Rapid, reversible alterations in spinach thylakoid appression upon changes in light intensity

Changes in light quantity and quality cause structural changes within the thylakoid membrane; long-term responses have been described for so-called 'sun' and 'shade' leaves. Many leaves, however, experience changes in irradiance on a time scale of minutes due to self-shading and sun flecks. In this study, mature, attached spinach leaves were grown at 300 mumol photons m(-2) s(-1) then rapidly switched to a different light treatment. The treatment irradiances were 10, 800 or 1500 mumol m(-2) s(-1) for 10 min, or 10 or 20 min of self-shading (about 10 mumol m(-2) s(-1) ). Image analysis of transmission electron micrographs revealed that a 10 min switch to a lower light intensity increased grana size and number per chloroplast profile by 10-20%. Returning the leaves to 300 mumol m(-2) s(-1) for 10 min reversed the phenomenon. Chlorophyll fluorescence measurements of detached, intact leaves at 77 K were suggestive of a transition from state 2 to state 1 upon shading. Diurnal ultrastructural measurements of granal size and number did not reveal a significant net change in ultrastructure over the time scale of hours. It is concluded that spinach chloroplasts can alter the degree of thylakoid appression in response to irradiance changes on a time scale of minutes. These ultrastructural responses are caused by biochemical and biophysical adjustments within the thylakoid membrane that serve to maximize photosynthesis and minimize photo-inhibition under rapidly fluctuating light environments.