Studies in the Physiology of the Onion Plant
- 1 January 1965
- journal article
- research article
- Published by Oxford University Press (OUP) in Journal of Experimental Botany
- Vol. 16 (1) , 128-144
- https://doi.org/10.1093/jxb/16.1.128
Abstract
Bulb development in the onion plant, which occurs in response to the stimulus of long days, has two main morphological features: swelling of the base of the pseudostem and the formation of scales from leaf initials produced at the stem apex. The swelling may be expressed as the ‘bulbing ratio’, i.e. the ratio of the maximum diameter at the base to the minimum at the neck, but this is a somewhat insensitive measure wider adverse light conditions and no ratio can be specified at which bulbing is first apparent. In the absence of bulbing, leaf initials have at all stages of development a blade longer than the leaf sheath, giving a ‘leaf ratio’ of blade/sheath at all times greater than unity. In scale formation, when bulb development begins, the sheath of a small initial starts to grow sooner than in foliage leaf formation and blade growth is suppressed; the leaf ratio therefore falls rapidly below unity and this is diagnostic of bulbing. Characteristic graphs of leaf ratio plotted against leaf number from the apex demonstrate the stage of bulbing reached and a fall in leaf ratio from near the apex in such a graph can indicate the beginning of bulbing before any actual scales (with ratios less than unity) have been formed. In normally grown plants, differentiation into foliage leaves or scales appears to take place when the initials reach about 1 mm in length. When the incidence of bulbing within a treatment is very variable, difficulties arise in averaging the leaf ratio curves for the different plants. Mean number of scales per plant, or mean minimum leaf ratio, may then be used as measures of bulbing. Heavy shading in winter was found to delay but not prevent the bulbing of those plants which survived. ‘Buds’, consisting of a portion of stem with the apex surrounded by leaf initials, but no scales, were dissected out from onion sets and cultured aseptically in tubes. Limited growth was obtained and rooting occurred in some cases. In the presence of sucrose, swelling of leaf sheaths and reduction of the leaf ratios took place; these responses increased with sucrose concentration. Glucose similarly stimulated bulbing but high osmotic pressure caused by mannitol or mineral nutrients had no effect. Rooting appeared to reduce the degree of bulbing, possibly by competing for carbohydrate supply. The bulb development in these sterile cultures was most unusual in occurring in short photoperiods and the possibility of a residue of ‘bulbing hormone’ being stored in sets is discussed.This publication has 2 references indexed in Scilit:
- Studies in the Physiology of the Onion Plant: V. AN INVESTIGATION INTO THE GROWTH SUBSTANCE CONTENT OF BULBING ONIONSJournal of Experimental Botany, 1962
- Studies in the physiology of the onion plant: II. Inflorescence initiation and development, and other changes in the internal morphology of onion sets, as influenced by temperature and day lengthAnnals of Applied Biology, 1944