Plants Adapting to Shade Conditions

Plants Adapting to Shade Conditions

Conditions of shade present low average light intensity for plants,

along with a difference in the quality of light (in terms of the

numbers of quanta of different wavelengths). Shade light contains more

far-red and infra-red quanta and fewer quanta in the range 400 -

680nm. Additionally, light quality and quantity can vary enormously

and rapidly due to the occurrence of sunflecks, (a valuable source of

light, yet potentially damaging.)

Adaptation to shade conditions can be determined genetically or be due

to acclimation. Obligate shade plants have evolved genetically to

exist in permanent shade and cannot acclimate to long-term levels of

high light intensity. However there are plants that show an element of

phenotypic plasticity and can acclimate to low (and others to high)

light intensities.

* Shade plants can absorb and utilize low light intensities more

efficiently than sun plants; they have a lower light compensation

point and achieve net carbon fixation at lower light fluxes. It is

their structural and biochemical characteristics which facilitate

these properties.

* At a structural level, shade plants have thinner leaves, with a

shallow layer of palisade mesophyll and patchy spongy mesophyll

containing more air spaces. They have fewer cells per unit area

than sun leaves. Less energy is needed for the construction and

maintenance of these thin leaves, making them energy efficient in

low light intensities and allowing the respiration rate to be

lower.

* At a biochemical level, shade plants have more light harvesting

pigments i...

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...lecks possess larger root systems than

plants that grow in deeper shade. This facilitates the uptake of

water.

* Photoinhibition and photo-oxidation occur at lower light

intensities in shade plants, as illustrated in the diagram in

Appendix 2. Contributing factors are that they possess less

ability to produce zeaxanthin, which mediates heat dissipation,

and contain less protective enzymes e.g. superoxide dimutase

(SOD), which destroys ROS by converting it to hydrogen peroxide.

Overall shade plants are adapted to possess a greater ability to

photosynthesise efficiently at lower light fluxes. Associated with

this, exists the increased likelihood of suffering from

photoinhibition. They have, however, also developed ways of coping

with this risk, presented to them by their environment.