Factors Affecting Rate of Transpiration

Two broad groups of factors govern how fast a plant transpires. External or environmental factors include atmospheric humidity (a very humid atmosphere sharply slows transpiration, while drier air speeds it up proportionately), temperature (rising temperature speeds up transpiration, until extreme heat instead forces stomata to close from flaccidity and stops transpiration altogether), light (which raises temperature, increases membrane permeability, and so raises transpiration in bright conditions while lowering it in dim light), wind velocity (a gentle breeze carries away the still, saturated air layer sitting just above the stomata and so speeds transpiration up, but very strong wind instead triggers stomatal closure and can actually cut water loss), and atmospheric pressure (transpiration increases as pressure falls, which is why hilly regions - despite their naturally lower atmospheric pressure favouring faster transpiration - often see this effect partly cancelled out by their generally cooler temperatures). Internal or plant factors include leaf area (a plant with more leaf surface transpires faster, which is exactly why many xerophytes evolve reduced leaf size) and leaf structure, where anatomical features such as sunken stomata, protective surface hairs, a thickened cuticle, and hydrophilic mucilage substances all help cut down water loss - taken to an extreme in some xerophytes, which dispense with true leaves altogether in favour of a flattened, leaf-like stem called a phylloclade (as in Opuntia), a limited-growth modified stem called a cladode or cladophyll (as in Asparagus), or a flattened, widened petiole called a phyllode (as in Acacia melanoxylon). When transpiration consistently outpaces water uptake, a plant passes through three recognisable stages of water stress: incipient wilting, where cell water content has already dropped but n …