Osuna Jessica L, Baldocchi Dennis D, Kobayashi Hideki, Dawson Todd E
Ecosystem Science Division, Department of Environmental Science, Policy and Management, University of California Berkeley, Berkeley, CA, USA Present address: Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, PO Box 888, L-103, Livermore, CA 94551, USA
Ecosystem Science Division, Department of Environmental Science, Policy and Management, University of California Berkeley, Berkeley, CA, USA.
Tree Physiol. 2015 May;35(5):485-500. doi: 10.1093/treephys/tpv023. Epub 2015 Apr 8.
The California Mediterranean savanna has harsh summer conditions with minimal soil moisture, high temperature, high incoming solar radiation and little or no precipitation. Deciduous blue oaks, Quercus douglasii Hook. and Arn., are winter-deciduous obligate phreatophytes, transpiring mostly groundwater throughout the summer drought. The objective of this work is to fully characterize the seasonal trends of photosynthesis in blue oaks as well as the mechanistic relationships between leaf structure and function. We estimate radiative load of the leaves via the FLiES model and perform in situ measurements of leaf water potential, leaf nitrogen content, an index of chlorophyll content (SPAD readings), photosynthetic and electron transport capacity, and instantaneous rates of CO2 assimilation and electron transport. We measured multiple trees over 3 years providing data from a range of conditions. Our study included one individual that demonstrated strong drought stress as indicated by changes in SPAD readings, leaf nitrogen and all measures of leaf functioning. In the year following severe environmental stress, one individual altered foliation patterns on the crown but did not die. In all other individuals, we found that net carbon assimilation and photosynthetic capacity decreased during the summer drought. SPAD values, electron transport rate (ETR) and quantum yield of photosystem II (PSII) did not show a strong decrease during the summer drought. In most individuals, PSII activity and SPAD readings did not indicate leaf structural or functional damage throughout the season. While net carbon assimilation was tightly coupled to stomatal conductance, the coupling was not as tight with ETR possibly due to contributions from photorespiration or other protective processes. Our work demonstrates that the blue oaks avoid structural damage by maintaining the capacity to convert and dissipate incoming solar radiation during the hot summer drought and are effective at fixing carbon by maximizing rates during the mild spring conditions.
加利福尼亚地中海稀树草原夏季条件恶劣，土壤湿度极低、气温高、太阳辐射量大且降水极少或无降水。落叶蓝橡树（Quercus douglasii Hook. and Arn.）是冬季落叶的专性深根植物，在整个夏季干旱期间主要通过蒸腾地下水来维持生存。这项工作的目的是全面描述蓝橡树光合作用的季节性趋势以及叶片结构与功能之间的机制关系。我们通过FLiES模型估算叶片的辐射负荷，并对叶片水势、叶片氮含量、叶绿素含量指数（SPAD读数）、光合和电子传输能力以及二氧化碳同化和电子传输的瞬时速率进行原位测量。我们在3年时间里对多棵树木进行了测量，提供了一系列条件下的数据。我们的研究包括一棵个体，其SPAD读数、叶片氮含量以及所有叶片功能指标的变化表明该个体遭受了严重的干旱胁迫。在经历严重环境胁迫后的一年里，有一棵个体树冠上的叶片生长模式发生了改变，但并未死亡。在所有其他个体中，我们发现夏季干旱期间净碳同化和光合能力下降。SPAD值、电子传输速率（ETR）和光系统II（PSII）的量子产率在夏季干旱期间并未显著下降。在大多数个体中，整个季节PSII活性和SPAD读数并未表明叶片结构或功能受到损害。虽然净碳同化与气孔导度紧密相关，但与ETR的相关性没有那么紧密，这可能是由于光呼吸或其他保护过程的影响。我们的研究表明，蓝橡树通过在炎热的夏季干旱期间保持转换和消散入射太阳辐射的能力来避免结构损伤，并在温和的春季条件下通过最大化光合速率有效地固定碳。
