an interesting tidbit from my biology textbook

gravyboots(7B)April 3, 2011

We are starting with plant physiology this quarter & this was in my text:

"... transpiration has an added benefit of cooling a plant's leaves. The evaporation of water from mesophyll cells consumes heat, thereby decreasing the leaf temperature. [One] can hold a leaf between thumb & forefinger to estimate its temperature; if the leaf doesn't feel cool, that means that transpiration is not occurring and it must be time to water."

Also of interest: the authors say that a 15m maple tree, with an estimated 177,000 leaves (= a surface area of 675 sq meters or 1.5 basketball courts) loses, on a hot summer day, 220 liters of water PER HOUR via evaporation from the leaves!


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rhizo_1 (North AL) zone 7

And that's why it's so important to foster a healthy soil/root system for our trees. Transpiration cannot work to its fullest extent unless that working system (soil/root) is all that it is supposed to be.

We don't typically have that kind of transpirational rate going on in the urban and suburban environment where soil/root systems are severely compromised. It does occur in healthy, undisturbed, natural locations.

    Bookmark   April 3, 2011 at 10:49PM
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I'd like to know more about this. What gets my attention is about feeling the leaf temperature, and knowing when to water. If I could tell when it is time to water based on leaf temperature, that would be great. I'd just aim my laser temperature deal at the leaves, and know if it was time. I'm having a tough time believing it though, so do expand on that.


    Bookmark   April 3, 2011 at 10:52PM
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MojaveLove(5 - IL)


    Bookmark   April 4, 2011 at 12:57PM
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karate626(7A Maryland)

Thanks for sharing!


    Bookmark   April 4, 2011 at 4:27PM
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tapla (mid-MI z5b-6a)

Experience greenhousemen often feel leaves of certain plants as a tactile test for water deficit/turgidity.

How much cooling a plant can benefit from depends on how much solar energy is available to vaporize water, the gradient difference between plant tissue and the surrounding air, including the boundary layer of air surrounding leaves and twigs (basically this means the difference between humidity levels inside and outside the plant, which impacts evaporation rates), and the resistance in the diffusion pathway (which means how quickly water can move in the plant from cell to cell & through inter-cellular spaces so it CAN evaporate).

It's important to realize that water deficits affect every aspect of plant growth, impacting anatomy, morphology, physiology, as well as biochemistry, and can be caused by either drought conditions or an imbalance of air:water in the root zone. Whenever soils hold excessive volumes of (perched) water, we can be sure that the O2 deprivation is negatively affecting the plant's ability to move water and to cool itself through transpiration is being affected.


    Bookmark   April 4, 2011 at 4:30PM
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Keeping all that in mind, and adding in the numerous variables of the individual environment and growing methodology, I think it's safe to say that another more tried and true method of testing for moisture is still wise... especially for us novices... like using a wooden skewer, or feeling the medium or the wicking, or judging by heft of a pot, or all of the above... ;-)

It is interesting, though... and it's always good for the grower to learn more where plants are concerned.

    Bookmark   April 4, 2011 at 5:34PM
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.... and, it doesn't work in the dark!

JodiK is right - more reliable & less open to interpretation is a method that allows one to see or feel the actual presence of water (or lack of it) in the plant's immediate environment.

But Joe, who knows... maybe with some tandem leaf feeling + skewer poking, you can in time develop some correlations for your plants?

The quote does give some insight to why dry winter homes can be so difficult for tropicals. Evaporation via leaves is the mechanism which pulls water & minerals up from the roots through the shoots & if the air is much, much drier than the leaf, the water gets pulled through more quickly.

Some types of plants even close stomata & shut down photosynthesis if conditions get too hot & dry; they rely on photorespiration instead, which makes less sugar/energy for the plant.

My understanding is that most tropicals have evolved as plants that can carry on a regular rate of photosynthesis even with stomata half closed to conserve water (meaning less CO2 coming in to make sugars) due to an additional enzyme that fixes CO2 but not oxygen - it just uses a little additional energy.

    Bookmark   April 4, 2011 at 10:14PM
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dellis326 (Danny)

Joe1980; It would be interesting to log what measurements you'd get.
Pick a plant or several plants and measure the temps at the same time, same leaves, everyday at least twice, like in the morning and evening, whether you've watered or not. log the readings. How long it's been since watering, the light conditions, air temperature and humidity.

I wonder if you could really determine an accurate watering schedule using this. You'd probably need to do it for each plant you own or at least the ones most important or most sensitive.

Probably not very practical for those of us with large collections but even with just a few select plants you could work something out. Even if you didn't do anything with the info I think it would be interesting to see how the plants cycle throughout the day(s) over time.

    Bookmark   April 5, 2011 at 9:11AM
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