Note: This program first aired on January 14, 2017.
We’ve been talking lately about the fact that water is life.
It plays a critical role in our most fundamental biological processes, without
it life could not go on. We’re told to drink water, maybe as much as 8 cups a
day, in order to have enough liquid on board to meet our needs. Plants use
liquid water too, but water vapor, the gaseous form of water is ultimately what
drives water movement from one part of a plant to another. Ice, water in its
solid crystalline form, is generally a liability to life. Ice forming in and
around cells in living tissue can result in mortal damage to those cells. So
yes, water is life, but the ways that water associates with life vary widely.
If you keep house plants or grow a garden, you know that
plants need water. Water is an ingredient in the fundamental biological
processes of photosynthesis and respiration, both of which plants partake in. When
we think of plants that live in severely water deprived environments, our minds
go first to the desert. Though there is little water in the desert environment,
there are plants adapted to take advantage and maximize the little water that
is there. Those adaptations encompass mechanisms to store water when it becomes
available, reduction of the normal water
loss that results from metabolic function, and enhanced ability to
photosynthesize in high heat. We all know what those plants look like—fleshy,
spine covered, leafless cacti. Plants adapted to survive with just barely
enough water.
There is another kind of water limiting environment out
there, it’s called winter. In winter, water is typically locked up in solid
form, unavailable for biological processes. Additionally, the cold air of
winter can hold less water vapor, so there is less gaseous water around as
well. The plants we see all around us here in the temperate zone reflect a
variety of ways to deal with this seasonal water stress, to deal with the fact
that water is life, but part of the year they can’t access it.
Plants deal with the inability to get water in winter in
many ways. Some just avoid the issue altogether, over wintering as a seed (in
the case of annual plants), or underground roots or bulbs. These strategies
enable the plant to lie dormant, and suffer no water loss during the time when
liquid water is scarce. Others, the deciduous woody plants, shed their leaves,
and for good reason. Leaves are the site of water loss in leafy vascular
plants. Water moves up through the plant from roots, which gather water from
the soil, to the leaves, where water is used in photosynthesis. It isn’t
pumped, but rather is pulled. As leave release water vapor, through pores on
the underside of leaves called stomata, it creates a vapor pressure gradient.
Water exits the leaves, creating a water void that must be filled, and is
filled, with water rising through the plant vascular tissue, called xylem to
replenish the water lost by the leaves. In the winter, water can’t be gathered
by the roots, as the soil moisture is frozen solid, so water can’t move up the
plant to replenish the leaves. Additionally, much of the vascular tissue of the
plant is frozen, compounding the barrier to water transport. If the plant had
broad leaves that continued to lose water vapor through stomata all winter
long, there would be no source of water to replenish the leaves. Desiccation
would result, and cell death. The shedding of leaves by deciduous trees is a
direct response to this water stress. By shutting down all transpiration,
deciduous plants reduce winter water loss to a minimum. Trees that don’t lose
their leaves in the winter, primarily the conifers in this part of the world,
have other ways of minimizing water loss in the winter. The needles of these
trees are in fact leaves, but they do not photosynthesize year round. In the
heart of winter, the stomata of these leaves are closed shut, and water loss
through transpiration drops to a minimum. In addition, the needles are encased
in a thick waxy cuticle that is not very permeable to water, another adaptation
to minimize water loss that could occur through diffusion of water vapor
through a permeable leaf surface.
Water is life, but in winter, even though we often have a
lot of precipitation, it is in short supply. Plants show a remarkable diversity
of adaptive strategies to deal with this seasonal water stress. We could learn
a lot from them.
References:
Peter Marchand’s Life in the Cold: An introduction to
winter ecology is a classic in the field. (University of New England Press)
