At first sight, there's nothing much to be said about photosynthesis in senescence.  It goes down.  That's it.  But look closer and it gets a bit more complicated and interesting.

Something that was noticed in the early physiological studies of senescence is that photosynthetic capacity generally begins to decline in leaf development earlier (sometimes much earlier) than symptoms of senescence become apparent.

How this happens is a bit of a puzzle.  To try to sort it out, physiologists have used various analytical tricks to resolve photosynthesis into different components and take a look at which of them is the limiting factor as the leaf develops.

A widely-used technique is infra-red gas analysis (IRGA) which measures CO₂.  By enclosing a leaf in a chamber attached to an IRGA instrument, the rate at which the gas is taken up in photosynthesis (or emitted in respiration) can be measured while environmental conditions are varied.

Useful values describing different aspects of photosynthetic performance can be obtained from the response curve of gas exchange rate (determined by IRGA) at different light or CO₂ levels.

For example, a combination of IRGA measurements and compositional analysis allows us to work out the net contribution to the plant’s carbon economy of a leaf over its lifetime.

In general, net photosynthesis per unit area at light saturation begins to decline around the time of full leaf expansion, usually well before visible yellowing commences.

The figure shows photosynthesis declining well before chlorophyll is lost from leaves of meadow fescue.  The interesting behaviour of a staygreen mutant is also presented.

Decreasing photosynthetic capacity is associated with diminishing leaf nitrogen content.  The CO₂-fixing enzyme rubisco represents the largest repository of protein nitrogen in the leaf so it seems logical that declining rubisco is the common factor.

The amount of a protein (or indeed any biomolecule in the cell) is determined by turnover – that is, the balance between the rate of synthesis and the rate of breakdown.  Over the years researchers have wrestled with the issue of rubisco turnover and come to the conclusion that there is little or no simultaneous synthesis and breakdown at any time in the life of the leaf.

That is, rubisco is synthesised at a high rate in young growing leaves.  Only when synthesis has stopped, at around full expansion, does protein breakdown start (by an as-yet unclear mechanism).

For example this study, by my friend Hiko Mae and colleagues, of rubisco in the 12th leaf of rice from birth to death shows that synthesis (green bars) and breakdown (yellow bars) overlap to only a very limited degree.  Notice how total amount (but not turnover pattern) of rubisco changes with increasing amount of nitrogen fertilizer fed to the plant.  It's this kind of N-responsive behaviour that suggests rubisco is bifunctional, acting as a  storage protein as well as an enzyme.

Data from Makino A, Mae T, Ohiro K (1984) Relation between nitrogen and ribulose-1,5-bisphosphate carboxylase in rice leaves from emergence through senescence. Plant and Cell Physiology 25: 429-437.

Some people think that the onset of rubisco decline (and the associated decrease in photosynthesis and leaf nitrogen) happens simply because synthesis has stopped.

Others (and I’m one of them) believe that it’s not enough just to stop making rubisco; the mechanism for degrading it must be positively promoted too.

IRGA, rubisco, nitrogen and pigment measurements tell us that the leaf continues to absorb light energy in the period between full expansion and the onset of yellowing, even though the capacity of the photosynthetic apparatus to use the light for fixing CO₂ is decreasing.  What happens to this unused energy?

Plants have a number of cunning ways of harmlessly disposing of excess light energy.  For example, they can re-emit the light as fluorescence and heat or use it to drive futile cyclic metabolic processes that lead nowhere.

The state of the photosynthetic apparatus can be probed non-invasively by measuring chlorophyll fluorescence.  The picture shows fluorescence imaging data obtained by Astrid Wingler from senescing Arabidopsis rosettes. F0=minimum fluorescence; Fm=maximum fluorescence; Fv/Fm=maximum photosynthetic efficiency; NPQ=photochemical quenching.

We can imagine that such defences against light damage will become more and more busy as CO₂ fixation by rubisco steadily goes downhill.  Eventually the point is reached when the light-dissipating apparatus is in danger of being overrun, threatening biochemical chaos by letting free radicals and reactive oxygen loose in the cell.

It may be that the cell is able to sense the approach of the tipping point and respond by initiating chlorophyll breakdown.  This way the absorption of excess light energy is reduced in a controlled way and viability is sustained.

After rubisco, the next most abundant source of protein nitrogen in the leaf is the chlorophyll-binding protein LHCP.  Removal of chlorophyll makes LHCP available for breakdown.

The intimate relationship between photosynthesis and nitrogen metabolism is of great agricultural and ecological significance, determining productivity, efficiency of nutrient use and integration of whole-plant growth and development.

It also raises a question about when exactly senescence starts.  Is it at the time CO₂ fixation capacity begins to turn down after leaf expansion?  Is it when decreasing nitrogen content becomes detectable?  Or loss of rubisco?  Or the onset of yellowing?

Or perhaps only senescence obsessives worry about such things!