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Turning Over a New Leaf WATER CONSERVATION VS. MAXIMIZING PHOTOSYNTHESIS BY KIRK ANDERSON, CURATOR OF GARDENS


Palo Verde Tree (Parkinsonia florida)


Desert plants and their adaptations to seasonally harsh environments have been the subject of much study and debate. Over the years evidence has emerged that shows that water conservation is not the ultimate adaptive stratagem most of us naturally assumed it would be. On the contrary, selecting for maximizing photosynthesis appears to be the goal for non-succulent plant structural modifi cations.


Succulents truly are the masters of effi cient water use in arid climates but almost all of the other plants from the tiniest annual to the largest desert tree have a feature in common that selects photosynthetic production over water savings. T ey have densely spaced stomata on both sides of their leaves (amphistomatic) as opposed to only on the bottom (hypostomatic). Stomata are the cells that open and close allowing for the uptake of carbon dioxide and the exhausting of the by-products of photosynthesis (oxygen) and respiration (CO2


process. Narrow cells loosely arranged perpendicular to the leaf surface allow for rapid absorption of CO2


.


It’s survival of the fi ttest. Maximizing carbon production via photosynthesis should provide the greatest resources for reproduction, surviving periods of drought and for competing with other plants for water and nutrients. Plants have optimal ranges of solar radiation absorption (light energy from the sun), temperature and water availability for maximum photosynthetic production. Adaptive measures strive to keep the plant functioning as close to these optimums as the existing conditions will allow.


and water vapor).T e high number of stomata brings with it the potential for a correspondingly high rate of water loss in the exceedingly hot, dry desert air due to transpiration. Conventional wisdom would dictate that these points of water loss would be limited rather than maximized in desert plants. T is trait enables them to cool off through transpiration as copiously as their more mesic dwelling cousins during favorable times of high water availability.


T e carbon dioxide absorbing tissue in the plant’s leaves are also specially confi gured for maximizing photosynthesis. CO2


10 www.LivingDesert.org


Desert plants do indeed have strategies for managing their water use and allocating their resources as any deviation from optimal ranges requires adjustments to compensate for those changes. T e key to success for non-succulent desert plants is not in their water saving capabilities but rather in their ability to continue to function in low water availability situations well beyond the threshold for plants from wetter climates. T e challenge to plants during the periods of high heat and extended drought is in maintaining non-lethal leaf temperatures while still absorbing suffi cient amounts of solar radiation to fuel maximum levels of photosynthesis.


is one of the limiting ingredients necessary to fuel the


Coping with surface temperatures that routinely reach 160° F. and dry air and desiccating winds that can suck moisture out of the soil at annual rates over one hundred times that which is received in years when rainfall may be nearly non-existent.


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