An article from British Bonsai magazine:

WATERING YOUR BONSAI: AN ART AND A SCIENCE


Bad watering practices can kill plants, but when you ask for advice,
how many times has someone said "watering is an art"? It may seem
like an art to people who know it's time to water but can't explain
why, but it's a science, too. You can do a better job caring for
your plants by trying to understand how plants absorb water.

We can think of a plant as a machine moving water out of the soil and
into the atmosphere. A schematic diagram of this machine would show a
root, a stem containing water-conducting tubes (called xylem elements)
and a leaf . The driving force is the loss of water from the leaf
pores (stomata); just as we perspire from pores in our skin, plants
transpire from pores in theirs. As water vapor escapes by
transpiration, leaf tissue dehydrates, and cell sap becoming more
concentrated in salts (ions). These salts cannot move out of the cell,
but cause water to move in (just as salt in your salt shaker will
=D2cake=D3 in humid weather; the salt can=D5t move out of the shaker, but
water vapor can condense on the salt). Because of the high ion
concentration in leaf cell sap, water in the veins of the leaf moves
into leaf cells. This creates suction in the xylem elements, pulling
more water up the stem from the roots. Water can be =D2pulled=D3 only
when it forms an unbroken strand (this is why you're supposed to cut
rose stems under water; if an air bubble enters a xylem element, the
transpiration pump stops working). As a healthy plant transpires and
water is pulled up the stem, the roots then become dehydrated, and
absorb water from the soil. Water moves from the soil to the roots up
the stem to the leaf, to the air, to the clouds, and then falls as rain
onto the soil to start the cycle over again.

Plants can control the rate of water loss through transpiration by
opening and closing leaf pores, by sinking the leaf pores in small pits
that keep water vapor from moving away too rapidly, by reducing leaf
area (the reason conifers have thin needles is to reduce
transpiration), or by developing white, wooly leaves like lamb's ears
and dusty miller.

So, what causes a plant to wilt? First, a plant will wilt if the soil
is too dry. This is easily remedied by watering the plant, but one
must make sure that all the soil surrounding the roots is moistened.
Outside, plants should be watered gently for long periods of time
rather than doused for a minute or two. Indoors, this means making
sure the soil in a pot gets wet to the core. Often, if a poorly
draining soil is used, water will run down the inside of the pot and
out the drainage hole without penetrating to the center of the root
ball. To make potting mix drain better, it is helpful to mix it with
sand, fine bark mulch, vermiculite, or perlite. Another reason for
poor water absorption in newly-repotted plants is the use of dry peat
moss as a component of soil mix; dry peat repels water. It is better
to wet peat thoroughly, kneading it with your hands, before adding it
to a potting mix. That way, it will wetten the first time you water.

Plants will wilt if there is too much salt in the soil, making the
water hard to extract. Remember, water goes into the plant only if the
plants cells have more dissolved ions than the soil water. The "salt"
in the soil can be excess fertilizer. Flushing the soil with excess
water will remove salt buildup.

Finally, a plant will wilt if the suction caused by transpiration
exceeds the water-pumping ability of the roots. This can occur if
plants are grown in a sunny, windy location. Full sun encourages high
rates of photosynthesis and wind blows water vapor away from the
vicinity of the leaf, both increasing transpiration. Evergreen plants
are in danger even in winter; a warm spell will cause the leaves to
photosynthesize and transpire even though the soil water is frozen and
unavailable to the roots. Indoor plants may suffer because the air in
most houses is very dry causing a plant to transpire heavily and wilt
even when the soil is moist.

Plants may suddenly wilt even when none of the aforementioned things
have happened. A one-time drought may kill enough roots to damage a
plant for the rest of the season. Root rot fungi may kill roots.
Certain root rotting fungi have spores that are so tiny that they can
spread through a plant by traveling in the xylem tubes. They will
eventually clog vessels. One sign of fungus wilt is the drooping of
leaves on one side of a plant, caused by the plugging up of a portion
of the xylem.

Besides suction, there is another way plants move water. Although
transpiration "pulls" water up the tree, that doesn't explain how maple
trees produce the "sap" that drips out in early spring, before leaves
are out, which is collected to make maple syrup. Some plants dump
sugars and salts into the water in the xylem elements. The presence of
these excess ions causes more water to enter the root xylem. This
excess water has nowhere to go but "up", so it is "pushed" up the plant
stem. So why don't all plants do this all the time? Root pressure can
only push water up about 20 feet. Trees could never grow very tall if
they used only this method. Root pressure, unlike transpiration pull,
is an active process that uses up a plant's energy. Prolonged bleeding
may weaken a sugar maple, so they are not tapped every year.

You may notice root pressure in action in your houseplants. Far from
wilting, your plants may be dripping water from the leaves (a condition
known as guttation) because they are using root pressure to get water.
This may mean that conditions are unfavorable for transpiration. You
might want to put these plants in a sunnier better ventilated location.