Tree Suppression and the Self-thinning Rule in a Monoculture of Pinus radiata D. Don

An experiment comparing different initial planting densitiesof Pinus radiata D. Don was established in 1940 at Mt. Burrin south-east South Australia. It had been measured periodicallyto age 40 years. It is shown that after some years the treesof each individual plot of the experiment segregated into agroup of ‘suppressed’ trees which grew little indiameter and a group of actively growing ‘dominant’trees. This is consistent with other work that has suggestedthat monocultures tend to produce a two-tiered canopy. The slopeof the relationship between the logarithm of the mean weightper tree and the logarithm of the density of the various plotsof the experiment (the ‘competition-density effect’)was computed at each age at which the experiment was measured,both for all live trees of the plots and for the dominants only.For this analysis tree weights were estimated using the allometricrelationship between weight and diameter at breast height. Forthe dominants, the slope rapidly declined with age until itapproached – 1.5, which is the limiting slope of the competition-densityeffect or the so-called ‘self-thinning rule’. Whenall live trees of the plots were used, the slope declined slowlywith age but remained far from – 1.5. Previously, theself-thinning rule has been seen as determining the way in whichmortality, i.e. self-thinning, occurs in stands. In the presentwork, the rule is found to apply to the larger, actively growingtrees of the stands soon after they start to suppress the smallertrees and long before substantial mortality has occurred inmany stands. Thus tree suppression, rather than mortality, isseen to determine the rule. The pattern of diameter growth oftrees of the various stands of the experiment is found to beconsistent with previous findings that skewness of the frequencydistribution of diameters of trees of stands increases withtime. Pinus radiata D. Don, radiata pine, self-thinning, competition     

Analysis of Shoot Apical Growth of Picea sitchensis Seedlings

During the first 100 days after sowing (March-June) the followingchanges took place at the terminal shoot apices of Picea sitchensisseedlings: plastochrones (T) decreased from over 24 h to 4 h;apical domes enlarged from less than 0·20 mm to 0·45mm diameter (D); the ‘projected’ area of tissuesproduced by the apical domes (i.e. viewed from above) increasedin amount from less than 0·012 to 0·024 mm² day^-1;about 15 per cent of this tissue was re-invested in the apicaldomes, the rest was used to produce primordia; and the volume-doublingtimes of the apical dome tissues decreased from over 150 h to50 h. After 100 days there was no further re-investment in theapical domes, but the domes did not decrease abruptly in size.Less tissue was produced per day, but the primordia were smallerso that the rate of primordia formation did not fall precipitously.Plastochrone ratios were inversely related to D, but the relationshipbetween T and D depended on whether T was decreasing or increasing.Progenies which were known to be fast growing tended to build-uptheir apical domes rapidly (i.e. have large ‘re-investmentratios’) and to be capable of producing small primordia.These attributes can evidently be evaluated on seedlings andcould help to lessen the cost of tree breeding progeny-testprogrammes. meristem, Picea sitchensis, Sitka spruce, growth, shoot apex     

A Study of the Growth of Four Provenances of Pinus contorta Dougl

A study was carried out on the growth of third- and fourth-yearseedlings of four provenances of Pinus contorta, two of whichwere from high elevations and two from coastal sites. Markeddifferences in height growth were observed, the two coastalprovenances showing significantly greater growth than the twomontane ones. These growth differences were due to differencesin the numbers of needle and stem ‘internode’ primordialaid down in the resting buds in the preceding year, and notto differences in ‘internode’ length in the annualextension shoots. The differences in height growth were paralleledby differences in dry weight. A greater relative height growthrate was shown by one coastal provenance and probably also bythe second, than by the montane provenances. The levels of reservematerials were similar in all provenances. These results arediscussed and it is concluded that the growth of montane provenancesmay be ‘sink limited’ rather than ‘sourcelimited’ when they are grown at lower altitudes.     

Pollen Tube Development and Characteristics of the Protein Emission in Conifers

When ripe, viable pollen of Pinus contorta, Pinus nigra, Piceasitchensis, Abies alba and Cedrus deodara is germinated on asuitable artificial substrate, the process of pollen tube growthbegins 12–36 h later, according to the species. The tubeemerges at the leptoma, a structurally specialized site in thepollen wall, and the early tube wall is continuous with thetwo microfibrillar intine strata within the grain. Later inpollen tube development, when cytoplasmic zonation has beenestablished, only the inner of these two wall layers is differentiated. Cytochemical methods show that, during hydration and throughoutthe period of tube growth in culture, proteins are releasedfrom the pollen grains; before germination most conspicuouslyfrom the leptoma, subsequently from the tube itself. The emissioncontains a number of hydrolytic enzymes and a non-catalyticmoiety. Gel-electrofocusing reveals that the hydrolases releasedfrom germinating Pinus contorta pollen include several acidphosphatase and esterase isozymes, and that there are differencesbetween the composition of the emission and the spectrum ofsoluble proteins extracted from the pollen grains before germination.Analysis by immunodiffusion demonstrates that two componentswith antigenic characteristics present in the quiescent pollengrains are represented in the emission. The possible utilityof the released components in the biology of conifer reproductionis discussed. Pinus contorta, Pinus nigra, Picea sitchensis, Abies alba, Cedrus deodara, lodgepole pine, Austrian pine, Sitka spruce, European silver fir, deodar, pollen tube, pollen germination, protein release, isoelectric focusing     

Effects of Gibberellin, Clone and Environment on Cone Initiation, Shoot Growth and Branching in Pinus contorta

Effects of photoperiod, temperature and gibberellin applicationwere clearly separated from clonal differences when small rootedcuttings, originating from two ‘floriferous’ treesof the same provenance of Pinus contorta Dougl., were treatedwith a simple micro-injection technique and grown in growthcabinets. Short days (10 h) for 11 weeks induced six times asmany female cones as long days (19.5 h), completely inhibitedfurther needle elongation after 5 weeks, and also led to enhancedbranching by year 3. Cool temperatures appeared to favour malecone initiation in year 1, and (in conjunction with long days)significantly promoted height in year 2. Injection with 1 mgGA_4/7 led to substantial and highly significant increases inbud growth in year 1, and in height, needle number and totalneedle length in year 2. Gibberellin treatment also increasedbranching but did not significantly affect internode elongationin year 2, nor cone initiation or needle elongation in year1. Highly significant clonal differences occurred, with onegenotype having longer leaves and buds in year 1, and up to80 per cent greater numbers and total lengths of needles inyear 2. Marked clonal differences in height growth, branchingand sex ratio were also found. The implications of both resultsand approach are discussed in relation to developmental physiology,genetic selection and advances in forestry research. leaf growth, height growth, branching, cone initiation, sex, photoperiod, temperature, controlled environments, gibberellin, vegetative propagation, clonal differences, Pinus contorta,, pine     

Modelling and Measuring Transpiration from Scots Pine with Increased Temperature and Carbon Dioxide Enrichment

Starting in 1996, individual trees of Scots pine (Pinus sylvestrisL.) aged 30 years, were grown in closed-top chambers and exposedto either normal ambient conditions (CON), elevated CO₂(approx.700 µmol mol^-1; Elev. C), elevated temperature (approx.2 °C and approx. 6 °C above the outside ambient temperatureduring the ‘growing season’ and ‘off season’,respectively; Elev. T) or a combination of elevated CO₂and warmertemperature (Elev. CT). Sap flow was monitored simultaneouslyby the constant-power heat balance method in a total of 16 trees,four for each treatment, over a 32 d period in summer 1998 (afterthe completion of needle expansion and branch elongation). Toquantify the contributions of crown and physical environmentalvariables to total crown transpiration, a ‘sun/shade model’was developed and used to partition the changes in transpirationto different sources. The results of the sap flow measurementsindicate that (1) total daily sap flow (E_tree.d) varied from0.15–3.41 kg per tree; (2) the treatment effect on E_tree.ddependedgreatly on the weather conditions; (3) the cumulative E_tree.dforthe 32 d dropped significantly by 22% relative to CON (P =0.038)under Elev. C and increased significantly by 21% (P =0.043)and 16% (P =0.048) under Elev. T and Elev. CT, respectively.In general, the modelled transpiration gave good agreement withthe sap flow results. The model computations showed that, ona typical sunny day in summer, the effect of treatment on crownstomatal conductance was responsible for approx. 80% of thechange inE_tree.d , while the increase in needle area and theeffect on total radiation absorption contributed only a smallpercentage. Furthermore, sunlit needles were responsible forover 60% of change in transpiration. The effect of the treatmentson E_tree.dwas larger at high temperature and vapour pressuredeficit but was not sensitive to incident daily radiation. Copyright2000 Annals of Botany Company Transpiration model, sap flow, CO₂and temperature elevation, environment-controlled chamber, Pinus sylvestris L.     

Studies of Growth and Flowering in Picea sitchensis (Bong.) Carr. 2. Initiation and Development of Male, Female and Vegetative Buds

Vegetative shoots from the base of the crown, and from partsof the tree likely to form male or female buds, were collectedfrom 40–years–old trees of Picea sitchensis (Bong.)Carr. throughout the 1973–4 annual growth cycle. The morphologyand growth rates of the terminal buds on these shoots were assessed. Bud scale primordia were formed most quickly in the female position,at an intermediate rate in the male position and most slowlyin the basal vegetative position during April, May and June.In July and early August the apical meristems swelled to formdomes and continued to grow at the same relative rates in themale, female and basal vegetative positions. Reproductive budswere first morphologically distinct in late August and sporangiaappeared in October. Dormancy, defined by the pause in apicalvolume increase, extended from mid-October to mid–March.Young strobili grew much faster than basal vegetative shootsof the same age between mid–March and bud burst in lateApril. Throughout the growth cycle, external changes in budsize reflected changes in size of the apical meristem, youngstrobihis or young vegetative shoot inside the bud. It is proposed that the rate of growth of an apical meristemmay be causally related to the type of bud which subsequentlydevelops from it. Sitka spruce, Picea sitchensis, bud development, morphology, growth of apical dome, flowering     

The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 1. Grown as Simulated Swards During a Regrowth Period

Simulated swards of each of two selection lines of Lolium perennecv. S23 with ‘fast’ and ‘slow’ ratesof ‘mature tissue’ respiration were establishedin growth rooms at 20/15 °C day/night temperatures and studiedover four successive regrowth periods of 46, 30, 26 and 53 daysduration. The ‘slow’ line outyielded the ‘fast’,both in harvestable shoot (above a 5 cm cut) and in root andstubble. Its advantage increased over successive regrowth periodsto 23 per cent (total biomass). Gas analysis measurements onthe entire communities (including roots), during the final regrowthperiod, showed that the ‘slow’ line had a 22–34per cent lower rate of dark respiration per unit dry weight.This enabled it to maintain its greater mass of tissue for thesame cost in terms of CO₂ efflux per unit ground area. Halfthe extra dry weight produced by the ‘slow’ line,relative to the ‘fast’, could be attributed to itsmore economic use of carbon. The rest could be traced to a 25per cent greater tiller number which enabled the ‘slow’line to expand leaf area faster (though not at a greater rateper tiller), intercept more light and fix more carbon, earlyin the regrowth period. Lolium perenne L., ryegrass, respiration, maintenance respiration, tiller production, simulated swards, canopy photosynthesis, carbon economy     

Patterns of Trunk Diameter, Tree Height and Crown Depth in Crowded Abies Stands

The size structure of trees in crowded, even-aged Abies (fir)stands of ‘Shimagare’ or ‘wave-regenerated’sub-alpine forests is analyzed. Tree-height distributions showconsistently smaller variation and less positive skewness thanthe distributions of trunk diameter and crown depth (tree heightminus height of the lowest branch). This difference is associatedwith changes in the relationships between trunk diameter, treeheight and crown depth as stands age. These, in turn, resultfrom self-pruning of the lower foliage crown due to competitionfor light in crowded stands. Abies, diameter-height curve, competition, size distribution, stand development, tree geometry, wave-regeneration     

Influence of Flooding on Net CO2Assimilation, Growth and Stem Anatomy of Annona Species

A series of experiments was conducted to assess net CO₂assimilationand growth responses to waterlogging of grafted and seedlingtrees in the genus Annona. Seedlings of A. glabra, A. muricataandA. squamosa L., and scions of ‘Gefner’ atemoya(A. squamosaxA. cherimola Mill.), ‘49-11’ (‘Gefner’atemoyaxA. reticulata L.), ‘4-5’ (‘Priestley’atemoyaxA. reticulata), A. reticulata grafted onto either A.glabra, A. reticulata orA. squamosa rootstocks were floodedfor up to 60 d. Soil anaerobiosis occurred on the third dayof flooding. Seedlings ofA. glabra and A. muricata, and thescions ‘49-11’, ‘Gefner’ atemoya, andA. reticulata grafted onto A. glabra rootstock were consideredflood tolerant based on their ability to survive and grow inflooded conditions. Scions of the normally flood-sensitive A.reticulata, ‘Gefner’ atemoya, and ‘49-11’tolerated root waterlogging when grafted onto the flood-tolerantspecies, A. glabra. In contrast, flooding of A. squamosa seedlingsand rootstocks, and A. reticulata rootstocks greatly reducedgrowth and net CO₂assimilation rates, and resulted in 20–80%tree mortality. Stem anatomical responses to long-term flooding(12 continuous months) were assessed in seedlings of A. glabraand A. muricata, and trees of ‘49-11’ grafted ontoA. glabra. Flooded trees developed hypertrophied stem lenticels,particularly in A. glabra, and enlarged xylem cells resultingin thicker stems with reduced xylem density. Flooding did notincrease air spaces in pre-existing xylem near the pith or inxylem tissue that was formed during flooding. Thus, flood tolerancedid not involve aerenchyma formation in the stem. Copyright1999 Annals of Botany Company Flood tolerance, net CO₂assimilation, photosynthesis, stem anatomy, shoot growth, anaerobiosis, Annonaceae.     

Simulation of the Structural Development of Warm-Temperate Rain Forest Stands

The change in all-sized dbh distribution with stand age wassimulated using data from secondary stands of different agesafter clear-felling in warm-temperate rain forests in YakushimaIsland, southern Japan. The cumulative basal area [cm² m^–2]of all trees larger than a given dbh x [cm] was used as an indexof onesided density effect, which primarily regulates the growthrate and mortality of the tree at x. The relative growth ratein dbh was expressed by a negative linear function of both thecumulative basal area and logarithmic dbh, irrespective of standage. Mortality was found to be positively related to the cumulativebasal area. From these empirical relations, the change in dbhdistribution during the course not only of stand developmentafter clear-felling, but also of gap regeneration within primaryforests, was successfully simulated using the continuity equation,eliminating ‘stand age factors’ from the model.Results of the simulation satisfied the —3/2 power lawof self-thinning. Competition, continuity equation, cumulative basal area, simulation, size distribution, stand dynamics, —3/2 power law, warm-temperate rain forest, Yakushima Island     

Integrated Analysis of Growth and Light Interception in Winter Lettuce II. Differences between Cultivars

‘Integrated growth analysis’ (emphasizing aspectsof crop and plant structure) and ‘light conversion analysis’(stressing the efficiency of interception and photosyntheticconversion of light) have been used to investigate the wintergrowth of different cultivars of butterhead and crisphead typesof glasshouse lettuce. Measurements from ‘Ambassador’ (large-framed butterhead),‘Renate’ (medium-sized butterhead) and ‘Cristallo’(crisphead) were made, statistical progressions were fittedto the primary data and hence estimates of all the analyticalcomponents were derived. Curves for crop growth rate, like those for most other components,followed a generally similar pattern for all three cultivars.In integrated growth analysis, the biomass curve for Ambassadorlay above the curves for the other cultivars. The weight advantagewas initially 60 per cent and it persisted with only a smallreduction (to 40 per cent) until the final harvest. Relativegrowth rate varied little between cultivars because differencesobserved in leaf area ratio were complementary to those seenin net assimilation rate. In light conversion analysis, differences in light interceptingefficiency between cultivars were not statistically significant,though Ambassador attained full interception 4 days earlierthan Renate and 6 days earlier than Cristallo. Differences inlight utilizing efficiency were small and non-significant exceptduring the post-rosette stage when the value for Renate waslower than that of either Ambassador or Cristallo. Deviationsaround the fitted curves were correlated with fluctuations inthe light regime. An assessment is made of the utility and limitations of thetwo procedures. It is concluded that both approaches can assistin analyzing the rate of dry matter production in crops or plantstands. Integrated growth analysis is advantageous when theneed arises to treat individual and population-based attributessimultaneously, while light conversion analysis provides a meansof explicitly incorporating the primary environmental variableinfluencing growth. Lactuca sativa L., lettuce cultivars, growth analysis, crop growth rate, relative growth rate, light interception, light utilization     

Shoot Apical Meristems of Picea sitchensis Seedlings Accelerate in Growth Following Bud-set

First-year seedlings of Picea sitchensis were induced to setbuds by transferring them from long days (17 h) to short days(10 h at 20 °C, 12 h night at 12 °C). After 20 shortdays, the seedlings stopped growing in height, the shoot apicaldomes began enlarging and the transverse growth rates of theshoot apical dome tissues increased. For about 25 days thereafterprimordia were produced faster within the developing buds thanat the apices of seedlings which had been kept in long daysto prevent bud-set. Accelerated primordia production after bud-setenables the largest possible buds to be formed during the frost-freeperiod before winter. Picea sitchensis, apical meristem, bud-set     

The Nitrogen Content of Plants and the Self-thinning Rule of Plant Ecology: A Test of the Core-skin Hypothesis

The ‘core-skin’ hypothesis postulates that secondarilythickened plants behave energetically as an inert ‘core’covered by an active ‘skin’, the ‘skin’being two-imensional, the ‘core’ three-dimensional.This would explain the ‘self-thinning ‘or‘–3/2’ rule of plant ecology, that is, the tendencyfor log (dry weight per plant) and log (number of plants perunit area) to progress along a straight line relationship, withslope = – 3/2’. The hypothesis was tested as follows. Plant nitrogen contentwas used as an estimate of the mass of ‘skin’ perplant, and dry weight as an estimate of the mass of the ‘core’.As plants mature the slope of the relationship between y = log(mass of nitrogen per plant) and x = log (mass of dry matterper plant) is expected to decline from an initial value of 1.0towards a final value of 0.66. The intercept of the relationshipis expected to reflect the intrinsic content of ‘skin’per unit of ‘core’. Genotypic variation in thisparameter should cause genotypic differences in the maximumattainable yield of biomass per unit area. The expectations were investigated by fitting the function y= p+qx+r exp – x to 30 sets of data on plant nitrogencontent, plant weight and time in 18 different vegetables. Simplelinear regressions of y on x were fitted to more limited setsof data on weights and nitrogen contents of mature trees. Theexpectations were, with some minor exceptions, confirmed. Nitrogen, yield, plant competition, self-thinning     

An Examination of the Differences in Dry Matter Production shown by Some Progenies of Pinus sylvestris L.

A study was carried out to determine the extent of the variationin growth rate between several selected progenies of Scots pine{Pinus sylvestris L.) and the physiological characteristicsunderlying such variation, using growth analysis and directmeasurement of photosynthetic rates. Differences in relativegrowth rate (RGR) could be demonstrated for first and secondyear seedling progenies, and these largely reflected differencesin net assimilation rate (NAR). The relative performance ofthe progenies in different experiments was not always the same,although the highest RGR was shown by the same progeny in bothfirst-year and second-year seedlings. For most progenies theobservations were extended to 14-year-old trees growing in fieldtrials established by the Forestry Commission. Comparisons ofthe relative height growth rates (RHGR) of these trees showedno significant differences between the progenies, apparentlybecause the growth rates were limited by the site conditions.Thus, the size differences at the end of the fourteenth yearprobably still reflected differences in RGR which occurred duringthe seedling stages in the nursery. The significance of theseobservations for the assessment of performance in tree breedingprogrammes is discussed.     

Annual Variations in Competition Symmetry in Even-aged Sitka Spruce

Understanding of the temporal development of competition symmetry,size hierarchy and the role of episodically supplied resourcesin forest stands is limited. Because of intrinsic differencesin the symmetry of competition for light and below-ground resources,it is hypothesized that the overall expression of competitionvaries as a consequence of variations in water availability.Historical growth and precipitation data from an unthinned standof Sitka spruce (Picea sitchensis(Bong.) Carr.) in Denmark areanalysed with respect to the effects of precipitation on competitionsymmetry. The shape and linearity of the size-increment relationshipare analysed, and the temporal development of diameter distributionand size hierarchy is modelled. The test value of lack of linearfit through the origin for the diameter/increment relationshipplotted against precipitation in the growth period forms a triangularenvelope, indicating a more curvilinear relationship for yearswith high water availability. This indicates that dominant treesapparently benefit more from increased precipitation than suppressedtrees—which is also shown in the analysis of the temporaldevelopment of the diameter distribution—and as a consequencethe size hierarchy increases. Finally, it is proposed that temporaland spatial variations in the availability of water and otherbelow-ground resources may have similar effects on competitionsymmetry. Copyright 2001 Annals of Botany Company Competition symmetry, size hierarchy, precipitation, temporal development, Picea sitchensis     

The Development of Desiccation-tolerance and Maximum Seed Quality During Seed Maturation in Six Grain Legumes

‘Physiological maturity’, i.e. the time when seedsreach their maximum dry weight during development, occurredwhen maturation drying on the parent plant in the field hadreduced seed moisture content to approximately 60 per cent infaba bean (Vicia faba L.), lentil (Lens culinaris Medic.), chickpea(Cicer arietinum L.), white lupin (Lupinus albus L.), soya bean(Glycine max [L.] Merr.) and pea (Pisum sativum L.) The onsetof desiccation-tolerance, i.e. the ability of seeds to germinatefollowing harvest and rapid artificial drying, coincided withphysiological maturity, except in pea where it occurred a littleearlier at about 70 per cent moisture content. Maximum seedquality as determined by maximum viability, minimum seedlingabnormalities and maximum seedling size occurred in pea, chickpeaand lupin when seeds were harvested for rapid drying at physiologicalmaturity; but for maximum seed quality in the other speciesmaturation drying had to proceed further - to about 45 per centmoisture content in soya bean and to about 30 per cent moisturecontent in lentil and faba bean seed crops. Much of this variationamongst the six species, however, was due to differences inthe variation in maturity within each seed crop. Results forindividual pods showed that peak maturity, i.e. maximum seedquality following harvest and rapid artificial drying, was achievedin all six species once maturation drying had reduced the moisturecontent of the seeds to 45–50 per cent. In pea, faba beanand soya bean there was a substantial decline in viability andan increase in seedling abnormalities when harvest was delayedbeyond the optimal moisture content for harvest.     

The Effects of Shading on the Yield Components of Six 'Leafless' Pea Genotypes

The effect of unshaded, 30 and 80 per cent shade conditionson the growth rate and yield composition of six ‘leafless’pea (afaf: stst) genotypes has been determined. No evidencefor a photoassimilate source limitation was found in the unshadedand 30 per cent shade treatments, but 80 per cent shade significantlyreduced the yield and the rate of d. wt and node productionof all the genotypes. Yield development was followed in each environment by determiningthe number of flowering nodes, the number of flowers producedper node, the pod number per node and the seed number per pod.The theoretical maximum for each of these yield components wasdetermined and compared with the actual level realized by theplant. Approximately 50 per cent of the theoretical potentialof each yield component was realized and this appeared to beindependent of the shade treatment. This stepwise reductionin potential yield throughout the development of the plant wassimilar to that reported for conventional pea plants. Pisum sativumL, ‘leafless’ peas, shading, growth rate, yield'components, source limitation     

Ageing in Woody Plants

The shoot systems of trees undergo characteristic changes withtime, as they increase in size and complexity. Certain of thesechanges, such as reduced annual growth increment, loss of apicaldominance, and changes in the geotropic responses, are easilyreversible, by pruning or by grafting shoots from older treeson to young stocks. These changes are referred to as ‘ageing’. This paper describes the changes occurring in Pinus sylvestrisand Larix leptolepis during ageing and reports an investigationinto the possible mechanisms controlling these changes. Pruningexperiments suggest that ageing is due to increased competitionfor nutrients between the various constituent shoots as thebranch systems increase in complexity, but the distributionof available nutrients is modified by apical dominance. Theseconclusions were supported by the results of experiments involvingthe injection of radioactive phosphorus (²²P), in associationwith the pruning treatments.     

Factors Affecting the Abscission of Reproductive Organs in Yellow Lupins (Lupinus luteus L.): III. ENDOGENOUS GROWTH SUBSTANCES IN VIRUS-INFECTED AND HEALTHY PLANTS AND THEIR EFFECT ON ABSCISSION

Extracts of small and mature-size lupin pods yielded four substancesaffecting the growth of wheat-coleoptile sections: one acidpromotor (A), two acid inhibitors(B and X), and one neutralinhibitor(Y). Inhibitor B was extremely active, however, coleoptile sectionsshowed no signs of toxic effects; they resumed growth at a rapidrate after rinsing them and adding ß-indolylaceticand (IAA) to the medium. 1 µg of IAA was required to counteractthe effect of ‘B’ extracted from 230 mg. Of tissue.On an equal fresh weight basis the inhibiting action of ‘B’in lupin pods was 500–1,500 times more potent than thatof ‘inhibitor ß’ in etiolated pea seedlings. Small pods of plants infected with pea-mosaic virus yielded3 times the amount of ‘A’ of healthy plants (equivalentto 1 µg. IAA 0.3 µg. IAA per 25 g. of tissue respectively),and approximately the amount of ‘B’. Mature podsof virus-infected plants again yielded more‘A’,but also 2? times more ‘B’ than pods of healthyplants. Healthy pods yielded more ‘A’ than virus-infectedpods, and there was no difference in ‘X’. A lupin abscission test was developed and the effects of proximaland distal application of -naphthyl acetic acid (NAA) are presented,and discussed with respect to results of other abscission tests. ‘A’ accelerated abscission when applied proximally,and delayed or prevented it when applied distally. ‘B’strongly accelerated abscission when applied in either way.A possible mechanism explaining the abscission-inducing effectof developing pods on later flowers is discussed in terms ofthe substances ‘A’ and ‘B’. The partlyprevented abscission observed on virus-infected plants was foundto agree well with the proposed mechanism.