However, under drought stress with stomata closed, the survival of plants depends on the barrier properties of the cuticle, which effectively control water loss. Under favourable environmental conditions, 94вЂ“99.7% of the gas exchange occurs through stomatal pores ( KГ¶rner, 1994). On leaf surfaces carrying stomata, the cuticle covers the outer epidermal cell walls and periclinal walls of guard cells, and it extends down the ventral walls forming the stomatal pore to the substomatal cavity ( Appleby and Davies, 1983 a Wullschleger and Oosterhuis, 1989 Osborn and Taylor, 1990 Pesacreta and Hasenstein, 1999). In hypostomatous leaves the upper (adaxial) surface is covered with a continuous cuticle. This dilemma between carbon uptake and water loss was solved by the evolution of stomata, often located only on the shaded lower (abaxial) leaf side.

However, a reduced permeability of the leaf surface for water simultaneously reduced CO 2 uptake.

The development of the вЂwaterproofвЂ™ cuticle was a prerequisite for the colonization of dry land, because the permanent water deficit in the atmosphere causes a constant loss of water from the plant surface into the atmosphere. The plant cuticle forms the outermost layer of leaves and fruits, separating the living cells from the atmosphere. Received 3 December 2003 Accepted 18 February 2004 Introduction The significantly higher permeability of the ivy cuticle on the lower stomatous leaf surface compared with the astomatous surface might result from lateral heterogeneity in permeability of the cuticle covering normal epidermal cells compared with the cuticle covering the stomatal cell surface.Д¬uticular transpiration, diffusion, helium, Knudsen diffusion, plant cuticle, pores, stomatal transpiration. However, mobility of 14CвЂђ2,4вЂђDB in reconstituted wax isolated from the lower leaf surface was 2.6 times lower compared with the upper leaf side.

In order to check whether this difference in resistance was due to different barrier properties of cuticular waxes on both leaf sides, mobility of 14CвЂђlabelled 2,4вЂђdichlorophenoxyвЂђbutyric acid ( 14CвЂђ2,4вЂђDB) in reconstituted cuticular wax isolated from both leaf surfaces was measured separately. Resistance to flux of water through the solid cuticle on the stomatous leaf surface was about 11 times lower than cuticular resistance on the astomatous leaf surface, indicating pronounced differences in barrier properties between cuticles isolated from both leaf surfaces.

As expected, water flux across the cuticle isolated from the astomatous leaf surface was not affected by the gas composition since there are no gasвЂђfilled pores. This approach allowed the flow of water across stomatal pores (вЂstomatal transpirationвЂ™) to be calculated separately from the flow across the cuticle (cuticular transpiration) on the stomatous leaf surface. This approach is based on the principle that the diffusivity of water vapour in the gas phase can be manipulated by using different gases (helium, nitrogen, or carbon dioxide) while diffusivity of water in the solid phase is not affected. Transpiration of cuticular membranes isolated from the lower stomatous surface of Hedera helix (ivy) leaves was measured using a novel approach which allowed a distinction to be made between gas phase diffusion (through stomatal pores) and solid phase diffusion (transport through the polymer matrix membrane and cuticular waxes) of water molecules.

Heli x crack simulator