Boundary layer, stomatal geometry and -spacing, in relation to drought tolerance in four Nicotiana tabacum L. cultivars

Abstract

Although numerous studies, both phenomenological and mathematical, have related gaseous interchanges at the plant leaf surface and atmospheric interface specifically with regard to drought tolerance, the important role of trichomes and the boundary layer has not been recognized. In this investigation the focus was restricted to the role of the boundary layer along with stomatal geometry and -spacing in determining the diffusional resistance to CO₂ and H₂O in four Nicotiana tabacum L. cultivars of different, but known, drought tolerance. Resistances were expressed both in terms of resistance components on a unit leaf area basis and in terms of dimensionless quantities. The abaxial leaf surface had the highest trichome frequency, more so in the drought tolerant than in the drought sensitive cultivars, being 9.7% and 6.9% higher than on the adaxial surface for TL33 and CDL28, as opposed to being 21.2% and 26.8% higher for GS46 and Elsoma, respectively. The frequency of the long glandular trichomes of epidermal cells, and consequently also the calculated stomatal index, was significantly (p < 0.01) higher on the adaxial surface of the drought tolerant cultivars. Both the fractional cavity and pore contributions to the overall entry resistance were higher for the drought tolerant cultivars, being 24.1 % (H₂O) and 20.6% (CO₂) for the stomatal cavity, and 9.5% (H₂O) and 7.4% (CO₂) for the stomatal pore contributions, respectively. The fractional contribution of the boundary layer was, however, on average 3.4% (H₂O) and 6.5% (CO₂) higher for the drought sensitive cultivars. From the results presented it is concluded that plant fitness for tolerating drought in this mesophytic crop plant may be maximized not by minimizing water loss or by maximizing the photosynthetic to transpiration ratio, but by maximizing the carbon gaining capacity of the leaf.