Winter bud content according to position in 3-year-old branching systems of 'Granny Smith' apple

An investigation was made of the number of preformed organs in winter buds of 3-year-old reiterated complexes of the 'Granny Smith' cultivar. Winter bud content was studied with respect to bud position: terminal buds were compared on both long shoots and spurs according to branching order and shoot age, while axillary buds were compared between three zones (distal, median and proximal) along 1-year-old annual shoots in order 1. The percentage of winter buds that differentiated into inflorescences was determined and the flowers in each bud were counted for each bud category. The other organ categories considered were scales and leaf primordia. The results confirmed that a certain number of organs must be initiated before floral differentiation occurred. The minimum limit was estimated at about 15 organs on average, including scales. Total number of lateral organs formed was shown to vary with both bud position and meristem age, increasing from newly formed meristems to 1- and 2-year-old meristems on different shoot types. These differences in bud organogenesis depending on bud position, were consistent with the morphogenetic gradients observed in apple tree architecture. Axillary buds did not contain more than 15 organs on average and this low organogenetic activity of the meristems was related to a low number of flowers per bud. In contrast, the other bud categories contained more than 15 differentiated organs on average and a trade-off was observed between leaf and flower primordia. The ratio between the number of leaf and flower primordia per bud varied with shoot type. When the terminal buds on long shoots and spurs were compared, those on long shoots showed more flowers and a higher ratio of leaf to flower primordia.     

Preformed and neoformed extension of shoots and sylleptic branching in relation to shoot length in Tsuga canadensis

Summary Shoot systems developed over 3 successive years were investigated on 55 understorey Tsuga canadensis (L.) Carr. trees. Paired comparisons of preformed-leaf content of terminal buds and numbers of leaves produced on new shoots showed that neoformed leaves were produced in large numbers. Parent-shoot character was not useful in predicting numbers of preformed leaves, was better related to total leaves produced, but left the majority of the variation unexplained. This reflected the capacity of any terminal bud to produce a shoot with more or less neoformation, depending on conditions for growth. All shoots over 6 cm long produced sylleptic shoots that bore from two to many leaves and were arranged in a mesitonic pattern along the parent. Some of the longer sylleptic shoots produced lateral buds or second-order sylleptic shoots. Monopodial second-year extensions of sylleptic-shoot axes followed an acrotonic pattern, as did proleptic shoots from the few lateral buds borne on the parent shoots. Such lateral buds were more frequent on shorter parent shoots: they typically occurred near the proximal and distal ends. Duration of shoot extension was positively correlated with shoot length: terminal buds became evident as shoot extension neared cessation.     

Modelling branching patterns on 1-year-old trunks of six apple cultivars

The structure resulting from branching on 1-year-old apple tree trunks was analysed in a set of apple cultivars with diverse branching and fruiting habits. Four different lateral types borne on successive nodes were observed when vegetative and flowering fates, as well as sylleptic and proleptic branching, were taken into account. The location and grouping of lateral types along the trunk were analysed for all cultivars, but are detailed for one cultivar only. This cultivar showed a succession of zones, each zone being characterized by its composition of lateral types. Statistical models-hidden semi-Markov chains-were built to take this structure into account and to characterize the cultivar's specific branching pattern. The models showed that most of the branching zones had a similar location in the different cultivars, even though zone composition and zone length differed among cultivars. On a more detailed scale, the nodes bearing a lateral, regardless of its type, were frequently followed by latent buds. The validity of the models and their biological interpretation are discussed with respect to parent shoot dynamics, hormonal gradients and competition between neighbouring buds.     

A preliminary investigation of the role of auxin and cytokinin in sylleptic branching of three hybrid poplar clones exhibiting contrasting degrees of sylleptic branching

Sylleptic branches grow out from lateral buds during the same growing season in which the buds are formed. This type of branching is present in poplar and in many tropical species. It results in the production of more branches, more leaves and expanded photosynthetic capacity and is thought to assist in increasing the overall growth and biomass of the tree at a young age. However, very little is known about the physiology of sylleptic branching in poplar, which is an extremely important source of fibre and fuel. In the present study of three hybrid poplar clones (11-11, 47-174 and 49-177) of Populus trichocarpa x P. deltoides exhibiting contrasting degrees of sylleptic branching, an analysis was carried out on parent shoot elongation and sylleptic branching, together with a preliminary comparison of the parent shoots' sensitivity to auxin (naphthaleneacetic acid) as a repressor of lateral bud outgrowth, and cytokinin (benzyladenine) as a promoter. Suggestive evidence was found for an inverse correlation between parent shoot sensitivity to auxin and the degree of sylleptic branching, as well as a partially positive correlation with respect to sensitivity to cytokinin. The present data are consistent with the hypothesis that auxin and cytokinin may play repressive and promotive roles, respectively, in the sylleptic branching of hybrid poplar.     

Some growth responses of apple seedlings to abscisic acid and growth stimulators

Antonovka seedlings were obtained from the embryos soaked in abscisic acid (ABA) alone, gibberellin A₄₊₇, benzyladenine or the mixtures of these regulators. The inhibitory effect of ABA on the growth of the seedlings was of temporary duration. The height of the 12 week-old seedlings was dependent only on the degree of seed after-ripening and not on the ABA treatment. The growth stimulators studied were unable to overcome the inhibitory effect of ABA observed in the early stages of seedling growth. The growth stimulators did not substitute, for cold treatment of apple seeds which is the only treatment so far known to overcome the dwarf conditions of the seedlings obtained from non-after-ripened embryos. ABA greatly suppressed the early stages of apple seedling growth (up to 6 weeks) during which they resembled physiological dwarfs. The highest concentrations of ABA (2·0 μg ml^?1) greatly modified the root system of the seedlings, and produced a larger percentage of seedlings with unbranched roots.     

Water relations of pine seedlings in relation to root and shoot growth

The effects of water stress on growth and water relations of loblolly and white pine seedlings were studied during series of drying cycles. As mean soil water potential decreased, growth of roots, needles, and buds decreased. Growth of roots during successive severe drying cycles was not uniform, however. A study of needle and root extension showed that of the total growth of roots for 3 7-day drying cycles, only 6% occurred during the third cycle, while needle extension was uniform for the 3 cycles. The difference in response of needles and roots to drying cycles may be attributed primarily to the effect of water stress on the growing region. When subjected to a severe stress, roots matured toward the tip and became dormant, resulting in less growth during subsequent drying cycles. The intercalary growing region of needles, however, was not altered seriously enough by the stress to cause a difference in amount of growth during each drying cycle.

Transpiration of loblolly pine was lower in the second drying cycle than in the first. Needle water potential after rewatering was as high as that of control plants watered daily; root resistance was apparently not important in restricting transpiration during a second drying cycle. Needle diffusion resistance of loblolly pine, measured with a low-resistance diffusion porometer, was slightly higher during the second drying cycle than during the first. In addition, many primary needles were killed during the first period of stress. These factors contributed to the reduction of transpiration during the second drying cycle. Diffusion resistance of Coleus increased and transpiration ceased during the first drying cycle while water potential remained relatively high. After rewatering, both leaf resistance and transpiration returned to the control level, presumably because the stress during the first period of drying was not severe. The diffusion resistances observed for well-watered plants were 30 to 50 sec·cm⁻¹ for loblolly pine, 3 to 5 sec·cm⁻¹ for Coleus, and 4 to 6 sec·cm⁻¹ for tomato. These values agree closely with those reported by other workers.

     

The effect of air temperature in successive periods of the growing season on sylleptic shoot formation in young apple trees

In two day-light phytotron rooms young apple trees were exposed to either 15 or 25°C in three successive periods of six weeks (Periods I, II and III) starting in the beginning of the growing season. A control group was kept outside throughout. Exposure to 15°C in either period had little effect on sylleptic growth; there was also hardly any difference with the outside treatment. At 25°C total sylleptic growth was greatly favoured especially when that temperature was applied in all three periods. However, 25°C given continuously did not affect sylleptic shoot number in Period II and III, but applied only in Period II or III greatly enhanced it. In a few cases there was a positive carry-over effect of 25°C given in Period I or II on sylleptic growth in the next period. No such carry-over effect was found for shoot number. Although temperature did influence growth of the parent shoot, that influence was much smaller than for sylleptic growth. The outgrowth of sylleptic shoots mainly occurred in periods when growth vigour of the parent shoot was highest. The distribution of sylleptic shoots along the parent shoot was greatly affected by the period in which the temperature of 25°C was applied suggesting that buds are only able to respond to outgrowth-inducing factors in a certain stage of development.     

Aphid-induced reduction of shoot and root growth in Douglas-fir seedlings

1. The short‐ and long‐term effects of photosynthate removal by sap‐sucking herbivores on plant growth were examined by experimentally manipulating densities of an aphid Cinara pseudotsugae (Wilson) on 2‐year‐old Douglas‐fir seedlings Pseudotsuga menziesii (Mirb.) Franco under greenhouse conditions. An 18‐week test was conducted to determine short‐term effects. Effects of long‐term aphid feeding were examined by exposing seedlings to aphid feeding for two consecutive growing seasons. A third experiment evaluated the extent of recovery during 1 year following a single season of aphid feeding. At least 35 seedlings were used in each test. 2. Volume and dry weight of both shoots and roots decreased significantly with increasing aphid feeding in all three experiments. 3. The most significant and severe aphid effect was reduced root tissue density, suggesting carbohydrate depletion due to translocation from roots to shoots. 4. There was no sign of recovery, of either root or shoot growth, during the year following one season of feeding. 5. The results of this study indicate that short‐term feeding by aphids can affect plant growth and structure for a relatively long time.     

MAX1 and MAX2 control shoot lateral branching in Arabidopsis

Plant shoots elaborate their adult form by selective control over the growth of both their primary shoot apical meristem and their axillary shoot meristems. We describe recessive mutations at two loci in Arabidopsis, MAX1 and MAX2, that affect the selective repression of axillary shoots. All the first order (but not higher order) axillary shoots initiated by mutant plants remain active, resulting in bushier shoots than those of wild type. In vegetative plants where axillary shoots develop in a basal to apical sequence, the mutations do not clearly alter node distance, from the shoot apex, at which axillary shoot meristems initiate but shorten the distance at which the first axillary leaf primordium is produced by the axillary shoot meristem. A small number of mutant axillary shoot meristems is enlarged and, later in development, a low proportion of mutant lateral shoots is fasciated. Together, this suggests that MAX1 and MAX2 do not control the timing of axillary meristem initiation but repress primordia formation by the axillary meristem. In addition to shoot branching, mutations at both loci affect leaf shape. The mutations at MAX2 cause increased hypocotyl and petiole elongation in light-grown seedlings. Positional cloning identifies MAX2 as a member of the F-box leucine-rich repeat family of proteins. MAX2 is identical to ORE9, a proposed regulator of leaf senescence ( Woo, H. R., Chung, K. M., Park, J.-H., Oh, S. A., Ahn, T., Hong, S. H., Jang, S. K. and Nam, H. G. (2001) Plant Cell 13, 1779-1790). Our results suggest that selective repression of axillary shoots involves ubiquitin-mediated degradation of as yet unidentified proteins that activate axillary growth.     

An analysis of mineral uptake in apple rootstock seedlings

Summary Eight families from biparental crosses of apple rootstocks and 12 families from open pollinated Malus spp. were analysed in 2 years for N, P, K, Ca and Mg content of the foliage. Highly significant differences were found between the families for all elements. There were no significant differences between the means of the biparental group and the open pollinated group. Ca and K content were significantly more variable in the open pollinated families compared with the biparental families. It is suggested that this increased variation could prove useful in breeding for efficiency of mineral uptake by apple rootstocks.     

The molecular and genetic regulation of shoot branching

The architecture of flowering plants exhibits both phenotypic diversity and plasticity, determined, in part, by the number and activity of axillary meristems and, in part, by the growth characteristics of the branches that develop from the axillary buds. The plasticity of shoot branching results from a combination of various intrinsic and genetic elements, such as number and position of nodes and type of growth phase, as well as environmental signals such as nutrient availability, light characteristics, and temperature (Napoli et al., 1998; Bennett and Leyser, 2006; Janssen et al., 2014; Teichmann and Muhr, 2015; Ueda and Yanagisawa, 2019). Axillary meristem initiation and axillary bud outgrowth are controlled by a complex and interconnected regulatory network. Although many of the genes and hormones that modulate branching patterns have been discovered and characterized through genetic and biochemical studies, there are still many gaps in our understanding of the control mechanisms at play. In this review, we will summarize our current knowledge of the control of axillary meristem initiation and outgrowth into a branch.

The key regulatory genes and the role of multiple plant hormones coordinate the process of axillary meristem initiation and subsequent growth into a branch.     

Reduced glutathione promotes callus growth and shoot development in a shoot tip culture of apple root stock M26

Reduced glutathione (GSH) was applied to prevent browning in shoot tip explants of apple (Malus pumila Mill.). Development was compared between shoot tips treated either by (a) dipping into 0.1 mm GSH solution prior to culture (dip treatment), (b) dipping into the GSH solution and transferring to a medium containing 0.1 mm GSH (dip-and-add treatment), or (c) without dipping or culturing with GSH (control). In the dip treatment, 100% of shoot tips developed into normal shoots after 120 days, while the results with the dip-and-add treatment and control were 50 and 40%, respectively. The results show that application of antioxidant GSH in the initial phase of culture promoted the normal development of shoot tips. Received: 10 October 1997 / Revision received: 12 January 1998 / Accepted: 24 January 1998     

Transitory growth control of apple seedlings with less persistent triazole derivatives

Paclobutrazol, triapenthenol (RSW0411), and BAS111 were applied to 4-week-old Delicious apple seedlings (Malus domestica Borkh.) as a root drench at 0.1, 1.0, and 10.0 mg per plant. Paclobutrazol eliminated shoot extension growth for 8 weeks at all three rates. RSW0411 controlled shoot elongation only at the highest rate. BAS111 produced the widest response, with shoot growth ranging from 38% to 93% of controls for the highest and lowest rates, respectively. Generally, leaf area decreased and leaf density increased with increasing rates of all chemicals. Root weight of plants treated with paclobutrazol nearly doubled but increased only slightly with RSW0411 and BAS111. Chemical analysis of the leaf tissue 8 weeks after treatment showed paclobutrazol levels highest, followed by RSW0411 and BAS111. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.     

Sterol composition of dwarf and tall Pisum sativum seedlings in relation to gibberellic-acid-enhanced shoot growth

Gibberellic acid (GA₃) stimulated shoot elongation in both dwarf and tall cultivars of pea, but more so in the dwarf cultivar. The sterol composition of shoots of both cultivars was similar, with sitosterol being the most abundant compound, followed by stigmasterol and campesterol. Cholesterol could not be detected. Following GA₃ application, levels of free sterols in whole shoots increased whereas glycoside levels tended to fall. The magnitudes of the changes in both classes of sterol were similar in both cultivars. Analyses of stems and leaves separately revealed a greater growth response to GA₃ in the former but no effect of the hormone on the sterol composition of either organ. It is concluded that GA₃ enhancement of shoot growth in pea is not mediated through quantitative changes in cell sterols.     

Auxin, cytokinin and the control of shoot branching

BACKGROUND: It has been known for many decades that auxin inhibits the activation of axillary buds, and hence shoot branching, while cytokinin has the opposite effect. However, the modes of action of these two hormones in branching control is still a matter of debate, and their mechanisms of interaction are equally unresolved. SCOPE: Here we review the evidence for various hypotheses that have been put forward to explain how auxin and cytokinin influence axillary bud activity. In particular we discuss the roles of auxin and cytokinin in regulating each other's synthesis, the cell cycle, meristem function and auxin transport, each of which could affect branching. These different mechanisms have implications for the main site of hormone action, ranging from systemic action throughout the plant, to local action at the node or in the bud meristem or leaves. The alternative models have specific predictions, and our increasing understanding of the molecular basis for hormone transport and signalling, cell cycle control and meristem biology is providing new tools to enable these predictions to be tested.