Plants and Light

Light is an extremely valuable source to all plants, without it, it is hard for them to photosynthesize, compete and reproduce Givnish 1988). Light availability often affects which plants will succeed and the life-history strategies (they choose to adopt (Valladares et al 97). Plants do not all receive the same amount of light, and this can vary greatly in certain conditions. Tropical Rain forests are an example of this; canopies cause the amount of light received by different plants to vary greatly. Plants at the top of the canopy receive a constant supply of intense light and understory plants receive very little light and what light they do receive can be sporadic and vary in intensity. (Chazdon and Pearcy 86). Living in a low light environment can be stressful as a plant needs to photosynthesize enough to have a positive carbon balance, therefore, plants in low light show adaptations to maximise the capture of light in the short periods available (Lei and lechowicz 97).

The plants in the understory find their light is dominated by a series of sunflecks and low intensity diffused light (Lei and lechowicz 97). Sudden changes in photon flux densities are often caused by sunflecks. Sunflecks are patches of light that hit the understory plants; they vary in intensity and in duration, depending on the angle of the sun and the vegetation above (Chazdon and Pearcy 1991). The structure of the canopy determines a sunflecks maximum photon flux density (Pearcy 90). However, they are not completely random and there are times of the day when they are more common than others such as in the middle of the day (Pearcy et al 85). Experiments have shown that on a clear day the 20-80% of photon flux density in the understory is from sunflecks (Pearcy 90). The pattern of sunflecks can play an ecological role in determining the succession of plants and which plants will out compete others (Leakey et al 04).

Phenotypic plasticity for certain features often those regarding capture of light in shade plants is often high and plants can adjust themselves to new light conditions (Valladares and Niinemets 08). This allows shade plants to adapt and utilise light efficiently. Table 1 highlights differences between sun and shade plants and many of the differences shown in shade plants are because of adaptations to maximise their survival in low light conditions

Some shade plants adopt morphological mechanisms such as having a higher leaf area, with thinner leaves compared to sun leaves resulting in a greater leaf area ratio and lower relative growth rate; this was shown in 50 cm conifer seedlings as leaf area ratio increased with increasing shade tolerance, leaf longevity also increased with shade tolerance (Valladares and Niinemets 08).

Shade plants can also adopt anatomical responses and their leaves contain more chloroplasts of greater volume; the chloroplasts contain a high thylakoid membrane and granal stacking as this allows them to capture more light than corresponding sun leaves of the same species (Maxwell et al 1999). The grana are often unusually arranged and this could be to maximise the inadequate light that hits the forest floor (Boardman 77).

Shade leaves have physiological and biochemical adaptations so that they can survive in low light environments. They have a high carboxylation capacity compared to light yielding abilities (Boardman 77). They contain low dark respiration points as they have lower amounts of Rubisco and increased chlorophyll (Lichtenthaler et al 1981). The low rubisco is adaptive as it allows plants to have low dark respiration points and, therefore, survive under extreme low light conditions (Bjorkman 1981). Shade plants differ in their economic use of light available, and invest more into safeguarding of its light harvesting apparatus (Boardman 77). The shade plants often contain a lower proportion of chlorophyll A to B (Boardman 77). The chloroplasts in shade leaves often include extra chlorophyll per unit weight (Lichtenthaler et al 1981). This is an adaptation to cope with extreme stress. Shade leaves also display greater maximum chlorophyll fluorescence (Lichtenthaler et al 1981).

The ability of a plant to use a sunfleck relies upon the timing and duration of the fleck and how long previously the leaf has been in the shade, the longer the period of shade the longer the period of induction to reach maximum rates of photosynthesis (Chazdon and Pearcy 86). It is clear that the carbon gain is not just dependent on the total amount of light absorbed but on the pattern of light; the duration and frequency is most important (Chazdon and Pearcy 86). Experiments have shown that plants responses are very variable, even plants of the same species can respond very differently , also short light flecks tend to be utilised more efficiently than longer ones (Chazdon and Pearcy 86). Shade plants utilise sunflecks more efficiently than sun plants (Valladares et al 97). Ogren and Sundin (96) state this could be due to the fact there is a higher electron capacity to carboxylation capacity in plants that grow in shade and are slow growing, as they are associated with diminished photosynthetic capacity.

Short sunflecks have been shown to be more effective than long sun flecks for shade plants. Leakey et al (04) carried out an experiment in which dipterocarp seedlings were put either under long sunflecks or short sunflecks which amounted to the same total daily photon flux density; long sunflecks contributed to 45% of the total photon flux density whereas short sunflecks contributed to 61% however the relative growth rate was 4 times greater in plants subjected to long sunflecks.

The temperatures that sunflecks create can cause extreme stress for plants. Sunflecks have been known to cause photoinhibition as the stress of them can result in limitation of carbon gain (Leakey et al 04). Leakey et al (04) carried out an experiment which demonstrated deactivation of enzymes responsible for RuBP can occur at high temperatures and this was seen in shade plants that were subject to high frequency light flecks of duration of 10 minutes or more.

Understory plants that grow in shade environments have been found to utilise sunflecks more efficiently than plants grown in sun environments that usually receive more of a constant supply of light (Valladares et al 97). This suggests that shade plants are adapted physiologically for these changes in light. Evergreen trees of the species Rubiaceae were looked at growing in different light conditions within the field; the understory, gaps and clearings (Valladares et al 97). These trees, therefore, ranged from receiving very little continuous light to a near continuous supply of light. The study showed that the understory plants showed the quickest induction during sunflecks which was within 4-8 minutes, and after 60 seconds the induction was higher than those trees that were found in the clearings and gaps, the understory species were also found to lose induction slower once the sunfleck had gone (Valladares et al 97). Kuppers and Schneider (93) state that only shade plants can become fully induced with sunflecks, sun plants require a continuous supply of light. Valladares et al (97) state this could be as in the understory the stomata are kept open as this will not cost as much in terms of transpiration as there will be lower surrounding temperatures due to less sunlight. Valladares et al 97 state that “there is probably a trade off between sunfleck utilisation and photosynthetic capacity.” The fact shade plants can utilise sunflecks more efficiently is due to morphological and physiological adaptations it is important as they rely on sunflecks as a light source more than sun plants, without the ability to do this we may see less growth and diversity in the understory. It is important to understand the rates of photosynthetic responses as they can show us how plants behave ecologically with regards to natural succession (Portes et al 08).

Kursar and Coley (1999) carried out an experiment using two shade plants, Ouratea and Hybanthus, which have differing lifespans, the first living for an average of 5 years compared to the latter living for life history. They state that “there appears to be a tradeoff between the ability to tolerate stress and the ability to rapidly exploit an increase in resources” (Kursar and Coley 99). The leaves from Ouratea the long lived plant showed a greater ability overall of its leaves to acclimate to the high light conditions and showed less damage of photoinhibiotion. However, Hybanthus the short leaved plant showed much more rapid development of new leaves, and much greater efficiency of photosynthesis and was more productive after being in high light conditions for 80 days (Kursar and Coley 99). Kursar and Coley (99) are showing that there is not one single mechanism that allow plants to take advantage of changes, plants respond differently as there are tradeoffs between different traits in each plant. For example although Ouratea did not respond as well in high light situation it has a much higher lifespan in low light, whereas Hybanthus has a much shorter life span in low light but can respond more quickly and take advantage of high light. Without these trade-offs and differing mechanisms there would not be as much diversity of shade plants.

Conclusion

Shade plants exhibit a wide variety of adaptations morphological, anatomical, physiological and biochemical that allow them to survive in stressful low light conditions. The adaptations are ecologically significant as they allow plants to survive and lead to a diverse range of plants growing in low light conditions that would not occur if plants did not have these mechanisms. The case studies studied have shown that there are trade-offs occurring in low light plants which allow them to utilise sunflecks more efficiently than equivalent sun plants. They show that low light plants are o successfully adapted so that they can utilise the rapid changes in photon flux density that occur in many ecosystems.

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