The Physiological function of epicuticular wax in coastal douglas-fir (Psedotsuga Menziesii (Mirb.) Franco)
Date
1995
Authors
Linger, Erin Christine
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Abstract
The surface of conifer needles is covered by amorphous and crystalline epicuticular wax and, in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), the crystalline wax completely occludes the stomata antechambers. It has been proposed that epicuticular wax limits cuticular water loss and provides an additional resistance to stomatal gas exchange. Under desiccating conditions, the wax may be a significant antitranspirant, but it may also act as a barrier to CO2 uptake thereby limiting photosynthesis. Experiments were carried out to determine the effects of epicuticular wax on cuticular resistance and gas exchange through the stomata.
Experiments were performed during the summers of 1993 and 1994 on coastal Douglas-fir seedlings grown in pots under ambient conditions. Epicuticular wax was removed from entire branches by applying a fast-setting dental impression compound to the needles, heating to 32°C and then gently removing the compound with tweezers. Treated needles remained green and physiologically active and electronmicrographs showed that the wax could be removed from stomata! antechambers without damage to the underlying tissue. Further examination of treated needles showed that epicuticular wax was not regenerated over time. Measurements of photosynthesis (A), stomatal conductance (gs), internal CO2 concentrations (ci) and instantaneous water use efficiency (WUE, the rate of A to transpiration, E) were then made using a portable open gas exchange system. It was expected that the increase in gs due to stomatal antechamber wax removal would produce two results: 1) an increase in A coupled with a decrease in WUE or 2) since stomata are very sensitive to changes in ci, stomata would partially close to compensate for the increase in ci. In other words, there would be no measurable change in the physiology of the treated needles. To determine the effect of changes in ci on gs, the gas exchange of a whole seedling was measured in a computer controlled whole plant cuvette.
Changes in cuticular resistance (re) due to wax removal were measured by suspending needles over saturated salt solutions of a known molality. Water loss over time was then determined gravimetrically.
Electronmicrographs showed that the wax removal technique produced two results. On some seedlings, greater than 50% of stomata were clear of stomatal antechamber wax, while on others, very little wax was removed. In the latter case, the wax crystals were smeared into an amorphous mass. When wax was removed from more than 50% of stomata, there was no significant change in gs and A before and after treatment. In contrast, unsuccessful wax removal resulted in a decrease in both gs and A. The whole-plant measurements showed that Douglas-fir seedling regulated gs in an attempt to maintain ci constant as ambient CO2 concentration (ca) was increased (dci/dca was 0.4).
Epicuticular wax removal resulted in an overall decrease in re in all but one of the seedlings. Generally, needles with the greatest degree of wax removal from stomatal antechambers had the most significant reduction (17 to 40%) in re.
In conclusion, successful removal of stomatal antechamber wax has shown that the wax does not limit productivity in coastal Douglas-fir. In fact, it appears that the tree
compensates for the loss of the physical resistance of the wax by adjusting its physiological resistance or by partially closing its stomata. However, the cuticular resistance measurements illustrate the importance of epicuticular wax, particularly amorphous wax, in limiting water loss through the cuticle.
Although it was not investigated in this study, the crystalline wax covering the rows of stomata may be essential for reducing wettability of needle surfaces thereby preventing adhesion and germination of fungal spores.