Ajustes fisiológicos e bioquímicos em plantas de palma de óleo (Elaeis Guineensis Jacq.) submetidas a eventos repetitivos de déficit hídrico.

Abstract

Oil palm (Elaeis guineensis Jacq.) plantations are prone to repetitive periods of water deficit and re-hydration due to distribution patterns and rains in the region. In some species, repetitive water deficit events trigger physiological and biochemical responses that culminate in the acclimating of photosynthetic machinery and improve plant responses to stress. This adjustment is known as stress acclimation, and has not yet been evaluated in oil palm. With that, this work aimed to evaluate the ability of oil palm in acclimating their photosynthetic machinery and antioxidant system in response of repetitive water deficit events. For this, the physiological responses of oil palm plants submitted to one (WD 1x), two (WD 2x) and three (WD 3x) water deficit events were compared to well-watered plants (control). Water deficit events were conducted until the predawn leaf water potential (Ψpd) of the stressed plants reached around –2.5 MPa. According to each treatment, the irrigation of stressed plants was resumed to induce the complete recovery of plant water status before a new water deficit event was applied. Treatment evaluations were performed during the induction of the third, second, and first stress events for WD 3x, WD 2x, and WD 1x treatments, respectively. A fast decrease in Ψpd was observed in the first seven days of stress, with averages of –1.81 MPa in WD 1x plants and –1.47 MPa in WD 2x and WD 3x plants. Dehydration of plants for seven days led to sharp decreases in the net CO2 assimilation rate (A), however, averages in WD 1x plants were lower than in WD 3x plants. The mesophyll conductance to CO2 was lower in WD 1x plants until day 21. The chloroplast concentration of CO2 decreased equally in WD 1x and WD 3x plants until day 14, being lower in WD 1x on subsequent days. The concentration of CO2 in the chloroplast decreased equally in WD 1x and WD 3x plants until day 14, being lower in WD 1x on subsequent days. WD 1x plants showed lower activity of ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) and lower regeneration rate of Ribulose 1,5 bisphosphate (Jmax) in parallel to higher rates of photorespiration and respiration than the other treatments. In addition, WD 1x plants showed lower average values of maximum quantum efficiency of photosystem II (PSII) (Fv/Fm) and maximum quantum efficiency of PSII at a given light intensity and this occurred in parallel with a higher concentration of malondialdehyde (MDA). Decreases in A of stressed plants were accompanied by increases in the substomatal to ambient CO2 ratio and significant reduction in carboxylation efficiency in WD 1x plants compared to WD 3x plants. Fv/Fm in plants evaluated at 28 days of evaluation did not differ between control and WD 3x plants, but it was 22.8% lower in WD 1x plants compared to the control. Significant reductions in ETR were also observed, linked to increases in NPQ in WD 1x plants. In relation to the control plants, the ETR/A was increased by 426.5% and 200% in WD 1x and WD 3x plants. The activity of SOD and GR increased by 124% and 270% in WD 3x plants, while 68.7% and 30% in WD 1x plants. H2O2 and MDA concentrations increased by 127% and 110% in WD 1x plants, with no effect on H2O2 and MDA in WD 3x relative to control. Thus, we conclude that oil palm suffered strong decreases in A mediated by stomatal and non-stomatal factors, as they were able to acclimatize their photosynthetic machinery, as well as modulate their antioxidant system and improve their physiological performance when subjected to repetitive events of water deficit.