Abstract：
Instead of red anthocyanins, birches synthesise colourless (to human eye), UV-absorbing flavonols during autumn senescence. To test if flavonols protect against insects, and if leaves with high or low amounts of flavonols differ in their photosynthetic functions, aphid-free and aphid-infested green and senescing birch leaves were collected from outdoor-grown trees and analysed. Photosynthetic parameters were greatly affected by the leaf chlorophyll content (i.e. the phase of senescence). Photochemical quenching and the amount of functional Photosystem I decreased linearly with chlorophyll content, while FV/FM (Photosystem II functionality) decreased strongly only at the end of senescence. Non-photochemical quenching of excitation energy (NPQ) increased towards the end of senescence. However, no significant differences in the total flavonol amounts, nor in individual flavonol species, were found between aphid-free and aphid-infested leaves, suggesting that flavonols play no role in defence against aphid herbivory. Interestingly, both green and senescing leaves with a high flavonol content showed low FV/FM values. High flavonol content slowed down PSII photoinhibition and improved recovery, but only in green leaves. Previously, we proposed that anthocyanins provide an additional sink for photosynthates at the nitrogen resorption phase during autumn senescence, and the present data may suggest that flavonol synthesis plays a similar role.
摘要：
而不是红花青素,桦木合成无色（人眼），秋季衰老期间吸收紫外线的黄酮醇。为了测试黄酮醇是否能抵御昆虫，如果黄酮醇含量高或低的叶子的光合功能不同，从户外种植的树木中收集了无蚜虫和蚜虫出没的绿色和衰老的桦树叶，并进行了分析。光合参数受叶片叶绿素含量（即衰老阶段）的影响很大。光化学猝灭和功能性光系统I的数量随叶绿素含量线性下降，而FV/FM（光系统II功能性）仅在衰老结束时急剧下降。在衰老末期，激发能（NPQ）的非光化学猝灭增加。然而,总黄酮醇量没有显著差异，在单个黄酮醇物种中也没有，在无蚜虫和蚜虫感染的叶子之间发现，这表明黄酮醇在防御蚜虫食草动物方面没有作用。有趣的是,黄酮醇含量高的绿色和衰老叶片均显示出低的FV/FM值。高黄酮醇含量减缓了PSII光抑制，提高了恢复，但只有绿叶。以前,我们提出花色苷在秋季衰老的氮吸收阶段为光合产物提供了额外的汇，目前的数据可能表明黄酮醇的合成也起着类似的作用。
