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Photosynthesis and Gas Diffusion in Leaves of Selected Crop Plants Exposed to Ultraviolet-B Radiation¹

ABSTRACT

This research was conducted as part of the federal interagency Biological and Climatic Effects Research (BACER) Program established to corroborate research findings related to UV-B radiation effects on plants and to expand the available data base.

Ultraviolet B (UV-B, 280–320 nm) radiation effects on gas diffusion through the upper (more exposed) vs. lower leaf surfaces of two snap bean varieties [Phaseolus vulgaris L. ‘Bush Blue Lake 290’ (BBL 290) and ‘Astro’] were compared. The bean plants were grown under greenhouse and growth chamber conditions, and irradiated for 4 weeks (6 hours/day) to 10 mW m^–2 of biologically effective UV (BEUV) radiation. Diffusive resistances of the upper (r_u) and lower (r_l) surfaces were determined at semi-weekly intervals. A distribution of mean r_u/r_l ratios vs. total leaf resistance (R) was plotted. Ultraviolet-B treatment appeared to increase r_u/r_l ratios only in BBL 290 leaves with R < 2 sec cm^–1 and R > 6 sec cm^–1. Chamber-grown plants were more susceptible to UV-B-induced visible injury than were greenhouse plants.

In addition, selected soybean [Glycine max (L.) Merr. ‘York’], cotton [Gossypium hirsutum L. ‘Gregg’], cucumber [Cucumis sativus L. ‘Poinsett’], clover [Trifolium pratense L. ‘Pennscott’], and wheat [Triticum aestivum L. ‘Monon’] varieties as well as BBL 290 snap bean were given extended greenhouse exposures ranging from 2 to 6 weeks (6 hours day^–1 or 24 hours day^–1) to 5–25 mW m^–2 BEUV. Foliar effects on CO₂ exchange rates (CER) of leaves irradiated throughout the expansion stage and on whole plant biomass production were compared with exposures producing visible injury. Threshold exposure doses required to depress CER and reduce plant dry matter correlated closely with those causing incipient visible injury.

Key Words: stomatal resistance • CO₂ exchange • stress physiology

¹ Contribution of the USDA-SEA-AR, Beltsville Agric. Res. Ctr., Beltsviile, Md.

² Plant Physiologist, Plant Stress Lab., Plant Physiology Inst., USDA-SEA, BARC, Beltsville, MD 20705.

Received for publication January 22, 1980.