ROLE OF BOUNDARY LAYER RESISTANCE AND WALL ULTRASTRUCTURE IN DETERMINING DIFFERENTIAL DROUGHT TOLERANCE IN TOBACCO

Abstract

ROLE OF BOUNDARY LAYER RESISTANCE AND WALL ULTRASTRUCTURE IN DETERMINING DIFFERENTIAL DROUGHT TOLERANCE IN TOBACCO. A holistic approach was followed to propose a new model for the leaf surface of tobacco to explain epidermal wall ultrastructure and permeability with the emphasis on drought stress. Nicotiana tabacum L. leaves of two drought sensitive and two drought tolerant cultivars were used. A triethylene glycol liquid substitution method together with a Flexible Image Process System was found excellent for studying the external micro-morphology of the leaf surface. Differences were found to exist between the leaf micro-morphology parameters quantified for the drought sensitive and drought tolerant cultivars. From these results, it is concluded that plant fitness for tolerating drought stress may be maximised by the carbon gaining capacity of the leaf. Contrary to most permeability studies, freshly harvested epidermal wall strips, with stomata and trichomes, were used. Results are discussed in terms of the limitations when using composite membranes with stomata and trichomes. The differential permeability coefficient values calculated for the transport of water in tobacco epidermal wall strips agreed with those reported for isolated cuticular membranes of various other species. The epidermal wall permeability coefficient seems to have merit as selection criterion for identifying drought tolerant genotypes. The cuticular membrane is not an amorphous layer but consists of small globules (referred to as nitis globules) which group together in the larger globules (referred to as nichrista globules). The tobacco epidermal cell wall has five layers consisting of a matrix and microfibrils with diameters of 4 - 10 nm. The fibrils of the cuticular membrane are possibly a pathway for water, wax and cutin transport. The possibility of resistance factors, other than the soluble cuticular lipids, contributing to water permeability during drought stress, was investigated.