CHAPTER 5 – WATER RELATIONS

Publisher Summary This chapter discusses water relations in plants. To absorb CO2 for photosynthesis, plants expose wet surfaces to a dry atmosphere and, in consequence, suffer evaporative water loss. The resultant cooling often accounts for considerable heat dissipation by leaves and is probably essential for maintaining equable temperatures for photosynthesis. Plants have, therefore, evolved leaves with an epidermis composed of a relatively impermeable cuticle and turgor-operated valves—stomata. The epidermis not only reduces rates of CO2 and water vapor exchange but also provides a means of controlling assimilation and transpiration through the size of the stomatal pores. Thus, stomata play a pivotal role in controlling the balance between water loss and carbon gain. For a given leaf, plant, or variety, the length and depth of stomata do not vary among stomata in mature tissues. Most of the changes in aperture are associated with changes in width. In practice, it is not possible to make a direct microscopic observation of stomata and, at the same time, preserve natural conditions.