Development of photosynthetic electron transport in Pinus silvestris

Abstract

AbstractPhotosynthetic electron transport activity has been measured in chloroplasts isolated from dark‐grown seedlings of Pinus silvestris L. and in chloroplasts isolated from seedlings subjected to illumination for periods of up to 48 h. Activities of photosystem 2, photosystem 1 and photosystem 2 plus 1 have been measured. Chloroplasts isolated from dark‐grown seedlings showed significant electron transport activity through both photosystems and through the entire electron transport chain from water to NADP. Illumination of the seedlings for only 5 min markedly promoted photosystem 2 activity. The artificial electron donor, diphenylcarbazide. promoted activity in chloroplasts from dark‐grown seedlings and in chloroplasts from seedlings illuminated for up to 30 min. In comparison to photosystem 2 and overall electron transport from water to NADP, photosystem 1 activity increased only slightly during illumination. Measurements of electron transport and fluorescence kinetics have confirmed that photosynthetic electron transport capacity is limited on the water splitting side of photosystem 2 in dark‐grown seedlings, whereas the primary and secondary electron acceptors of photosystem 2 are fully synthesized and functioning in darkness.Polyethylene glycol must be used as a protective agent when isolating photoactive chloroplasts from secondary needles of conifers. However, the presence of polyethylene glycol, when isolating chloroplasts from dark‐grown pine cotyledons, caused a total inhibition of the activity of photosystem 2. The failure of others to show a substantial electron transport activity in chloroplasts from dark‐grown Pinus silvestris might depend on their use of polyethylene glycol in the preparation medium and/or on their use of suboptimal reaction conditions for the electron transport measurements.