Stomatal and Cuticular Transpiration

 

Throughout the period of growth, plant transpiration at the leaf level happens in a proportion of about 90 % through the stomata and 10 % through the cuticle. Stomata are found on the epidermis of unsuberized leafs and stems.

Stomatal Transpiration

Stoma represent special pores in the epidermis of leaves and other organs which are surrounded by two guard cells with a special shape (specialized parenchymal cells).

Fig. 1 . A stoma (photon microscope image)

and are the basic structures participating in water removal. Guard cells are characterized by:

• a prolonged shape in monocotyledonous plants and a semi oval shape in dicotyledonous plants;
• a thickened inner cell wall;
• chloroplasts containing chlorophyll.

Mature leaves can contain between 50 and 500 stomata. The majority of cultivated plants with the horizontal positioning of the leaves contain stomata only on the bottom side of the leaf and are called hipostomatic. Plants with leaves positioned more or less vertically contain stomata on both sides of the leaf and are called amphystomatic.

Aquatic plants containing stomata on their top parts, are called hyperstomatic. In most of the plants cultivated in the temperate zones, guard cells are located in the leaf epidermis. Plants growing in wet zones have the stomata exposed on cells located outside of the epidermis to stimulate transpiration.

In drought-resistant plants stomata are buried in the leaf mesophyll. On average 10,000 stomata can be
found on 1 cm2, making up 1–2 % of the foliar surface. Transpiration of branches and stems is very reduced compared to that of the leaves and happens at the level of lenticels.

Lenticels correspond to a lack of cohesion between the suberized cells or to an absence of suberization. They make only up to 2 % of the suberized surface (see Chap. 9—Elimination of substances in plants). Plants can also regulate transpiration intensity and the volume of eliminated water.

This regulation is carried out by the alteration of guard cell shape that contributes to opening and closing of the stomatal pore (the osteole). The mechanism of such changes in shape is based on cell turgidity variation. The following types of deformations can be distinguished in stomata (Figs. 3.7
and 3.8):

Fig. 3.7 Physiological deformations of guard cells Fig. 3.8 Different states of the stoma

Passive, conditioned by changes in the turgidity state occurring in neighboring cells surrounding the guard cells;

• Active, caused by turgidity state changes taking place directly in the guard cells. Passive movements are hydropassive (those of opening or closing stomatal pores), they are determined by changes in water content. Active movements can be both hydroactive (those of opening and closing), dependent on water content and photoactive (also of opening and closing) caused by light.
• Hydro-passive closure of stomata is related to the mechanical pressure exerted by the neighboring cells of the epidermis under full turgidity. Hydropassive opening occurs when this pressure is released during weak water deficit conditions.
• Hydro-active opening and closure of the stomata occur when transpiration levels become greater than water absorption by the roots and when the decrease in guard cell turgidity reaches critical levels. The photo active opening and closure is caused mainly by the photosynthesis process occurring in guard cells.
• Internal and external environmental factors influence directly or indirectly the process of transpiration by affecting the turgidity of guard cells, which, therefore, lead to closing or opening osteoles.
• Osteole opening and closure depend on humidity, CO2 concentration in the atmosphere and in the intercellular space, on the ionic balance in guard cells, on the phytohormone amount (cytokinin stimulates osteole opening and abscisic acid—its closure). The degree of water supply, the deficit in relative humidity of the atmosphere, the wind speed etc. are important factors as well.

Cuticular Transpiration

The cuticle is a barrier to water loss, because it is rich in hydrophobic substances, derived from the oxidation of fatty acids, and is less permeable for water. Its permeability varies considerably depending on the age and thickness of the leaf. The cuticle may also contain microscopic perforations or less hydrophobic areas, where water vapor diffusion is possible.

Slight transpiration can also take place through thin cuticles. This type of transpiration is prevalent in young plants (50 %), which have not developed yet a thick cuticle, but it can also happen in mature plants during senescence (40 %), when the epidermis and the cuticle of the leaf begin to deteriorate.

During vegetation the rate of cuticular transpiration does not exceed 10 % of the total transpiration. During winter, transpiration and water elimination from reproductive organs and buds happens through the cuticle.

The intensity of cuticular transpiration is determined by the structure and the thickness of the cuticle and is 10–20 times lower than the intensity of transpiration through stomatal pores. For example, it is very insignificant in conifers and magnolia, which have a very thick and rigid cuticle. It is also less important in the life of cultivated plants.