Small leaves of sacred datura plants buffer ambient thermal variation more efficiently than large leaves due to a smaller boundary layer, allowing higher amounts of transpiration.

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How a boundary layer works. Artists: Neil Dold, Abraham Khan, Julie Kohn, and Chrissy Kaufmann

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"Do leaf microclimates buffer ambient thermal variation, such that eggs on leaves rarely experience extremes? Mahan and Upchurch argue that plants are 'limited homeotherms', i.e. that plants control transpiration to optimize temperature for cell function (Mahon and Upchurch, 1988). Because photosynthesis, growth and reproduction rely on enzymes with species-specific optimal temperatures, plants that can lower their leaf temperatures may avoid the cost of shutting down these processes in the heat of the day (Hatfield and Burke, 1991; Zangerl, 1978). While a leaf's minimum temperature is environmentally controlled, its high temperature is subject to greater control by the leaf itself. This control stems from stomatal behavior. Open stomata allow for increased transpiration and evaporative cooling, particularly when the vapor pressure difference between leaf and air is high. Alternatively, stomatal behavior may act to limit water loss; leaves with closed stomata may be warmer than air temperature due to solar heating (Field et al., 1982). In the same way that insect heat balance depends on size, so does leaf heat balance. Because leaf size affects boundary layer thickness, leaves even within the same plant may vary in their ability to thermoregulate and thus provide different microhabitats

"Our leaf temperature data support the microclimatic buffering hypothesis; leaf temperatures were usually cooler than ambient, especially during the hottest part of the day. Interestingly, although both size classes protect eggs, larger leaves were consistently warmer during the day than their matched small leaves. Indeed, only small leaves show limited homeothermy as defined by Mahan and Upchurch (Mahan and Upchurch, 1988). This pattern likely reflects differences in the physiology and morphology of smaller D. [Datura] wrightii leaves, which have higher transpiration rates during the day than large leaves (G. Barron-Gafford, unpublished) and higher boundary layer conductance (Jones, 1992)." (Potter et al. 2009:3448-9, 3452)


"Boundary layer – The boundary layer is a thin layer of still air hugging the surface of the leaf. This layer of air is not moving. For transpiration to occur, water vapor leaving the stomata must diffuse through this motionless layer to reach the atmosphere where the water vapor will be removed by moving air. The larger the boundary layer, the slower the rates of transpiration. 

"Plants can alter the size of their boundary layers around leaves through a variety of structural features. Leaves that possess many hairs or pubescence will have larger boundary layers; the hairs serve as mini-wind breaks by increasing the layer of still air around the leaf surface and slowing transpiration rates. Some plants possess stomata that are sunken into the leaf surface, dramatically increasing the boundary layer and slowing transpiration. Boundary layers increase as leaf size increases, reducing rates of transpiration as well. For example, plants from desert climates often have small leaves so that their small boundary layers will help cool the leaf with higher rates of transpiration." (Library of Crop Technology Lessons website)

Journal article
Insect eggs protected from high temperatures by limited homeothermy of plant leavesJournal of Experimental BiologyOctober 16, 2009
K. Potter, G. Davidowitz, H. A. Woods

Web page
Transpiration - Factors Affecting Rates of Transpiration

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