Istituto di Fisica Applicata Carrara, IFAC, Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, I-50019 Sesto. F.no, Firenze, Italy.
J Plant Physiol. 2011 Feb 15;168(3):204-12. doi: 10.1016/j.jplph.2010.07.016. Epub 2010 Sep 20.
Flavonoids have recently been suggested to have the potential to serve as antioxidants other than effective UV attenuators in photoprotection. Here, we tested the hypothesis that flavonoids accumulate in response to "excess light" in the presence or in the absence of UV radiation. In a UV exclusion experiment, we grew Ligustrum vulgare plants outdoors under 30% or 100% sunlight irradiance, by cutting-off the whole UV waveband. These plants were also exposed to UV irradiance or supplied with 125 mM NaCl at the root zone. Leaves of plants under 100% sunlight irradiance suffered from excess light, which was exacerbated greatly by root zone salinity stress. Salinity stress repressed the activities of antioxidant enzymes, particularly in full sunlight, and led to severe leaf oxidative damage. Dihydroxy B-ring-substituted flavonoids, namely quercetin 3-O- and luteolin 7-O-glycosides, accumulated steeply in response to sunlight irradiance in the absence of UV radiation. UV radiation and root zone NaCl increased, to a similar degree, the concentration of these flavonoids, which have a great potential to scavenge various forms of reactive oxygen. Treatment-induced changes in leaf phenylpropanoid concentration affected antioxidant activities to a greater extent than the UV-screening capacities of leaf extracts. Early responses to an abrupt increase in sunlight irradiance included a steep increase in the concentrations of quercetin derivatives and cyanidin 3-O-glucoside, with the latter negligibly absorbing in the UV-spectral region. In contrast, effective UV attenuators, such as hydroxycinnamates and monohydroxy B-ring flavonoids, were unresponsive to the light treatments. Overall, these findings lead to the hypothesis that flavonoids may have an important antioxidant function in photoprotection. This hypothesis is further corroborated by the large distribution of quercetin and luteolin derivatives in the vacuoles of mesophyll, not only in the corresponding compartments of epidermal cells, but also in full sunlight-treated leaves in the absence of UV radiation. Future experiments aimed at evaluating the relative contribution of flavonoids within the complex antioxidant defense systems operating in the leaf are needed to help conclusively address the relevance of their antioxidant functions in photoprotection.
类黄酮最近被认为除了作为有效的紫外线衰减剂之外，还有作为抗氧化剂的潜力，从而起到光保护的作用。在这里，我们检验了这样一个假设，即在存在或不存在紫外线辐射的情况下，类黄酮会响应“过量的光”而积累。在一个紫外线排除实验中，我们将女贞属植物在户外生长，用紫外线滤光片去除整个紫外线波段，使其分别处于 30%或 100%的阳光辐照度下。这些植物还暴露于紫外线辐射或在根部供应 125 mM NaCl。在 100%阳光辐照下的植物叶片受到过量光的影响，而根部盐胁迫则大大加剧了这种影响。盐胁迫抑制了抗氧化酶的活性，特别是在全光照下，导致叶片严重的氧化损伤。二羟基 B 环取代的类黄酮，即槲皮素 3-O-和木樨草素 7-O-糖苷，在没有紫外线辐射的情况下，随着阳光辐射的增加而急剧积累。紫外线辐射和根部 NaCl 以相似的程度增加了这些类黄酮的浓度，这些类黄酮具有清除各种形式活性氧的巨大潜力。叶片苯丙烷浓度的处理诱导变化比叶片提取物的紫外线屏蔽能力更能影响抗氧化活性。对阳光辐射急剧增加的早期响应包括槲皮素衍生物和矢车菊素 3-O-葡萄糖苷浓度的急剧增加，而后者在紫外线光谱区域几乎不吸收。相比之下，有效的紫外线衰减剂，如羟基肉桂酸和单羟基 B 环类黄酮，对光照处理没有反应。总的来说，这些发现导致了这样一个假设，即类黄酮在光保护中可能具有重要的抗氧化功能。这个假设进一步得到了槲皮素和木樨草素衍生物在质体中大量分布的支持，不仅在表皮细胞的相应隔室中，而且在没有紫外线辐射的全光照处理的叶片中也是如此。需要进行未来的实验来评估类黄酮在叶片中复杂抗氧化防御系统中的相对贡献，以帮助明确其在光保护中的抗氧化功能的相关性。
