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.


2012 HGCA Recommended List.

Now that we have hard time to digest the new HGCA Recommended List for a few hours I thought it worth adding a few of my own thoughts. The first thing I would like to mention is the innovative way we could keep briefed on the release of the RL, this year I watched in anticipation via Twitter. Several press members were commenting on Tuesday from the press briefing but alas there was a press embargo until the Wednesday – nothing better to raise the excitement levels to tell people something but nothing. Then mid-way through the morning of Wednesday 16th November @HGCATweets tweeted a link to take us directly to THE NEW 2012 HGCA RECOMMENDED LIST on www.hgca.com. Sadly I was teaching at the time and I could not access it until lunchtime, but I have to say what a fantastic way HGCA and AHDB have for keeping us bang up to date with real cutting edge information.  It’s not that long ago we used to be sat in the office eagerly awaiting the screaming banshee that was the fax machine (for younger readers – ask  your dad!) to begin churning out the release from NIAB hq.   To see the whole RL appear on your screen in full within seconds of it being released is a fantastic development.

In tomorrow’s blog I will begin to give my own comments on the varieties but today I will  comment on the list in general. The information published on the list can be quite daunting however if you simply concentrate on looking at the varieties you are interested in it does reduce the cognitive load, and you can begin to be less confused.

It’s worth noting the agronomic characteristics of the varieties as they appear on the list, however I would like to you be aware that these varieties have been grown in a relatively restricted areas and have been exposed to less disease pressures than they will be. Once varieties begin to be grown on farm scale in a much wider geographical spread the diseases that affect the varieties will become more prevalent. As a result the disease ratings can often go down from say a 7 to a 6 or even a 5 over a few years.  For example a new variety could be rated at say a 9 for mildew, meaning so far it has been seen to be fairly resistant of mildew in trials, so far.  However once the variety is grown wider then it may become exposed to new strains of mildew and be affected by more disease pressure & as a result the rating may drop from a 9 to an 8.

That said, the ratings published and are genuine and it is clear that varieties are being bred with much better all round disease profiles. Increasing the disease ratings will result in reduced fungicide inputs.

You will also notice that the yields are published as a percentage (%) of control figure, not as a yield. This is so we can look at how a variety performs or yields when compared to the mean of a range of varieties (the controls) over a number of years.

  • If the control yield figure quoted is 10.3 this means the mean of the control was 10.3 tonnes per hectare
  • If an individual variety yield score is quoted as  100, this means that this variety will have performed at 100% of the controls i.e. 10.3 tonnes/ha.
  • If the yield figure is say 106 then the variety has performed 6 % better than control i.e. at 10.92 tonnes/ha

Finally I would like to applaud the HGCA RL committees for removing more varieties than they promoted, as surely the RL has to be about the varieties being the best available and with developments in agronomic characteristics’ being bred in to new varieties it is right that older varieties with diminishing ratings or being outclassed by newer ones do indeed fall off the list.  That’s not to say that these older varieties will not still perform on farm, of course they will and they will continue to be grown for a few years yet.  To me the RL is about the top performing varieties.  If all varieties continue to be listed we will continue to have an ever lengthening RL and this serves no-one any good.

One only has to look at the Forage Maize listing system to see how easy it is get things wrong.

I hope you find this useful, please do comment if you so wish.

Thanks for reading



2012 HGCA Recommended List

Summaries of the new HGCA Recommended Lists are available at www.hgca.com/varieties.


There are five new winter wheat varieties.

Crusoe is a nabim Group 1 variety for the UK. It has asimilar yield to Gallant but with better resistance to Septoria tritici, mildew and the rust diseases. The variety is also approved for the ukp exportcategory.

Torch is recommended for the UK as a nabim Group 3 variety.It is high yielding, has good resistance to brown rust and is resistant toorange wheat blossom midge. It is also approved for the uks export category.

Horatio has been added as a soft-milling feed variety whichhas distilling potential and is also approved for the uks exportcategory.

KWS Gator is a hard-milling feedvariety with a high yield in both first and second wheat situations with orangewheat blossom midge resistance; it has good resistance to yellow rust but mayrequire attention to manage brown rust.

Relay has been added to the list for the East and Westregions as a hard-milling feed variety. It is high yielding in both first and second wheat situations and has good resistance to yellow rust.

Seven winter wheat varieties have been removed from the list; Cassius,Gladiator, Glasgow, Istabraq, Kingdom, Oakley and Robigus.

One new spring wheat variety has been added to the list for spring and autumn sowing.

KWS Alderon is a hard feed variety with good resistance toyellow and brown rust.


Three new winter barley varieties, two malting and one feed, have been added.

SY Venture and Archer are high yieldingmalting varieties with good resistance to lodging and the barley mosaicviruses.

For the feed market, the 6-row variety KWS Meridian is aconventional variety yielding similar to hybrid 6-row varieties. It has a good spectrum of disease resistance and screenings levels have been good despite a moderate specific weight.

One winter barley variety has been removed from the List; Boost.

There are three new spring barley varieties, all under test for malt brewing andmalt distilling: Odyssey, Chronicle and Overture.All three are high yielding and have good resistance to mildew and Rhynchosporium. Chronicle appears to have stiffer straw.

Forensic, Scout, Panther and Publican have been removed from the spring barley list.

Oilseed rape

There are three new winter oilseed rape varieties for the East/Westregion.

DK Expower is a restored hybrid variety with a good yieldand very good resistance to Phoma stem canker (9 rating), though attention willbe required to protect the variety from light leaf spot. Thorinis a new restored hybrid semi dwarf variety bringing together a good yield withshort stiff stems. For this variety, attention will be required to manage Phomastem canker.

One open pollinated variety has been added to the East/West list. DKCamelot has a good yield and is shorter and earlier compared withother open pollinated varieties, but will require attention for Phoma stemcanker.

There are three new additions for the North region – allrestored hybrid varieties. Cracker brings together goodresistance to the two main disease threats in the region; light leaf spot (9rating) and clubroot. It also has good resistance to lodging. Palaceshows good resistance to light leaf spot and has a stiff stem, but is latematuring. Artoga also has good resistance to light leaf spotand is earlier maturing.

Three varieties have been removed from the East/West region; Excel, DKSecure and NK Bravour. Five varieties have been removed from the North region;Emerson, DK Secure, Hornet, NK Bravour and NK Grace.


No new varieties have been added to the winter or spring oat RecommendedLists. Drummer and Leven have been removed from the spring oat list.

ukp bread wheat is a blend of semi-hard varieties to suit both EU andnon-EU bread making.

uks soft wheat is a blend of soft extensible varieties, well knownthroughout the EU for its biscuit making and bread making characteristics, andis also suitable for blending with hard high protein wheats.

HGCA Pocketbooks to download

The 2012 UK Recommended Lists will be published this week. In preparation for this its worth pointing out the excellent pocket book of the varieties can be downloaded from the below link.

These pocketbooks are excellent reference points for anyone wishing to refer to simple, summarised variety data.
I will give my comments on the new RL late in the week. In the meantime, why not have a look at the varieties here.

“We found it hard to find anything negative to say about this College” – Agricultural study – the facts

Well, i think I may have to put my mortgage on a horse, buy the lottery and invest in Euro’s. Such are my recent found powers of prediction.  Hot on the heels of my blog about the diploma minefield, the Farmers Weekly have only published a survey about Agricultural Colleges.  I genuinely had no idea this was in the offing.

It’s nice to read the comments from our current and past students.  It’s also heartening to read the comments so positive about other agricultural colleges, its great to read that most are once again On The Up as it were.

Although I have no facts to bear this out, I have a feeling in my (soil?) water that Ag Colleges are going to have another resurgence. The scale of University fees is, inevitably, going to put many future learners (Yes, that word again), sorry STUDENTS off studying agricultural subjects at uni.  Therefore the provision at traditional Ag Colleges will be looked at by many next-gen agriculturalists and farmers. 

We as colleges must be ready to deliver the requisite courses, and I am sure we will be.

Please excuse my indulgence but I feel i should include the snippets from Reaseheath, if I may.

The basics?

All the traditional agricultural courses plus some more unusual ones such as adventure sports. Opportunity to study up to foundation degree level. About 95% of students find employment or move on to higher education. £3.5m recently invested into the college’s dairy and livestock facilities. There’s also a demonstration anaerobic digester on its 300ha college farm. Recent £35m spend across campus. The farm has commercial pig, sheep and dairy units – the pride of which is the former Genus MOET herd of high merit dairy cows, now renamed Reaseheath Holsteins. The dairy is so high-tech it’s like something out of a sci-fi film. Over 350 agricultural students, 95 of whom are studying degree-level courses.


What the students love about it?

Beautiful country estate feel. Experienced (in a know-what-they’re-talking-about-not-fresh-out-of-college themselves way) staff. The college farm has a real mixed feel, with a bit of everything, and plenty of practical sessions. Very strong on dairy. Good at arranging work experience.


Can’t all be wonderful, though, can it?

In common with Sparsholt, however much we cajoled we couldn’t dig up a bad word about this college.


A student (or recently left) student’s experience?

“Through the college I’ve already got a list of employers looking to take people on, so have no worries about getting a job. Getting involved in showing the Longhorn cattle was a really great experience in my first year. The college farm is really cutting-edge; it’s given me such a lot to think about when I go home to the family farm.”


Social life and extra-curriculum activity?

Regular events, such as Thursday night discos, in the student bar. Only a few minutes’ drive from the smart market town of Nantwich. Bigger nights out to Chester and Shrewsbury.