What is photoacclimation?

Acclimation of the photosynthetic process to alterations in the light environment is common to bacteria, algae and plants. In higher plants, it is displayed by all species, its extent differing depending up on habitat distribution. This process, called photoacclimation, is an exceedingly diverse set of responses which affects many aspects of the biochemistry, physiology and morphology of the plant. Photoacclimation may be a source of limitation of photosynthetic capacity in crop plants. In the field it has been shown that the failure of plants to fully acclimate to full sunlight gives rises to large decreases in radiation use because of light-saturation of photosynthesis.

Two levels of photoacclimation

It has proved useful to consider two levels of photoacclimation – that occurring in the composition of the chloroplast (chloroplast level acclimation) and that which affects features such as leaf thickness, cell size, chloroplast number and position (leaf level acclimation). Whilst these responses are integrated to provide the net change in photosynthetic activity of a leaf, the two levels are expressed to different extents in different plant species, and are almost certainly controlled by different signal transduction pathways. For example, leaf level acclimation may involve participation of a blue light receptor and also appears to be controllable systemically. On the other hand, regulation of chloroplast composition almost certainly involves metabolic signals generated within the chloroplast itself. It is also clear that chloroplast level acclimation is a heterogeneous process that involves a hierarchical response to incremental differences in light level. A detailed analysis of the response of the contents of light harvesting and reaction centre proteins has suggested that it is useful to consider acclimation to low light as a different response from acclimation to high light. Acclimation to low light is addressing the need to maintain a high quantum yield and to allocate resources towards limiting processes e.g. synthesis of light harvesting complexes rather than electron transport complexes or carbon fixation enzymes. Acclimation to high light is concerned with both maximising the utilisation of light, but also with providing photoprotection from the increasing tendency for damage by excess light (Horton et al 2002). Thus at high irradiance there is a decrease in content of light harvesting complexes, and increase in electron transport and carbon fixation components, thereby raising photosynthetic capacity. As this response itself becomes saturated, then the levels of photoprotective responses increase. The latter include an increase in capacity of non-photochemical quenching, an increase in the content of xanthophyll cycle carotenoids, and induction of enzymatic reactions, which scavenge reactive oxygen species.

The new approach

The seriousness of the problem and the enormity of the challenge suggests a radical change is needed in the contribution that scientific research makes to technology and development. Ultimately, science and technology are responsible for the perilous position faced by mankind. A change in outlook, motivation and philosophy is needed. We need a new era of scientific investigation and a new generation of young scientists that have these central global issues as the focus - an integrated approach shared by the physical, mathematical, biological and environmental sciences. This new community of science will strive towards the common goals of prosperity and security for all of mankind. Whilst there is still a place for competition between individuals and institutions to help drive the pursuit of excellence essential for the level of scientific progress required, its worst excesses (that are the route to the most distasteful aspects of science, unethical and fraudulent practice), based on ego and greed will be replaced by humility and philanthropy. But how may this culture change be fostered?

How is photoacclimation regulated?

There is little in the way of detailed knowledge of how photoacclimation is regulated. Chloroplast level acclimation does not appear to involve photoreceptors, but there is no compelling evidence relating to whether all of the chloroplast level acclimation responses are initiated through activation of the same sensing mechanism, although this seems unlikely. Redox control via the plastoquinone pool, or the thioredoxin systems have been implicated, and there is evidence for signalling via a secondary product of reactive oxygen species, hydrogen peroxide. The existence of redox control allows not just differences in light intensity and quality to be integrated, but also responses to temperature. It has long been recognised that input (light) and output (carbon assimilation) of the photosynthetic system are linked by feedback and feed forward metabolic controls, based upon redox state and energy state, which strive to bring them into balance. Thus, it is not surprising to find that photoacclimation is similarly regulated. Indeed, whilst acclimation provides a course control over this balance, metabolic regulation provides the fine-tuning. Thus, at a given acclimation set point, regulatory mechanisms extend the dynamic range of light intensity over which this balance can be achieved. More recently, it was recognised that in fact the complete sequence of events from light absorption to growth and development are integrated into these control networks. Processes are diverse as leaf movements and chloroplast re-locations add to chloroplast and leaf level acclimations, and help establish balance in the photosynthetic system. Control at the level of transcription, translation and protein turnover have been identified in the photoacclimation response, and co-ordination of the expression of both nuclear and chloroplast genomes are involved. A further level of complexity is that photoacclimation involves co-ordination of the biosynthesis of various apoproteins and the bound pigment co-factors, chlorophylls a and b, and several specific carotenoids. Little is known of the downstream processes that connect light perception with altered protein level. Examination of mutants defective in central signalling pathways, showed that the acclimation signalling may involve input into the common network nodes, identified in a wide range of plant cell signalling.

Photoacclimation and leaf development

Photoacclimation and leaf development are inextricably linked. Leaf level acclimation occurs by modulation of the processes of leaf development, and is programmed at the very early stages of cells division and expansion. The major features of leaf level acclimation, such as leaf thickness, are hence set and can not be reversed if the leaf is subsequently exposed to changes in irradiance. Instead, the next leaves to be produced are acclimated to the new light environment. On the other hand the responses at the chloroplast level are more dynamic – e.g. contents of LHCII can reversibly change within a few days of a change in irradiance. However, even at the chloroplast level, there appears to be less dynamic range for the acclimation response to a transition in irradiance compared to continuous exposure during growth.