Transpiration

Plants are found in almost every ecosystem in the world from deserts, to temperate forests, and down to the bottom of the sea. As a consequence of natural selection plants have evolved a stunning diversity of adaptations to deal with different environmental challenges. One of the key challenges plants face is maintaining proper hydration. Water is a critical resource that they depend upon for photosynthesis, structural support, and the transportation of nutrients and other important molecules. One way that plants can control their water balance is by regulating a process known as transpiration, which is essentially the evaporation of water from the aerial parts of a plant. This loss of water occurs primarily from pores on the leaves called stomata. But how does the water get here?

To answer this question let's look more closely underground. Here water enters the plants roots by osmosis then moves all the way up to the leaves through a vascular tissue called xylem. This channel of water is known as the transpiration stream. Because water molecules stick to each other and to the xylem walls, when it evaporates from stomata water from lower in the transpiration stream is pulled upwards to take its place…resulting in upward flow from the roots. Now let's take a closer look at a stoma. Each stomatal pore is bordered by two guard cells that can expand to open the pore and contract to close it. Plants open their stomata to take in carbon dioxide for photosynthesis and to release oxygen gas. Loss of water through transpiration is an unavoidable side effect of this process.

This trade off presents a particular challenge for plants living in arid environments and so they have evolved strategies to reduce their water loss as much as possible. One way they can do this is by growing leaves with small surface areas, presenting a small area over which transpiration can occur. This is why the leaves of desert plants such as creosote are relatively small. But going one step further plants from arid environments also have fewer stomata per unit area on their leaves allowing them to minimize water loss through transpiration.

In contrast plants that inhabit environments with plentiful water like rain forests can afford to lose a lot of water through transpiration. Such plants like this taro for example often develop leaves with large surface areas that increase their capacity to intercept light to fuel photosynthesis. These plants also have a high density of stomata compared to plants from arid habitats allowing them to maintain high rates of photosynthesis and to support large leaves and stems.

In this lab you'll measure transpiration rates and examine the frequency of leaf stomata in diverse plant species to reveal how plants from different habitats solve the problem of regulating transpiration.