Enhancing Photosynthesis

Plants don’t necessarily operate at their full potential. Let’s make them, says Peter Horton.

Peter Horton

To provide more crop yield on less land with fewer inputs undoubtedly requires alteration to the fundamental physiological attributes of plants. Included in these is the increase in efficiency of photosynthesis, recently identified by BBRSC as a focus of special interest and subject of a previous post on this blog.

The relationship between photosynthesis and crop yield is controversial. On the one hand, the interception and conversion efficiency of solar radiation by plants is directly proportional to biomass accumulation. On the other, linking photosynthetic activity at the leaf level (the pre-occupation of the plant scientist) to crop yield per unit land area (the concern of the farmer) has proven very difficult.

The reasons for this difficulty are numerous and at least in part result from the complexity of the system.

25 years ago researchers, including myself, first tried to set out some elements of this complexity, describing the various sub-stages of photosynthesis, from light capture by the chlorophyll-protein complexes in plant thylakoid membranes, to the electron transport processes, carbon assimilation, carbohydrate synthesis and partitioning, and product accumulation in the grain – the part that we most often eat.

The key idea was that each of these was connected not only by the fluxes between them, but by the presence of various feed-back and feed-forward regulatory processes, which tuned photosynthesis to external environmental factors, developmental processes and metabolic constraints. This network of interactions buffered the effects of internal and external change, providing balance and homeostasis, a universal feature of all biological systems. Such a model provides a means to analyse processes including stress tolerance and exemplifies the challenges presented to the plant breeder when wishing to ‘improve photosynthesis’ – where to intervene, what to change, what will be the consequences to name a few considerations.

Light the way

This formulation was redefined to provide a context for the work done by my group at the University of Sheffield on rice photosynthesis in collaboration with the International Rice Research Institute. This work revealed some striking insights, mainly how poor photosynthesis was in the field, even under conditions widely regarded as optimum.

In general, in many leaves, for significant periods of the day, photosynthetic activity was far below capacity. Causative factors included: closure of the stomata shutting off the supply of carbon dioxide to the leaves; reduction in the efficiency of light collection by the chloroplasts; and feedback from the accumulation of carbohydrate products of photosynthesis.

The conclusion from this study is important but so far widely ignored: There is enough photosynthetic activity in the existing cellular machinery to sustain a much larger yield if only plants could be induced to perform at their full potential.

So why don’t plants perform at their full potential?

Optimal operation

One reason why photosynthetic activity is not maximally expressed is inappropriate optimisation. Put simply, stability and survival (a low risk strategy) in the natural environment are driving forces of evolution, not necessarily high growth rate and photosynthetic rate (a high risk strategy) or high grain yield. Photosynthesis is held back below its potential because growth is optimised in the face of the particular properties of the plant’s habitat. Therefore, we have to consider the evolution and basic biology of each crop species.

Particularly important is that the environment is never constant- there are fluctuations in levels of sunlight, temperature and rainfall. Plants record, memorise and (try to) predict their environments to ensure that they always have enough energy storage from photosynthesis to power their growth and development. For example, plants have to determine the size of their reproductive sinks (i.e. grain capacity) in advance, predicting what the photosynthetic rate will be to give maximum grain filling. Over-estimation of future photosynthesis results in poor grain filling and/or poor quality grain; under-estimation of future photosynthesis results in a decrease in the efficiency of solar energy use and losses of potential productivity. Trade-offs inevitably result from optimisation of the internal regulatory mechanisms involved (dynamic range, kinetics, precision), and this readily explains the apparent under-performance of photosynthesis.

A particularly clear example of how optimisation points may differ in different plant genotypes is our observation that stress tolerant varieties of bean have a low growth rate under favourable conditions, whereas others have high yield under favourable conditions but suffer badly when grown under stress. Consequently, there may be opportunities for the breeding of higher yielding crops by tailoring regulatory responses to specific agricultural scenarios, where man’s intervention has moderated some of the environmental constraints on productivity, by irrigation, provision of fertilisers and elimination of weeds.

A key point is that optimisation will vary according to plant species or variety, the climate and season, the agronomic practice, the locality and so on. Thus, significant benefits will come from understanding at the molecular and genetic levels how to alter the optimisation of the biochemistry and physiology of individual leaves, their performance in the whole plant, and the way individual plants interact in the crop canopy.

Indeed, such knowledge may also be necessary to offset the inherent conservatism of plants that could thwart current attempts to increase photosynthetic efficiency, and hence yield, by manipulation of with the basic biochemical processes of carbon assimilation.

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