Light Effects on Apical Dominance

Reducing light intensity from 18·0 klx to 5·4klx promoted main-stem growth in Phaseolus vulgaris but leftlateral extension unaffected. Shading individual laterals orapices of main stems promoted elongation of the shaded portion.Increasing day length from 8 to 16 or 24 h did not greatly affectplant growth if full-intensity and full-spectrum light was used.If, however, the additional light was supplied by incandescenttubes containing a high proportion of infra-red light, growthof main stems but not laterals was induced in proportion tothe length of the additional light period. It is suggested thatthe increased growth of the main stem in response to infra-redlight prevented the equivalent response of the laterals becauseof the enhanced ability of the main stem to suppress lateralelongation.     

The Mechanism of Apical Dominance in Coleus

The development of lateral buds in isolated stems of Coleus blumei is inhibited by low concentrations of indoleacetic acid or other auxins, just as in other plants. The inhibition can be fully reversed by kinetin, about 3 times as much kinetin as IAA being needed. However, the outgrowth of the same lateral buds on intact Coleus plants is sensitive to environmental conditions, well-nourished plants in full daylight often showing little inhibition by applied auxin. It is shown that (a) the solvent used for IAA, (b) the light intensity and (c) the nitrogen and phosphorus nutrition, all control the sensitivity of the buds to auxin inhibition. Using water instead of lanolin, lowering the light intensity or decreasing the supply of either nitrogen or phosphorus all increase the degree of apical dominance.     

Hormones and Apical Dominance in the Fern Davallia

Lateral buds of the fern Davallia trichomanoides are releasedfrom inhibition by the removal of the main shoot apex. However,auxin is not capable of substituting for the apex in decapitatedshoots nor can auxin in shoot tips be detected by bioassay orextraction and chromatography. Expanding leaves of this speciescontain auxin, but these organs are not responsible for inhibitionof lateral bud growth. The response of lateral buds to an exogenouslyapplied cytokinin does not result in initial bud break. It isconcluded that the hormonal factors known to govern apical dominancein seed plants are not responsible for the regulation of differentialbud expansion in this fern.     

Gravimorphism in Trees: 1. Effects of Gravity on Growth and Apical Dominance in Fruit Trees

The effect of shoot-orientation on the vegetative growth andapical dominance of rootstocks of apply, cherry, plum, and blackcurrantwas investigated. In all the species studied the total annualextension growth of horizontally grown trees was markedly lessthan that of vertical trees, and the effect was accentuatedwhen the plants were rotated. In cherry and plum the reducedtotal growth was due both to a smaller number of internodesand also to reduced internode length; in apple and blackcurrantthe difference in extension growth was mainly due to differencesin internode length. In experiments with laternal shots trainedin various positions it was found that the apical dominancenormally exhibited by the uppermost shoot is only manifestedwhen it occupies a vertical position. The normal apical dominancerelationships can be reversed if the uppermost shoot is grownhorizontally and the second shoot vertically. Gravitationaleffects appear to play a role in the normal apical dominancerelationships of the tree. In all species there is a markedtendency for laterals to grow out on the upper side of horizontaland arched plants. The effects of training arched trees into various positionsare considered in relation to current concepts regarding therole of auxin in apical dominance and the redistribution ofauxin in horizontal organs; it is concluded that the outgrowthof laterals in arched shoots cannot be interpreted primarilyin terms of these current concepts. The observed results suggestthat the outgrowth of lateral buds occurs at the nearest pointto the roots at which the shoot is diverted from the verticalposition, and that nutrients are diverted to the highest upwardlydirected meristem. It is proposed that the term gravimorphismshould be applied to these diverse effects of gravity on plantgrowth.     

Effect of Root Position on Measured Root Position on Measured Root Membrane Potentials

Root membrane potentials were measured by interposing a plantroot between two KCI solutions of different concentrations.The potentials measured across the two calomel electrodes werefound to depend on the position of the root. The potentialswere found to be lower when the root tip rather than the rootbase was in contact with the more concentrated solution. Thisindicates that the two parts of the root do not have the samein transport properties. Using an approximate theoretical treatmentthe observed potentials could be accounted for.     

乙烯在豌豆植株顶端优势中的作用

用人工合成细胞分裂素BAP和CPPU处理豌豆植株叶腋可诱导处理部位侧芽的生长,同时伴有大量乙烯产生;用乙烯合成抑制剂AVG处理或植株去顶同样可导致创芽生长,但乙烯释放量却明显少于对照,表明侧芽的生长与乙烯释放量的多少无关。而3种物质处理后诱导产生的侧芽的数目、长度及其鲜重与处理部位内源IAA含量的增加则呈正相关。     

Senescence of Inhibited Shoots of Peas and Apical Dominance

When peas of the cultivar Radio have their epicotyl removedand are selected to have two unequal shoots, the smaller ofthese shoots is inhibited and starts senescing when the plantsare about 17 days old. This senescence is a correlative phenomenon:removal of the dominant shoot prevents it in every case. Indole-3-acetic acid partially replaces the dominant shoot incausing the senescence of the inhibited shoot. Kinetin, whenapplied to the inhibited shoot, prevents its senescence andcauses renewed growth. The control of this senescence, therefore, resembles the inhibitionof lateral growth. It differs from it in occurring only whenthe plants have reached a definite age. The age at which senescencestarts is the same under very different conditions; it doesnot depend on a lack of nutrients or the maturation of the leaves.It might be associated with the development of flowers.     

Some Observations on Factors Controlling Apical Dominance in the 'Rogue' Tomato

The rogue tomato differs from the normal plant in that it exhibitsa lesser degree of apical dominance. Grafting techniques andmeasurements of the endogenous levels of growth substances inthe two types have been used in order to establish whether thisdifference is due to an altered hormonal balance in the roguetype. The results suggest that root-produced cytokinins play no rolein the control of apical dominance in the tomato plant, andthat lateral bud out-growth is influenced by a balance betweenapically-produced auxin, abscisic acid produced at the sitesof bud development and cytokinins synthesized within the budsthemselves. Lycopersicon esculentum L., tomato, apical dominance, abscisic acid, auxins, cytokinins, growth regulation     

Cytokinin/Auxin Control of Apical Dominance in Ipomoea nil

Although the concept of apical dominance control by the ratioof cytokinin to auxin is not new, recent experimentation withtransgenic plants has given this concept renewed attention.In the present study, it has been demonstrated that cytokinintreatments can partially reverse the inhibitory effect of auxinon lateral bud outgrowth in intact shoots of Ipomoea nil. Althoughless conclusive, this also appeared to occur in buds of isolatednodes. Auxin inhibited lateral bud outgrowth when applied eitherto the top of the stump of the decapitated shoot or directlyto the bud itself. However, the fact that cytokinin promotiveeffects on bud outgrowth are known to occur when cytokinin isapplied directly to the bud suggests different transport tissuesand/or sites of action for the two hormones. Cytokinin antagonistswere shown in some experiments to have a synergistic effectwith benzyladenine on the promotion of bud outgrowth. If theratio of cytokinin to auxin does control apical dominance, thenthe next critical question is how do these hormones interactin this correlative process? The hypothesis that shoot-derivedauxin inhibits lateral bud outgrowth indirectly by depletingcytokinin content in the shoots via inhibition of its productionin the roots was not supported in the present study which demonstratedthat the repressibility of lateral bud outgrowth by auxin treatmentsat various positions on the shoot was not correlated with proximityto the roots but rather with proximity to the buds. Resultsalso suggested that auxin in subtending mature leaves as wellas that in the shoot apex and adjacent small leaves may contributeto the apical dominance of a shoot. (Received September 24, 1996; Accepted March 16, 1997)     

植物的顶端优势和水分上升机理新说

植物体有一个能量系统,由植物顶端、由植物顶端、形成层及其初生韧皮部和木质部、传递细胞等组成。这些细胞含线粒体量多质好,摘顶等刺激这些地方ＡＴＰ复合酶能产生大量能量,通过胞间连丝引起连锁反应并刺激和激活激素、营养物质等而产生生命活动、项端优势等。植物细胞的原生质是一种液晶,只需极少能量便能产生很大的作用。除去顶芽,刺激线粒体能产生大量能量供给侧芽,激活侧芽激素等而使侧芽生长,由于能量作功,逐渐减少,     

The Effect of Hadacidin on Bud Development and its Implications for Apical Dominance

Hadacidin, which is known to inhibit the enzyme adenylo-succinate synthetase, was applied locally to lateral buds on decapitated Pisum sativum L. plants. In controls the buds grew out normally, but the treated buds were almost completely inhibited. After a delay, the buds overcame the inhibition, and their subsequent outgrowth could be further hastened by the local application of a cytokinin. The inhibition, unlike that due to IAA, was not transported to a second bud, and it could be largely reversed by kinetin or dimethylallylaminopurine. Adenine itself produces little or no reversal, however. There was some reversal by aspartic acid, which is known to reverse, at least partially, the hadacidin inhibition of the isolated enzyme. It is deduced that lateral bud development is dependent on the synthesis of a cytokinin, which may take place by a biochemical route similar, but probably not identical, to that for adenine. It follows that this synthesis takes place locally in the bud itself.     

木本植物的构型及其在植物生态学研究的进展

一般认为,木本植物的植株结构由枝系和根系两个亚系统构成[5,38].其地上部分的枝或茎的顶端分生组织和侧生分生组织通过不断重复的、持续的活动产生新的分枝,构成了复杂的枝系结构和多样的形态特征.     

Reproductive Phenology of Woody Species in a North Australian Tropical Savanna1

Interspecific and interannual variation in reproductive phenology was quantified for 50 common species of trees and shrubs from a mesic savanna near Darwin, northern Australia. The presence of buds, flowers, and fruit was noted over a 30-month period, from September 1992 to February 1995. Surveys were undertaken at monthly intervals for the less common species, and at bimonthly intervals for ten of the common trees and tall shrubs. The majority of species flowered each year at about the same time. There was no evidence of sub-annual or continuous regimes of reproductive phenology. There was no supra-annual carryover of seed-bearing fruit in the canopy of any species. The peak flowering periods were the mid to late dry season (July–August) and the transition between the dry season and the wet season (October–November). The two dominant trees–Eucalyptus miniata and E, tetrodonta– flowered during the dry season, thereby providing resources for some elements of the vertebrate fauna. Flowering and fruiting were uncommon at the end of the wet season (February/March), although two species that flower and fruit at this time (E. porrecta and Terminalia ferdinandianas may provide resources to consumers at a time when floral or fruit resources are otherwise scarce. Because the peak of reproductive activity takes place during the late dry season, fruit maturity and seed dispersal have occurred prior to the onset of the rainy season for most species, and germination and seedling establishment potentially may take effect in response to the first rains. Late dry season fires, which tend to be extensive and intense, are a potential threat to the floral and fruit reserves within these savannas.     

Positive Effects of Plant Genotypic and Species Diversity on Anti-Herbivore Defenses in a Tropical Tree Species

Despite increasing evidence that plant intra- and inter-specific diversity increases primary productivity, and that such effect may in turn cascade up to influence herbivores, there is little information about plant diversity effects on plant anti-herbivore defenses, the relative importance of different sources of plant diversity, and the mechanisms for such effects. For example, increased plant growth at high diversity may lead to reduced investment in defenses via growth-defense trade-offs. Alternatively, positive effects of plant diversity on plant growth may lead to increased herbivore abundance which in turn leads to a greater investment in plant defenses. The magnitude of trait variation underlying diversity effects is usually greater among species than among genotypes within a given species, so plant species diversity effects on resource use by producers as well as on higher trophic levels should be stronger than genotypic diversity effects. Here we compared the relative importance of plant genotypic and species diversity on anti-herbivore defenses and whether such effects are mediated indirectly via diversity effects on plant growth and/or herbivore damage. To this end, we performed a large-scale field experiment where we manipulated genotypic diversity of big-leaf mahogany (Swietenia macrophylla) and tree species diversity, and measured effects on mahogany growth, damage by the stem-boring specialist caterpillar Hypsipyla grandella, and defensive traits (polyphenolics and condensed tannins in stem and leaves). We found that both forms of plant diversity had positive effects on stem (but not leaf) defenses. However, neither source of diversity influenced mahogany growth, and diversity effects on defenses were not mediated by either growth-defense trade-offs or changes in stem-borer damage. Although the mechanism(s) of diversity effects on plant defenses are yet to be determined, our study is one of the few to test for and show producer diversity effects on plant chemical defenses.     

Light and Hormone Interaction in Apical Dominance in Phaseolus vulgaris L.

Gibberellic acid (GA₂), kinetin, and indole-3yl-acetic acid(IAA) each at four concentrations (0, 0.5, 5, and 50 µM)were applied alone and in all possible combinations to rootsof Phaseolus vulgaris L. grown under four different light regimes(7000, 14 000, 21 000, and 28 000 lx). GA₃ increased growthof main stem and laterals but reduced apical dominance, especiallyin the absence of, or at low kinetin concentrations. A highlevel of kinetin lowered GA₃ induced growth of main stems and,to a lesser extent, laterals. Kinetin greatly reduced apicaldominance, especially in the absence of, or at low GA₃ concentrations.IAA slightly reduced growth of main stems and laterals and slightlyincreased apical dominance. Generally the magnitude of the IAAeffects were less than those of GA₃ or kinetin and there wereless interaction between IAA and other factors than betweenGA₃ or kinetin and other factors. Light affected growth of bothmain stem and laterals but the effect was dependent on GA₃ andkinetin levels and the interactions were complex. Generallya hormone balance seems to be operative with gibberellin-promotinggrowth of main stem and laterals and cytokinins and possiblyauxins preventing excessive elongation. Differential responsesbetween main stem and lateral may be due to different localhormone concentrations and over-all responses may be temperedby light intensity.     

Apical Dominance is not Due to a Lack of Functional Xylem and Phloem in Inhibited Buds

Mature sieve tubes were located in inhibited cotyledonary budsof soybean plants. They were connected to the pholem systemof the stem and were shown to be functional by observing theirability to transport a phloem-mobile tracor. The associatedxylem system was also shown to be functional by using a decolourizedbasic dye tracer. The inhibited buds in the axils of the primaryleaves also contained sieve tubes and xylem elements which wereconnected to their counterparts in the stom. It is concludedthat in soybeans the inhibition of bud growth due to apicaldominance cannot be caused by an incomplete or non-functionalvascular system.