The loss of water from a
leaf is a simple physical process of evaporation from the cell surfaces,
which have to stay wet so that gases can dissolve and pass in and
out. It is given the name transpiration because the plant has
some control over it.
The holes through which
air enters the leaf, thousands of them per square inch and mostly
in the lower surface, are called stomata, and they can close
in response to too much water loss by the leaf. They don't always
stop in time, and the leaf wilts, usually closing the stomata more
tightly. This is a mixed blessing: it stops most of the water loss,
but with the stomata tightly closed the exchange of carbon dioxide
and oxygen with the outside air cannot take place. Without this, some
of the photosynthesis reactions cannot go on, so the energy-trapping
function of the leaf is lost.
How should you, as a grower, respond if your plants are wilting? First
of all, don't automatically reach for the hose, particularly in the
middle of the day. Or, if you do go for the hose, use it to mist the
leaves rather than adding water to the root area. What you are probably
seeing is water loss too fast for the roots to take in enough to meet
the demand, rather than dry soil. Of course, the soil may also be
dry, and that can be remedied, but do check first before watering.
You don't want to drown the roots.
Wilting happens in nature,
just as it does in our nurseries and gardens. The rate of water loss
depends on air movement over the surface of the leaf, and on the temperature
(is anyone else old enough to remember what made a good drying day
when we still hung clothes outside on a clothesline?). Many plants
have evolved ways of changing the amount of sunlight that reaches
their leaves, which, of course, affects the temperature of the inside
of the leaf. Some of the most dramatic happen in the bananas, and
if you are ever around a clump in hot sunshine, you will see that
in the middle of the day the blade of the leaf folds down along the
centre part. This means that the sun's rays strike it at an angle,
a glancing blow if you like, which stops the leaf from overheating,
and keeps the energy reaching the chloroplasts to a level which they
I know a load of trivia
about leaves, and could do pages of unrelated facts for you. How is
it that I am never invited onto a quiz show that would let me turn
this clutter of information into real money!
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