Many processes have been described in the control of shoot branching. Apical dominance is defined as the control exerted by the shoot tip on the outgrowth of axillary buds, whereas correlative inhibition includes the suppression of growth by other growing buds or shoots. The level, signaling, and/or flow of the plant hormone auxin in stems and buds is thought to be involved in these processes. In addition, RAMOSUS (RMS) branching genes in pea (Pisum sativum) control the synthesis and perception of a long-distance inhibitory branching signal produced in the stem and roots, a strigolactone or product. Auxin treatment affects the expression of RMS genes, but it is unclear whether the RMS network can regulate branching independently of auxin. Here, we explore whether apical dominance and correlative inhibition show independent or additive effects in rms mutant plants. Bud outgrowth and branch lengths are enhanced in decapitated and stem-girdled rms mutants compared with intact control plants. This may relate to an RMS-independent induction of axillary bud outgrowth by these treatments. Correlative inhibition was also apparent in rms mutant plants, again indicating an RMS-independent component. Treatments giving reductions in RMS1 and RMS5 gene expression, auxin transport, and auxin level in the main stem were not always sufficient to promote bud outgrowth. We suggest that this may relate to a failure to induce the expression of cytokinin biosynthesis genes, which always correlated with bud outgrowth in our treatments. We present a new model that accounts for apical dominance, correlative inhibition, RMS gene action, and auxin and cytokinin and their interactions in controlling the progression of buds through different control points from dormancy to sustained growth.

Roles for Auxin, Cytokinin, and Strigolactone in Regulating Shoot Branching

Prediction of mineral nutrient status of trees by foliar analysis

Introduction. 79 Control of Carbohydrate Movement from the Leaf 80 Leaf Position and Assimilate Distribution 82 The Role of Vascular Connections in the Pattern of Distribution 83 The Demand for Assimilates 84 Environment and Assimilate Distribution 88 Temperature .... 88 Light 89

The control and pattern of movement of carbohydrates in plants

Two weeks after anthesis, when the grain is filling rapidly, the rate of photosynthesis by flag leaves of wheat cv. Gabo was between 20 and 30 mg CO2 dm(-2) leaf surface hour(-1) under the conditions used. About 45% of flag-leaf assimilates were translocated to the ear, and only about 12% to the roots and young shoots.On removing the ear, net photosynthesis by the flag leaves was reduced by about 50% within 3-15 hours, and there was a marked reduction in the outflow of (14)C-labelled assimilates from the flag leaves.Subsequent darkening of all other leaves on plants without ears led to recovery of flag-leaf photosynthesis, as measured by gas analysis and (14)CO2 fixation, and to increased translocation of assimilates to the roots and young shoots. Minor changes in the rates of dark respiration accompanied these major, reversible changes in photosynthetic rate.After more than a week in continuous, high-intensity light, the rate of photosynthesis by flag leaves of intact plants had fallen considerably, but could be restored again by a period in darkness, or by inhibiting photosynthesis in the ears by spraying them with DCMU. The inhibition of ear photosynthesis increased translocation of labelled assimilates from the flag leaf to the ears, without affecting leaf sugar levels.The application of TIBA to the culm below the ear inhibited auxin movement throught the culm, but had no influence on flag-leaf photosynthesis.These results suggest that, at least in this system, photosynthesis by the flag leaf is regulated directly by the demand for assimilates from the flag leaf and not indirectly through action in the leaf of auxins produced by the "sink" organs.

Effect of assimilate utilization on photosynthetic rate in wheat.

CHAPTER 5 – Mineral Nutrition of Ectomycorrhizae

The role of indolylacetic acid (IAA) in influencing the movement of radiophosphorus from a remote locus has been investigated in the decapitated stems of Pisum sativum (var. Meteor) and disbudded shoots of Populus robusta. It has been shown after only a few hours that the radioisotope accumulates at the region where the auxin was applied. By steam-girdling experiments with pea, it was established that auxin-induced transport occurs via the phloem and other obserbations suggest that this effect may be independent of growth. When tri-iodobenzoic acid was applied together with IAA at the stump or to the surface of the stem between the IAA source and the injection point of the tracer, the transport of 32P along the stem was greatly reduced. Of a number of auxin-type substances tested, only naphthoxyacetic acid gave a comparable response to that obtained with IAA, whilst other growth regulators such as kinetin or gibberellic acid did not stimulate 32P transport.

C. R. Davies

Papers

Auxin-directed transport of radiophosphorus in stems