Abstract: Water use is a key process of photosynthetic carbon sequestration in plants. δ¹³C is a key indicator for long-term water use efficiency (WUE), reflecting the adaptive ability of plants to stressful environments. It is thus of great significance to reveal the long-term relationship between plants and environment. In this study, we examined the effects of drought and shading on instantaneous WUE and δ¹³C in Pinus massoniana seedlings under four treatments, including drought (30% saturated water content of soil moisture), shading (30% of full light intensity), drought + shading (30% saturated water content of soil moisture and 30% of full light intensity), and CK (70% saturated water content of soil moisture and full light intensity). The instantaneous WUE in the current-year needles significantly increased by 67.96% and 60.78% under drought and combined stress, respectively. δ¹³C significantly increased by 14.35% under drought, but did not significantly change under shading and combined stress. However, the δ¹³C values did not significantly change in the 1-year-old needles under three treatments. This indicated that the changes of instantaneous and long-term water use efficiency were not synchronized in P. massoniana, with WUE responded earlier to drought. The δ¹³C value in sink organs was higher than that in the source needles under all treatments. Moreover, δ¹³C in stems and roots had significant correlation with that in needles, which was significantly positively correlated with that in 1-year-old needles. The δ¹³C value in sink organs was also mainly influenced by the supply of source needles. These results implied that the changes in WUE caused by changing environments could further affect δ¹³C in each sink organ, which would provide a reference for the understanding the mechanism underlying the responses of P. massoniana seedlings to climate change.

Key words: drought, shading, water use efficiency, δ¹³C, Pinus massoniana seedling