High irradiance and water stress induce alterations in pigment composition and chloroplast activities of primary wheat leaves

Remarkable changes were observed in chlorophyll (Chl) (a+b), carotenoids (Car), and protein content of leaves and fluorescence emission, polarisation, excitation energy transfer, lipid peroxidation and DCPIP photoreduction activity in isolated chloroplasts of wheat leaves grown under moderate irradiance (MI, 15 W m(-2), control) and subsequently exposed to high irradiance stress (HIS, 250 W m(-2)), water stress (WS, 5% aqueous polyethylene glycol-4000 solution) and HIS+WS simultaneously, during mature and senescence phase. In the stress exposed samples the Chl, Car and protein contents and kinetics of Hill activity significantly declined: Decrease in excitation energy transfer and increase in membrane polarisation and accumulation of malondialdehyde (MDA) in chloroplasts were also observed. The effect was more pronounced when the seedlings were treated with HIS+WS simultaneously. These observations suggest additive and a possible synergetic action of HIS and WS causing faster loss of pigments and protein content, intense changes in membrane properties including photochemical function, compared to samples exposed to either of the stresses individually.