential effect of phosphorus supplying on the plant physiological responses to drought, since such element is essential to photosynthetic processes. This study was carried out in a greenhouse where sugarcane plants (Saccharum spp.) cv. RB 86-7515 were grown. The cultivar RB 86-7515 is drought tolerant and has high crop yield, being adapted to sand soils and cultivated in the states of São Paulo and Parana, Brazil. One single-node stalk segment was planted in plastic pots (20 L) filled with soil (red-yellow ultisol). Soil fertilization was done according to the chemical analysis, except for the phosphorus (P) supply. Regarding this element, plants were subjected to the original P concentration in soil (36 mg dm-3) or supplied with the equivalent to 100 kg P2O5 ha-1 using CaH4(PO4)2. Plants were irrigated daily until the beginning of drought treatment. After 90 days of shoot emergence, the water deficit was started at the phenological phase II (tillering). Plants were irrigated with the equivalent to 100% (control) or 10% (water deficit) of daily evapotranspiration. Sugarcane plants were subjected to water deficit for ten weeks. At the end of this period, light response curves of leaf gas exchange and other physiological and growth variables were performed in each treatment. The experiment was performed in a completely randomized design, with four repetitions. Data were subjected to two-way analysis of variance (p<0.05) and the mean values were compared by Tukey s test (p<0.05). To assess changes in system network, we evaluated the occurrence of photosynthetic system modulation under distinct water regimes and phosphorus supplying via the concept and measurement of global connectance, Cg. The set of results from CV and Cg analyses indicated adjustments in the relationships among the elements of the leaf gas exchange network that support the high drought tolerance of the sugarcane cultivar RB 86-7515. Such adjustments enabled the homeostasis of both photosynthesis and plant growth under water deficit. Moreover, P-supplying improved the sugarcane acclimation capacity by affecting plant characteristics related to water status and photosynthetic performance and causing network modulation under water deficit.