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 length

Previous papers have dealt with the effects of five factors on the external aspects of flowering, bulbing and ripening in onions grown from sets and also the production of sets from seed and their storage through the winter; the present communication is concerned with changes in the internal morphology of onion sets and plants from the same experiments. The previously recorded effects on flowering of high and low temperatures are confirmed and further elucidated by data from dissections, which show the changes occurring at the growing point. Thus high temperature throughout the first season's growth from seed to set exerted an after-effect, completely preventing initiation of inflorescences in the following spring. This effect may, however, have been due to the smaller size of the sets produced at high temperature. High temperature for the first 8 of the 22 weeks' experimental storage period produced a similar after-effect in delaying and greatly reducing inflorescence initiation; here there was no question of a size effect being involved. High temperature throughout the 22 weeks not only completely prevented such initiation during treatment, but also practically inhibited it subsequently; when given during the latter part of storage only, it at least delayed initiation of inflorescences. High temperature during the growth from set to mature onion showed a direct effect in suppressing the emergence of inflorescences already initiated in the sets, apart from the effect of bulbing. (Bulb development, which occurs under the stimulus of long days if the temperature is not too. low, also inhibits such emergence.) Inflorescences prevented from emerging, whether by high temperature or bulbing, were displaced by the axillary bud which carried on the further development of the plant. Low temperature for the first 8 weeks of storage exerted an after-effect in reducing the incidence of inflorescence initiation. When cold storage was given for the whole 22 weeks or the last 8 weeks only, initiation was prevented during treatment, but whereas in the former case subsequent initiation was greatly reduced, in the latter a rapid flush of initiation occurred on return of the sets to normal temperature. The effects of high- and low-storage temperatures on formation of new leaf initials were mainly direct; the only after-effect was a tendency for sets from treatments preventing leaf initiation to produce initials more rapidly on return to normal temperatures. (In the case of late cold storage this resembled the effect just noted for inflorescence initiation.) Sets produced at low temperature were prevented from forming leaf initials by continuous high-temperature storage and those produced at high temperature by continuous cold storage; this suggests some degree of acclimatization to low and high temperature respectively. It appears that the 1 to 2 leaf primordia formed at normal temperatures between mid-October and mid-March are initiated mainly during the latter part of that period. The number of swollen bulb scales in an onion set is nearly constant at 3, irrespective of set size. Even the unswollen leaf initials vary little in number over a considerable range of set size, and therefore the better yields given by large sets (if they do not bolt) must be due rather to large size of parts than to high number of leaf initials ready for emergence. The size effect on flowering is likewise not connected with number of leaf initials present in the sets; it may be a matter of the number of emerged leaves in the previous season. There appears to be a minimal total number (12-14) of leaves which must be initiated before an inflorescence can be formed.