摘要:研究树木叶片非结构性碳水化合物（Nonstructural carbohydrate，NSC）组分的季节变化是掌握树木碳代谢规律的基础，也有利于判断以往研究仅凭生长季单次或几次（<5次）的取样方法是否存在一定局限性。以秦岭东段栓皮栎（Quercus variabilis Blume）优势群落为研究对象，在其分布的海拔上下限（650 m和970 m），于2016年5月至2017年5月，通过月尺度周期性取样（共计9次），测定栓皮栎叶片NSC及其组分含量，并观测同期叶片物候变化。结果显示：（1）栓皮栎叶片NSC及其组分季节变化差异显著（P < 0.05），可溶性糖、淀粉和NSC变异系数分别为20.99%、52.28%和25.96%；（2）整体而言，栓皮栎叶片NSC最小值在展叶初期（3月末-4月初，5%左右），最大值在展叶末期（5月上旬，12%左右），之后NSC呈持续下降趋势。不同海拔NSC极值出现时间略有不同，叶片物候可能是影响年内极值的主要原因。（3）栓皮栎叶片NSC组成以可溶性糖为主（65%），这可能是树种在暖温带所采取的生长策略。（4）海拔对栓皮栎叶片NSC及其组分影响差异不显著，低海拔栓皮栎叶片NSC及其组分含量略大于高海拔。研究结果，栓皮栎叶片NSC含量存在明显的季节波动，适当加大NSC采样频率对于正确理解树木碳代谢十分必要。



Abstract:Nonstructural carbohydrate (NSC) in plant tissues is a reservoir for carbon that fluctuates seasonally depending on the balance between supply via photosynthesis and demand for growth and respiration (the source-sink dynamics concept). High NSC concentration in tree tissue is generally interpreted as excessive supply relative to demand at that moment. Research into the seasonal dynamics of NSC concentrations in tree tissues throughout the year is indispensable to gain insight into tree carbon supply status and nutrient metabolism, and thus an understanding of the tree growth mechanism. To determine the seasonal dynamics, i.e., the fluctuation and amplitude of NSC content in tree tissues, we conducted a field observational study by monitoring the NSC concentrations in leaves of the Chinese cork oak (Quercus variabilis Blume), and synchronously observing the phenology of those trees at some unevenly-aged secondary oak forests growing at its lower and upper distribution limits (650 m to 970 m a.s.l.) in the east Qinling Mountain range. Sampling intervals were set semimonthly/monthly during the leaf unfolding period from March to May, and monthly/bimonthly during the tree's full growing season from June to November. The time series of observation and sampling spanned from May 2016 to May 2017. The results reviewed some facts and phenomena, and a few inferences could be drawn. (1) The NSC concentrations in the oak tree leaves varied significantly with seasonal rhythms (P < 0.05). The coefficients of variation for soluble sugar, starch and NSC in leaves were 20.99%, 52.28%, and 25.96%, respectively. (2) The minimum and maximum concentrations of NSC in tree leaves were observed in mid-March/April and mid-May, respectively, with a slight difference between the two altitudes. At the lower elevation site, the minimum and maximum contents of NSC in oak leaves appeared in late March (5.69%) and early May (12.29%), respectively; whereas at the upper elevation, the minimum and maximum contents of NSC occurred in mid-April (4.54%) and early May (12.76%), respectively. After that, the NSC concentrations in oak leaves continuously declined from May to November throughout the growing season. The minimum and maximum NSC content could be partly explained by the simultaneous leaf phenological dynamics. During the period of bud break and leaf development (from March to April), canopy rebuilding consumed much of the carbohydrate reserves. Considering the deficit in carbon supply via the initial photosynthetic event coupled with the high carbon demand for growth, it was reasonable that the NSC concentration in Q. variabilis leaves reached their minimum during this period of leaf flourishing. (3) Soluble sugar was the major contributor to total NSC in oak leaves, accounting for ca. 65% of the total. As soluble sugars performed immediate functions whereas starch acted mostly as reservoirs for future use, it could be inferred that the Q. variabilis, a deciduous tree species growing typically in the warm temperate zone, developed this physiological feature naturally as its life strategy to survive the harsh winters of the temperate climate. (4) However, the NSC concentrations in leaves did not vary significantly with elevational changes. Generally, the NSC concentrations in leaves at the lower elevation sites were higher than that at the upper elevational sites. In conclusion, seasonal fluctuations of NSC in leaves of Q. variabilis varied significantly with the tree's phenological rhythms, particularly in the spring when the new growing season began. Therefore, increasing the sampling frequency of NSC and reducing the interval time is necessary for further understanding of NSC metabolism in relation to tree growth.





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