Enumerated below are the environmental factors affecting transpiration in plants. These factors are described as "environmental" to distinguish them from those genetic factors which are inherent in the plants. They are also described as external factors, that is, outside of or external to plants. Specifically they are climatic factors, that is, they are elements of the climate.
These factors are the same climatic factors which can either promote or inhibit plant growth and development and, eventually, crop productivity.
It is important in crop agriculture to be able to familiarize with these environmental factors affecting transpiration in order to be guided on the proper timing of farm activities. Likewise, it should be essential in devising strategies to reduce evapo-transpirational water loss or otherwise balance transpiration rates with water absorption and dry matter production. The ultimate objective is to achieve a net surplus of biomass, that is, increased crop production.
Thus, for example, transplanting late in the afternoon and providing artificial shade to newly transplanted seedlings in open sun are sound practices to improve plant survival. A stable and abundant water supply is necessary to maximize production in crops grown under conditions that favor rapid evaporation. Otherwise, a strategy has to be effected, like using plastic mulches, to minimize excessive water loss from the soil without curtailing plant exposure to sun.
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In general, plants transpire fastest under the following climatic conditions: (a) bright day, (b) dry air, (c) moist soil, (d) warm temperature, and (e) windy day.
The various environmental factors affecting transpiration, particularly the rate of transpiration, are briefly discussed below. They operate in influencing transpiration, however, interconnectedly.
1. Light. Light, specifically light intensity, is probably the most obvious among the environmental factors affecting transpiration in plants. It has a controlling effect on the opening of the stoma through which water primarily escapes in gaseous state. In general, transpiration rate is high during daytime, particularly when light is bright, than during night time.
The stomata are typically open during daytime,
allowing the entry of CO2 and the exit of O2. The opening of the stomata
likewise enables the escape of water as water vapor in the process of stomatal
transpiration. Except in CAM plants, the stomata are close at darkness between
sunset to sunrise.
2. Relative Humidity. This environmental factor affects transpiration by regulating stomatal movement and atmospheric demand. At high RH (moist air), the stoma tends to close and thus limit the exit of water vapor from the plant. Further, high RH means that the water-potential gradient (also water vapor concentration and vapor pressure gradient) from plant to the atmosphere will be minimal compared to when RH is low. In addition, at high RH the atmosphere contains more water and has low atmospheric demand, meaning that it has limited capacity to absorb more water.
At 50% relative humidity at a temperature of 20°C, water potential Ψw of the atmospher is -93.5 MPa but at 90% RH, water potential will be -14.2 MPa. On the other hand, the typical water potential of the leaves of a small tree that grows with sufficient soil moisture will be -1.5MPa (Moore et al. 2003). In both RH, transpiration occurs whereby water vapor moves outward from higher to lower water potential or from less negative to more negative water potential values, i.e., from Ψw= -1.5MPa to Ψw= -93.5 MPa (at 50% RH) and from Ψw= -1.5MPa to Ψw= -14.2 MPa (at 90% RH).
Transpiration rate will be faster at 50% than at 90% RH. At 50% RH, the water potential gradient is more steeper (93.5 MPa – 1.5 MPa = 92 MPa) compared to 90% RH (14.2 MPa - 1.5 MPa = 12.7 MPa). Low RH also favors faster transpiration due to stronger atmospheric demand.
But as long as the stomata are open, transpiration occurs, even at saturated condition of 100% RH. In this case the expelled water vapor readily condenses (Hopkins 1995).
3. Temperature. The rate of transpiration is fastest when air temperature is between 20°C to 30°C (Moore et al. 2013) . At these temperatures the stomatal apertures or opening are generally widest. In general, the stomata close at temperatures about 0°C and progressively increase in aperture up to about 30°C (Devlin 1975).
In addition, there are fleshy or relatively thick leaves, stems and fruits which, upon exposure to sunlight, results to internal temperatures which exceed that of sorrounding air. The difference may reach 10°C , according to Moore et al. (2013). This wide difference in temperature would result to a steeper water potential (and vapor pressure) gradient between the plant organ and the external environment. Consequently, it will favor rapid rate of transpiration.
Temperature as an environmental factor affecting transpiration also relates to water potential and relative humidity. The relationship of temperature (T, in °K), relative humidity (RH, in %) and water potential (Ψw, in pascal) is shown in the following mathematical equation provided by Hopkins (1995):
Ψw = 1.06 T log(RH/100)
Applying the equation, an increase in temperature will decrease water potential. Thus at the same RH of 50%, Ψw at temperature of 20°C or 293.15°K is -93.5 MPa but lower (more negative, Ψw = -96.7 MPa ) at higher temperature of 30°C. Increase in atmospheric temperature will therefore steepen further the plant-air water potential gradient.
4. Soil Water. Where the supply of water from the soil is limiting, the rate of transpiration tends to slow down. This is more pronounced where other conditions, such as bright light and warm temperature, favor escape of water from the plant. In this case, water deficit within the plant may occur leading to the closing of the stomata which is manifested by wilting of leaves.
Conversely, the entry of water into the guard cells opens the stoma. This allows the passage of water in the process of stomatal transpiration.
5. Air. Also one of the environmental factors affecting transpiration is air, particularly atmospheric CO2 concentration. In an update (posted July 15, 2013 in this site's blog), researchers found that high concentration of CO2 in the atmosphere (now 400 ppm) enhances photosynthesis while limiting the extent or duration of stomatal opening. As a result, trees had reduced rate of transpiration, more efficient use of water, and faster growth. Commercial growers have in fact practiced CO2 fertilization by supplying this gas into the air in greenhouses to induce more rapid growth (http://www.sciencedaily.com/releases/2013/07/130710141845.htm).
6. Wind. This environmental factor affects transpiration by removing that thin moist layer of air, called boundary layer, which lies next to the surface of a leaf. This moist air causes a lesser water potential gradient from the leaf resulting to reduced rate of transpiration. This layer also reduces light penetration into the leaf. But with wind, this boundary layer is replaced with drier air thus increasing water potential gradient and enhancing transpiration (Moore et al. 2003). However, strong wind may cause excessive loss of water from leaves leading to stomatal closure.
(Ben G. Bareja 2013, edited Apr. 25, 2019)