Relative contributions of photochemical and non‐photochemical routes to excitation energy dissipation in rice and barley illuminated at a chilling temperature

Abstract

The mechanistic basis for differential sensitivities to chilling‐induced photoinhibition among two rice (Oryza sativa L.) cultivars (an Indica and a Japonica type) and one barley cultivar (Hordeum vulgare L. cv. Albori) was examined. When leaf segments were exposed to moderate illumination at 4°C, a sustained decrease in the photochemical efficiency of photosystem (PS) II measured as the ratio of variable to maximal fluorescence (Fv/Fm) was observed for several hours. An analysis of fluorescence quenching revealed a sudden drop in PSII‐driven electron transport rate (ETR) and a rapid rise in the reduction state of the primary electron acceptor QA upon exposure to chilling in moderate light. There was no appreciable difference in the level of non‐photochemical quenching (NPQ) nor in the xanthophyll cycle activity between Japonica rice and barley. However, barley was capable of sustaining a higher ETR, thereby keeping a lower reduction state of QA throughout the chilling for 6 h. The Indica rice was characterized by the lowest ability to develop the xanthophyll cycle‐associated NPQ, particularly the fast relaxing NPQ component (qf), accompanied by the highest reduction state of QA and photoinhibitory quenching (qI). It is concluded that the lower susceptibility of barley to chilling‐induced photoinhibition than Japonica rice is attributable to its higher potential to dissipate excess light energy via a photochemical mechanism, whereas Indica rice is more sensitive to photoinhibition at a chilling temperature than Japonica rice, due primarily to its lower capacity to develop an efficient NPQ pathway.