Abstract:It has been shown that fluorescence yield of chloroplasts at wavelength near 684 nm can be regulated by Mg+2. But on the other hand trypsin abolishes this Mg+2-induced fluorescence change. Pretreatment of chloroplasts with Mg+2 protects the Mg+2 induced chlorophyll a fluorescence change from trypsin. It is found that the protective effect of Mg+2 pretreatment of chloroplasts against trypsin is concentration dependent (at the range btween 1.5-12 mM Mg+2). At a 12 mM MgCl2 concentration it appears that trypsin has no effect on Mg+2-induced fluorescence change. Similar results were obtained with the determination of chlorophyll contents in the chloroplasts. Plants grown at different temperature produce chloroplasts which show similar characteristics. But lower fluorescence and chlorophyll change are found in the Low temperature grown plants. The above results support the evidence that pretreating the chloroplasts with Mg+2 causes a conformafional change in LHCPs and protecting the functional group that acts as regulator to the fluorescence change against trypsin. It is also proposed that pretreatment of chloroplasts with Mg+2 also protects the connection of chlorophyll molecule with protein against trypsin in thylakoid membrane.