Xu Liukang, Baldocchi Dennis D
Ecosystem Science Division, Department of Environmental Science, Policy and Management, 151 Hilgard Hall, University of California at Berkeley, CA 94720, USA.
Tree Physiol. 2003 Sep;23(13):865-77. doi: 10.1093/treephys/23.13.865.
Understanding seasonal changes in photosynthetic parameters and stomatal conductance is crucial for modeling long-term carbon uptake and energy fluxes of ecosystems. Gas exchange measurements of CO2 and light response curves on blue oak leaves (Quercus douglasii H. & A.) were conducted weekly throughout the growing season to study the seasonality of photosynthetic capacity (Vcmax) and Ball-Berry slope (m) under prolonged summer drought and high temperature. A leaf photosynthetic model was used to determine Vcmax. There was a pronounced seasonal pattern in Vcmax. The maximum value of Vcmax, 127 micromol m(-2) s(-1), was reached shortly after leaf expansion in early summer, when air temperature was moderate and soil water availability was high. Thereafter, Vcmax declined as the soil water profile became depleted and the trees experienced extreme air temperatures, exceeding 40 degrees C. The decline in Vcmax was gradual in midsummer, however, despite extremely low predawn leaf water potentials (Psipd, approximately -4.0 MPa). Overall, temporal changes in Vcmax were well correlated with changes in leaf nitrogen content. During spring leaf development, high rates of leaf dark respiration (Rd, 5-6 micromol m(-2) s(-1)) were observed. Once a leaf reached maturity, Rd remained low, around 0.5 micromol m(-2) s(-1). In contrast to the strong seasonality of Vcmax, m and marginal water cost per unit carbon gain (partial partial differential E/ partial partial differential A) were relatively constant over the season, even when leaf Psipd dropped to -6.8 MPa. The constancy of partial partial differential E/ partial partial differential A suggests that stomata behaved optimally under severe water-stress conditions. We discuss the implications of our findings in the context of modeling carbon and water vapor exchange between ecosystems and the atmosphere.
了解光合参数和气孔导度的季节性变化对于模拟生态系统的长期碳吸收和能量通量至关重要。在整个生长季节每周对蓝橡树（Quercus douglasii H. & A.）叶片进行二氧化碳气体交换测量和光响应曲线测定，以研究在夏季长期干旱和高温条件下光合能力（Vcmax）和Ball-Berry斜率（m）的季节性变化。使用叶片光合模型来确定Vcmax。Vcmax存在明显的季节性模式。Vcmax的最大值为127微摩尔·米-2·秒-1，在初夏叶片展开后不久达到，此时气温适中且土壤水分充足。此后，随着土壤水分剖面耗尽以及树木经历超过40摄氏度的极端气温，Vcmax下降。然而，尽管黎明前叶片水势极低（Psipd，约-4.0兆帕），Vcmax在仲夏仍逐渐下降。总体而言，Vcmax的时间变化与叶片氮含量的变化密切相关。在春季叶片发育期间，观察到较高的叶片暗呼吸速率（Rd，5 - 6微摩尔·米-2·秒-1）。一旦叶片成熟，Rd保持在较低水平，约为0.5微摩尔·米-2·秒-1。与Vcmax的强烈季节性相反，m和单位碳增益的边际水分成本（∂E/∂A）在整个季节相对恒定，即使叶片Psipd降至-6.8兆帕。∂E/∂A的恒定性表明气孔在严重水分胁迫条件下表现最佳。我们在模拟生态系统与大气之间的碳和水汽交换的背景下讨论了我们研究结果的意义。
