‘Sun leaves’ and ‘shade leaves’

There are many marked differences between sun and shade leaves on a single individual, and these differences include leaf thickness, leaf size, leaf shape, and leaf chlorophyll density.

Because the conductance of ambient carbon dioxide for photosynthesis is one of the principal factors driving the entire photosynthetic process, leaves exposed to the sun will make use of the high light availability by increasing mesophyll thickness (Davis, 2015; Kozlowski et al., 1991; McMillen & McClendon, 1983). This increase in mesophyll thickness means increased inter-cellular spaces that in turn facilitate increased carbon dioxide conductance into the chloroplasts, thus allowing for greater rates of photosynthesis per leaf unit area (Terashima et al., 2001) – light-saturated photosynthesis per unit area or unit of chlorophyll is about 1.5 times greater in sun leaves than in shade leaves, as a result (Kozlowski et al., 1991; McMillen & McClendon, 1983). Conversely therefore, shade leaves are typically much thinner.

In response to the more shaded conditions however, shade leaves adopt numerous ‘strategies’ to improve their functionality. Shade leaves will usually not only be larger in size and reside at a more horizontal angle as they cannot ‘track’ the sun so effectively (Nobel, 1976), but also have more chloroplasts per leaf unit area (Givnish, 1988) – these adaptations ensure photosynthesis can still operate efficiently, as the shade leaves are maximising usage of the little light available to them. Shade leaves will also, where the leaf is of a species where lobing is a common occurrence, be less markedly lobed than on sun leaf counterparts. Such reduced lobing means the leaf has greater surface area for where photosynthesis can occur, and given that lobing is typically a means of improving leaf aerodynamics and shade leaves are likely to be sheltered (Karban, 2015), distinct lobing on shade leaves is an inefficient and ineffective trait.

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Here we can see oak leaves basking in sunlight, whilst further beneath leaves sit within the shade. How will this environmental difference impact upon their morphology?

Leaf size is not only beneficial in terms of light-absorptive capacity, however. Sun leaves are also smaller because, in being so, they have reduced surface area where transpiration can occur. This helps to regulate water loss where leaf (and ambient) temperatures will be higher (Givnish, 1988; Nobel, 1976) – leaf thickness also aids with this moisture-retention necessity.

Whilst not directly related to leaf physiology, plants that are subjected to low light intensities often grow more rapidly, producing longer inter-nodes (the part of the stem between each leaf). Rapid growth may help the shoot to reach light. Sun leaves on the other hand will have short inter-nodes, as they do not need to ‘reach’ towards light so readily (Pieters, 1974).

Ultimately, for leaves to be efficient, they must be able to utilise around 20% of full sunlight. Having ‘tiers’ of leaves improves ‘net’ efficiency, as the numerous tiers of leaves can all, if optimally positioned and retained, capture 20% of full sunlight with ease. Where leaves have less than 2% of sunlight however, the cost of maintaining the leaf outweighs the benefits (Davis, 2015), and thus the shaded leaf will be shed in spite of its adaptations.

Sources:

Davis, M. (2015) A Dendrologist’s Handbook. UK: The Dendrologist.

Givnish, T. (1988) Adaptation to sun and shade: a whole-plant perspective. Functional Plant Biology. 15 (2). p63-92.

Karban, R. (2015) Plant Sensing & Communication. USA: University of Chicago Press.

Kozlowski, T., Kramer, P., & Pallardy, S. (1991). The Physiological Ecology of Woody Plants. UK: Academic Press.

McMillen, G. & McClendon, J. (1983) Dependence of photosynthetic rates on leaf density thickness in deciduous woody plants grown in sun and shade. Plant Physiology. 72 (3). p674-678.

Nobel, P. (1976) Photosynthetic Rates of Sun versus Shade Leaves of Hyptis emoryi Torr. Plant Physiology. 58 (2). p218-223.

Pieters, G. (1974) The growth of sun and shade leaves of Populus euramericana “robusta” in relation to age, light intensity and temperature. Ph.D thesis: Landbouwhogeschool te Wageningen.

Terashima, I., Miyazawa, S., & Hanba, Y. (2001) Why are sun leaves thicker than shade leaves?—Consideration based on analyses of CO2 diffusion in the leaf. Journal of Plant Research. 114 (1). p93-105.

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‘Sun leaves’ and ‘shade leaves’

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