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Waxes and Cuticle: Form, Function and Development.
Peter M.A. Toivonen. |
| Abstract - Waxes are an important component of a seedling's stress resistance capability. Surface waxes have several functions which help to control water loss under exposed conditions which occur on an outplanting site. In addition, good cuticular wax development has implications on the handling and storage of seedlings as they are lifted, held and then shipped for planting on logged sites. The form and function of cuticular wax is relatively complex and manipulation requires some understanding. The generally cited function of waxes is provide a water resistant barrier to the cuticle, however if this were the only function, surface waxes could be replaced with straightforward surface coating technologies. However this is not the case. Therefore the emphasis will be on the enhancement of waxy development by seedlings. This talk is designed not to expound a volume of data, rather to utilize what limited information there exists and to stimulate discussion and perhaps research towards manipulating surface wax development in nursery-grown seedlings. |
Wax development occurs within the cuticle (cuticular wax) and on the surface of the cuticle (epicuticular wax). Cuticular wax is integral to the subcuticular cell wall (cutin) and provides a waterproof quality to inhibit water loss from the leaf surface. Epicuticular wax is extruded or migrates from the epidermal cells and is crystalline in nature. The epicuticular wax occurs as a three-dimensional mat on the surface of the leaf and it may also fill (occlude) stomatal cavities. The major function of the surface mat is to is to scatter and reflect light, while the stomatal occlusions can significantly reduce water loss from the stomates.
Cuticular wax can be enhanced by growing plants under mild water or heat stress conditions. The degree of wax development can be affected by the level of stress applied. This wax is an integral part of the cutin structure, occurs as discontinuous plates and these plates, in aggregate, act to make the cutin water repellant.
Surface wax is extruded in a crystalline form, often resulting in complex and ornate patterns in the leaf surface. The fusion of aggregates of these crystalline structures can form into very thick protective mats which can serve also as protection against physical injuries. The inclination angle of these wax structures is important for their function. The main function of the crystalline surface is to scatter and reflect light. The waxes will scatter UV and blue light and the bloom of most plants which have a well developed epicuticular surface appears to be white to bluish in colour, depending on the specific reflective characteristics of the wax. The effect of scattering is to reduce the heat loading from sunlight. The reduction of heat loading results in lower water use rates and lower leaf temperatures. Plants exposed to high sunlight or to UV will produce thick, well developed surface wax structures.
Waxes are essential to keeping moisture in the needles at all times. Waxes can provide barriers to development of decay moulds in stored seedlings. Waxes (both cuticular and surface) are essential conserving seedling moisture contents when planted out in stressful sites, until new root development can increase the water uptake capabilities of the seedling. Because of the nature of wax development and structure, new wax development cannot be expected to occur on existing needles of recently outplanted seedlings. We have made observations of spruce seedlings grown in crowded styro-blocs to the 2-0 stage. Needles from the exposed portions of the seedlings had obvious waxy "bloom" and were generally preserved once outplanted. Needles from the lower portions of the seedling, which had been in deeply shaded conditions were generally dropped within a short time after outplanting. This illustrates the incapability of the existing needles to respond to a changed environment and also the severe handicap a seedling could have when trying to establish in a stressful environment. Lost needles represent lost growth and survival potential.
Can seedlings be grown in such a way as to enhance wax development and thereby increase stress resistance? The short answer is yes, however there are at least two qualifications to this answer. Firstly, the "energy" expense of wax production to the seedling must be considered and, second, the ability of the seedling to produce wax at limited periods of time during development must be considered. The production of one molecule wax requires a minimum of 5 times more energy than for the production of one molecule of primary sugar from photosynthesis. This is an expensive process to the plant and management manipulation using stress is used then this fact must be taken into account. If stress treatments are to be applied, then the degree of stress must low enough not to limit photosynthetic activity. In addition, it is known that seedlings can only produce wax within a finite developmental window. Usually wax production ceases before the needles are fully expanded. Therefore it is not reasonable to expect a seedling with little wax to produce protective wax after it is transplanted into the field. Surface wax development must occur during the entire production cycle of the seedling (i.e. from seedling emergence to lifting).
There are no definitive formulas or techniques for the enhancement of cuticle wax. However there are several reports which give a good starting point as to what might work. Four which have some potential are; 1. enhanced exposure to high light, 2. managed water stress during seedling production, 3. slowing of growth through root restriction during seedling production, and 4. application of abscisic acid. I will make comments on each of these.
1. Exposure to high light.
High light can leads to both heat and water stress in seedlings. It is known that high light will particularly enhance production of epicuticular wax. The result is that needle surface can more effectively scatter and reflect UV and blue light. When UV and blue light are absorbed, the major part is converted into heat. UV and blue light are highly energetic and as a result, can cause greater heat build up than longer wavelengths (red, yellow or green light). It is critical to minimize the absorption of these highly energetic wavelengths, especially when seedlings will be exposed to stressful conditions of an outplant site. The greater the levels of light exposure during seedling development, the better the epicuticular wax layer. Therefore high light levels with UV and blue wavelengths are required to elicit good wax development. Glass and most plastics will not transmit UV light. Seedlings grown in enclosed greenhouses will have much less potential for wax development than those grown in open compounds. The density of seedling spacing in the styro-blocs will influence the degree of wax development over the entire seedling. In crowded blocks, lower needles will have little or no wax development. If the seedling spacing allows full light exposure to the whole seedling, then wax development will be extensive for all the needles on that seedling. Therefore consideration has to be made for modification of seedling spacing density, taking into account the loss of production efficiency and weighing that against improvements in seedling survival. Research is required to allow this decision to be made on an informed basis.
2. Water stress.
Water stress has been demonstrated to enhance wax development in many plants. The stress that is applied to enhance wax development must be moderate. If the stress is too great, photosynthetic activity is impaired and so is the production of waxes. Because wax development ceases in mature needles, water stressing must be done continuously as the seedlings grow and develop. This requires some research to evaluate the stress levels required. However, this stressing approach cannot be done without regard to the previous discussion on high light levels. Densely crowded seedlings will maintain extremely high humidities within the lower canopy within the centre of the block. Therefore seedling spacing must also be such that good air circulation and exposure exists for the whole seedling canopy.
3. Root restriction.
Several reports have shown that root restriction can enhance wax production of plants. This procedure is different than water stressing, in that root restriction impacts on nutrient uptake as well as water. The result is a slowing in seedling growth rate. With this slow growth rate wax production is enhanced and the wax surface structure is thicker and much better developed. The surface structure on slow growing plants has been found to be much greater in resistance to damage. This resistance to damage could be very useful especially in the lifting, storage and transplanting phases of seedling production, when a lot of handling occurs. As with the previously discussed approaches, the degree of restriction required must be researched. The resultant reduction in seedling growth rate must also be evaluated on a cost/benefit basis. Will the cost of producing slower growing seedlings be outweighed by a resultant improvement in survival?
4. Abscisic acid.
Abscisic acid is known to mimic some of the effects of water stress. In fact, many of the effects of water stress are associated with endogenous production of abscisic acid by the plant. There is limited evidence that application of abscisic acid sprays can enhance wax production. However, the degree of response is less than expected with water stress for the examples cited in the literature. The use of abscisic acid should be approached cautiously, since hormone applications may have longer term effects on growth in the field. Before it is used, research involving quantification of effects on wax development as well as subsequent growth in the field must be undertaken.
In summary, the potential to improve seedling survival through enhancement of wax development exists. There is however, little information on waxes and conifer seedlings. Therefore application of this strategies to improve wax development will require some research. In general, a current recommendation would be to try to increase seedling spacing in styro-blocs as much as possible and to use as long an interval between waterings as the nursery manager is comfortable.
Clark, J.B. and G.R. Lister. 1975. Photosynthetic action spectra of trees. II. The relationship of cuticular structure to the visible and ultraviolet spectral properties of needles from four coniferous species. Plant Physiol. 55:407-413.
Eglinton, G., and R.J. Hamilton. 1967. Leaf epicuticular waxes. Science. 156:1322-1335.
Jones, H.G. 1983. Plants and Microclimate. A Quantitative Approach to Environmental Plant Physiology. Cambridge University Press, Cambridge. Chapter 10. Drought and drought tolerance. pp. 212-237.
Juniper, B.E. 1959. Growth, development and effect of the environment on the ultra-structure of plant surfaces. J. Linn. Soc. (Bot.) 367:413-419.
Kramer, P.J. 1969. Plant and Soil Water Relationships: A Modern Synthesis. McGraw-Hill Book Company, New York. 482 pp.
Vidaver, W., W. Binder, R.C. Brooke, G.R. Lister and P.M.A. Toivonen. 1989. Assessment of photosynthetic activity of nursery grown Picea glauca (Moench) Voss seedlings using an integrating fluorometer to monitor variable chlorophyll fluorescence Can. J. For. Res. 19:1478-1482.
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