Kitao Mitsutoshi, Yazaki Kenichi, Tobita Hiroyuki, Agathokleous Evgenios, Kishimoto Junko, Takabayashi Atsushi, Tanaka Ryouichi
Hokkaido Research Center, Forestry and Forest Products Research Institute, Sapporo, Japan.
Department of Plant Ecology, Forestry and Forest Products Research Institute, Tsukuba, Japan.
Front Plant Sci. 2022 Oct 28;13:1006413. doi: 10.3389/fpls.2022.1006413. eCollection 2022.
Leaves of fullmoon maple () turn brilliant red with anthocyanins synthesis in autumn. Based on field observations, autumn coloring mainly occurs in outer-canopy leaves exposed to sun, whereas inner-canopy leaves remain green for a certain longer period before finally turn yellowish red with a smaller amount of anthocyanins. Here, we hypothesized that outer-canopy leaves protect themselves against photooxidative stress anthocyanins while simultaneously shading inner canopy leaves and protecting them from strong light (holocanopy hypothesis). To test this hypothesis, we investigated photoinhibition and leaf N content during autumn senescence in leaves of pot-grown seedlings of fullmoon maple either raised under shade (L0, ≈13% relative irradiance to open) or transferred to full sunlight conditions on 5 (LH1), 12 (LH2), or 18 (LH3) Oct, 2021. Dry mass-based leaf N (N) in green leaves in shade-grown seedlings was ≈ 30 mg N g in summer. N in shed leaves (25 Oct to 1 Nov) was 11.1, 12.0, 14.6, and 10.1 mg N g in L0, LH1, LH2, and LH3 conditions, respectively. Higher N was observed in shed leaves in LH2, compared to other experimental conditions, suggesting an incomplete N resorption in LH2. F/F after an overnight dark-adaptation, measured on 19 Oct when leaf N was actively resorbed, ranked L0: 0.72 > LH3: 0.56 > LH1: 0.45 > LH2: 0.25. As decreased F/F indicates photoinhibition, leaves in LH2 condition suffered the most severe photoinhibition. Leaf soluble sugar content decreased, but protein carbonylation increased with decreasing F/F across shade-grown seedlings (L0, LH1, LH2, and LH3) on 19 Oct, suggesting impaired photosynthetic carbon gain and possible membrane peroxidation induced by photooxidative stress, especially in LH2 condition with less N resorption efficiency. Although the impairment of N resorption seems to depend on the timing and intensity of strong light exposure, air temperature, and consequently the degree of photoinhibition, the photoprotective role of anthocyanins in outer-canopy leaves of fullmoon maple might also contribute to allow a safe N resorption in inner-canopy leaves by prolonged shading.
元宝枫（）的叶片在秋季因花青素合成而变成鲜艳的红色。根据实地观察，秋季叶片变色主要发生在树冠外层暴露于阳光下的叶片，而树冠内层叶片在较长一段时间内保持绿色，最终才会因花青素含量较少而变成黄红色。在此，我们假设树冠外层叶片通过花青素保护自身免受光氧化胁迫，同时为树冠内层叶片遮荫，保护其免受强光照射（全树冠假说）。为了验证这一假说，我们研究了盆栽元宝枫幼苗叶片在秋季衰老过程中的光抑制和叶片氮含量，这些幼苗要么在遮荫条件下培育（L0，相对开阔地约13%的光照强度），要么于2021年10月5日（LH1）、12日（LH2）或18日（LH3）转移到全日照条件下。夏季，遮荫培育的幼苗绿叶中基于干重的叶片氮含量（N）约为30毫克氮/克。在落叶期（10月25日至11月1日），L0、LH1、LH2和LH3条件下落叶中的氮含量分别为11.1、12.0、14.6和10.1毫克氮/克。与其他实验条件相比，LH2条件下落叶中的氮含量更高，这表明LH2条件下氮的再吸收不完全。在10月19日叶片氮积极再吸收时，经过一夜暗适应后测量的F/F值排序为：L0: 0.72 > LH3: 0.56 > LH1: 0.45 > LH2: 0.25。由于F/F值降低表明存在光抑制，LH2条件下的叶片遭受的光抑制最为严重。10月19日，随着遮荫培育的幼苗（L0、LH1、LH2和LH3）F/F值的降低，叶片可溶性糖含量下降，但蛋白质羰基化增加，这表明光合碳同化受损，且可能存在光氧化胁迫诱导的膜过氧化，尤其是在氮再吸收效率较低的LH2条件下。尽管氮再吸收的受损似乎取决于强光暴露的时间和强度、气温以及光抑制程度，但元宝枫树冠外层叶片中花青素的光保护作用也可能有助于通过延长遮荫时间使树冠内层叶片安全地进行氮再吸收。
