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

The mechanistic basis for differential sensitivities to chilling-induced ph otoinhibition among two rice (Oryza sativa L.) cultivars (an Indica and a J aponica type) and one barley cultivar (Hordeum vulgare L. cv. Albori) was e xamined. When leaf segments were exposed to moderate illumination at 4 degr ees C, a sustained decrease in the photochemical efficiency of photosystem (PS) II measured as the ratio of variable to maximal fluorescence (F-v/F-m) was observed for several hours, An analysis of fluorescence quenching reve aled a sudden drop in PSII-driven electron transport rate (ETR) and a rapid rise in the reduction state of the primary electron acceptor Q(A) upon exp osure to chilling in moderate light, There was no appreciable difference in the level of non-photochemical quenching (NPQ) nor in the xanthophyll cycl e activity between Japonica rice and barley, However, barley was capable of sustaining a higher ET, thereby keeping a lower reduction state of Q(A) th roughout the chilling for 6 h, The Indica rice was characterized by the low est ability to develop the xanthophyll cycle-associated NPQ, particularly t he fast relaxing NPQ component (qf), accompanied by the highest reduction s tate of Q(A) and photoinhibitory quenching (qI), It is concluded that the l ower susceptibility of barley to chilling-induced photoinhibition than Japo nica rice is attributable to its higher potential to dissipate excess light energy via a photochemical mechanism, whereas Indica rice is more sensitiv e to photoinhibition at a chilling temperature than Japonica rice, due prim arily to its lower capacity to develop an efficient NPQ pathway.