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Effects of Leaf Aging Upon Stomatal Resistance in Bean Plants¹

The effect of leaf age on leaf resistance to gaseous diffusion was separated from three factors inherently related to leaf age: (a) progressive shading of lower leaves, (b) plant development, and (c) changes in stomatal size and density. Bush beans (Phaseolus vulgaris L., ‘Contender’) were grown and studied in an environmental chamber under constant conditions. Measurements of leaf resistance, light level, and stomatal density were made throughout the life of the plant (10 to 60 days from plant emergence). It was found that in a fully expanded leaf, exposed to constant light, the diffusive resistance of the whole leaf increased continuously with age until leaf abscission. The same was found true for leaves that were continuously shaded at 15% of the full light level (1,150 µ;einsteins m^-2 s^-l), but the resistances were higher. The rate of change in leaf resistance with time varied among the leaves within a plant, but the differences were eliminated when the time scale was expressed as the fractional lifespan of each leaf.

Further, the effects of correlated factors such as shading, stage of plant development, and stomatal densities were negligible; and the leaf age effect occurred not only during the latter stages of senescence, when the leaf was becoming chlorotic or necrotic, but as a continuous process, beginning at leaf maturity and continuing until leaf abscission.

Key Words: Leaf resistance • Stomatal density • Stomatal size • Light level • Porometer • Xeromorphism • Phaseolus vulgaris

² Former graduate research assistant, Dep. of Biology, professor and associate professor, Dep. of Soil and Crop Sciences, Texas Agric. Exp. Stn., Texas A&M Univ. Present affiliation of senior author: Pepperdine Univ., Natural Science Division, Malibu, CA 90625.

Received for publication September 7, 1976.