Redox states of photosystems I and II in irradiated leaves of wheat seedlings grown under different conditions of nitrogen nutrition

Changes in the redox states of photosystem I (PSI) and PSII in irradiated wheat leaves were studied after growing seedlings on a nitrogen-free medium or media containing either nitrate or ammonium. The content of P700, the primary electron donor of PSI was quantified using the maximum magnitude of absorbance changes at 830 nm induced by saturating white light. The highest content of P700 in leaves was found for seedlings grown on the ammonium-containing medium, whereas its lowest content was observed on seedlings grown in the presence of nitrate. At all irradiances of actinic light, the smallest accumulation of reduced Q(A) was observed in leaves of ammonium-grown plants. Despite variations in light-response curves of P700 photooxidation and Q(A) photoreduction, the leaves of all plants exposed to different treatments demonstrated similar relationships between steady-state levels of P700(+) and Q(A)(-). The accumulation of oxidized P700 up to 40% of total P700 content was not accompanied by significant Q(A) photoreduction. At higher extents of P700 photooxidation, a linear relationship was found between the steady-state levels of P700(+) and Q(A)(-). The leaves of all treatments demonstrated biphasic patterns of the kinetics of P700(+) dark reduction after irradiation by far-red light exciting specifically PSI. The halftimes of corresponding kinetic components were found to be 2.6-4 s (fast component) and 17-22 s (slow component). The two components of P700(+) dark reduction were related to the existence of two PSI populations with different rates of electron input from stromal reductants. The magnitudes of these components differed for plants grown in the presence of nitrate, on the one hand, and plants grown either in the presence of ammonium or in the absence of nitrogen, on the other hand. This indicates the possible influence of nitrogen nutrition on synthesis of different populations of PSI in wheat leaves. The decrease in far-red light irradiance reduced the relative contribution of the fast component to P700(+) reduction. The fast component completely disappeared at low irradiances. This finding indicates that the saturating far-red light must be applied to determine correctly the relative content of each PSI population in wheat leaves.