Chloroplasts isolated from the primary leaves of wheat seedlings lost 22 % of their chlorophyll and 35 % of the carotenoids after 4 h of incubation under a medium irradiance (MI) (400 pmol m'^ s'*, PAR). After a 3 h MI exposure, the photosystem (PS) 2 activity was completely lost while photosystem 1 (PSI) activity was reduced to 32 % of that, found initially. Chloroplasts isolated from the leaves of seedlings exposed to a high irradiance (1500 pmol m-^ s"') contained a higher concentiation of zeaxanthin (z) and lost pigments and photochemical activities at a relatively slower rate. When the z concentration in chloroplasts incubated under MI was enhanced several times by ascorbate (Asc), the losses of pigment contents and photosystem activities were significantly reduced. However, when the Asc-induced synthesis of z was inhibited by dithiothreitol, protection of the pigment contents and photochemical activities disappeared. Thus, z can photoprotect the chloroplasts during incubation under irradiation.
Aging induced loss of chlorophyll (Chl), carotenoids (Car) and proteins was studied in suspensions of isolated wheat chloroplasts incubated in the light or dark. Faster rate of pigment loss in the light was due to photodestruction of pigments. Compared to Chl, a greater loss of Car in the light was due to photoprotective action of Car towards Chl resulting in its own destruction. The total protein content was lowered more in the light than in dark. Soluble proteins of chloroplasts declined by 36 %, whereas membrane proteins showed a loss of only 8 % after 6 d of light incubation.