Spraying low concentrated (0.5-5.0 mM) solutions of NaHSO3 on Satsuma mandarin (Citrus unshiu Marc.) leaves resulted in enhancement (maximal about 15 % at 1 mM NaHSO3) of net photosynthetic rate (PN) for 6 d. The potential photochemical efficiency of photosystem 2 (PS2, Fv/Fm) and the quantum yield of PS2 electron transport (ΦPS2) were increased under strong photon flux density (PFD). The slow phase of millisecond delayed light emission (ms-DLE) was increased, showing that the transmembrane proton motive force related to photophosphorylation was enhanced. We also observed that low concentrations of NaHSO3 promoted the production of ATP in irradiated leaves. We suggest that the increase in PN in Satsuma mandarin leaves caused by low concentrations of NaHSO3 solution may have been due to the stimulation of photophosphorylation and, hence, the increase in photochemical efficiency through speeding-up of PS2 electron transport. Photoinhibition of photosynthesis in leaves was modified by NaHSO3 treatment under high PFD. Hence the increase in leaf dry mass seems to be associated with the mitigation of photoinhibition caused by strong PFD. and Y.-P. Guo ... [et al.].
The effect of high irradiance (HI, photosynthetically active photon flux density of 1 300 µmol m-2 s-1) on net photosynthetic rate (PN), chlorophyll fluorescence parameters, and xanthophyll cycle components were studied in fruit tree bayberry leaves. HI induced the photoinhibition and inactivation of photosystem 2 (PS2) reaction centres (RCs), which was characterized by decreased PN, maximum yield of fluorescence after dark adaptation (Fm), photochemical efficiency of PS2 (Fv/Fm) and quantum yield of PS2 (ΦPS2), and increased reduction state of QA (1-qP) and non-photochemical quenching (NPQ). Initial fluorescence (F0) showed a decrease after the first 2 h, and subsequently increased from the third hour exposure to HI. Furthermore, a greater increase in the ratio (Fi-F0)/(Fp-F0) which is an expression of the proportion of the QB non-reducing PS2 centres, whereas a remarked decrease in the slope of Fi to Fp which represents the rate of QA reduction was observed in leaves after HI exposure. Additionally, HI caused an increase in the pool size of the xanthophyll cycle pigments and sustained elevated contents of zeaxanthin (Z), antheraxanthin (A), and de-epoxidation state (DES) at the end of the irradiation period. During HI, decreased Fm, Fv/Fm, ΦPS2, NPQ, slope of Fi to Fp, V+A+Z, and DES, and increased F0, 1-qP, ratio (Fi-F0)/(Fp-F0), and V were observed in dithiothreitol (DTT)-fed leaves compared to control ones under the same conditions. Hence photoinhibition caused by HI in bayberry was probably attributed to inactivation of PS2 RCs, and photoprotection from photodamage were mainly related to the xanthophyll cycle-dependent heat dissipation in excess photons. and Y.-P. Guo ... [et al.].