关键词: 水分胁迫, 光合作用, 光合产物分配, δ13C值, N13CR, 根冠比

Abstract:

Changing the allocation of carbohydrates to various organs is a central mechanism by which plants cope with temporally or spatially varying environments. Hence, a primary objective of eco-physiological research is to understand when and how this process is affected by specific external conditions. Chinese fir (Cunninghamia lanceolata) is one of the most important timber species in Southern China due to its fast growth and good timber quality. Due to large-scale afforestation/reforestation activities, Chinese fir stands, most of which are plantations, have expanded rapidly since the 1950s, and, in particular, since the 1980s, with both the area and standing volume having more than doubled. Therefore, to understand the carbon allocation in Chinese fir in response to varying soil moisture conditions, we studied the photosynthetic response and changes in photosynthate allocation of 2-year-old Chinese fir seedlings under different water treatments. The experiment was conducted in the Subtropical Forestry Experimental Center of the Chinese Academy of Forestry located in Fengyi of Jiangxi Province. Eighty potted 2-year-old Chinese fir seedlings were grown for one growing season under two water treatments: a water stress treatment in which one third the normal water supply was applied and a control (normal water management). The net photosynthetic rate (P n) in response to photosynthetic photo flux density (PPFD) were measured using a LI-6400 portable photosynthesis system from 8∶00 to 11∶00 on clear days in late June. Air temperature and relative humidity in the chamber were maintained at (25±1) ℃ and 70%±5%. Carbon allocation in seedlings under the two water treatments was measured in the morning of early July using a 13 C pulse labeling technique. The initial CO 2 concentration in the labeling chamber was about 1 000 μmol·mol -1 and each labeling lasted 40 minutes. The current-needles, 1-year old needles, branches, stems, fine roots and coarse roots were sampled on day 1, day 3, day 7 and day 21 following labeling. Samples were dried, grinded to a powder, combusted, and then analyzed on an isotope ratio mass spectrometer to measure the stable carbon isotope ratio. Our results showed that water stress did not alter the photosynthetic characteristics of Chinese fir seedlings. The δ 13 C values and net 13 C ratio (N 13 CR), i.e., the ratio of the net increment of 13 C to the natural total carbon, of the seedlings decreased in the water-stress treatment. The effect of water deficit on the δ 13 C values and N 13 CR in shoots was more significant than in roots. The shoot biomass under water stress was reduced remarkably, while little changes were found for root biomass. Water stress had a more significant effect on current-needles than other organs. The δ 13 values, N 13 CR and dry weight of current-needles under water stress were lower than in the control. The more rapid decline of N 13 CR in current-needles of water-stressed seedlings 21 days following labeling indicated that there was an increase in the export of photosynthetic products. The growth decline of the current needles under water stress caused a decrease in leaf area resulting in a reduction in total photosynthesis. Under water stress, more photosynthetic products were transferred to belowground biomass, especially to the fine roots. As a result, carbon allocation patterns were altered and higher root:shoot ratios were found in seedlings experiencing water stress!in comparison to seedlings under conditions of normal water management.

Key words: Water stress, Photosynthesis, Carbon allocation, δ13, N 13 CR, Root:Shoot ratio