To All A comparison of Opuntia productivity to C3 and C4 plants, under both natural and cultivated conditions, was done in the early 90's by Park Nobel as part of a long term investigation of the physiology of Opuntia. I am writing from a place where I don't have access to the papers (but I do have the references: Nobel, PS, E Garciamoya, and E Quero. 1992. The high annual productivity of certain agaves and cacti under cultivation. Plant, Cell and Environment 15(3). pp329-335. Nobel, P. S. (1991). Achievable productivities of certain CAM plants - basis for high values compared with C3 and C4 plants. New Phytologist 119:183-205.). >From what I can remember, the productivity of CAM plants was remarkably high. >Remember what happened in Australia! Your assumptions throughout here seems >to be that productivity in deserts is limited by physical factors and that may >not be true.
The real problem here is that we are anthropomorphizing the idea of stress. The idea needs to be one that can be applied to specific situations, not cofining it to a generalization like "deserts are stressful environments." Stress seems possible in all environments, native or otherwise. Are not some of our native oaks under much stress now in their native habitat with the addition of sudden oak death and gypsy moths? Perhaps stress needs to be tied to phylogeny as much as to environment. As for the theoretical basis for engineering or selecting for drought-tolerance, there seems to be much that might be done to me. Drought adaptations found in some lineages might be transplanted into other lineages through engineering. Fundamental changes might be considered, such as engineering rubisco's ability to discriminate between CO2 and O2. I am not a plant physiologist but I think lots of genetic variation with regard to productivity under drought conditions exists and, if I am right, then there is a basis for hoping that a particular plant species might be induced to maintain productivity at low levels of water availability. I may be wrong (and perhaps I missed the beginning of this thread) but I seem to detect a worry that we are trying to expand biofuel agriculture onto land now not utilized for agriculture. This seems like a separate issue to me and one that deserves its own thread. Phil Ganter Biological Sciences Tennessee State University On 12/21/11 12:42 AM, "Wayne Tyson" <[email protected]> wrote: Ecolog: Additional responses to Merran: I very much appreciate Merran's thoughtful response. I have made similar observations, but I do not know of any studies which have settled this matter. I remain open to enlightenment. I will attempt to do justice to Merran's contributions, but am doubtful that I can accomplish that in one or two emails. I hope that Merran and others, particularly some apparently highly qualified individuals who have contacted me off list with some very provocative ideas. In terms of the evolution of C4 plants from C3's and the abundance of the former in the tropics, I see further fertile fields for research. This may open a whole additional can of worms, but might it be that C4 evolved via a mutation that ALSO worked in more mesic circumstances rather than arising only in xeric environments? But this is too much and too distracting for now, and perhaps worthy of a spin-off thread? Later. "A saguaro is bigger than a sagebrush, but it took longer for it to get that way?" --Merran Exactly! And with respect to KY bluegrass and buffalo grass, I presume that productive potential (quantity) is not as important as persistence under stress. But are not "lawns" under continuous luxury-consumption (quantity) conditions by definition? I presume that Merran will stress his buffalo grass, and not have to mow down the excess biomass as much (quality, in Merran's eyes, not quantity, which is irrelevant in that context. And context is everything, eh? Let us not neglect RATE calculations, especially if we're going to get picky (nothing wrong with that)? I forgot to mention unit/time, and thanks to Merran for correcting my oversight. I hope someone will clear up the confusion about how "selection hasn't allowed plants to create the same biomass with less water" as Merran also points out. Any if it has, which plants they are and how much more efficient they are in producing more units of biomass IN LESS TIME OR THE SAME TIME as less efficient non-drought-tolerant plants. WT ----- Original Message ----- From: "Merran" <[email protected]> To: <[email protected]> Sent: Tuesday, December 20, 2011 8:16 PM Subject: Re: [ECOLOG-L] Plant Physiology Drought tolerance Re: [ECOLOG-L] course and symposium on plant breeding for drought tolerance > Isn't drought tolerance defined by a plant's water use efficiency? C4 > plants have the ability to fix 2 or 3 times more carbon with the same > amount of water not because they use less water in photosynthesis, but > because they limit photorespiration and the amount of water lost through > their stomatas. So they do fix more more carbon with less water, but > unless the climatic conditions are perfect I don't think the advantage is > really that great. I'm fairly sure that the tropics have a greater > abundance of C4 plants than the American deserts, and saltbushes (C4, > right?) are not usually that much larger than sagebrushes.. There must be > other limiting factors. > > It's my understanding as well that CAM photosynthesis is not the same as > C4 > photosynthesis -- I've read that it is a different, even more > efficient process. It occurs in desert succulents and allows the plants > to > open their stomatas only at night, thus losing far less water to > transpiration. The CO2 is stored as an acid and metabolised the next day. > These plants can breath in up to 40 times more Carbon dioxide than C3 > plants with the same water loss. > However efficient these plants are, they are also very slow-growing > -- something that I have never fully understood. I think that there's a > low limit to their acid-storing capabilities. So they lose less water in > exchange for performing less photosynthesis each day, but are still > creating the same biomass with less water? A saguaro is bigger than a > sagebrush, but it took longer for it to get that way? I'm guessing that > this will not be the technique they are teaching at the CSU symposium. > > If I've got any of this wrong, some one please let me know. > > Surely there must be ways to raise a plant's water use efficiency aside > from changing the photosynthetic process. I mean, I just spent my morning > picking out which variety of Buffalo Grass to replant my Kentucky > Bluegrass > lawn with. How about making the plant hairier? Give it a smaller leaf > size and orient the leaves directly upwards. Make the leaves waxy > with stomatas that don't open fully. Give it stem pleats (such as in > cacti) that create shade. But it's my understanding that many of these > adaptations also limit CO2 intake and therefore biomass production. I > don't know if these adaptations will actually let you breathe in more CO2 > for the amount of water lost in transpiration. Anyone? > > Maybe I'm completely off base but it seems confusing to me to suggest that > selection hasn't allowed plants to create the same biomass with less > water. Thank you for this conversation -- writing this email really made > me think. > > Merran > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1415 / Virus Database: 2108/4092 - Release Date: 12/20/11 >
