A recording porometer with detachable cups operating on four separate leaves

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Abstract
A new form of recording resistance porometer is described which enables records of stomatal behaviour to be obtained concurrently from four different leaves; the porometer cups are auto-matically detached between readings, exposing the leaf surfaces to the ambient air in order to avoid artifacts due to changes in carbon dioxide concentration. The readings are recorded for the four leaves seriatim, the cup for each remaining attached for 3 min. out of every 1/2 h. The results of several of the preliminary experiments carried out to test the new instrument are presented: (1) Continuous records for periods up to 6 days appear to have no serious effect on the functioning of the stomata under normal conditions. With Taraxacum officinale slightly greater stomatal opening was recorded on the fifth day than on the first; for wheat a slight falling off occurred on the third and fourth days, possibly due to ageing of the leaves. (2) The stomata of similar leaves on different plants which have been subjected throughout to the same sequence of conditions show remarkable uniformity of behaviour at any one time; a great increase in the precision of comparisons of experimental treatments should thus be achieved by applying them to such leaves and recording the results concurrently. (3) Experiments with plants of T. officinale, wheat and Xanthium pensylvanicum subjected to increasing water strain have shown a marked closing response to the process of taking a reading (which involves forcing air through the leaf). This closing response is first seen some time before visible wilting occurs and for X. pensylvanicum has been shown to be mainly if not entirely due to a greatly increased sensitivity to carbon dioxide. The biological implications of this are discussed. The stomata (e.g. of T. officinale) may not re-open fully for 2 or 3 days after re-watering, when the plant is recovering from water strain, but the enhanced sensi-tivity to readings is only shown while water is actually in short supply, and not during the period of recovery.

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