Stomatal Control of Photosynthesis ofEucalyptus globulusLabill. Trees under Field Conditions in Portugal

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Abstract
Pereira, J. S., Tenhunen, J. D. and Lange, O. L. 1987. Stomatal control of photosynthesis of Eucalyptus globulus Labill. trees under field conditions in Portugal.—J. exp. Bot. 38: 1678–1688. Stomatal behaviour of adult leaves of Eucalyptus globulus trees was studied under field conditions in Portugal. In the absence of severe plant water stress stomata were open when the summed total of photosynthetically active photon flux density incident on both leaf surfaces was above 100 μmol m⊟2s⊟1 and leaf conductance to water vapour reached 245 mmol m ⊟2 s⊟1 on a total (both epidermes) leaf area basis. The stomata of both leaf epidermes responded similarly to changes in solar radiation and water stress. Water stress resulted in decreasing daily maxima in leaf conductance as predawn leaf water potential decreased. Maximal leaf conductance decreased to less than 50 mmol m ⊟2 s ⊟1 when predawn leaf water potential decreased below —1·0 MPa. At similar values of predawn leaf water potential stomata were more closed as the leaf to air water vapour partial pressure difference increased. The effect of increasing air dryness on stomata was greatest at high predawn leaf water potential. Daily maxima in photosynthetic rates and in leaf conductance were linearly related to one another in spring and summer. Both decreased with increase in leaf water stress. In autumn and winter, increases in leaf conductance occurring under natural conditions during the course of the day were not necessarily accompanied by increases in net photosynthesis. Stomata were more closed in the afternoon than in the morning at the same rates of net photosynthesis, temperature or leaf to air water vapour partial pressure difference.

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