关键词: CO2浓度倍增, 土壤干旱, 柠条, 羊柴, 生物量, 碳含量, 分配

Abstract:

Atmospheric CO 2 concentrations are expected to double around the middle part of the 21 st century. Plant growth might be favored by CO 2 enrichment, but water limitation is a common stress for plant growth and productivity. At present, only a few studies have looked at the combined effects of CO 2 enrichment and drought on plant ecophysiology. This experiment was conducted to investigate the responses of two dominant desert shrubs, Caragana intermedia and Hedysarum mongolicum, in western China to the interaction of doubled CO 2 levels and soil drought in large environmental growth chambers (19 m 2). In this paper, we employed different methods, including allometry and carbon isotope discrimination, to examine the effects of water availability on carbon allocation and stable carbon isotope composition (δ 13 C) of the two desert shrubs under two CO 2 concentrations. The objectives included the following: 1) to investigate the effects of soil drought and CO 2 enrichment on plant biomass and δ 13 C; 2) to investigate the effects of soil drought and CO 2 enrichment on the allocation of dry matter and carbohydrates; and 3) to elucidate the adaptive strategies of C. intermedia and H. mongolicum to soil drought under doubled atmospheric CO 2 concentrations. Compared to ambient CO 2 concentrations, doubled CO 2 concentrations did not improve the leaf water status, but soil drought significantly reduced the leaf relative water content (RWC). Doubled CO 2 concentrations enhanced plant growth under well-watered conditions but increased root growth under drought conditions resulting in an increase in root to shoot ratios. Soil drought significantly reduced plant biomass and increased root to shoot ratios, especially for H. mongolicum. The δ 13 C values were reduced at doubled CO 2 concentrations but increased under drought conditions. By plotting the leaf δ 13 C values against the root δ 13 C values, it was possible to assess carbon allocation and incorporation into roots in relation to present biomass. There was a significant and linear relationship between leaf δ 13 C and root δ 13 C values, and the slope of H. mongolicum was greater than that of C. intermedia indicated a higher plasticity in the ability to change carbon allocation patterns. This resulted in higher root to shoot ratios in H. mongolicum under drought conditions. The results indicated that both C. intermedia and H. mongolicum had a higher tolerance to severe water deficits under doubled CO 2 conditions. Decreases in precipitation might accompany with future increases in atmospheric CO 2 concentrations in the region dominated by these two species, suggesting that distribution ranges of C. intermedia and H. mongolicum might be constrained. Our results suggest that H. mongolicum has a higher tolerance to environmental stress than C. intermedia. Future work should emphasize how to enhance the drought tolerance of plants in semiarid region under conditions of CO 2 enrichment.

Key words: Doubled atmospheric CO2, Soil drought, Caragana intermedia, Hedysarum mongolicum, Biomass, Carbon content, Carbon allocation, Global climate change