The Effects of Growth Substances on the Growth of Tapinanthus bangwensis (Loranthaceae) in vitro

The growth response of Tapinanthus bangwensis (Engler and K.Krause) Danser to exogenous growth substances, indole-3yl-aceticacid (IAA), kinetin, and gibberellic acid (GA₃), not only varieswith the concentrations of each of these substances in the mediumbut follows a similar trend in each case. Kinetin, IAA, andGA₃ were found to enhance growth but not by very appreciableamounts, kinetin being the most effective growth promoter. Highconcentrations of these substances (5.0 and 10.0 ppm) exceptGA₃ were toxic to the parasite. Gibberellic acid induced theproduction of accessory leaves at all the concentrations used.Kinetin was also effective in inducing accessory leaves butonly at a concentration of 1.0 ppm. Some of the lower concentrationsof the growth substances affected significantly some aspectsof the parasite's growth and development. Nevertheless, thegrowth in vitro was still slower than that in vivo.     

The Relationship between the Growth of the Primary Leaf and of the Coleoptile in Seedlings of Avena and Triticum

Partial inhibition of extension growth of the primary leaf occurswhen whole Triticum seedlings are immersed in aerated solutionsof IAA but is replaced by growth promotion when sucrose is addedto the external solution. In seedlings in which the coleoptilehas been excised, IAA increases the growth of the leaf bothwith and without additional sucrose. Inhibition of the leaf by moderate concentrations of IAA nolonger occurs when the seedling is detached from the endosperm.Sucrose added to the external solution raised the percentageelongation of the coleoptile almost to the level of that attainedin intact seedlings without additional carbohydrate. It alsoenabled the leaf to show a positive growth response with IAA. The results indicate that in intact seedlings treated with IAAthe growth of the primary leaf is markedly diminished owingto diversion of carbohydrate to the coleoptile if the growthof the latter is promoted as a result of the treatment. Whenthe competition of the coleoptile for carbohydrate is diminishedor eliminated, acceleration of the growth of the primary leafby IAA becomes apparent. In addition to the endogenous rhythm, with a period close to24 hours, induced in the growth-rate of the coleoptile whenseedlings of Avena are transferred from red light to darkness,a similar rhythm, with a slightly longer period, is inducedin the growth-rate of the primary leaf. This rhythm persistsin elongating leaves so long as they remain within the coleoptile.It can be recorded for at least 100 hours in deseeded seedlings. When intact seedlings of Avena are immersed for one hour inrelatively high concentrations of IAA and then transferred todistilled water for 18 hours, the elongation of the coleoptileis greater and the inhibition of the leaf is less than whenthey are transferred to humid air. Sections of the leaf of Triticum showed a slight increase inelongation in concentrations of IAA up to 5 mg./l., but no evidencewas obtained that sections of leaf and coleoptile exert any.influenceon each other's elongation when floated together on solutionsof IAA.     

Changes in the Levels of Endogenous Growth Regulators during Development of the Flowers of Chrysanthemum morifolium

Gibberellin-like substances and an auxin similar to IAA weredetected by bioassays in extracts of flowers of Chrysanthemummorifolium. The activity of these substances was shown to reacha maximum early in the development of the flower when its relativegrowth-rate was at a maximum, and then to decline with the relativegrowth-rate. The leaves of lateral flowering shoots were found to containgibberellins similar to those detected in the flowers whilea different gibberellin, which appeared to decrease in activitywith the age of the shoot, was detected in the stem. An auxinsimilar to indol-3yl-acetic acid (IAA) was also detected inthese stems. Growth-promoting substances were not detected inthe old stems and leaves from the main shoot. Gas-liquid chromatographyrevealed the presence of a number of additional gibberellinsin the flowers. The chemical nature of the growth substances is discussed inrelation to their biological and chromatographic behaviour.     

The Effect of Root Formation on Photosynthesis of Detached Leaves

The CO₂ exchange of fully expanded detached primary leaves ofdwarf bean (Phaseolus vulgaris) with roots on the petioles hasbeen measured. Rates of apparent photosynthesis and respirationincreased as roots grew, decreased when roots were removed,and increased again as root regenerated. Rates of photosynthesisof different leaves were highly correlated with the dry weightof root on their petioles. Photosynthesis and respiration weredecreased when root growth was restricted by kinetin, and wereincreased when root growth was stimulated by IAA. Photosynthesisof an attached leaf declined with time while that of a comparabledetached leaf increased. The results suggest that photosynthesisis correlated with the size of the roots that are a sink forphotosynthates.     

Chloroplast Protein Synthesis During Barley Leaf Growth and Senescence: Effect of Leaf Excision

Chloroplast protein synthesis was measured during the expansion,maturity and senescence of the oldest leaf of barley, Hordeumvulgare L., var. Hassan. A maximum rate of protein synthesisoccurred near the end of the expansion stage 9 d after sowing.Protein synthesis increased again at the beginning of senescenceand reached a new maximum at day 14 after sowing. Detachmentand incubation of leaves in the dark stimulated chioroplastprotein synthesis by fully expanded or by senescent leaves butnot by expanding leaves. If the detached leaves were kept inthe light, chloroplast protein synthesis was stimulated in fullyexpanded but not in senescent leaves. Short treatments (18 h)of leaf segments with growth substances in either light or indarkness, significantly changed the rate of protein synthesisshown by chloroplasts. The relationship between chloroplastprotein synthesis and leaf senescence is discussed. Key words: Hormones, light, maturity     

The Control of Leaf Senescence in Kleinia articulata by Photoperiod

Experiments with attached and detached leaves of K. articulatahave shown that senescence rates are determined by the daylengthprevailing during early growth (leaf-expansion stage) and thatthis effect is lasting, long days leading to early leaf death.Daylength also affects longevity after full expansion has occurred,long days hastening senescence. Tests with numerous plant-growthregulators have revealed beneficial effects of gibberellic acid,while kinetin is detrimental to survival at 50 ppm. Entire detachedleaves and isolated petioles behave similarly to leaves on theplant, but leaf discs do not behave in the same way. Other parametersaffected by daylength include: leaf shape, the capacity of leavesto form roots, and enlargement of mesophyll cells normal tothe leaf surface.     

The Dynamics of Leaf Growth and Photosynthetic Capacity in Capsicum frutescens L.

In Capsicum frutescens L. cv. California Wonder the specificleaf weight (dry weight per unit laminar area) at leaf unfoldingis three times higher in the eighth leaf than in the first leafproduced. Intermediate leaves exhibit a trend between the twoThe change in specific leaf weight during laminar expansionis greatest in leaf 1 and least (sometimes zero) in leaf 8.Large changes in specific leaf weight during laminar expansionare associated with a large degree of palisade cell expansion,while leaves showing smaller rates of change have less palisadecell expansion but cell division is more evident. At leaf unfoldingthe fraction I protein content per unit laminar area is higherin upper than in lower leaves. Ribulose diphosphate carboxylaseactivity per unit laminar area and ¹⁴CO₂ fixation per unit laminararea have a similar pattern of development in all leaves andshow no correlation with the changes in specific leaf weight.The peak of activity in all leaves occurs when the laminar areais 10 cm². These results are compared with previous data onlaminar expansion and are seen as in accord with current ideason leaf growth.     

PRIMORDIAL LEAF AND PHYTOHORMONE EFFECTS ON EXCISED SHOOT APICAL MERISTEMS OF COLEUS BLUMEI BENTH.

Coleus blumei Benth. apical meristems and apical meristems +1, +2, +3 primordial leaf pairs were cultured to examine phytohormone influences on development and correlative effects of developing primordial leaves on in vitro responses. The meristem with no phytohormones or low levels of IAA could not develop in vitro. At least 0.1 mg/l IAA and optimumly 1-2 mg/l IAA were required for development into complete plants. IAA from 0.1 to 3 mg/l also resulted in root development with no apparent leaf or shoot formation. Levels of IAA higher than 3 mg/l were inhibitory to development. Kinetin, as a substitute for naturally occurring cytokinins, alone (0.0003 to 3 mg/l) resulted in development of rosettes of leaves. In the presence of IAA (***1 mg/l) and kinetin (0.003 mg/l) plants, rosettes, individual leaves with roots, and roots developed from isolated meristems. Glutamine and adenine sulfate both appeared inhibitory to meristem development. With +1, +2, +3 developing primordial leaf pairs left attached to the apical dome, three pairs were required for plant formation in the absence of phytohormones. In the presence of IAA, two pairs of primordial leaves resulted in plant formation; whereas, with IAA and low levels of kinetin one pair of primordial leaves was enough. Higher levels of kinetin were inhibitory to plant development with primordial leaves present. ABA appeared to be inhibitory to development of meristems and meristems +1, +3 primordial leaves at low concentrations and resulted in death at ***1 mg/l. Developing primordial leaves appear to supply the apical meristem with a balance of phytohormones during growth. Meristem development into a plant first involved formation of leaf primordia. Establishment of a bipolar axis with root formation followed.     

Growth Regulator Interactions in the Growth of the Shoot System of Avena sativa Seedling: II. THE GROWTH OF THE FIRST LEAF AND THE COLEOPTILE1

1. Studies have been made of the growth in culture medium of thecomponent parts of compositesegments excised from 3 to 7-day-oldAvena sativa seedlings and comprising portions of coleoptileand first leaf bases and various lengths of first internodetissue.
2. The effects of various concentrations of gibberellicacid (GA)and indole-3- acetic acid (IAA) alone and in combinationhavebeen studied on the growth of these organs.
3. Both GA andIAA stimulate the growth of coleoptile base tissuebut in combinationtheir joint effects are less than additive.No synergism occurs.
4. The growth of the first-leaf base is greatly stimulated byGAbut is inhibited by IAA. In combination, the stimulatoryeffectof GA (up to 1 0 p.p.m.) may be virtually eliminatedby evenlow concentrations of IAA (0.01 p.p.m.).
5. The inclusionof first internode tissue in the segments considerablyincreasesthe growth of first leaf base tissue but has no consistenteffecton the growth of coleoptile base tissue. The presenceof firstinternode tissue also greatly increases the degreeof growthstimulation invoked by GA but does not influence thedegreeof IAA inhibition. It is postulated that the first internodetissue is the source of an unknown growth factor necessary forGA action in the first leaf and potentiating the action of endogenousgibberellin.
6. Kinetin, adenine sulphate, glutarnine, glutarnicacid, asparagine,glycine, arginine, histidine, lysine, aneurin,and pyridoxinewill not simulate the effects of this unknowngrowth factorin the growth of leaf tissue. Like IAA, kinetinvirtually eliminatesthe GA stimulation of leaf growth.
7. Astudy of extracts of internode tissue in various solvents,analysedby paper partition chromatography and assayed by thegrowthof the first leaf base, has indicated the presence ofgrowthinhibitors and gibberellin-like substances but has failedtoisolate the postulated endogenous GA-synergist.
8. The implicationsof these results for growth correlations andthe hormone controlof shoot growth in Avena sativa seedlingsis discussed.

     

Developmental Physiology of Sugar-beet: IV. EFFECTS OF GROWTH SUBSTANCES AND DIFFERENTIAL ROOT AND SHOOT TEMPERATURES ON SUBSEQUENT GROWTH

The effects of three growth substances, viz. indol-3yl-aceticacid (IAA), gibberellic acid (GA₃), and kinetin (KIN), and differentialshoot and root temperatures on growth of sugar-beet (Beta vulgarisL.) plants have been studied. IAA, GA₃, and KIN were applied in aqueous lanolin at differentconcentrations (50 ppm to 5000 ppm) to decapitated sugar-beetplants at the eight-leaf stage, one group also having alternateleaves removed. The growth substances significantly increasedthe dry weights of the plants when all the leaves were present,which was mainly explained by the large increase in roots. Thegrowth substances probably stimulated cambial activity and hencethe mobilization of substrates resulting in a bigger root whena relatively large leaf area existed. The failure of the plantsto respond to treatments following the removal of alternateleaves suggests that under such conditions the growth substanceshave hardly any major effect on the production of substrates;rather they influence growth by regulating the movement of substratesby altering the ‘sink strength’ if the supply ofsubstrates is not limiting. It could also be that the rootsproduce sufficient growth substances to maintain half the leavesat maximum expansion and maximum photosynthesis. Treatment withgrowth substances would therefore have little effect. When allthe leaves were present, they are limited by insufficient growthsubstances. All combinations of root and shoot temperatures of 17 and 25°C were imposed on plants decapitated at the eight-leafstage, one group also having each alternate leaf removed. Leaf8 expanded most at shoot and root temperature of 25 °C whereasother leaves had the largest areas at shoot and root temperatureof 17 °. When all the leaves were present root growth wasmaximal at shoot temperature of 17°C and root temperatureof 25 °C, but when alternate leaves were removed maximumroot growth occurred at shoot and root temperatures of 25 °C.Generally, a higher concentration of soluble carbohydrates wasfound in the roots and leaves when either the shoot or rootor both were kept at 17 °C. Concentrations of nitrogen,phosphorus, and potassium in different organs were less at 17°C than at higher shoot or root temperatures and decreasedwith age.     

Regulation of Leaf Shape in the Solanifolia Mutant of Tomato (Lycopersicon esculentum) by Plant Growth Substances

The solanifolia mutant (sf/sf) of tomato (Lycopersicon esculentum)produces leaves consisting of leaflets with entire margins,unlike the lobed leaflets of normal plants. Normal plants treatedwith gibberellic acid (GA₃) produced leaves with entire marginswhereas mutant plants exposed to 2-chloroethyl-trimethyl ammoniumchloride (CCC)—an inhibitor of gibberellin biosynthesis—producedlobing of leaflets. The leaf area of the mutant was significantlygreater than that of the normal, but was not significantly differentfrom GA₃-treated normal leaves. Similarly, in CCC-treated mutantleaves the leaf area was not different from that of normal untreatedleaves. These observations suggest that the sf/sf mutation affectsthe leaf shape through its effect on endogenous gibberellinsand/or inhibitory substances. Leaf shape, Lycopersicon esculentum, plant growth substances, tomato     

Treatments that Enhance Flowering in the Post-inductive Period of a Short-day Plant, Chenopodium rubrum L.

Flowering was promoted in young plants of Chenopodium rubrumL. by application of growth inhibitors such as 5-fluorodeoxyuridine(FUDR) and (2-chloroethyl) trimethylammonium chloride (CCC),growth substances (indol-3yl-acetic acid, IAA), by the removalof roots and by drought. All the treatments were effective onlyduring the post-inductive period and at the threshold levelof photoperiodic induction. The response of plants was strictlytime-dependent. The experimental data indicate that the stimulationof flowering is usually accompanied by inhibition of leaf initiationand growth. The treatments probably produced variation in thequantitative expression of flowering by causing a shift in emphasisin the development of leaf and bud primordia at the shoot apex.The dynamic analysis of differentiation of the shoot apex indicatesa correlation between the morphological stage of the shoot apexand its responsiveness to the treatments.     

Positive and Negative Messages from Roots Induce Foliar Desiccation and Stomatal Closure in Flooded Pea Plants

Flooding the soil for 5–7 d caused partial desiccationin leaves of pea plants (Pisum sativum. L. cv. ‘Sprite’).The injury was associated with anaerobiosis in the soil, a largeincrease in the permeability of leaf tissue to electrolytesand other substances, a low leaf water content and an increasedwater saturation deficit (WSD). Desiccating leaves also lackedthe capacity to rehydrate in humid atmospheres, a disabilityexpressed as a water resaturation deficit (WRSD). This irreversibleinjury was preceded during the first 4–5 d of floodingby closure of stomata within 24 h, decreased transpiration,an unusually large leaf water content and small WSD. Leaf waterpotentials were higher than those in well-drained controls.Also, there was no appreciable WRSD. Leaflets detached fromflooded plants during this early phase retained their watermore effectively than those from controls when left exposedto the atmosphere for 5 min. Stomatal closure and the associated increase in leaf hydrationcould be simulated by excising leaves and incubating them withtheir petioles in open vials of water. Thus, such changes inflooded plants possibly represented a response to a deficiencyin the supply of substances that would usually be transportedfrom roots to leaves in healthy plants (negative message). Ionleakage and the associated loss of leaf hydration that occurswhen flooding is extended for more than 5 d could not be simulatedby isolating the leaves from the roots. Appearance of this symptomdepended on leaves remaining attached to flooded root systems,implying that the damage is caused by injurious substances passingupwards (positive message). Both ethylene and ethanol have beeneliminated as likely causes, but flooding increased phosphorusin the leaves to concentrations that may be toxic. Key words: Pisum sativum, Flooding, Foliar desiccation, Stomata, Ethylene     

HORMONAL REQUIREMENTS OF EXCISED DIANTHUS CARYOPHYLLUS L. SHOOT APICAL MERISTEM IN VITRO

Whereas a medium containing kinetin alone enabled a few Dianthus caryophyllus L. apical meristem dome explants to develop into rooted plants, the highest frequency of plants was obtained in one containing supplements of both IAA and kinetin. In an unsupplemented medium, continued development required that explants have 2 pairs of primordial and a pair of expanding leaves. Kinetin alone caused production of many new leaves, but the development was significantly less than when it was furnished in combination with IAA. IAA given alone caused meristem explants to develop primarily callus, roots, and a few leaves. Gibberellin and abscisic acid were without promotive effects on leaf and shoot formation. A balance of hormonal substances, synthesized in young leaf structures and relocated to the meristem, is proposed as the fundamental mechanism that regulates new leaf initiation in the shoot apex.     

Leaf and Tiller Production of Prairie Grass (Bromus willdenowii Kunth) and Two Ryegrass (Lolium) Species

A detailed morphological study of three prairie grass cultivars(Bromus willdenowii Kunth) was conducted under ‘vegetative’and ‘reproductive’ growth conditions (short andlong photoperiods) and at different temperatures. Perennialryegrass (Lolium perenne L.) and Westerwolds ryegrass (Loliummuhiflorum Lam.) were compared during vegetative growth. Prairie grass had higher leaf appearance rates (leaves per tillerper day) and lower site filling (tillers per tiller per leafappearance interval) than the ryegrass species. Tillering rates(tillers per tiller per day) were also lower, except under vegetativeconditions at 4C. Low tiller number in prairie grass was notdue to lack of tiller sites but a result of poor filling ofthese sites. Lower site filling occurred because of increaseddelays in appearance of the youngest axillary tiller and lackof axillary tillers emerging from basal tiller buds. In prairiegrass, no tillers came from coleoptile buds while only occasionallydid prophyll buds develop tillers. Low tiller number in prairiegrass was compensated for by greater tiller weight. Prairiegrass had more live leaves per tiller, greater area per leafand a high leaf area per plant. Considerable variation between cultivars was found in prairiegrass. The cultivar ‘Bellegarde’ had high leaf appearance,large leaves and rapid reproductive development, but had lowlevels of site filling, tillering rate, final tiller numberand herbage quality during reproductive growth. ‘Primabel’tended to have the opposite levels for these parameters, while‘Grasslands Matua’ was intermediate and possiblyprovided the best balance of all plant parameters. prairie grass, Bromus willdenowii Kunth, perennial ryegrass, Lolium perenne L., Westerwolds ryegrass, Lolium multiflorum Lam., temperature, photoperiod, leaf appearance, leaf area, tillering, site filling, tillering sites, yield     

Measuring the RGR of Individual Grass Plants

Vegetative growth of grasses was analysed by dry mass increaseof growing leaves.Holcus lanatuswas grown in a controlled environmentand leaf extension rates of leaf numbers 5–10 of the maintiller were monitored daily. Leaf appearance and leaf extensionrates (LER) of leaves 5–7 enabled the prediction of thefinal length and dry mass of leaf 8 during its growth. A linearincrease of leaf mass per unit leaf length (LML) of leaf 8 wasobserved during growth. After harvest the daily increase indry mass of growing leaves was calculated from the LER and correspondingincrease in LML. The relative growth rate (RGR) of the maintiller showed day-to-day fluctuations and was gradually reducedby 50% over a 16-d period. The RGR of the shoot was maintainedby tillering. The RGR of a single (grass) plant can be calculatedfrom four parameters only: LER, LML, leaf appearance and tillering.Variation of RGR over a period can be reconstructed after harvestand the impact of these four parameters on RGR can be established.Copyright1998 Annals of Botany Company. Relative growth rate, grass, leaf growth,Holcus lanatus.     

Hormone Interaction in Apical Dominance in Phaseolus vulgaris L.

Gibberellic acid (GA₃), kinetin, and indole-3yl-acetic acid(IAA) were applied to roots of Phaseolus vulgaris under twodifferent light intensities and when either young or old leaveswere removed In all cases GA₃, promoted stem and lateral growth,especially when light intensity was reduced. Promotion by GA₃,of stem growth under reduced light was reduced if IAA and kinetinwere present; promotion of lateral growth under reduced lightwas reduced if IAA was added and eliminated if kinetin or kinetinplus IAA were added to GA₃. Removal of young and mature leavesreduced main stem growth; removal of young leaves promoted,and of mature leaves reduced, lateral shoot growth. We suggestthat shoot growth and apical dominance are governed by the balanceof hormones present in elongating internodes. There may be twoways of modifying this balance; firstly by altering light, temperature,or nutrients, or by applying hormones generally to the plant.Secondly, local modifications can be made by removing apicesor young leaves, or applying hormones in lanolin to specificareas. Knowledge of both the general and local conditions maybe necessary for a complete understanding of apical dominance.     

DEVELOPMENTAL POTENTIAL OF EXCISED PRIMORDIAL AND EXPANDING LEAVES OF COLEUS BLUMEI BENTH.

Coleus blumei Benth. primordial leaves 1 through 4 and expanding leaves 5 to 8 were isolated and cultured to examine the effects of auxin and kinetin on development. Without the plant growth regulators in the medium, expanding leaves 7 and 8 developed as leaves; younger leaf primordia did not develop. With 0.01 to 5.0 mg/1 IAA, 2–7% of the youngest pair of primordial leaves (1 and 2) developed as roots. Small leaf blade development occurred on IAA at 0.5 to 5.0 mg/1 with 10–12% of the explants, and shoots developed from 2% of the youngest primordia explants at 3 mg/1 IAA. With 2–28% of the second set of primordial leaves (3 and 4), a leaf with a root developed with 0.01 to 5.0 mg/1 IAA. At 3.0 mg/1 IAA, in addition to leaf formation, 2% of the explants formed a rosette of leaves and 1% formed a shoot. With the highest level of IAA (5 mg/1), 2% of the explants formed a root. Expanding leaves 5 through 8 developed mostly into leaves without petioles on IAA and kinetin. Plant development occurred from 2% of the youngest primordial leaves on 0.03 mg/1 kinetin; otherwise, these primordia on 0.003 to 2 mg/1 kinetin developed into abnormal leaves. Primordia 3 and 4 developed into normal appearing leaves at levels of kinetin between 0.03 and 2 mg/1. At lower levels the leaves were abnormal.     

Morphology of Head Formation of Cos Lettuce (Lactuca sativa L. cv. Romana) 2. The Development of Spiral-leaved Heads

The morphogenesis of a malformation occurring in cos lettuceheads (Lactuca sativa L. cv. romana) during and at the end ofthe heading period, is described. Abnormal leaf growth resultedin twisting of the midrib of the affected leaf around its ownaxis, giving the leaf a spiral-like look. Twisting began atthe outer leaves and then, mainly by affecting successive orsometimes alternating leaves, spread inwards in the directionto the younger leaves. Accelerated midrib elongation accompaniedleaf twisting. The external appearance of this malformationwas influenced by the location and number of the twisted leaves,and by the degree of twisting of the individual leaves.     

Increase in DNA Content of Primary Leaves of Phaseolus vulgaris upon Decapitation

Changes in DNA content of bean (Phaseolus vulgaris) primaryleaves after decapitation were investigated. When apical budswere removed at 11 d, DNA content per leaf increased by about20% at 15 d and then decreased in parallel with the controls.The RNA and chlorophyll contents, fresh weight, and leaf areaexpressed on a single leaf basis changed in the same manneras the DNA content in response to decapitation. But when bothapical and lateral buds were removed, all these values continuedincreasing during the test period. Thus, growing lateral budsand apical buds have the same effect on the DNA change in primaryleaves as that due to ageing of the leaves. Cell number perleaf was not increased by any treatment, indicating that theobserved increase in the DNA content of primary leaves is ascribableto an increase in DNA per cell. Next, the whole shoots above the nodes of primary leaves wereremoved at various ages. The response of primary leaves to decapitationvaried according to their age. With age, they lost the abilityto increase their fresh weight, area, and chlorophyll contentbut not their DNA and RNA contents in response to decapitation.Decapitation stimulated chloroplast replication only withinthe period in which chloroplasts were replicating in controlleaves, but it induced chloroplast enlargement at any age. Therefore,the increase in DNA content after decapitation may be partiallydue to an increase in the amount of chloroplast DNA. When stems were heat-girdled above the nodes of the primaryleaves, these leaves showed responses similar to but smallerthan those to decapitation. The senescence of primary leavesseems to be controlled by the distribution of substances whichare transported from the roots.