CHLOROPLASTS IN THE STOMATA OFALLIUM CEPAL.

Abstract

The stomata of Allium cepa are fully motile (Loftfield, I921) and exhibit a genuine response to light and, by inference, to changes in carbon dioxide tension (Heath, I952). They are, however, completely starch-free, as demonstrated by a number of investigators from Parkin (I899) to Heath. The former, having examined both alcohol fixed and living epidermis of some eight species of onion, including A. cepa and A. schoenoprasum, reported: 'No chloroplasts could be recognized, nor could I certainly detect any green colour in the guard cells.' He noted, however, that refractive granules were sometimes present. Heath also was unable to recognize chloroplasts in the stomata of A. cepa (var. Ebenezer) and obtained negative results with certain microchemical tests for chlorophyll (saturated KOH and the phaeophytin test: Molisch, I92I), but did not refer to the Molisch reaction (reduction of AgNO3 by chloroplasts: Molisch, I9I8). It may be noted that Sayre (I926) found that the stomatal plastids of Rumex patientia, though obviously green, did not give the microchemical tests (sic) for chlorophyll, a result he regarded as inconclusive in view of the small amount of pigment present. Apparently neither Heath nor Sayre made use of the fluorescence microscope, with which the stomatal plastids of a number of species have been found to exhibit the red fluorescence characteristic of chlorophyll. Moreover, Heath was apparently not aware that chloroplasts have been reported to occur in onion guard-cells. In I927 Linsbauer wrote of A. cepa (pp. 53I-2): 'Die Chloroplasten, 20-25 an der Zahl, liegen zumeist der Riickenwand an und sind auffallend klein und bleich. Nichtsdestoweniger geben sie die Molische Chlorophyll Reaktionen und reduzieren AgNO3.' In a footnote he added that the guard-cell chloroplasts showed typical fluorescence under the fluorescence microscope, commenting that Sayre's observations, recorded above, certainly did not apply to Allium. Again, Hofler (I939), in a study of silver nitrate reduction by guard-cell plastids, obtained positive results with A. cepa and A. schoenoprasum (cf. Parkin, above) as well as with Vicia faba, Helianthus, Tradescantia, Galium and Pelargonium zonale. Hofler and also Linsbauer (1926, for Chrysanthemum) observed that localized blackening of the plastids was accompanied by a general reduction (browning) of silver nitrate throughout the protoplast in open but not in closed stomata, suggesting an increase in reducing substances during opening of 'starch' and 'starch-free' stomata alike. Unfortunately, Hofler presented no experimental data for Allium, merely stating that the results obtained were similar to those for Vicia. It is clear, none the less, that the results of Linsbauer and Hofler are not in accord with those of Parkin and Heath. Since the point at issue has a critical bearing on stomatal