Seasonal Responses Of δ¹³C Values Of Stem Phloem Water Soluble Sugars Of Pinus Massoniana And Cunninghamia Lanceolata To Meteorological Factors

The phloem acts as the main pathway of long distance transport and allocation of nutrients, and as a conduit of inter-organ communication in vascular plants. As a carrier of the dynamic photosynthetic and post-photosynthetic carbon isotope fractionation signal,813C value of stem phloem water soluble sugars can be a reliable indicator of whole canopy stomata conductance (Gs) to reflect plant responses to the environmental variables in the plant canopy. Therefore, quantifying the stable carbon isotope fractionation of stem phloem and the extent such fractionation would be impacted by outer environmental variables is the key to understanding plant stem phloem carbon pool discrimination, and interactions between plants and outer environmental parameters. However, nearly all current studies of phloem carbon isotope fractionation of conifer trees had been limited to only several conifers in temperate forests, and there is little information about the impact of abundant availability of water and light and higher temperature on different conifer tree species in subtropical ecosystem. Given the importance of exploring plant responses and adaption to outer environmental variables, there was a need for more studies of phloem stable carbon isotope fractionation of additional plant groups in additional ecosystem. The paper measured813C value of stem phloem water soluble sugars ofPinus massoniana and Cunninghamia lanceolata for6or7days in each of four seasons, and related such values to the daily mean solar radiation(Sr), relative humidity(Rh), water vapor pressure deficit(VPD) and air temperature(Ta) calculated on the sampling day and for1-7days before the sampling day, meanwhile combining precipitation(PPT) data for further analysis, in order to reveal the dynamics of stable carbon isotope discrimination of newly assimilated photosynthates in P. massoniana and C. lanceolata stem phloem during different time scale (i.e. seasonal and whole-year), and its responses to the studied meteorological factors in subtropics in a day-to-day and seasonal time scale.The study showed that1) mean δ13C values of stem phloem water soluble sugars of P. massoniana fluctuated between-26.78and-25.82‰, with an average of-26.33±0.62‰during the whole year. Phloem water soluble sugars of P. massoniana were enriched in13C in autumn (with an average of-25.97±0.69‰), winter (with an average of-26.29±59‰) and summer (with an average of-260.48±0.63‰), but depleted in spring (with an average of-26.58±0.47‰). Seasonal mean813C values of P. massoniana varied significantly (P≤0.05).2) mean813C values of stem phloem water soluble sugars of C. lanceolata fluctuated between-29.26‰and-27.47‰, with an average of-28.77±0.65‰during the whole year. Phloem water soluble sugars of C. lanceolata were enriched in13C in winter (with an average of-28.55±0.51‰), spring (with an average of-28.67±0.91‰) and autumn (with an average of-28.80±0.60‰), but depleted in summer (with an average of-29.00±0.43‰). Seasonal mean δ13C values didn’t varied significantly (P≤0.05).3) Solar radiation and air temperature were the dominant meteorological factor in each season and water vapor deficit and relative humidity exerted the greatest influence on δ13C values of stem phloem water soluble sugars in the whole year.4) Short-term weather signifantly influenced813C values of stem phloem water soluble sugars of P. massoniana and C. lanceolata (P≤0.05), but with a time lag.5) Long-term climate resulted in a different seasonal plant response pattern between813C values of stem phloem water soluble sugars and the environmental variables studied, as well as the time lag with the shortest lag in summer.6) δ13C values of stem phloem water soluble sugars of C. lanceolata and the environmental variables exhibited seasonal decoupling in the season of autumn and winter.7) δ13C values of stem phloem water soluble sugars and their responses to the studied meteorological factors were different between two species.The results suggested that813C values of stem phloem water soluble sugars of P. massoniana and C. lanceolata can be a sensitive record of short-term weather and long-term climate change, thus helping to understand and predict the impacts of the weather and climate dynamics on stable carbon isotope discrimination of plant phloem water soluble sugars.