Responses of stomata to environmental factors-experiments with isolated epidermal strips of Polypodium vulgare : II. Leaf bulk water potential, air humidity, and temperature.

Stomatal apertures of isolated and suitably conditioned epidermal strips of Polypodium vulgare are described as the stomata respond to the influences of temperature, air humidity, and water potential at the epidermal inner walls. Water stress as a result of reduced water potential in the substomatal airspace leads to narrower stomatal pores when water potential falls below -8 bar. Water potentials above this threshold value show minor influence. Stomatal responses to such water stress strongly interact with the responses to humidity changes in ambient air and to temperature. The linear dependence of stomatal apertures on the vapor saturation deficit of the air (closing) is shifted to lower values (more closed) by lower leaf bulk water potentials.Stomatal behavior depending on the temperature factor seems to be reversed by higher water stress. Without water stress, rising temperatures between 20 and 28° C are accompanied by further opening of the pores, whereas an increase of temperature within this range leads to narrowing of the stomata under the influence of lower water potentials within the substomatal airspace. It can be demonstrated that stomatal aperture values of Polypodium vulgare depending on temperature always describe optimum curves. With no water stress, closing does not occur before rather high temperatures are reached and above a broad range of maximal opening. Water stress, on the other hand, results in more pronounced narrowing of stomatal pores and shifts the onset to considerably lower temperatures.