Disruption of the Polar Auxin Transport System in Cotton Seedlings following Treatment with the Defoliant Thidiazuron

Abstract

The effect of the defoliant thidiazuron (TDZ) on basipetal auxin transport in petiole segments isolated from cotton (Gossypium hirsutum L. cv LG102) seedlings was examined using the donor/receiver agar block technique. Treatment of intact seedlings with TDZ at concentrations of 1 micromolar or greater resulted in a dose-dependent inhibition of 14C-IAA transport in petiole segments isolated 1 or 2 days after treatment. Using 100 micromolar TDZ, the inhibition was detectable 19 hours after treatment and was complete by 27 hours. Both leaves and petiole segments exhibited a marked increase in ethylene production following treatment with TDZ at concentrations of 0.1 micromolar or greater. The involvement of ethylene in this TDZ response was evaluated by examining the effects of two inhibitors of ethylene action: silver thiosulfate, 2,5-norbornadiene. One day after treatment, both inhibitors effectively antagonized the TDZ-induced inhibition of auxin transport. Two days after TDZ treatment both inhibitors were ineffective. The decrease in IAA transport in TDZ treated tissues was associated with increased metabolism of IAA. The transport of '4C-2,4-dichlorophenoxyacetic acid was also inhibited by TDZ treatment. This inhibition was not accompanied by increased metabolism. Incorporation of TDZ into the receiver blocks had no effect on auxin transport. The ability of the phytotropin N-1-naphthylphthalamic acid to stimulate IAA uptake from a bathing medium was reduced in TDZ-treated tissues. This reduction is thought to reflect a decline in the auxin efflux system following TDZ treatment.The timing and extent of leaf abscission is largely determined by the physiological interplay between the leafand the subtending abscission zone. To a large extent, this interplay involves alterations in the levels and activities of endogenous growth substances (1, 6, 9).The manipulation of leaf and other organ abscission continues to be an important aspect of modern agricultural and horticultural practice. Although many chemical treatments have been devised to alter abscission to suit individual needs, all suffer from inherent problems, chief of which is inconsistent performance. It can reasonably be expected that future improvements in the deliberate manipulation of abscission will depend on increased fundamental knowledge of the physiology of the abscission process itself.To this end, a series of investigations has been initiated in this laboratory with the goal of developing a more thorough understanding of the physiology of leaf abscission through the systematic study of the action of a registered defoliant. TDZ' 2 is a recently introduced cotton defoliant. It is unique among defoliants in that it stimulates the premature shedding of seemingly healthy, turgid leaves (so-called 'green-drop'). It has been shown that TDZ treatment of cotton results in a massive and sustained increase in ethylene production and that ethylene is, at least partially, responsible for the subsequent leaf fall (16, 17).It is generally thought (1, 9) t...

Effect of thidiazuron and gibberellic acid on leaf yellowing of cut stock flowers

“…Studies carried out on cotton and Pelargonium cuttings using TDZ showed an increase of ethylene production [20][21][22]. In our experiments ethylene production was stimulated only when TDZ was combined with GA 3 .…”

Interaction of 1-Methylcyclopropene and Thidiazuron on Cut Stock Flowers Vase Life

“…The ethylene produced by TDZ has been used for promoting leaf abscission in cotton plants [18]. Experiments performed on cotton demonstrated that TDZ induced abscission of young and mature leaves.…”

Ethylene-Induced Leaf Abscission in Cotton Seedlings

“…Both IAA and synthetic auxins are thought to operate through the same receptor/signal transduction systems (5,8). On the other hand, 2,4-D and 1-NAA are metabolically more stable in cotton tissues (3,16). Thus, it is possible that IAA is more readily metabolized in the younger leaf tissues (including the abscission zone) and this difference contributes to the enhanced abscission response of younger leaves both in situ and as explants.…”

“…The basipetal transport of ['4C]IAA was determined using the donor/ receiver method described previously (16). After a 2 h transport period (dark/27°C), the radioactivity in the receiver blocks was determined.…”

“…In the leaf blade and subtending petiole, ethylene reduces both the endogenous levels and transport of the abscission inhibitor IAA (2,4,12,16), while in the abscission zone, ethylene stimulates both the de novo synthesis and secretion of cell wall hydrolytic enzymes such as cellulase (13,18). Accordingly, any one or a combination of these sites of action may be responsible for the differential abscission response of cotton seedlings to ethylene.…”