The effects of daminozide on the levels of Indol-3yl-acetic acid and gibberellins in radish (Raphanus sativus L.) in relation to the control of storage root growth

The levels of indol-3yl-acetic acid and gibberellins were determined in shoots and storage roots of radish (Raphanus sativus L.) at various times during the vegetative growth cycle of control plants and plants in which the root to shoot ratio was modified by daminozide treatment. In control plants the onset of storage organ growth was preceded by a change in the hormone root to shoot ratio to favour the root. There was a general reduction in hormone levels in daminozide-treated plants but the pattern of their distribution in roots and shoots was very similar to that in control plants. Thus the effects of daminozide on increased storage root growth cannot be explained in terms of altered root to shoot hormone ratios.     

Synergistic effects of plant growth retardants and IBA on the formation of adventitious roots in hypocotyl cuttings of mung bean

The synergistic effect of plant growth retardants, such as daminozide, paclobutrazol and triadimefon, and of indole-3-butyric acid (IBA) on the formation of adventitious roots in hypocotyl cuttings of mung bean was studied. The three retardants and IBA all stimulated adventitious root growth, but IBA was the most effective. However, mixtures of the retardants with IBA have proven generally more effective than IBA alone in promoting adventitious root formation. When IBA was applied to the hypocotyls one day after cutting preparation followed by the growth retardant on the second day, there were even more adventitious roots produced than if applied in the reverse order. The effectiveness of the treatments were in the order, IBA followed by growth retardant, IBA + growth retardant together, and IBA alone.Abbreviations IBA indole-3-butyric acid - GA gibberellin     

Daminozide and prohexadione have similar modes of action as inhibitors of the late stages of gibberellin metabolism

One of the effects of daminozide is to retard shoot growth in certain plant species, but its mode of action is unclear. Prohexadione, an acylcyclohexanedione, also causes retardation of shoot growth, but it is active in all plant systems tested so far. This inhibition has been shown to be a result of competition with the natural co-substrate, 2-oxoglutarate, at the active site of hydroxylases involved in the later stages of the gibberellin (GA) biosynthesis pathway. In order to determine the mode of action of daminozide in relation to prohexadione, the potencies of the two retardants as inhibitors of 3β-hydroxylation of GAs in cell-free systems from pumpkin ( Cucurbita maxima ) endosperm and French bean ( Phaseolus coccineus ) cotyledons were studied. Several compounds, related to or representing structural links between daminozide and prohexadione, were also included in this investigation. Daminozide was found to inhibit only the bean 3β-hydroxylase to a significant degree, whereas prohexadione inhibited both the bean and pumpkin enzymes.
Further information was obtained from the GC-MS analysis of GAs found in the newly formed parts of the shoots of peanut seedlings treated with the two compounds. Both growth retardants inhibited the formation of GA₁, whereas its immediate precursor, GA₂₀, accumulated. Furthermore, levels of GA₈ (2β-hydroxy GA₁) were also reduced, but by a smaller relative amount.
These results clearly indicate for the first time that daminozide has the same mode of action as prohexadione in distinct plant species, namely to inhibit the 3β-hydroxylase and, to a lesser extent, the 2β-hydroxylase. This is probably due to the structural similarity of the two inhibitors with 2-oxoglutarate.     

Interactions of daminozide, 2, 2′-dipyridyl,and gibberellins A3, A9 and A20, in stem extension inChrysanthemum morifolium Ramat: an indication that daminozide may not inhibit the responses to GA9 or GA20 by blocking gibberellin hydroxylation

Interactions between the growth retardant daminozide (a substituted succinamic acid) and a subsequent application (1 or 10 g) of either gibberellin A₃, A₉ or A₂₀, on stem extension inChrysanthemum morifolium cv. Bright Golden Anne, indicated that pre-treatment of plants with daminozide largely prevented the response to GA₂₀ as well as to GA₉. The daminozide-GA₃ interaction on total stem length was dependent upon the dose of GA₃ such that, by flowering time, 1 g of GA₃ had virtually eliminated the retardant effect, while 10 g of GA₃ increased stem length to a value similar to that achieved by control shoots receiving 10 g of GA₃. In contrast, prior application of 2, 2-dipyridyl (an inhibitor of hydroxylation in some plant and animal systems) had no significant influence on the time courses of response to any of the GAs. In the absence of daminozide (and 2, 2-dipyridyl) all three GAs were very active in promoting internode extension soon after their application. If 2, 2-dipyridyl can block hydroxylation reactions in chrysanthemum tissues, the results do not support the hypothesis that daminozide restricts GA₉- (or GA₂₀-) induced stem elongation by preventing the hydroxylation of GA₉.     

Daminozide inhibits ethylene production in apple fruit by blocking the conversion of methionine to aminocyclopropane-1-carboxylic acid (ACC)

At harvest, fruit from apple trees sprayed with daminozide (+daminozide) had lower levels of aminocyclopropane-1-carboxylic acid (ACC) and produced significantly lower amounts of ethylene than untreated (–daminozide) fruit. Flesh discs from the fruit of +daminozide and –daminozide trees were fed precursors of ethylene to determine how daminozide inhibits ethylene production. ACC was metabolized to ethylene regardless of treatment. Methionine (MET), however, was only converted to ethylene by –daminozide fruit, and only after the fruit had been maintained at 4 °C for 5 months. +Daminozide fruit failed to convert MET to ethylene at harvest, as well as after cold storage. When daminozide was added to the incubation media of flesh discs it did not inhibit ethylene production or the conversion of ACC to ethylene. The addition of daminozide did, however, inhibit the metabolism of exogenous MET to ethylene. Aminooxyacetate acid (AOA) blocked both the endogenous production of ethylene and that from MET feeds. Daminozide inhibits ethylene production by preventing the conversion of MET to ACC, but it does not appear to act as a simple competitive inhibitor of ACC synthase activity.Abbreviations ACC aminocyclopropane-1-carboxylic acid - AVG aminoethoxyvinylglycine - AOA aminooxyacetic acid - CH cycloheximide - MET methionine - PUT putrescine Author for correspondence     

Inhibition of flowering in Chinese cabbage by applying heat and growth retardants to transplants

A procedure was developed to reduce premature bolting and flowering in Chinese cabbage grown in the field during the winter. This involved (a) the use of geothermic water as an inexpensive heat source for heating the transplants in the nursery; (b) application of the growth retardants daminozide (Alar) and paclobutrazol (PP333) to the heated plants before removing them to the field; and (c) application of daminozide to the plants in the field. This procedure markedly reduced stem elongation and the length of the internal stem in relation to the head without affecting the size of the head. Furthermore, application of daminozide in the field minimized the severity of tipburn, a common physiological disorder in Chinese cabbage fields. The result of such treatment was the production of high-quality Chinese cabbage heads which otherwise are unobtainable during the winter.Contribution from the Agricultural Research Organization, The volcani Center, Bet Dagan, Israel. No. 1172-E, 1984 series.     

The use of growth retardants to improve microtuber formation by potato (Solanum tuberosum)

The growth retardant chlormequat stimulated microtuber formation by a recalcitrant cultivar of potato (Solanum tuberosum), but reduced microtuber fresh weight in a cultivar that tuberised readily in its absence. Inhibition of microtuber growth by high concentrations of chlormequat was confirmed using a different in vitro system where all cultivars tuberised in the absence of growth retardants.Alternative growth retardants were tested. Daminozide also had a detrimental effect on microtuber fresh weight, but ancymidol and paclobutrazol did not inhibit microtuber growth at the concentrations required for stimulation of tuberisation by recalcitrant cultivars. In addition, 10^-5 M ancymidol and paclobutrazol inhibited premature sprouting of microtubers in vitro.Abbreviations BA benzyladenine - DMSO dimethyl sulphoxide - FW fresh weight - PAR photosynthetically active radiation - SE standard error