Photoinhibition and xanthophyll cycle activity in bayberry (Myrica rubra) leaves induced by high irradiance

The effect of high irradiance (HI, photosynthetically active photon flux density of 1300 mu mol m(-2) s(-1)) on net photosynthetic rate (P-N), 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 (F-m), photochemical efficiency of PS2 (F-v/F-m) and quantum yield of PS2 Phi(PS2) and increased reduction state of Q(A) (1 - q(P)) and non-photochemical quenching (NPQ). Initial fluorescence (F-0) 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 (F-i - F-0)/(F-p - F-0) which is an expression of the proportion of the Q(B) 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 F-m, F-v/F-m, OPS2, NPQ, slope of F-i to F-p, V+A+Z, and DES, and increased F-0, 1 - q(P), ratio (F-i - F-0)/(F-p - F-0), 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.