The strategies of acclimation to growth light intensity in two exotic herbaceous species with different ecological traits in Xishuangbanna,China

Acclimation to growth light regimes and the strategies of photoprotection were explored in two exotic herbaceous species, Eupatorium adenophorum Spreng. and Amomum villosum Lour. E. adenophorum, a notorious invasive species, spreaded into China in 1940s, and now distributes in all provinces of southwest China. It is sun species, but can grow and develop normally in shade environment. A. villosum, being introduced to Xishuangbanna intentionally from Yangchun county, Guangdong province, is a perennial herb and is considered as being an obligate shade plant that occurs in the understorey of humid subtropical and tropical forest. Its seeds are important materials for Chinese medicine. Both E. adenophorum and A. villosum influence biodiversity, structure, and function of ecosystem invaded badly, but the impact of the former on ecosystem is much greater. The experimental materials were cultured under three light regimes for about 10 months. Lamina mass per unit area (LMA), chloroplast pigments content, the maximum net photosynthetic rate (P_(max)), and chlorophyll fluorescence parameters were measured during the dry and hot season in 2003 at Xishuangbanna, Yunnan province, China. In this study we pay attention to (1) the ability and the strategies they acclimate to different light regimes and (2) are this ability associated with their invasiveness? And how? Under lower growth light regimes the diurnal photoinhibition of photosynthesis was not serious in E. adenophorum (12.5% and 36% relative irradiance) and in A. villosum(12.5% RI), as judged by the maximum efficiency of photosystem (PS) Ⅱ photochemistry, quantum yield of PSⅡ non-cyclic electron transport. With the increase of growth light intensity photoinhibition intensified in the two species, but photoinhibition was much more serious in A. villosum grown under 36% RI than in E. adenophorum under 100% RI. With the increase of growth light intensity, P_(max), LMA, non-photochemical quenching efficiency (NPQ), and carotenoid content per unit area increased; while chlorophyll content per dry mass decreased. E. adenophorum could acclimate to large extent of environmental light regimes through changes of morphological and physiological characteristics. This might be associated with its strong invasiveness. Reaction centres of PS Ⅱ inactivated reversibly in E. adenophorum grown under 100% RI, as judged by initial fluorescence (F_o). Inactivated but not photodamaged reaction centres of PSⅡ can dissipate excessive light energy too. Thermal dissipation, as judged by NPQ, increased with the increase of diurnal and growth light intensity in both of the two species. But thermal dissipation was lesser in E. adenophorum, even grown under 100% RI, than in A. villosum. The biggest diurnal NPQ was 1.6 only, smaller than that of other species in Xishuangbanna. E. adenophorum, a sun species, protected photosynthetic apparatus from photodamage mainly through utilizing more light energy by photosynthesis. In A. villosum P_(max) could increase with the increase of growth light intensity too, but its value and increment were small. In contrast the value and increment of thermal dissipation were large in A. villosum grown under 36% RI, with the biggest diurnal NPQ value of 5.4. F_o decreased significantly with the increase of diurnal light intensity in the morning, and maintained at low level in the afternoon, indicating large amount thermal dissipation too. A. villosum, a shade species, protected photosynthetic apparatus from photodamage mainly through thermal dissipation. Morphological plasticity and its ability of physiological acclimation to light were small in A. villosum. This was consistent with its understorey distribution.