Saturday, January 14, 2017
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.
Peter Marchand’s Life in the Cold: An introduction to winter ecology is a classic in the field. (University of New England Press)