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Effect of Windbreaks and Soil Water Potential on Stomatal Diffusion Resistance and Photosynthetic Rate of Sugar Beets (Beta vulgaris)¹

The stomatal resistance (r.) to diffusion of water vapor and CO₂ through leaves of sugar beets grown in well irrigated open plots and plots sheltered by windbreaks was calculated from measurements of stomatal density and aperture.

Mean daily stomatal resistance in both sheltered and exposed situations increased markedly as the soil water potential decreased from –035 to –0.52 bars. As potential decreased, increases in the stomatal resistance occurred earlier in the day. Superimposed upon this increase in resistance were cyclical changes which were attributed to the readjustment of stomatal apertures necessary for the maintenance of a balance between water uptake and loss. The results suggest that, even under well-irrigated conditions, climatic stresses characteristic of the Plains environment may induce partial slomatal closure during afternoon hours on many days.

The mean stomatal resistances were found to be independent of CO₂ concentration, air temperature, water vapor pressure, and wind speed, except in an extreme cise where the passage of a front caused high winds and the influx of dry air. In this case, the stomatal resistance increased sharply.

Mid-day values of stomatal resistance averaged 1.4 sec cm^–1. The mean difference in stomatal resistance between the open and sheltered plots can explain a 6% decrease in the resistance to CO₂ flux and therefore a 6% increase in photosynthetic rate in shelter. The extreme observed difference in r. can explain a 26% increase in photosynthetic rate in shelter.

These results occurred under conditions of ample litigation. The data suggest that photosynthetic rate should be even more strongly favored where plants grown on dryland are sheltered from wind.

Key Words: Diurnal stomatal activity

² Instructor and Professor, Agricultural Climatology, Department, of Horticulture and Forestry, University of Nebraska, Lincoln, Nebraska 68503.

Received for publication May 10, 1969.