Development of the Stem Tuber in Kohlrabi

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Abstract
Stem swelling in kohlrabi (KR) normally occurred between the third and fifth node and followed initiation of meristematic activity in pith parenchyma cells; this usually began about 7 weeks after germination and was associated with the death of parenchyma cells in the centre of the pith lower down the stem. Cell division began just above this region, at about the third node and zones of dividing cells extended upwards and laterally, giving rise to more centres of active division. Carbohydrate analyses showed that fructose, glucose, starch, and an unidentified sugar accumulated in the stem. Melezitose appeared in the stem and raffinose in the leaves only after visible bulbing had occurred. Sucrose content fluctuated in both leaves and stems and in the latter was positively correlated with hours of sunshine on the day before sampling. The contents of gibberellin-like substances in the leaves of KR were compared at two stages of development with those of marrowstem kale (MSK). Gibberellins were present in both acidic and neutral fractions in KR tissues and the amounts were similar to those in MSK. Stem swelling in KR could not be attributed to a gross deficiency of gibberellins. Inter- and intra-varietal approach-grafts were made with KR and MSK plants. With KR as ‘scion’ partner, stem elongation of the scion was not greatly affected by the nature of the stock, but stem swelling was much reduced on MSK stocks. Stem swelling in MSK scions was greater on KR stocks than on MSK. Retention of stock leaves and buds above the union tended to inhibit elongation of the scion partner and also inhibited stem swelling in KR scions on MSK stocks. Application of gibberellic acid to leaves of MSK stocks from which the terminal bud had been removed, increased both stem elongation and swelling in KR scions. KR scions top-grafted on defoliated MSK stocks showed vigorous stem elongation without swelling. MSK scions top-grafted on KR stocks showed reduced stem elongation with abnormal stem thickening. It is suggested that high auxin levels may prevent bulbing in KR and that gibberellin-like substances may facilitate both stem elongation and bulbing when auxin levels are low. Applying gibberellic acid (GA3) or indole-3-acetic acid (IAA) alone or in combination, to young intact plants of KR increased stem height and checked stem swelling. Application of benzyladenine (BA) at 10−4 M accelerated swelling and checked stem elongation. GA3 applied with BA (10−4 M) increased both stem height and girth, both with or without added IAA. It was concluded that the requisite conditions for stem swelling included the presence of a kinin2 and a low level of auxin. High levels of auxin prevented stem swelling, whether applied as IAA to young intact plants or as natural auxins from leaves and buds of MSK plants grafted to KR scions. Although applied gibberellins usually tended to prevent swelling, native gibbereltins appeard to play a minor role in controlling its initiation which probably occurs when the auxin/kinin ratio in the plant reaches a critical low level.

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