Studies in Stomatal Behaviour

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Abstract
Xanthium pennsylvanicum exhibited a small stomatal opening (‘night opening’) towards the end of a long night at 27° C. Experiments are described in which a temperature increase from 27° to 36°, given during the period of night opening, caused the stomata to open widely for several hours. The degree of opening first achieved was comparable with that observed in light of 1,000 lux, but high temperature was less efficient than light for maintaining opening. Openmg was greater in mature than in young leaves. The opening did not appear to be due to water strain under the high temperature. It was found that temperature-induced opening was much greater after a long night (16 hours) than after a short night (four hours). From this it is deduced that the opening is affected by the endogenous rhythm which occurs in darkness (this was studied in previous work). The results obtained here contrast sharply with some obtained previously (and confirmed here) in which different temperatures were given throughout the night. The differences can probably be explained in terms of temperature effects on the endogenous rhythm. The stomata retained their normal responses to carbon dioxide and carbon dioxide-free air during temperature-induced opening. Thus the opening must occur in spite of any temperature stimulation of respiration. An experiment on the effect of temperature on stomatal closure in response to darkness is also described. Closure was significantly slower at 36° than at 27° C.

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