Dai J, Gao H, Dai Y, Zou Q
Department of Plant Science, Shandong Agricultural University, Tai'an, Shandong, P.R. China.
Plant Biol (Stuttg). 2004 Mar-Apr;6(2):171-7. doi: 10.1055/s-2004-817845.
Excitation energy dissipation, including the xanthophyll cycle, during senescence in wheat flag leaves grown in the field was investigated at midday and in the morning. With progress of senescence, photosynthesis (Pn) and actual PSII photochemical efficiency (PhiPSII) decreased markedly at midday. The decrease in extent of Pn was greater than that of PhiPSII. However, there was no significant decline in Pn and PhiPSII observed in the morning, except in leaves 60 days after anthesis. The kinetics of xanthophyll cycle activity, thermal dissipation (NPQ), and q (f) observed at midday during senescence exhibited two distinct phases. The first phase was characterized by an increase of xanthophyll cycle activity, NPQ, and q (f) during the first 45 days after anthesis. The second phase took place 45 days after anthesis, characterized by a dramatic decline in the above parameters. However, the qI, observed both at midday and in the morning, always increased along with senescence. A larger proportion of NPQ insensitive to DTT (an inhibitor of the de-epoxidation of V to Z) was also observed in severely senescent leaves. In the morning, only severely senescent leaves showed higher xanthophyll cycle activity, NPQ, q (f), and qI. It was demonstrated that, at the beginning of senescence or under low light, wheat leaves were able to dissipate excess light energy via NPQ, depending on the xanthophyll cycle. However, the xanthophyll cycle was insufficient to protect leaves against photodamage under high light, when leaves became severely senescent. The ratio of (Fj - Fo)/(Fp - Fo) increased gradually during the first 45 days after anthesis, but dramatically increased 45 days after anthesis. We propose that another photoprotection mechanism might exist around reaction centres, activated in severely senescent leaves to protect leaves from photodamage.
研究了田间生长的小麦旗叶衰老过程中包括叶黄素循环在内的激发能耗散情况，分别在中午和早晨进行了测定。随着衰老进程，中午时光合作用（Pn）和实际PSII光化学效率（PhiPSII）显著下降。Pn下降的幅度大于PhiPSII。然而，早晨时Pn和PhiPSII没有显著下降，除了开花后60天的叶片。衰老期间中午观察到的叶黄素循环活性、热耗散（NPQ）和q（f）的动力学表现出两个不同阶段。第一阶段的特征是开花后前45天叶黄素循环活性、NPQ和q（f）增加。第二阶段发生在开花后45天，其特征是上述参数急剧下降。然而，中午和早晨观察到的qI总是随着衰老而增加。在严重衰老的叶片中还观察到更大比例的NPQ对DTT（V向Z脱环氧化的抑制剂）不敏感。早晨，只有严重衰老的叶片表现出较高的叶黄素循环活性、NPQ、q（f）和qI。结果表明，在衰老初期或弱光条件下，小麦叶片能够通过依赖叶黄素循环的NPQ耗散过剩光能。然而，当叶片严重衰老时，在强光下叶黄素循环不足以保护叶片免受光损伤。（Fj - Fo）/（Fp - Fo）的比值在开花后前45天逐渐增加，但在开花后45天急剧增加。我们提出在反应中心周围可能存在另一种光保护机制，在严重衰老的叶片中被激活以保护叶片免受光损伤。
