Seasonal and diurnal patterns in leaf gas exchange of Eucalyptusglobulus trees growing in Portugal

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Abstract
Gas exchange of adult leaves of Eucalyptus globulus Labill. trees growing in Portugal was monitored during weekly periods between September 1982 and August 1983. Photosynthetic carbon assimilation rates and leaf water use efficiency were highest (maximum instantaneous values, ca. 12 .mu.mol CO2 m-2 s-1 and ca. 7 mmol CO2 mol H2O-1, respectively) in the spring, somewhat reduced in the winter, and strongly depressed by the middle of the summer, when severe drought conditions prevailed. Diurnal patterns of variation in stomatal conductance and net photosynthesis rates showed a marked seasonal variation. With the transition from winter to spring and summer, as the environmental conditions became warmer and drier, there was an increasing tendency for a midday depression in gas exchange rates as well as a decreasing capacity in the afternoon for recovery to the same net photosynthesis rates measured in the morning. Midday depression in gas exchange and stomatal conductance occurred in leaves positioned both vertically and horizontally inside the measurement cuvettes. Leaf dark respiratory rates also changed with the season; the temperature necessary to evoke similar rates increased with the transition from winter to summer.
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