Photoprotective And Antioxidative Mechanisms In Woody Plants In The Yuanjiang Hot-dry Valley, Yunnan, SW China

Savanna climates are characterized with a distinct dry season that can last six months or so, and with hot weather the year round. However, in the savanna valleys in southwestern China, the hot weather coincides with the rainy season, but the dry season is separated by a chilly period from November to February and a warm period from March to April. In this study, the foliar membrane thermostability, the changes in photosynthesis, activities of antioxidant enzymes and non-enzymatic antioxidants, and contents of some key osmoletes were examined in five tree species—the evergreen Cyclobalanopsis helferiana (CH), chilly–dry-season deciduous tree Symplocos racemosa (SR) and warm-dry-season deciduous trees Terminthia paniculata (TP), Woodfordia fruticosa (WF), and Wendlandia tinctoria var. intermedia (WT) in October 2003, February and April 2004. The changes in photosynthesis, water and energy utilization were also tested in October 2005, February and April 2006, in which a serious drought occurred. The aim of this study was to elucidate the photoprotective and antioxidative mechanisms to counteract the different seasonal multiple stresses in the main woody species in Yuanjiang savanna valley. The main results of this study are as follows:(1) The savanna species examined had strong heat tolerance ability with the leaf semi-lethal temperature of 51–52?C.(2) All species especially CH and WF had quite high photosynthetic rates (Pn)—their maximum Pn ranged from 12 to 18 ?mol m–2 s–1 in the rainy season.(3) The results of the present study showed that the present savanna species especially the evergreen CH were sensitive to the chilly-dry season. For all species in this season, Pn, actual quantum yield of photosystem II (ФPSⅡ) and content of chlorophyll decreased sharply compared with those in the rainy season, yet the midday non-photochemical quenching (NPQ) increased to the highest level. For all species, the content of chlorophyll was linearly correlated with Pn. In the chilly season, the pool size and De-epoxidation state (DPS) of the xanthophylls cycle increased significantly, and the content of malondialdehyde (MDA), and production rate of superoxide radical were highest in all species especially CH. NPQ was positively corrected with DPS for each species.(4) For the first time, the present dissertation described rather comprehensively the difference in antioxidative mechanism between the evergreen and deciduous savanna trees. Although they all had high activity of SOD, the initial enzyme of water-water cycle, the deciduous trees especially the drought-deciduous species WF had much higher activities of other enzymes of the water-water cycle than the evergreen species. The contents of non-enzymatic antioxidants were not significantly different between the evergreen and deciduous savanna trees.(5) For the first time, the present dissertation reported the compensation relation between the antioxidant system and xanthophyll cycle. The present savanna plants had exceptionally high activity of antioxidant system but rather smaller pool size of xanthophylls cycle. The xanthophyll cycle not only plays the major role in heat dissipation but also functions in antioxidation. The antioxidant function of xanthophylls cycle of the savanna plants was compensated by the antioxidant system.(6) For all species in the chilly season, the content of proline, the main osmolete, increased significantly and the content of soluble carbohydrate reached the highest level. In a relative warm winter, photosynthesis in TP andФPSⅡat midday and the content of chlorophyll in both WT and WF was not significantly reduced compared to in the rainy season.(7) It was found that the content of soluble carbohydrate was positively correlated with both contents of of anthocyanin and ascorbate for all species. This revealed the central role of soluble carbohydrate in linking the processes of photosynthesis, antioxidant system and osmoregulation.(8) Although the savanna species differed in protection mechanism against stresses, in all seasons in the year with a normal dry season, they were able to maintain some degree of Pn, relatively constant and high Chl a/b ratio and predawn Fv/Fm, and exceptionally high contents ofβ-carotene. These indicated that the savanna species were able to maintain the stability of photosynthetic apparatuses in the normal year through the regulation of pigments and antioxidant system.(9) These species had the ability to maintain a significant midmorning Pn (4–10μmol m–2s–1) even at serious water deficit (water potential was lower–3MPa) and they showed different mechanism in water utilization. WF had stronger tolerance to drought stress than other study species.(10) The study species showed substantial differences in sensitivity to drought and hot-induced photoinhibition as indicated by the decreases in midday Fv/Fm and the values of 1–qp. WF had stronger tolerance to midday photoinhibition than other study species.(11) These species exhibited different process to release excess light energy in PSⅡ: WF carried that out mainly via a photochemical mechanism, whereas CH mainly developed the efficient NPQ pathway. The other species showed both mechanisms.(12) These savanna species retained high contents of zeaxanthin (Z) and antheraxanthin (A) at night in the chilly and the warm-dry seasons, but which was not involved in heat dissipation. This is probably a consequence of the combination of water deficit with high or low temperature and the adjustment of photosynthetic apparatuses.(13) Under severe water deficit, dissipating excess energy via the xanthophyll cycle of the present savanna species must have an advantage to save carbon and water over other pathways of heat dissipation such as transpiration and VOC emission.In summary, all study species had strong ability in coping with hot and drought-induced stresses but sensitive to the conditions in the chilly period. They showed substantial differences in photoprotective and antioxidative mechanisms under the different combined multiple stresses in the Yuanjing savanan valley.