Heliophage


My Ada Lovelace day post: Constance Hartt
March 24, 2009, 7:05 am
Filed under: Farming, Plant physiology, Warning: contains molecules

Sugar cane in Hawaii

Sugar cane in Hawai'i

Cheryl made me aware of the excellent idea of a synchronised posting about women in technology in honour  of Ada Lovelace (my image of whom, for what its worth, was set irevocably and doubtless unreliabley by Bruce Sterling and Bill Gibson in The Difference Engine). I said I’d join in, and my subject is Constance Hartt, about whom I know very little, but whose work is of fundamental importance to people trying to understand the evolution of photosynthesis over the past 30 million years or so, and also to opening up the possibility of radical improvements to various crops.

Hartt was a laboratory researcher at  the Hawaiian Sugar Planters Association Experiment Station, and her assiduous work on the biochemistry of sugar cane in the 1930s and 1940s convinced her that, for that plant at least, the primary product of photosynthesis  is malate, a four carbon sugar. Later carbon-14 studies showed that she was right — and led to an interesting conundrum. Why did some plants — most plants, indeed, and almost all algae — make a three carbon sugar, phophoglycerate, while sugar cane and, it later became clear, various other grasses made a four-carbon sugar?

The answer lies in the process of photorespiration. The enzyme which fixes carbon into phosophglycerate, rubisco, is very ancient and rather easily confused — left to itself it will sometimes grab oxygen molecules rather than carbon dioxide molecules, and instead of making phosphoglycerate makes phosphoglycolate. This is no good to man nor beast nor, most tellingly, plant: recycling the phosphoglycolate made accidentally in this process of photorespiration into a form of carbon that can be used for further photosynthesis takes energy, and thus making less phosphoglycolate in the first place is a good thing. The malate-initiated photosynthesis that Hartt was instrumental in discovering is an evolutionary response to that problem: malate is part of a clever biochemical/physiological supercharger that concentrates a great deal more carbon dioxide into the cells where rubisco is doing its thing, thus making it less likely to commit that costly error with the oxygen.This supercharging system is known as C4 photosynthesis, the 4 denoting the number of carbons in malate; the regular sort of photsynthesis is called C3 in contrast.

C4 photsynthesis confers various advantages: in particular, it makes plants more efficient in their use of water. The mechanisms that concentrate carbon dioxide mean that the pores through which it is taken up, the plant’s stomata, don’t have to be as wide open as they would be otherwise, and thus less water is lost. C4 plants resist various sorts of stress better, including  direct sunlight and salty ground. The mechanism has evolved independently many, many times over the past 30 million years or so, mostly but not entirely in the grasses, which either have a propensity for the sorts of physiological re-design that is required or are particularly prone to finding themselves in the sort of niches where this approach helps, or both. Sugar cane is not the only domesticated or agriculturally relevant example — there’s also maize and sorghum, and for energy crops switch grass and miscanthus, among others. There is now considerable interest in building the pathway into some grasses that have not learned it naturally — most importantly rice. C4 rice, with higher water use efficiency and other extra hardiness, might have considerably higher yields than traditional varieties while needing less water (my colleague Emma wrote a little about this not so long ago, though her words are behind the Nature paywall).

This knowledge and potential all flows from the work of Hartt and her colleagues in Hawai’i. It was small scale stuff, and more or less by defintition the team was isolated form the mainstream. Their work was for some time almost forgotten, and may still not be as well remembered as it should be; the elucidation of the C4 pathways took place in Australia a decade or so later, and that work tended, afterwards, to eclipse the discovery work done in Hawai’i. The secondary sources that I have say little about Hartt, other than noting the devoted careful work she invested in the subject, and giving the impression that the team she worked in, led by a sweet sounding Quaker called Hugo Kortschak, was a friendly and happy one.

Do I think she is a great unsung scientist? Well unsung, yes. Great, probably not. But whenever one looks into the history of science — or indeed into the way it goes today — one sees that you do not need to be great to matter, to discover, to move the story on, or to fulfill yourself through it. She and her colleagues, tucked away far from the mainstream, trying to do some good, discovered something of profound importance for science, and perhaps, in time, for technology and humanity. What more is needed?

Many more Findingada posts to be found at this central site, or  via twitter, or by searching for AdaLovelaceDay09 on delicious or Technorati

Update: Gary has some wise words on the subtleties of C3 and C4. His point that C4 plants tend to be protein poor is a good one (though in a higher CO2 world that might even out a bit, as the rubisco content in C3 plants will drop whereas in C4 you’d expect it to stay the same, ceteris paribus) and reminds me of Arnold Bloom‘s idea that photorespiration might help with nitrate assimilation. His bigger point is that ceteris paribus is a poor way to see the world, and that to concentrate on any single factor, such as C3 v C4, is to overlook a great deal that you should probably be paying attention to. And that’s true.

Image from Flickr user _Wiedz, used under a creative commons licence

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3 Comments so far
Leave a comment

If it were possible, would genetically modifying C3 crops into C4 crops increase their resilience to climate change?

If so, it may be worth doing, in order to protect the agricultural prospects of some areas.

Comment by Milan

Milan — its an interesting question. If drought is likely to be an issue, then oing C4 will often be a help, other things being equal. But C4 plants don’t get the carbon-dioxide fertilization effects of higher CO2, so in some temperate climates staying C3 will be an advantage. Can’t offhand think of any circumstances in which going from C4 to C3 on purpose would make sense, but maybe there are some…

Comment by Oliver

From what I have read, such a genetic modification would be extremely challenging – far more ambitious than anything achieved so far.

That being said, it does seem like sustaining agriculture in a world that is a few degrees hotter may well require the breeding or engineering of new crops.

Comment by Milan




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