Stomatal patchiness in Mediterranean evergreen sclerophylls

Abstract

Midday depression of net photosynthesis and transpiration in the Mediterranean sclerophylls Arbutus unedo L. and Quercus suber L. occurs with a depression of mesophyll photosynthetic activity as indicated by calculated carboxylation efficiency (CE) and constant diurnal calculated leaf intercellular partial pressure of CO2 (Ci). This work examines the hypothesis that this midday depression can be explained by the distribution of patches of either wide-open or closed stomata on the leaf surface, independent of a coupling mechanism between stomata and mesophyll that results in a midday depression of photosynthetic activity of the mesophyll. Pressure infiltration of four liquids differing in their surface tension was used as a method to show the occurrence of stomatal patchiness and to determine the status of stomatal aperture within the patches. Liquids were selected such that the threshold leaf conductance necessary for infiltration through the stomatal pores covered the expected diurnal range of calculated leaf conductance (g) for these species. Infiltration experiments were carried out with leaves of potted plants under simulated Mediterranean summer conditions in a growth chamber. For all four liquids, leaves of both species were found to be fully infiltratable in the morning and in the late afternoon while during the periods leading up to and away from midday the leaves showed a pronounced patchy distribution of infiltratable and non-infiltratable areas. Similar linear relationships between the amount of liquid infiltrated and g (measured by porometry prior to detachment and infiltration) for all liquids clearly revealed the existence of pneumatically isolated patches containing only wide-open or closed stomata. The good correspondence between the midday depression of CE, calculated under the assumption of no stomatal patchiness, and the diurnal changes in non-infiltratable leaf area strongly indicates that the apparent reduction in mesophyll activity results from assuming no stomatal patchiness. It is suggested that simultaneous responses of stomata and mesophyll activity reported for other species may also be attributed to the occurrence of stomatal patchiness. In Quercus coccifera L., where the lack of constant diurnal calculated Ci and major depression of measured CE at noontime indicates different stomatal behavior, non-linear and dissimilar relationships between g and the infiltratable quantities of the four liquids were found. This indicates a wide distribution of stomatal aperture on the leaf surface rather than only wide-open or closed stomata.

Contribution of diffusional and non-diffusional limitations to midday depression of photosynthesis in Arbutus unedo L.

“…Similar results were observed in A. unedo, throughout an entire year, by Beyschlag et al (1992), although in the same species and similar environmental conditions Raschke and Resemann (1986) observed a broad temperature optimum for photosynthesis. These latter authors concluded that rising leaf temperature could account for not more than 10% of the observed decline in A max at midday.…”

Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses?

“…Analogously, patchy stomatal behaviour at the canopy scale (e.g., inactivity of a proportion of sun leaves), should provide decreasing water‐use efficiency at the canopy level. Stomatal patchiness has been identified as a common phenomenon in Mediterranean species (Beyschlag et al ., 1992). Yet, the impact of stomatal patchiness on whole canopy gas exchange has never been assessed (Beyschlag & Eckstein, 1998).…”

Inverse modeling of seasonal drought effects on canopy CO2/H2O exchange in three Mediterranean ecosystems

“…On the other hand, large drought‐induced reductions of photosynthetic capacities of more than 70%, as estimated for the Puéchabon and Arca di Noè site, seem unlikely since in drought‐adapted species only minor changes in mesophyll capacities have been found in laboratory studies [ Kaiser , 1987; Chaves , 1991; Cornic , 1994]. Instead, patchy stomatal closure over a large portion of the leaf surface has been observed for Mediterranean species as a consequence of drought [ Beyschlag et al , 1992; Beyschlag and Eckstein , 1998] and also for Quercus ilex L. [ Genty and Meyer , 1995] so that Roupsard et al [1996] concluded that stomatal closure is probably the main factor reducing CO 2 availability in the chloroplasts during drought (p. 252).…”

“…Harley and Tenhunen, 1991;Lawlor, 1995], (2) the induction of stomatal patchiness (i.e., nonnormal distribution of stomatal apertures [cf. Beyschlag et al, 1992;Beyschlag and Eckstein, 1998;Buckley et al, 1999]). The first hypothesis has meanwhile received support from leaf-level studies [Xu and Baldocchi, 2003].…”