The effects of Mn-deficiency on CO2 assimilation and excitation energy distribution were studied using Mn-starved maize leaves. Mn-deficiency caused about 70 % loss in the photon-saturated net photosynthetic rate (PN) compared to control leaves. The loss of PN was associated with a strong decrease in the activity of oxygen evolution complex (OEC) and the linear electron transport driven by photosystem 2 (PS2) in Mn-deficienct leaves. The photochemical quenching of PS2 (qP) and the maximum efficiency of PS2 photochemistry (Fv/Fm) decreased significantly in Mn-starved leaves under high irradiance, implicating that serious photoinhibition took place. However, the 'high-energy' fluorescence quenching (qE) decreased, which was associated with xanthophyll cycle. The results showed that the pool of de-epoxidation components of the xanthophyll cycle was lowered markedly owing to Mn deficiency. Linear electron transport driven by PS2 de-creased significantly and was approximately 70 % lower in Mn-deficient leaves than that in control, indicating less trans-thylakoid pH gradient was built in Mn deficient leaves. We suggest that the decrease of non-radiative dissipation depending on xanthophyll cycle in Mn-starved leaves is a result of the deficiency of trans-thylakoid pH gradient. and C. D. Jiang, H. Y. Gao, Q. Zou.
Violaxanthin de-epoxidase (VDE) is localised in the thylakoid lumen of chloroplasts and catalyses de-epoxidation of violaxanthin into antheraxanthin and zeaxanthin. Tobacco vde gene was inserted into a binary vector pCAMBIA1301 with the hygromycin resistant gene for selection in antisense and overexpressed ways. Two constructs with antisense and overexpressed vde gene were introduced in tobacco (Nicotiana tabacum L.) using Agrobacterium tumefaciens strain LBA4404, PCR and Southern blot analyses demonstrated that the exogenous gene was integrated into genome of tobacco plants. VDE activity assay and HPLC analysis of pigments showed that the vde gene was expressed in the overexpressed transformants, whereas suppressed in the antisense ones. The chlorophyll fluorescence measurements proved that the contents of VDE in transgenic plants have a significant function in non-photochemical quenching. and Ying Deng ... [et al.].
Glycine betaine (GB) is an effective compatible solute that improves the tolerance in plants to various stresses. We investigated the effects of 2 mM GB applied to the roots of a tobacco (Nicotiana tabacum L.) cultivar on enhancing photosynthesis under low-temperature (LT) stress (5/5 °C, 12/12 h, 300 µmol m-2 s-1) and in the subsequent recovery (25/18 °C) from the stress. The net photosynthetic rate, intrinsic efficiency measured as the ratio of variable to maximum fluorescence, and actual efficiency of the photochemistry of photosystem 2 as well as the ATPase activity in the thylakoid membrane decreased, and a distinct K step in the fluorescence transient O-J-I-P appeared under cold stress. Exogenous GB alleviated the decrease in all these parameters. The LT-stress induced the accumulation of 33-66 kDa polypeptides and decreased the proportion of unsaturated fatty acids in the thylakoid membrane. In plants subjected to LT-stress, GB protected these polypeptides from damage and enhanced the proportion of unsaturated fatty acids. An increase in non-radiative energy dissipation (NPQ) may be involved in the improvement of the function of the thylakoid membrane by GB since exogenous GB protected violaxanthin de-epoxidase and enhanced NPQ. and C. Wang ... [et al.].
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 (PN), maximal quantum yield of PSII photochemistry (Fv/Fm), and effective quantum yield of PSII photochemistry (ΦPSII) 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. and X. S. Song ... [et al.].