Cactus grow slowly when grown under drought conditions. For clarity, please keep in mind what constitutes drought conditions for a Sonoran Desert cactus would be water heaven for many Mojave Desert cactus species. Based on personal experience gained from 30+ years of gardening in the Mojave Desert... Cactus are water efficient when their growing environment demands water efficiency. When grown under more water-luxuriant conditions, all the cactus species with which I am familiar respond with faster growth and attain greater size compared to cactus grown under native conditions. The plants often become water junkies. The excess growth tends to overwhelm the internal support systems of branching cactus such as Cylindropuntia and the plant falls over. Reducing water availability back to a level comparable with native growing conditions tends to severely shock the cactus plant and necrosis to part or all of the plant has been the usual result.
________________________________ From: Merran <pantscr...@gmail.com> To: ECOLOG-L@LISTSERV.UMD.EDU 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
