transpiration

(redirected from Transpiration ratio)
Also found in: Dictionary, Thesaurus, Encyclopedia.

transpiration

 [trans″pĭ-ra´shun]
discharge of air, vapor, or sweat through the skin.

tran·spi·ra·tion

(tran'spī-rā'shŭn),
Passage of watery vapor through the skin or any membrane.
See also: insensible perspiration.
[trans- + L. spiro, pp. -atus, to breathe]

transpiration

(trăn′spə-rā′shən)
n.
The act or process of transpiring, especially through the stomata of plant tissue or the pores of the skin.

tran′spi·ra′tion·al adj.

tran·spi·ra·tion

(trans'pir-ā'shŭn)
Passage of water vapor through the skin or any membrane.
See also: insensible perspiration
[trans- + L. spiro, pp. -atus, to breathe]

transpiration

the loss of water vapour from the inside of a leaf to the outside atmosphere, via STOMATA and LENTICELS. Transpiration exerts a considerable upward pressure in the stem and is thought to be part of the explanation of how water ascends from roots to leaves.

The rate at which transpiration proceeds depends upon several physical factors:

  1. (a) the Water Vapour Pressure at the MESOPHYLL cell surface inside the leaf. The evaporating surface is saturated and will have a VAPOUR PRESSURE (WVPsatn) that is highly affected by ambient temperature. For example, at 20 °C the WVPsatn = 2.34 kPa, at 10 °C the WVPsatn = 1.23 kPa.
  2. (b) the Water Vapour Pressure in the outside air (WVPa ir ), the maximum value being equal to WVPsatn at that temperature. If there is the same temperature inside and outside the leaf, the rate of flow of water vapour between the surface of the mesophyll cell and the outside is the difference between WVPsatn and WVPair; i.e:WVPdiff= WVPsatn - WVPair Thus the greater the WVPdi ff value, the greater the diffusion gradient and the higher the transpiration rate.
  3. (c) the size and number of stomatal pores per unit area of a leaf. The smaller the pore diameter, the greater the resistance to water vapour diffusion. The presence of'vapour shells’ over each stoma creates a boundary layer of high Water Vapour Pressure in still air, which will slow down the transpiration rate since it increases the WVPa ir value. This effect is most important when the pore diameter is large. In moving air, the vapour shells cannot form and thus transpiration rates increase.
  4. (d) the stomatal and leaf structure, which are modified in some XEROPHYTES to reduce transpiration rates.
  5. (e) a constant supply of water from the roots.
References in periodicals archive ?
Effect of relative soil water content (RSWC) on the normalized transpiration ratio (NTR), stomatal conductance to water vapor ([g.sub.s]) and leaf net C[O.sub.2] assimilation rate ([A.sub.N]).
Adequate levels of nitrogen and phosphorus increase the size and depth of root systems and lower the transpiration ratio. For example, work in Texas showed that unfertilized (nitrogen) sorghum produced a 190-pound yield per inch of water used, while sorghum fertilized with 240 pounds/acre of nitrogen produced a 348-pound yield per inch of water used.
The normalization was done by dividing daily transpiration of individual stressed plants by the mean daily transpiration of the well-watered control plants to produce a daily transpiration ratio. In Exp.