The relationships between photochemical reflectance index (PRI) and photosynthetic status in radish species differing in salinity tolerance

Since photosynthesis is highly sensitive to salinity stress, remote sensing of photosynthetic status is useful for detecting salinity stress during the selection and breeding of salinity-tolerant plants. To do so, photochemical reflectance index (PRI) is a potential measure to detect conversion of the xanthophyll cycle in photosystem II. Raphanus sativus var. raphanistroides is a wild radish species closely related to domesticated radish, and is distributed throughout the coastal regions of Japan, where it is thought to be salt tolerant. In this study, we raised wild and domesticated radishes under various salt conditions and assessed growth, photosynthetic status, and PRI. When grown at mild salt stress (50 mM NaCl), wild radish leaves showed photosynthetic activity levels comparable to control plants, whereas the photosynthetic activity of domesticated radish was suppressed. This result suggests that wild radishes are more salt-tolerant than domesticated radishes. Although photosynthetic rate and the photochemical quantum yield were significantly correlated with PRI in both species, the PRI resolution was insufficient to distinguish differences in salt tolerance between wild and domesticated radish. Wild radish had a lower maximum quantum yield (Fv/Fm) when grown under moderate salt stress (200 mM NaCl), suggesting chronic photoinhibition. The relationship between non-photochemical quenching (NPQ) and PRI was significant when leaves with chronic photoinhibition were eliminated but this relationship was not significant when they were included. In contrast, the relationship between photosynthesis and PRI was significant regardless of whether leaves displayed chronic photoinhibition or not. We conclude that PRI is useful to detect relatively large reductions in photosynthetic rate under salinity stress, and that care should be taken to evaluate NPQ from PRI.