Gibberellin increase auxin

That gibberellin alone would produce normal cell division and enlargement in potato buds remains to be seen, as does a clear demonstration that the rise in level of endogenous gibberellins actually precedes sprouting. Whether the increase in gibberellin precedes the reported increase in auxin as rest terminates is also of... 

Although in the latter case gibberellin appears to be primarily responsible for the increased mitotic activity and auxin for differentiation of the newly formed cells, auxin may also be involved in the primary cambial activation mechanism (25). After the discovery of the gibberellins several studies showed conclusively that auxin and gibberellin have separate, sometimes additive and sometimes syner-... 

In reproductive growth, BR has been shown to promote the growth of pollen tubes in vitro, at a concentration an order of magnitude below those of auxin or gibberellin (41). Abe (42) has recently reported correlative changes in the levels of BRs in lily pollen as it matures, with increases in two of the more active BRs. Taken together with the response of pollen tubes in vitro (41), his data suggest active BRs are present in mature pollen in adequate amounts to contribute to the control of pollen tube extension in vivo. 

Another important factor of hormone-hormone interaction is the hormonal control of hormonal movement and polarity. Thus, various cytokinins have been reported to increase the polar movement of indole acetic acid and vice versa. Gibberellin treatment was shown also under cer- bain circumstances to increase the basipetal auxin movement. On the other hand, abscisic acid decreases the auxin movement and ethylene, according to some evidence, the gibberellin movement, perhaps through a promotion of conjugation of the auxins with aspartate and of the gibberellins with glucose (cf. Eef. 27). 

Effects on Gibberellins. Exposure to cold enhances subsequent GA biosynthesis in certain seeds (Ross and Bradbeer 1971). The increase appears to arise from de novo GA biosynthesis and cannot be inhibited by concentrations of ABA which inhibit germination (Arias et al. 1976) and suppress apparent GA biosynthesis in maize shoots (Wareing et al. 1968). Cold increases GA in potato tubers (Thomas and Wurr 1976), while low temperature during tuber germination increases both GA and auxin activity in leaves of potato (Romanova et al. 1975). In corn roots, on the other hand gibberellin production or export is severely inhibited by prolonged exposure to cold (Atkin et al. 1973). 

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