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1. On the basis of new theoretical considerations developed by us a new and reliable method has been elaborated for the rapid determination of water deficit in plant cultures. 2. In paper-chromatograms of plants optimally supplied with water free proline is shown to be present in very small amounts, usually only as traces. Comparative experiments showed that in cases of water shortage the proline content of the leaves of the plant strikingly increases. This commonly occurs in every phase of development of mono- and dicotyledons. 3. Drought-resistant plants synthetize more proline than varieties less resistant to water deficit when equally water deficient. 4. By a method which does not require chromatography the procedure of tests has been accelerated so that leaf extract containing amino acid mixture was dropped onto a chromatogram paper and immediately treated with isatine. In this test the effect of slight, permanent or medium water deficit upon proline content is so great that extracts react with isatine to give an intense dark blue colour which suppresses the reddish-brown colour of other amino acids. 5. To avoid the necessity of preparing isatine reagent frequently, because it loses its amino acid sensitivity very rapidly, a new isatine solution has been formulated. Our isatine-paper indicator prepared with this solution keeps its reactivity with amino acids 5 to 6 months. 6. An unusually high proline concentration occurs also if the salt content of irrigation water or also of the soil is high. The increase of the proline level indicates therefore physiological dryness. [ABSTRACT FROM AUTHOR]