DIFFERENTIAL SUSCEPTIBILITY OF PHOTOSYSTEM-II TO LIGHT STRESS IN LIGHT-ACCLIMATED PEA LEAVES DEPENDS ON THE CAPACITY FOR PHOTOCHEMICAL AND NONRADIATIVE DISSIPATION OF LIGHT

The importance of photoprotective strategies for Photosystem (PS) II f unction under all light regimes was determined from the content of fun ctional PS II, measured by repetitive flash yield of oxygen evolution, in leaves of pea (Pisum sativum L.) grown under 50 (low light), 250 ( medium light), and 650 (high light) mu mol photons m(-2) s(-1). The mo dulation of PS II functionality in vivo was induced in 1.1% CO2 after infiltration of leaves with water (control), nigericin (a lipophilic u ncoupler) or dithiothreitol (DTT, inhibitor of violaxanthin to zeaxant hin conversion) through the cut petioles of leaves of 22-24 day-old pl ants. In all nigericin-treated pea leaves, photoinactivation of PS II was greatly increased with increasing photon exposure (mel photons m(- 2)). This nigericin-induced increase of photoinactivation of PS II was greater than that induced by DTT in medium- and high-light pea leaves , but comparable in low-light peas. In low-light peas during steady-st ate photosynthesis, the development of non-photochemical quenching of chlorophyll fluorescence (NPQ, measured as F-m/F'(m)-1) was lowest, ac companying the highest reduction state of PS II (measured by photochem ical quenching, qp) Further, the susceptibility of PS II to light stre ss, estimated as (1-qp)/NPQ [Park et al., Plant Cell Physiol., 36 (199 5) 1163-1167], was highest in low-light peas. The decline of chlorophy ll fluorescence from maximum (F-m) to steady-state level during induct ion phase of chlorophyll fluorescence was retarded by nigericin treatm ent, with a greater inhibitory effect in high- and medium- than low-li ght pea leaves. From these results, we suggest that the greater suscep tibility of low-light peas to light stress than medium- and high-light peas is ascribed to lower capacities for both the utilization of abso rbed light by photosynthesis and for non-radiative dissipation of abso rbed light as heat.