If photosynthesis requires six molecules of CO2, and 12 of water, what happens if the plant receives only, say, 5 molecules of CO2, 11 of water? Does the plant attempt to store the volumes of co2 and H2o until a sufficiency is reached, or can you get a situation where plants only semi-photosynthesize? Or, even worse, if a plant absorbs insufficient co2 or H2o, does the plant automatically die - meaning the amounts of water and co2 absorbed have been in waste?
A chemical reaction can only take place if the correct number of molecules are available. There is no such thing as a half reaction. However as there are about 10^23 (ie 100,000,000,000,000,000,000,000) molecules in each gram of water, and something similar with CO2, the question of whether three or four molecules are left over generally does not play a big part. I assume the plant can store a certain amount of CO2, as it can dissolve in water. And the plant certainly stores vast amounts of water, just squeeze any green plant and you will get water coming out. The answer to your last question is certainly yes. If you starve a plant of water it will certainly die. And I suspect that if you starve it of CO2 it will also die. But that effect is not as easy to see. I am not sure of the details, but it is worth pointing out that the result of photosynthesis is for the plant to make sugars and starches. These are the real food. They are transported to the different parts of the plant and there burnt (releasing CO2). Its this process that actually makes the plant cells. So I suspect that it is when these are exhausted the plant dies. This is how the plant can survive overnight in the dark when there is no photosynthensis. The plant will actually keep growing overnight as it uses up the starches and sugars it made during the day. In the old days nurses used to take flowers out of hospital wards at night because they burn oxygen and give off CO2. But then they realised the effect was too small to suffocate patients overnight.
Commercial growers can raise the CO2 level to increase growth rate (usually by burning Paraffin I think). Presumably Brownian motion, or somesuch, increases the "Pressure" of CO2 across the plant's membranes so they take up more. But they need "enough of everything else" to be able to make use of it, as Pete said. I thought Mr Avagadro was 6-ish times 10^23, or is that a different factoid and I am misremembering Chemistry form nearly half a century ago? :(
You have a very good memory Kristen - I had to look it up :DOH:. The figure is 6.02 x 10^23. But that is for one mole, which you get when you add the atomic weight of two hydrogen atoms (1 each) and one oxygen atom (16) to give 18. Thus you get 6.02 x 10^23 molecules in 18 grams of water. However I thought a factor of 2 or 3 or 6 didn't really matter too much in such a huge number. I saw an interesting program on television some time ago about a sugar beet growing enterprise. They had huge machines to pick the sugar beet and process the sugar from it. All the pulp left over was burnt, and provided the heat for a massive greenhouse where they grew tomatoes. But as you say they also vented the exhaust fumes, which were high in CO2, into the greenhouse, which increased the growth rate of the tomatos by something like 40% and doubles the yield. They said the greenhouse provided about 10% of the tomatoes grown in the UK. Thats some efficiency. Products and Services, tomatoes There is a Youtube video here.
+ 2) use of flueless oil stoves 3) burning propane/natural gas in special burners 3) dry ice 4) liquid CO2 from 'Growing Tomatoes' by Ian G. Walls I knew I'd seen that somewhere!
Some of the science in this thread goes way over my head, but I've picked up bits and pieces over the years from books and telly programs. I believe the point of raising CO2 is not so much that more CO2 makes plants grow better in its own right, but that given the right conditions, the photosynthesis process becomes more efficient than is typically the case in our gardens. There's something about the law of the single limiting factor (or something like that). It means that if you give a plant ideal measures of all bar one of the things it needs, then it will still only photosynthesize at the rate allowed by the most limited factor. I.e. if you provide ideal heat, ideal light, ideal water and nutrients, but CO2 is too low, then all those 'ideals' will be wasted because the lack of CO2 is the limiting factor. Likewise if you provide ideals of everything but only half the light, then the light will be the limiting factor and again all the other 'ideals' are wasted. Commercial growers know this, and go to great lengths to make sure that all the factors are as close possible to ideal levels. By artificially adding heat, light, water and nutrients, CO2 would be the limiting factor, so they add that too. This is, I understand, all controlled so tightly that they have computers and lots of sensors and other such magic to make sure everything is as close to ideal as possible.
Clueless - I think you are absolutely right. Plants need light, oxygen, water, CO2 and plant feed etc. But there is an optimum level for each. If they get too little they don't grow as well, and if they get too much they either ignore the surplus or actually suffer. For instance too little heat slows down growth rates. But if it gets too hot, the plant can suffer as well. There is an ideal temperature. Extra CO2 improves the growth rate but only up to a certain point. Also I have read that when light levels go above a certain value, which is different for each plant, the plant saturates and cannot use the extra. I have noticed that different years benefit different plants. For instance, this year my Lupins which I planted last autumn are rubbish. About 2 or 3 years ago they were brilliant. I suspect they have been short of water this time. But other plants don't seem to have been effected in the same way.
Funnily enough I've noticed a similar thing, but a bit more subtle. I've noticed that clover in grass seems to go through a 3 or 4 year cycle where the clover seems to be winning, then the grass seems to be winning, and then back again. I have a theory for this (which might extend to other plants). Clover fixes nitrogen. Grass is very competitive when nitrogen levels are high, so after a year of clover doing well, the grass starts to do very well. The grass swamps out the clover, and over time the grass gets the upper hand. The clover loses territory and thus fixes less nitrogen. The grass uses up all the nitrogen fixed by the clover, and its growth rate is reduced. Clover takes its opportunity in the weakened grass, and once again gains the upper hand, fixing nitrogen once again, and the so the cycle goes. Its just a theory I have based on a casual observation. I might be way off the mark.
Excellent thread I really do learn something everyday on here. Now I am off to breath over my struggling plants to give them some more CO2 :WINK1:
In post #7 above, I missed one of the methods by which CO2 levels can be increased - 5) Straw bale culture system - CO2 from the decomposing straw. Which got me wondering if there was something else growing using hotbeds provided other than heat .....
I think I must have been influenced by our politicians - after all there seems to be little difference to them between saving the world and causing the greatest financial crash for decades. I put it down to the great drive for tolerance. Clueless - you may well be right. I am a great believer in cycles. Something goes too far in one direction which sets up a situation which reverses it.
