Periodic drought fluctuation is a common phenomenon in Northwest China. We analyzed the response of Chinese dwarf cherry (Cerasus humilis) seedlings, a dwarf shrub species with considerably strong adaptabilities, exposed to water stress (WS) by withholding water for 21 d, and subsequent recovery of 7 d. Leaf relative water content (LRWC), net photosynthetic rate (P N ), maximal quantum yield of PSII photochemistry (F v /F m ), and effective quantum yield of PSII photochemistry (Φ PSII ) decreased with increasing water deficit. In contrast, the nonphotochemical quenching of Chl fluorescence (NPQ) significantly increased, as well as the amounts of violaxanthin (V) + antheraxanthin (A) + zeaxanthin (Z). In the whole levels, the photosynthetic pigment composition did not display significant changes in WS seedlings. However, the de-epoxidation state of xanthophyll cycle pigments [(Z+0.5A)/VAZ ] generally exhibited higher values in WS seedlings. The significant inhibition of de-epoxidation by dithiothreitol (DTT) and negligible changes of epoxidation of Z by glucosamine (Gla) were both observed; the slight but stably upregulated transcript level of violaxanthin de-epoxidase (VDE) and downregulated zeaxanthin epoxidase (ZEP) expression profile were found during WS period, indicating that they were regulated on post-transcript levels. VDE activity, via the accumulation of Z and A, which confers a greater capacity of photoprotection, appears to contribute to the survival of severely stressed plants.

2 Recently, there has been increasing attempts for photosynthetic bacteria (PSB) as foliar fertilizer in China. The biomass, net photosynthetic rate (Pn), maximal photochemical efficiency of PS II(Fv/Fm) and antioxidant isoenzymes metabolism were examined in Chinese dwarf cherry (Prunus humilis Bunge) seedlings with or without exogenous PSB application in this study. PSB sprays resulted in a significant accumulation of fresh weight and leaf area of Chinese dwarf cherry plants, accompanying with a marked increase of Pn. However, Fv/Fm had negligible changes. The exogenous PSB also induced the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX), and a general increase of their isoenzymes activities. The increased level was the most significant in chloroplasts [such as Cu/Zn-SOD, Fe-SOD and stroma APX (sAPX)]. Our results demonstrated that PSB played a favorable role in growth and photosynthesis and improved the antioxidative capability in plants.