Physio-Biochemical Responses of Oil Palm (Elaeis guineensis Jacq..) Seedlings to Mannitol- and Polyethylene Glycol-Induced Iso-Osmotic Stresses

The aim of this investigation was to comparatively examine the physio-biochemical responses of oil palm seedlings to mannitol- and PEG-induced iso-osmotic stresses. The water content of osmotically stressed oil palm seedlings decreased, but the proline content and the electrolyte leakage of the seedlings increased with decreasing water potential (Psi(w)). However, the responses varied with the strength of osmotic stress and type of osmotic agent. Relative electrolyte leakage (REL) was negatively correlated to chlorophyll content in the osmotically stressed leaves. Chlorophyll a (Chl(a)), chlorophyll b (Chl(b)), total carotenoids (Cx+c) and total chlorophyll (TC) in the seedlings were significantly reduced by osmotic stress, subsequently reducing maximum quantum yield of PSII (F-v/F-m) and photon yield of PSII (Phi(PSII)), thereby lowering net-photosynthetic rate (P-n) and inhibiting growth. Physio-biochemical parameters, inducting REL, F-v/F-m, and Phi(PSII) in oil palm seedlings were reduced more greatly by PEG-induced osmotic stress than by mannitol-induced stress. A deterioration in morphological characters, including leaf chlorosis, leaf burn, and green leaf area reduction were demonstrated in oil palm seedlings under osmotic stress induced by either mannitol or PEG. However, the toxic symptoms in oil palm seedlings under PEG-induced stress were severer than in those under mannitol-induced iso-osmotic stress, especially under severe osmotic stress.