Photosynthetic Characteristics and Photoprotective Mechanisms in Highland Barley

Abstract

As a local crop of Qinghai-Tibet Plateau, highland barley (Hordeum vulgare L.) has acclimated to the typical habitat with strong light and low O2 to CO2 ratio. Low O2 to CO2 ratio may have a great influence on photorespiration which plays an important role in protecting photosynthetic apparatus against photoinhibition. In this study, we hope to know whether photorespiration plays an important role in photoprotection of highland barley and what are the main photoprotective mechanisms of high-land barley. Gas exchange, fluorescence quenching kinetics and reflectance spectrum were investigated to explore the photosynthetic characteristic and allocation of excitated energy in two highland barley cultivars. Results showed that light saturation point of highland barley was about 1 000 μmol m-2 s-1. Photorespiration rate (Pr) increased with light intensity increasing while it did not change significantly above 500 μmol m-2 s-1. The percentage of photorespiration to total photosynthesis (Pr/Pm) declined gradually with the increase of light intensity. The efficiency of open centers of photosystem II (Fv'/Fm'), actual photosystem II efficiency (ΦPSII) and photochemical quenching (qP) all declined with the increase of light intensity; non-photochemical quenching increased with light intensity increasing, indicating that more and more excited energy dissipated as thermal dissipation. Spectrum analysis showed that the change of photochemical reflectance index (△ PRI) increased significantly when fully dark-adapted plants were suddenly exposed to light. On these bases, we concluded that photorespiration is not the main pathway for highland barley to relieve strong light stress; thermal dissipation relying on xanthophyll cycle may play an important role in dissipating excessive excited energy in highland barley.