Effects of Auxin, Auxin-Transport Inhibitors and Mineral Nutrients on Apical Dominance in Pharbitis nil

Summary An analysis and comparison was carried out in Pharbitis nil on the effects of treatments with nutrients (N, P and K), auxin and the auxin transport inhibitors, triiodobenzioc acid (TIBA) and 2-(1-pyrenol) benzoic acid (PBA), on lateral bud outgrowth in decapitated/intact upright plants and/or in plants with inverted shoots. Although it could not be unequivocally determined whether or not the initiation of bud outgrowth was dependent on a nutrient signal, it was clearly demonstrated that the continued outgrowth of the lateral bud was dependent upon an continuous supply of nitrogen and phosphorous via the root system. There was also evidence, though not conclusive, for a significant role of auxin in the control of Apical Dominance. IAA treatment of decapitated shoots delayed but outgrowth. Stem treatment with the auxin transport inhibitors TIBA and PBA promoted lateral bud outgrowth. The shoot-inversion approach as employed here allowed for the analysis of interaction between the shoot apex and lateral bud both before and after the release of Apical Dominance.

a reappraisal of the role of abscisic acid and its interaction with auxin in Apical Dominance

• Background and Aims Evidence from pea rms1, Arabidopsis max4 and petunia dad1 mutant studies suggest an unidentified carotenoid-derived/plastid-produced branching inhibitor which moves acropetally from the roots to the shoots and interacts with auxin in the control of Apical Dominance. Since the plant hormone, abscisic acid (ABA), known to inhibit some growth processes, is also carotenoid derived/plastid produced, and because there has been indirect evidence for its involvement with branching, a re-examination of the role of ABA in Apical Dominance is timely. Even though it has been determined that ABA probably is not the second messenger for auxin in Apical Dominance and is not the above-mentioned unidentified branching inhibitor, the similarity of their derivation suggests possible relationships and/or interactions.

• Methods The classic Thimann–Skoog auxin replacement test for Apical Dominance with auxin [0·5 % naphthalene acetic acid (NAA)] applied both Apically and basally was combined in similar treatments with 1 % ABA in Ipomoea nil (Japanese Morning Glory), Solanum lycopersicum (Better Boy tomato) and Helianthus annuus (Mammoth Grey-striped Sunflower).

• Key Results Auxin, Apically applied to the cut stem surface of decapitated shoots, strongly restored Apical Dominance in all three species, whereas the similar treatment with ABA did not. However, when ABA was applied basally, i.e. below the lateral bud of interest, there was a significant moderate repression of its outgrowth in Ipomoea and Solanum. There was also some additive repression when Apical auxin and basal ABA treatments were combined in Ipomoea.

• Conclusion The finding that basally applied ABA is able partially to restore Apical Dominance via acropetal transport up the shoot suggests possible interactions between ABA, auxin and the unidentified carotenoid-derived branching inhibitor that justify further investigation.

naa restores Apical Dominance in the axr3 1 mutant of arabidopsis thaliana

Strong evidence for a role of auxin in Apical Dominance is provided by the classic Thimann-Skoog experiment (Proceedings of the National Academy of Sciences, USA19: 714–716, 1933) wherein exogenous auxin applied to a decapitated shoot represses outgrowth of the next lower lateral bud. Although Apical Dominance in most herbaceous species can be restored by this auxin treatment, such is not the case with wild-type Arabidopsis thaliana. In the present study, it has been demonstrated that Apical Dominance can be partially or fully restored with exogenous auxin (1% naphthaleneacetic acid, NAA) applied to the decapitated shoot of the axr3-1 mutant which is thought to be hypersensitive to auxin. A similar repressive response to auxin (1 μM NAA) was also shown in an in vitro assay with detached nodes. The role of AXR3 as a gene mediating auxin response is thus supported.

growing competitive or tolerant significance of Apical Dominance in the overcompensating herb gentianella campestris

As a compensatory response to herbivory, plants may branch vigorously when the growth of dormant meristems is triggered by shoot damage. Undamaged plants, on the other hand, often restrain branching, and this limitation on growth can be considered a cost of tolerance to herbivory. Restrained branching is caused by Apical Dominance and may, alternatively, be associated with fitness benefits in competitive environments that favor fast vertical growth. To test these hypotheses regarding selection for restrained branching, we compared the performance of two subspecies of the biennial grassland herb Gentianella campestris; the tall, Apically dominant ssp. campestris and the short, multi-stemmed ssp. islandica, which shows reduced Apical Dominance. For both subspecies, we manipulated the height of surrounding vegetation (competition) and damage intensity in grasslands of differing productivity (high, medium, low), and examined population growth rates using matrix population models combined with life table response experiments. In the absence of damage, ssp. campestris exhibited a higher population growth rate than ssp. islandica in the tallest vegetation, however with the growth rate still being below one. In the medium and low productivity environments where the vegetation was shorter, the population growth rate of ssp. islandica was considerably higher than that of ssp. campestris as long as no more than about 50% of the plants were damaged. When plants were damaged, the Apically dominant ssp. campestris showed a positive population growth rate (λ > 1) and often overcompensatory seed production in all productivity levels, while ssp. islandica showed no compensation and therefore the population was predicted to decline (λ < 1). We conclude that restrained branching in Gentianella cannot be selected for by competition alone, but that episodes of Apical damage are required to maintain the trait. Furthermore, because of the costs of restrained branching, Apical Dominance should be selected against in grasslands where competition and disturbance are low.

overcompensation and adaptive plasticity of Apical Dominance in erysimum strictum brassicaceae in response to simulated browsing and resource availability

In the cases where overcompensation has been observed in monocarpic herbs, overcompensation is associated with an Apically dominant shoot architecture of intact plants, increased lateral branching following herbivory, and increased reproductive success as a consequence of damage. The compensatory continuum hypothesis expects overcompensation to be more prevalent in resource rich environments compared to poor environments. This is paradoxical since in resource rich conditions the intact plants should branch most vigorously and hence any further increase in branch number should lead to lower seed yield. An explanation could be that Apical Dominance is rather insensitive to changes in resource availability, and that overcompensation is possible in conditions where plants experience meristem limitation (due to Apical Dominance) in relation to available resources. We explored the branching patterns and fitness responses of tall wormseed mustard (Erysimum strictum ) to simulated browsing, soil nutrients, and competition in common garden. Competition increased Apical Dominance and reduced plant fitness whereas fertilization had the reverse effects. Simulated browsing increased lateral branching and had little impact on plant fitness. Fitness overcompensation was observed only among plants grown in competition and in the absence of fertilization � / the most resource poor treatment combination in the experiment. The results contradict both with the compensation continuum and the assumption that Apical Dominance shows no or very little plasticity in relation to growing conditions. Because directional selection gradients on branch number were invariantly positive irrespective of growing conditions, we propose that, in spite of phenotypic plasticity of Apical Dominance, the plants appear to be meristem rather than resource limited, and that meristem limitation is strongest in conditions where intact plants produce fewest lateral branches. Our results deviate from the compensation continuum because resource availability affected compensation ability more strongly through phenotypic plasticity of shoot architecture rather than via changes in resource availability per se.

cost of Apical Dominance in two monocarpic herbs erysimum strictum and rhinanthus minor

Apical Dominance is advantageous in conditions favoring rapid growth in height and unbranched architecture. The cost of Apical Dominance, on the other hand, should be expressed in conditions where fecundity increases along with an increasing number of branches. Apical damage can be used to measure such costs: when suppressed meristems are released from Apical inhibition, the vegetative and reproductive productivity of initially unbranched plants should increase owing to the regrowth and increased branch development following damage. We studied these regrowth responses in two monocarpic herbs, Erysimum strictum P. Gaertn., B. Mey., and Scherb. and Rhinanthus minor L., after both Apical damage (10% of the shoot cut) and more extensive damage (50 and 75% cutting). Both species tolerated Apical damage, although severe damage had detrimental effects on the performance of both, especially R. minor. Apical damage had positive effects on most of the measured performance parameters of Erysimum. However, the succes…

the role of Apical Dominance in paradormancy of temperate woody plants a reappraisal

Summary Paradormancy, also known as summer dormancy, is the temporary dormancy which precedes winter or endodormancy in temperate woody plants. Apical Dominance, the control exerted by the shoot apex over the outgrowth of the lateral buds, is thought to play a primary role in the paradormancy of these current lateral buds which normally do not grow out until the following spring after over-wintering. In the present study, to test Apical Dominance, periodic decapitation and defoliation beginning with early spring flushing and extending through the growing season were carried out on eleven tree species (white ash, green ash, red oak, black walnut, pignut hickory, shagbark hickory, sweetgum, cottonwood, sugar maple, hybrid silver/red maple and white pine) ranging in age from 6 to 50–60 years. Significant decapitation and defoliation release of current lateral buds into growing branches was found only in silver/red hybrid maple and red oak, respectively. In a greenhouse study of 1-year-old sugar maple and green ash seedlings, a significant response was found only in the latter. The lack of response in most of the older species analyzed indicates that other inhibitory influences may be overriding that of Apical Dominance and suggests that Apical Dominance does not play the primary role in paradormancy in some temperate woody species.