Physiological and Ecological Studies in the Analysis of Plant Environment: VI. The Constancy for Different Species of a Logarithmic Relationship between Net Assimilation Rate and Light Intensity and its Ecological Significance

In this further study of light as an environmental factor theeffects of shading on the growth of Helianthus annuus, Fagopyrumesculentum, Trifolium subterraneum, Tropaeolum majus, Lycopersicumesculentum, Vicia faba, Pisum sativum, Hordeum vulgare, Solanumdulcamara, and Geum urbanum have been investigated. It has been established that the reductions in the net assimilationcaused by shading are similar for all ten species. The net assimilationrate during the season of active growth is linearly relatedto the logarithm of the light intensity. When similar experimentsare conducted late in the autumn and the relative growth rateis very low, the logarithmic relationship no longer holds. From the data it is possible to obtain precise estimates ofthe compensation point. The mean values of the compensationpoint ranged from 0·06 to 0·09 of daylight foreight species, while for V. faba and H. vulgare somewhat higherfigures were obtained—0·14 and 0·18 of daylight. Flctuations in the net assimilation rate in full daylight showedno correlation with variations in the value of the compensationpoint. From these results it is concluded that species cannot be groupedinto ‘sun’ or ‘shade’ plants, eitheron the basis of differences in the value of the compensationpoint or on the grounds that there are large variations in theeffects of shading on net assimilation rate.     

Physiological and Ecological Studies in the Analysis of Plant Environment: VII. An Analysis of the Differential Effects of Light Intensity on the Net Assimilation Rate, Leaf-Area Ratio, and Relative Growth Rate of Different Species

Since relative growth rate is the product of net assimilationrate and leaf-area ratio (leaf area/plant weight), it followsthat if the effects of shading on both net assimilation rateand leaf-area ratio can be expressed mathematically, then therelationship between light intensity and relative growth ratecan be derived from the product of the two mathematical expressions. For all the ten species investigated in field and pot cultureexperiments, it has been found that during the early vegetativephase both the changes in leaf-area ratio and net assimilationrate, over the range of 0·1 to full daylight, are linearlyrelated to the logarithm of the light intensity. In consequence,the relationship between relative growth rate and the logarithmof light intensity—being the product of the two linearregressions—is curvilinear. For species of shady habitats (Geum urbanum, Solamun dulcamara)neither the levels of assimilation rate nor the ‘compensation-point’values are very different from those of the eight species fromopen situations (e.g. Hordeum vulgare, Pisum sativum, Fagopyrumesculentum). Nevertheless the intensity at which growth rateis maximal varies between species: it is 0•5 for G. urbanum,0•7 for H. annuus, full daylight for F. esculentum, whilefor Trifolium subterraneum the calculated value is 1·8daylight. Such specific differences can be largely accountedfor in terms of the differences in leaf-area ratio at the differentlight levels. On the basis of this analysis of the light factor, a ‘shade’plant is best redefined as a species in which a reduction ofthe light intensity causes a rapid rise in the leaf-area ratiofrom an initial low value in full daylight: for a ‘sun’plant the converse definition holds.     

An Analysis of the Influence of Plant Density on the Growth of Vicia faba: II. THE SIGNIFICANCE OF COMPETITION FOR LIGHT IN RELATION TO PLANT DEVELOPMENT AT DIFFERENT DENSITIES

In a further analysis of the changing pattern of developmentinduced in Vicia faba by varying the density, it has been foundthat a reduction from a high to a low density has little influenceon the subsequent development unless such thinning is delayeduntil the flowering phase. By this time, save for widely spacedplants, the level of self shading within the population hasbecome marked. In fact, at high densities (55–65 plants/metre²)during the early-ripening phase the light intensity at groundlevel may fall to 0.03 daylight while a considerable proportionof the plant—up to 38 per cent.—may receive lessthan o-1 daylight. At low densities (11-I2 plants/metre²) theminimum intensity at ground level is 0.14 daylight and lessthan 3 per cent, of the shoot is subjected to o-1 daylight.In pot experiments, using a range of screens, it was establishedthat the compensation point is about o-I daylight. Thus, asthe density is increased the light gradient between the apexand the base becomes progressively steeper and the proportionof the leaves not actively assimilating correspondingly greater. To assess the ways in which such a light gradient operates,experiments were carried out in which either the apex or theinflorescences or leaves were removed over different sectionsof the stem or various parts of the shoot shaded and detailedrecords made of development, particularly of flower- and pod-production.Removal of the upper leaves or shading the apex primarily increasesthe rate of pod abscission after the flowers have set. Partialremoval of the inflorescences, especially at the lower nodes,has an opposite effect, while decapitation, though it augmentsthe percentage of flowers which produce immature pods, subsequentlycauses fewer pods to reach maturity. Shading of the lower nodesreduces at these nodes the number of mature pods but may resultin more pods maturing at the upper nodes. It is concluded that when the light gradient is such as to restrictthe internal supplies of substrates the growth of those organswith the least competitive ability, e.g. the newly formed pods,is arrested. It is at this phase that the factors controllingabscission come into play and that abscission is dependent upona balance between the levels of auxins and the production ofan abscission factor.     

Phenotypic Plasticity of Grass Root Anatomy in Response to Light Intensity and Nutrient Supply

The phenotypic plasticity of axial root anatomy was investigatedin response to the availability of above- and below-ground resourcesusing eight grass species of the genera Bromus and Poa. In a7-week garden experiment nutrients were varied by a factor offive and light treatments of 100, 30 and 5·5% daylightwere applied. Both nutrients and light influenced root structure.The effect of nutrients was largely explained by plant size,but this was not the case for light. Shading to 30% daylightled to a higher proportion of stele, larger stelar cells andlarger xylem vessels. This can be understood either as an increasedneed for high transport capacity in the shade, where leaf areais larger but root mass lower than in full daylight, or as anincreased resistance against desiccation, which is more of ahazard in open sites. Under 5·5% daylight, tissue massdensity was reduced due to a lower proportion of stele, thoughxylem characteristics were not influenced when a correctionfor the effect of root cross-sectional area was applied. Thisresponse may be interpreted as a mechanism to maintain rootfunction with a lower investment in biomass when growth is limitedby low irradiance. The results show that the response of a plantto resource limitation is not restricted to those organs responsiblefor the acquisition of that resource. Furthermore, the qualitativeresponse to shading depends on the absolute level of irradiance.For this reason, care is needed when comparing the results ofshading experiments conducted under different irradiances. Copyright2001 Annals of Botany Company Bromus, hydraulic conductivity, light, nutrients, phenotypic plasticity, Poa, Poaceae, root anatomy, stele     

Physiological and Ecological Studies in the Analysis of Plant Environment: IX. Adaptive Changes in the Vegetative Growth and Development of Helianthus annuus induced by an Alteration in Light Level

Pot experiments were carried out in which sunflowers in theearly vegetative phase were first grown for a period under threelevels of light (1·0, 0·5, and 0·24 daylight).Subsequently pots from each light group were subdivided intothree so that in a second period plants could be subjected tothe nine possible combinations of the same three light intensitiesbefore and after transference. During the post-transference period of adaptation to eithera higher or a lower intensity the net assimilation rate is logarithmicallyproportional to the light received and there is no residualeffect of the initial light treatments. Eight days after transferencethe leaf-area ratios (total leaf area/total plant weight) ateach light level become adjusted to a new equilibrium irrespectiveof the large initial differences in the ratio induced by thepre-transference intensities. In both periods there is an inverseand logarithmic relationship between the leaf-area ratio andfalling light intensity; consequently, the greater the degreeof shading in the pre-transference period, the higher are themean ratios in the second period. Since the relative growthrate is the product of the net assimilation rate and the leaf-arearatio, the variations in the leaf-area ratio in the post-transferenceperiod induced by the initial light treatments are reflectedin the relative growth rates. Thus plants transferred from alower to a higher light intensity are leafier and initiallygrow faster than plants maintained at the higher level in bothperiods: the converse conditions lead to a reduction in thegrowth rate. Shading depresses the growth of the roots, but the relativegrowth rate is dependent on the light intensity in both thepre-transference and post-transference periods. With transferencefrom daylight to 0·24 daylight, the roots during theperiod of adjustment may lose weight, while the growth rateis maximal when plants are moved from the lowest to the highestintensity. In two out of the three experiments the relativegrowth rate of the shoot in the post-transference period isof the same order at all light intensities and is largely independentof the light received in the initial period. In terms of leaf weight, decreasing the light intensity decreasesthe relative growth rate and there is no consistent after-effectof the initial light treatments. The rate of expansion in leafarea tends to be highest at the intermediate level of 0·5daylight and over all the post-transference intensities therates are maximal for those plants which received initiallyfull daylight. The ratio of leaf area to leaf weight is inversely and logarithmicallyproportional to the light level. After transference the slopesof the regressions are independent of the initial light treatments,but the mean ratios are inversely correlated with the initialdegree of shading. These adaptive changes to a variation in the light level arediscussed with particular reference to the control of growthexerted by growth-regulating substances. It is concluded thaton the basis of existing knowledge no adequate interpretationis yet possible.     

Physiological and Ecological Studies in the Analysis of Plant Environment: XIII. A Comparison of the Effects of Seasonal Variations in Light Energy and Temperature on the Growth of Helianthus annuus and Vicia faba in the Vegetative Phase

The influences of seasonal changes in light radiation and temperatureon the vegetative growth of Helianthus annuus and Vicia fabahave been investigated in the east of Scotland by pot experiments,carried out in the open at weekly intervals between June andSeptember in 1956 and May and October in 1957. To minimize theeffects of ontogenetic drift pots containing plants of a similarmorphological status were selected from batches sown every fewdays. At the beginning and end of each experiment replicatedand paired pots were harvested and the dry weights of the leaves,stems, and roots together with the leaf areas determined. Fromthese data weekly values for net assimilation rate, leaf-arearatio (ratio of leaf area to plant weight), and relative growthrate were calculated. Simultaneously, records were kept of the diurnal changes inair temperature and of light energy by means of an integratingphotometer. Multiple regressions linking light and temperature with netassimilation rate, leaf-area ratio, and relative growth ratewere calculated separately for each year. A significant ‘time-of-season’trend was largely eliminated by including an additional variable,the initial leaf-area ratio. In the individual regressions thevariance accounted for was very high, ranging from 75 to 97per cent. The results demonstrated that for both species the net assimilationrate and relative growth rate were positively dependent on lightand temperature. The leaf-area ratio of both species was negativelyaffected by light, but only for V. faba was there a positiverelationship between the leaf-area ratio and temperature. H.annuus grew faster than V. faba during the major part of theseason, largely because of its higher leaf-area ratio. The results are compared with prior investigations in Englandand elsewhere.     

Specific Daylength Effects on Leaf Growth and Dry-matter Production in High-latitude Grasses

Plants of Phleum pratense L. cv. Engmo (origin 69° N) andBromus inermis Leyss. cv. Löfar (origin 64° N) raisedin short days gave large and significant increases in d. wt,height and leaf area upon exposure to continuous light, comparedwith 8 h short days, at essentially identical daily inputs ofradiant energy (8 h summer daylight ± low intensity extension).The increases in both plant height and leaf area were causedmainly by increases in the dimensions of the leaf sheaths andblades, which, in turn, were the result of increased cell sizeand number. However, there was a pronounced interaction betweentemperature and daylength such that the greatest photoperiodicstimulation occurred at lower growing temperatures (     

Growth and Dry-weight Distribution in Callistephus chinensis as Influenced by Lighting Treatment

Plants of two cultivars of Callistephus chinensis (Queen ofthe Market and Johannistag) were grown in 8 h of daylight perday with one of the following treatments given during the 16h dark period: (a) darkness—‘uninterrupted night’,(b) I h of light in the middle of the dark period—a ‘nightbreak’, (c) I min of light in every hour of the dark period—‘cycliclighting’, (d) light throughout—‘continuouslight’. The plants receiving uninterrupted dark periods remained compactand rosetted in habit with small leaves, while leaf expansion,stem extension, and flower initiation were promoted in all threeillumination treatments (b, c, d). Although these three treatmentsproduced similar increases in leaf area, continuous light wasthe most effective for the promotion of both stem growth andflower initiation while cyclic lighting was generally more effectivethan a I-h night break. Continuous light also caused more dry matter to be divertedto stems at any given vegetative dry weight and it was shownthat the stem weight ratio of both varieties was correlatedwith stem length.     

Interrelationships between Light Intensity and the Physiological Effects of 2:4-Dichlorophenoxyacetic Acid on the Growth of Helianthus annuus

The interrelationships between light intensity and the effectsof sodium 2:4-dichlorophenoxyacetate on the growth of Helianthusannuus in the early vegetative stage have been studied by subjectingtreated and control plants to a range of light intensities from,1.0 to 0.12 daylight. The growth regulator in varying amountswas applied either as droplets of aqueous solution to the firstor second pairs of leaves or as an overall spray. When the amountof the compound is such as to cause small but significant reductionsin the relative growth rate, the leaf-area ratio and the ratioof leaf area to leaf weight are likewise depressed but the netassimilation rate is relatively unaffected. Between 0.25 daylightand full daylight the proportionate changes in the relativegrowth rate, leaf-area ratio, and net assimilation rate inducedby sub-lethal amounts of the growth regulator are not greatlymodified by the level of light. If the intensity is reducedfurther to 0.12 daylight, then the reactions of the shaded plantsdiffer markedly from those of unshaded plants. For example,when the plants are shaded both before and after a spray application,the concentration required to cause a 50 per cent. mortalityis one-tenth of that demanded for plants receiving full daylight.For less phytotoxic amounts the percentage reductions, relativeto the controls, induced in the growth rate of the shoot aremore dependent on the intensity after than before the applicationof either a spray or measured droplets. These greater depressions in shoot growth at 0.12 daylight afterthe application are linked with comparable depressions in therate of growth of the leaves of which the first pair are moresensitive. Similarly, the growth rate of the first internodeis also depressed more under shade conditions, but that of thesecond internode at low doses may be greatly increased. Forboth the leaves and internodes shading before as against shadingafter the application may have different significant effectson the changes in growth caused by the growth regulator. Undersome conditions the interactions between the light intensitybefore and after the application and the quantity of the growthregulator are exceedingly complex. By using sodium 2:4-dichloro-5-iodophenoxyacetatelabelled with radioactive I¹³¹, it has also been establishedthat the rate of penetration into leaves is accelerated by ahigh light intensity after the application, but that the intensityreceived prior to the application causes no significant effect.Evidence was also obtained that transport to the shoot fromthe treated leaf is more dependent on the light intensity afterthe application—the transport being greater in full daylight.It is concluded that a number of factors must be involved inbringing about these differential effects and their relativeimportance is discussed.     

Responses to shading of naturalized and non-naturalized exotic woody species

Background and Aims

Recent studies have suggested that responses to shading gradients may play an important role in establishment success of exotic plants, but hitherto few studies have tested this. Therefore, a common-garden experiment was conducted using multiple Asian woody plant species that were introduced to Europe >100 years ago in order to test whether naturalized and non-naturalized species differ in their responses to shading. Specifically, a test was carried out to determine whether naturalized exotic woody species maintained better growth under shaded conditions, and whether they expressed greater (morphological and physiological) adaptive plasticity in response to shading, relative to non-naturalized species.

Methods

Nineteen naturalized and 19 non-naturalized exotic woody species were grown under five light levels ranging from 100 to 7 % of ambient light. For all plants, growth performance (i.e. biomass), morphological and CO₂ assimilation characteristics were measured. For the CO₂ assimilation characteristics, CO₂ assimilation rate was measured at 1200 μmol m^–2 s^–1 (i.e. saturated light intensity, A₁₂₀₀), 50 μmol m^–2 s^–1 (i.e. low light intensity, A₅₀) and 0 μmol m^–2 s^–1 (A₀, i.e. dark respiration).

Key Results

Overall, the naturalized and non-naturalized species did not differ greatly in biomass production and measured morphological and CO₂ assimilation characteristics across the light gradient. However, it was found that naturalized species grew taller and reduced total leaf area more than non-naturalized species in response to shading. It was also found that naturalized species were more capable of maintaining a high CO₂ assimilation rate at low light intensity (A₅₀) when grown under shading.

Conclusions

The results indicate that there is no clear evidence that the naturalized species possess a superior response to shading over non-naturalized species, at least not at the early stage of their growth. However, the higher CO₂ assimilation capacity of the naturalized species under low-light conditions might facilitate early growth and survival, and thereby ultimately favour their initial population establishment over the non-naturalized species.     

Differences in the Interacting Effects of Light and Temperature on Growth of Four Species in the Vegetative Phase

A comparative study, employing the concepts of growth analysis,has been made of the varying responses in the early vegetativephase of Gossypium hirsutum, Helianthus annuus, Phaseolus vulgaris,and Zea mays to combinations of light intensity (1.08, 2.16,3.24, 4.32, and 5.4 x 10⁴ lx—photoperiod 14 h) and constantdiurnal air temperatures (10, 15, 20, 25, 30, and 35 °C).Depending on the combination of treatments, the temperatureof the internal tissues departed from air temperature by 6.9to 1.4 °C: so only the internal temperatures are cited here. For each species there are complex interactions between theeffects of light and temperature on the net assimilation rate,the leaf-area ratio, and the relative growth-rates of plantweight and leaf area. The magnitude of the changes induced bythe two factors vary both with the growth component and thespecies. The temperature responses are maximal up to 20–5°C while at the highest temperatures they may be negative.The temperature coefficients for leaf-area ratio are consistentlyless than those of the other three components: here betweenspecies the coefficients over 10–20 °C vary by a factorof 9.6, 5.4, and 5.1 for the rates of gain in plant weight andleaf area and the net assimilation rate, while the orderingwithin each growth component is species dependent. Under conditions of optimal temperature the relative growth-rateand net assimilation rate progressively increase, accordingto the species, up to either 4.32 or 5.4x 10⁴ lx. The leaf-arearatio is always largest at the lowest intensity. The level oflight at which the rate of gain in leaf area reaches a maximumranges from 2.16x 10⁴ lx for Phaseolus to between 4.32 and 5.40x10⁴ lx for Gossypium. The highest relative growth-rate and net assimilation rate ofHelianthus exceed those of Zea substantially. Indeed the maximalassimilation rate for Helianthus of 2.10 g dm^–2 week^–1is the highest ever recorded under field or controlled conditions.Possible reasons for this reversal of the photosynthetic potentialsof the two species observed by previous workers are discussed.     

The Importance of Light Intensity for Pollen Tube Growth and Embryo Survival in Wheat x Maize Crosses

The success of Triticum aestivumxZea mays crosses, used to producewheat doubled haploids, is influenced by light intensity. Toexamine the basis for this response, pollen tube growth, embryosurvival and indicators of photosynthetic rate were measuredin two wheat cultivars (‘Karamu’ and ‘Kotuku’)crossed with maize at two irradiance levels (250 or 750 µmolm^-2s^-1, PAR). Pollen tube growth was significantly affectedby light intensity in ‘Karamu’ plants but not in‘Kotuku’ plants, despite both cultivars being pollinatedby the same maize source. The percentage of pollen tubes reachingthe cavity between the ovarian wall and integuments, or in themicropyle of ‘Karamu’ plants at high light intensity(65%) was nearly three-times greater than that at low lightintensity (22%). Thus, either low light intensity can affectthe maternal wheat plant in a way that inhibits pollen tubegrowth and/or high light intensity may promote pollen tube growthin ‘Karamu’ plants. Significant differences in ratesof electron transport in plants grown at the two light intensitiesindicated that the rate of photosynthesis may also have an effecton pollen tube growth. These results have importance for improvingthe efficiency of wheat x maize crosses and other wide cerealcrosses. Copyright 2001 Annals of Botany Company Intergeneric hybridization, light intensity, pollen tube growth, embryo survival, Triticum aestivum, wheat,Zea mays , maize     

Effects of Natural and Artificial Light in Arctic Latitudes on Long- and Short-day Plants as Revealed by Growth Analysis

The effects on growth and flowering of two short-day and twolong-day plants when grown under different conditions of illuminationare described. The plants fully investigated were Kalanchoeblossfeldiana and Xanthium pennsylvanicum and the annual varietiesof Hyoscyamus niger and Beta vulgaris. Wintex barley, Iberisumbellata, and tomato were also grown in some selected treatments.The conditions investigated comprised continuous full daylight(24 hours), full daylight for the whole of the daily photoperiodand full daylight for half the photoperiods, the other halfconsisting of either daylight reduced by shading or light fromincandescent lamps or fluorescent tubes (daylight-matching type),all of the same low intensity. Two lengths of photoperiod wereused for each species, one nearly optimal for flowering, theother closer to the critical day-length; and the order of thelow and high light treatments was varied. These factors werecombined factorially. Data were collected (or derived) for the following characteristics,though not always for all the species grown: dry weights, leafareas, heights, water contents, epidermal cell sizes, net assimilationrates, times to flowering, leaf-number increments until flowering,numbers of inflorescences, stomatal apertures, and leaf postures. Among other effects, the data revealed that in all four speciesinvestigated the adverse effects on over-all growth to be expectedfrom reduction of the daily photoperiod or of the intensityof illumination are in fact minimized. This compensation waseffected mainly by large increases in leaf areas, even thoughin all cases half the daily photoperiod consisted of full daylight.There are indications that increased efficiencies (net assimilationrates) may also have been involved. The leaf-area increasesappear to have been due to increased cell size rather than cellnumber and a close positive correlation with water content wasfound. The most striking among the effects on flowering was the failureof sugar-beet to bolt when half of its photoperiod (totals of20 and 14 hours) consisted of light from fluorescent lamps.The flowering of barley and Hyoscyamus was also delayed considerablyunder these conditions. The deficiency of red in the spectrumof the fluorescent light is believed to have been the cause.By contrast, the flowering of Iberis, a crucifer, was not affected.     

Isozyme Variation in Wild and Cultivated Species of the Genus Trifolium L. (Leguminosae)

Isozyme patterns for PGI (phosphoglucoisomerase), MDH (malatedehydrogenase), EST (esterase) and SOD (superoxide dismutase)were evaluated in 36 accessions of eight Trifolium species,namely T. incarnatum, T. polymorphum, T. pratense, T. repens,T. resupinatum, T. riograndense, T. subterraneum and T. vesiculosum.Similarity between species and accessions was estimated by Jaccard’ssimilarity index based on presence or absence of bands. TheUPGMA method was utilized for the groupings and dendrogram constructionwith the aid of the NTSYS-PC program. Interspecific similaritywas low (J 0.351). The dendrogram presented eight groups, eachone corresponding to a taxonomic species. Most of the accessionsof the same species grouped together at J > 0.50, exceptT. riograndense, T. repens and T. pratense. These three speciesshowed the lowest similarity between their accessions, reflectinghigher intraspecific variation. Some accession-specific bandswere identified. The species groupings are consistent with traditionaltaxonomic species delimitation. Therefore isozyme patterns,especially when several systems are employed, are useful andreliable biochemical markers for the taxonomic delimitationand characterization of Trifolium germplasm. Copyright 2000Annals of Botany Company Genetic diversity, germplasm characterization, isoenzymatic variability, biochemical markers, isozymes,Trifolium     

Characteristics of Sugar, Amino Acid and Phosphate Release from the Seed Coat of Developing Seeds of Vicia faba and Pisum sativum

After removal of the embryo from developing seeds of Vicia fabaL. and Pisum sativum L., the ‘empty’ ovules werefilled with a standard solution (pH 5.5). Seed coat exudatesof both species were collected during relatively long experiments(up to about 12 h) and the concentration of sugar (mainly sucrose),amino acids and phosphate in the exudate measured. A discussionis presented on the amino acid/sugar ratio and the phosphate/sugarratio in the seed coat exudate. A pretreatment (15 min) withp-chloromercuribenzenesulphonic acid (PCMBS) reduced the releaseof sugar, amino acids and phosphate from broad bean seed coats.After excision of ‘empty’ ovules of Vicia faba andPisum sativum from the maternal plant, 2–4 h after thistreatment a strong difference became visible between sucroserelease from excised seed coats and sucrose release from attachedseed coats. Similarly, when the rate of phloem transport ofsucrose into an ‘empty’ ovule of Vicia faba or Pisumsativum was reduced by a sub-optimal mannitol concentrationin the solution, a reduced rate of sugar release from the seedcoat could be observed. Excision and treatment with a sub-optimalmannitol concentration reduced the release of amino acids toa lesser extent than for sucrose. These treatments did not reducethe rate of phosphate release from the seed coat. Key words: Seed development, Seed coat exudate, Phloem transport     

Physiological and Ecological Studies in the Analysis of Plant Environment: XII. The Role of the Light Factor in Limiting Growth

Previous investigations in southern England on twenty-two herbaceousspecies have demonstrated that for widely spaced plants thediurnal solar radiation limits the net assimilation rate ofall species and restricts the relative growth rate of many.In examining how far these limitations apply to other environmentsit is now shown that in the subtropics and tropics the levelsof net assimilation rate and relative growth rate can greatlyexceed those so far recorded for cool temperate regions, andthese differences are attributed to the higher insolation andtemperatures. From a variety of evidence it is concluded that as the distancebetween plants is reduced 8O the net assimilation rate is progressivelydiminished even in regions of high insolation through the enhancedmutual shading. In consequence levels of light which may besupra-optimal for relatively isolated individuals may yet limitthe dry-matter production of a dense population. There is anoptimal ratio of leaf area to ground surface (leaf-area index)for the maximal exploitation of the incoming radiation in carbonfixation by the population and this optimum will vary with thespecies and the light intensity. Where other environmental factorsare favourable, light may limit dry-matter production everywhere. On an annual basis dry-matter production will be dependent ontwo components—the length of the ‘growing season’and the period over which the leaf-area index remains optimal.In the tropics the highest annual rate of production so farrecorded is 7⁸ tonnes/hect. produced by Saccharum officinarumandin north-east Europe 23.5 tonnes by Fagus sylvatica. Over shortperiods the rate of dry-matter production can attain 3⁸g./m.²/dayand the utilization of solar energy can be as high as 4.2 percent., or 9.5 per cent, for the range 4, 000–7, 000 A. Although information on the productivity of natural communitiesis still ex-ceedingly scanty, an attempt has been made to interpretthe general pattern in terms of the length of the growing season,the level of solar radiation, the magni-tude of the leaf-areaindex of the whole community, and the period over which theleaf canopy remains green. It is postulated that in any regionthe vegetation reaches a dynamic equilibrium when there is themaximum exploitation of the incoming radiation to produce thegreatest production of dry matter.     

Photosynthesis and photorespiration in the genus Oryza

Photosynthetic gas exchange has been surveyed in 22 of the 23species currently placed in the genus Oryza and constitutingthe wild relatives of cultivated rice. Unimproved, wild germplasmof a number of species showed light-saturated assimilation ratesin atmospheric air at least as great as cultivars and elitebreeding material of the cultigen O. satlva. One of these specieswas O. australiensis, different accessions of which were significantly(P=0.001) superior in assimilation rate to the 0. satlva genotypestested, including representatives of the Indica, Japonica andJavanica subgroups. Amongst species, assimilation rate was correlatedpositively with light saturation and with carboxylation efficiency.The wild species fell into two distinct groupings accordingto whether they originated from sun or shade habitats, withthe higher assimilation rates being associated with the sunspecies. Assimilation rates were also higher in diploids thanin tetraploids and this was associated with the fact that allsun species are diploids and all tetraploids are shade species.The carbon dioxide compensation concentrations ranged from 28to 43µmol mol^–1 with the two lowest values (28 and32µmol mol^–1) coming from accessions of O. rufipogon.The mean value for the absolute quantum yield of photosynthesismeas ured on attached leaves was 0.060. There was a large rangein the activity of phosphoenolpyruvate (PEP) carboxylase witha number of species having rates several times those of C₃ species.Some species with the highest assimilation rates were assessedfor photorespiratory losses and these were generally around30% and similar to O. satlva cultivars. However, a range ofO. rufipogon accessions had photorespiration rates significantly(P=0.01) lower than the O. sativa genotypes tested. No speciesin the genus possessed C₄ photosynthetic metabolism though somedid overlap with compensation concentrations and phosphoenolpyruvatecarboxylase activities reported for C₃–C₄ intermediatespecies. The potential value of wild relatives to the improvementof cultivated rice is discussed. Key words: Oiyza, photorespiration, photosynthesis, rice, wild rice     

Responses of CO2 assimilation to changes in irradiance: laboratory and field data and a model for beans (Phaseolus vulgaris L.)

The responses of net CO₂ assimilation to sudden changes in irradiancewere studied in Phaseolus vulgaris L. in the laboratory andthe field. For irradiance changes between 50 µmol m^–2s^–1 to 350 µmol m^–2 s^–1 in the laboratory,assimilation rate increased with half-times of 2.7 and 4.1 minin well-watered and water-stressed plants, respectively. Ina field experiment with a change in irradiance from 400 to 1200µmol m^–2 s^–1 the response was faster (half-time=c.1.2 min). In all cases when irradiance was returned to a lowvalue, assimilation declined rapidly with a half-time of approximately1 min, which approached the time resolution of the gas-exchangesystem. The corresponding changes in stomatal conductance in responseto both increasing and decreasing irradiance were much slowerthan the assimilation responses, indicating that biochemicalprocesses, rather than CO₂ supply, primarily determined theactual rate of assimilation in these experiments. The conceptof stomatal limitation to photosynthesis is discussed in relationto these results. A simple model for assimilation in a fluctuating light environmentis proposed that depends on a steadystate light response curve,an ‘induction lag’ on increasing irradiance, andan induction-state memory. The likely importance of taking accountof such induction lags in natural canopy microclimates is considered. Key words: Models, Phaseolus vulgaris, photosynthetic induction, CO₂ assimilation, stomatal limitation, sunflecks, water stress     

Effect of Shade on Leaf and Cell Size and Number of Epidermal Cells in Garlic (Allium sativum)

Garlic cultivars ‘Bangladesh Local’ and ‘Fructidor’were grown under field conditions in the south-east of Englandand subjected to different shading treatments. The effects ofshading on final leaf size were related to cell numbers anddimensions. An increase in light intensity reduced leaf lengthand size of epidermal cells. In the cv. Fructidor, cell lengthwas reduced, but in the cv. Bangladesh Local, both length andwidth of cells were reduced. The stomatal index decreased withincreasing light intensity in both cultivars. Leaf thicknessincreased with light intensity, resulting in corresponding gainsin leaf d. wt per unit area. Data from cv. Fructidor growing in full sunlight indicated thatthe epidermal tissue adjacent to the abaxial leaf surface hada greater number of cells than that next to the adaxial surface,but no significant differences were found in the correspondingcell depths. The size of epidermal cells in both epidermisesdecreased from the base to tip of the leaf due to reductionin cell length and width. The influences of environmental factorson leaf growth in general, and those related to shade, are alsodiscussed. Allium sativum L. cv. Bangladesh Local, Allium sativum L. cv. Fructidor, epidermal cells, garlic, light intensity, shade, stomatal index     

Biochemical Limitations to Carbon Assimilation in C3 Plants--A Retrospective Analysis of the A/Ci Curves from 109 Species

Species-specific differences in the assimilation of atmosphericCO₂ depends upon differences in the capacities for the biochemicalreactions that regulate the gas-exchange process. Quantifyingthese differences for more than a few species, however, hasproven difficult. Therefore, to understand better how speciesdiffer in their capacity for CO₂ assimilation, a widely usedmodel, capable of partitioning limitations to the activity ofribulose-1,5-bisphosphate carboxylase-oxygenase, to the rateof ribulose 1,5-bisphosphate regeneration via electron transport,and to the rate of triose phosphate utilization was used toanalyse 164 previously published A/C_i, curves for 109 C₃ plantspecies. Based on this analysis, the maximum rate of carboxylation,Vc_max, ranged from 6µmol m^–2 s^–1 for the coniferousspecies Picea abies to 194µmol m^–2 s^–1 forthe agricultural species Beta vulgaris, and averaged 64µmolm^–2 s^–1 across all species. The maximum rate ofelectron transport, J_max, ranged from 17µmol m^–2s^–1 again for Picea abies to 372µmol m^–2 s^–1for the desert annual Malvastrum rotundifolium, and averaged134µmol m^–2 s^–1 across all species. A strongpositive correlation between Vc_max and J_max indicated that theassimilation of CO₂ was regulated in a co-ordinated manner bythese two component processes. Of the A/C_i curves analysed,23 showed either an insensitivity or reversed-sensitivity toincreasing CO₂ concentration, indicating that CO₂ assimilationwas limited by the utilization of triose phosphates. The rateof triose phosphate utilization ranged from 4·9 µmolm^–2 s^–1 for the tropical perennial Tabebuia roseato 20·1 µmol m^–2 s^–1 for the weedyannual Xanthium strumarium, and averaged 10·1 µmolm^–2 s^–1 across all species. Despite what at first glance would appear to be a wide rangeof estimates for the biochemical capacities that regulate CO₂assimilation, separating these species-specific results intothose of broad plant categories revealed that Vc_max and J_maxwere in general higher for herbaceous annuals than they werefor woody perennials. For annuals, Vc_max and J_max averaged 75and 154 µmol m^–2 s^–1, while for perennialsthese same two parameters averaged only 44 and 97 µmolm^–2 s^–1, respectively. Although these differencesbetween groups may be coincidental, such an observation pointsto differences between annuals and perennials in either theavailability or allocation of resources to the gas-exchangeprocess. Key words: A/C_i curve, CO₂ assimilation, internal CO₂ partial pressure, photosynthesis