Physiological and Ecological Studies in the Analysis of Plant Environment: X. An Analysis, of the Effects of Seasonal Variation in Daylight and Temperature on the Growth of Helianthus annuus in the Vegetative Phase

To determine on a quantitative and mathematical basis the effectsof seasonal changes in the levels of daylight and temperatureon vegetative growth and development in two years pot experimentsin the open were carried out at successive weekly intervalsbetween May and September. So as to minimize errors arisingfrom ontogenetic drifts the procedure adopted was to sow atintervals of a few days throughout the season batches of potswith seed of Helianthus annuus and to select pots containingplants of a standard morphological status for the start of eachweekly experiment. At the beginning and end of the week halfthe pots were harvested, the plants divided into root, stem,and leaf, the leaf area estimated, and the dry weights determined.The diurnal changes in air temperature were continuously recordedwhile the amount of daylight, excluding infra-red and ultravioletradiation, was measured with a specially constructed integratingrecorder. From the biological data for each week twelve variables werecalculated, namely the relative growth rates of both the wholeplant and the individual parts, the proportion by dry weightof the individual parts (root-, stem-, and leaf-weight ratios),the ratio of leaf area to total plant weight (leaf-area ratio),the rate of leaf expansion, the ratio of leaf area to leaf weight,and the net assimilation rate on the criteria of leaf area andweight. The main independent variables considered were the meanweekly temperature, the mean daily maximum minus the mean nightlyminimum temperature, the total amount of light per week, andthe time of year when the individual experiment was undertaken. Multiple regression analyses showed that (i) save for the stem-weightratio the data for the two years could be pooled, (ii) the fluctuationin diurnal temperature was of little account, (iii) transformationof the light data to either logarithms or square roots did notimprove the fit and (iv) for some of the dependent variables,e.g. leaf-area ratio, the ‘time of year’ effectwas significant but could be eliminated if the equation wasmodified to predict the value at the end of the week, giventhe initial value and the light and temperature data. The final series of multiple regressions revealed that (i) theleaf-weight ratio is not controlled by either the amount oflight or mean temperature, (ii) the relative growth rate ofthe root and the root-weight ratio are positively linked onlywith temperature, (iii) the rate of leaf growth either in areaor weight together with the net assimilation rate (area basis)are positively dependent on light alone, (iv) the net assimilationrate (weight basis) and the relative growth rates of the wholeplant and the stem are directly and positively correlated withboth temperature and light, and (v) the leaf-area ratio, theratio of leaf area to leaf weight and the stem-weight ratioare depressed by increasing light but augmented by rising temperature.In the individual regressions for net assimilation rate (areaand weight), the relative growth rates of the whole plant, stemand leaf weight, and the ratios of stem weight and leaf areato leaf weight the percentage variation accounted for rangedfrom 47 to as high as 91 per cent. The implication of these findings in relation to experimentsin controlled environmental chambers are discussed.     

The Vegetative Growth of Helianthus annuus and Phaseolus vulgaris as Affected by Seasonal Factors in Freetown, Sierra Leone

Sand-culture experiments were carried out in full daylight atsuccessive weekly intervals between March and December 1969,to investigate the effects of seasonal changes in climatic factorson the growth of Helianthus annuus and Phaseolus vulgaris inFreetown. Values for a number of growth parameters were calculatedfrom the dry weights of the leaves, stems, and roots, and fromthe leaf areas. Simultaneously the diurnal changes in climaticfactors were recorded. Multiple regressions linking light, temperature, and relativehumidity with some of the growth parameters were calculated.The total variance accounted for in the regressions of relativegrowth-rate, net assimilation rate, and leaf weight ratio onlight, temperature, and relative humidity ranged from 51 to60 per cent in P. vulgaris. In H. annuus relative humidity wasless important; the percentage proportion of total varianceaccounted for in the regression of leaf weight ratio (and leafarea ratio in both species) on light and temperature was notsignificant. The results showed that H. annuus grew faster than P. vulgaris,but the latter's growth was depressed less by the dull weatherof the rainy season. The relative growth-rates of both specieswere positively dependent on light and temperature while theirnet assimilation rates were negatively dependent on relativehumidity, and their leaf area ratios negatively dependent onlight. All parameters used except leaf area ratio and leaf areato leaf weight ratio showed seasonal variations correspondingto dry and rainy seasons. The initial and final values of leafarea ratio and weight ratios were always different but did notvary in the same direction in both species. The results are discussed in relation to similar work done elsewhere.     

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.     

Effects of Temperature on the Growth and Development of Lemna minor, under Conditions of Natural Daylight

The effects of temperature on the growth and development ofLemna minor in the open have been studied in the east of Scotlandby means of four water baths constructed to maintain constantwater temperatures of 12.5, 17.5, 22.5, and 27.5 °C whensubjected to natural insolation. Experiments were conductedat weekly intervals between August and November in 1958 andMay and July in 1959. At the beginning of every experiment,for all temperature treatments, 134 fronds were placed in eachof six containers. From the initial and final samples, the weightsof roots and fronds together with frond (leaf) area were measured,so that weekly values for net assimilation rate, leaf-area ratio,and relative growth-rate could be calculated. Daily solar radiationwas recorded by means of bimetallic radiation recorder. In 1958 linear regressions of a satisfactory fit were obtainedwhen the data for net assimilation rate, leaf-area ratio, andrelative growth-rate were calculated on the logarithms of theradiation for each temperature. Since radiation remained relativelyconstant in 1959 it was not possible to evaluate very reliablythe effects of radiation on the growth parameters but only occasionally,notably for the final leaf-area ratio (12.5 °C) were thelines for 1958 and 1959 significantly different. Single lineswere fitted to the points for both years. In all the regressions,apart from that for final leaf-area ratio (12.5°C) the proportionof the variation accounted for ranged from 87 to 97 per cent. The results showed that the net assimilation rate was positivelylinked with radiation and was optimal at 17.5 °C, thoughthe rise from 12.5 to 17.5 °C was not significant. At thehigher temperatures (22.5 and 27.5 °C) there was a significantnegative effect of temperature on the net assimilation rate.The leaf-area ratio and relative growth-rate were positivelydependent on radiation and reached the highest values at thehighest temperatures. The maximum growth-rate recorded amountedto no less than 0.39 g.g^–1 day^–1. The results are discussed in relation to those for other aquaticand terrestrial plants.     

The Influences of Seasonal Changes in Solar Radiation and Air Temperature on the Growth in the Early Vegetative Phase of Zea mays

At weekly intervals from May to September over 2 years the growthof separate batches of Zea mays (Swiss hybrid Orla 266) wasrecorded for individual plants in pot experiments, togetherwith the corresponding weekly means of solar radiation and themean, maximum, and minimum diurnal air temperatures. To reducesampling errors the plants were ranked on the bases of initialgrain size and leaf number. To minimize differences in stageof development at the end of 21 days from sowing the size wasadjusted in 1965 by switching the pots in and out of a glasshouse:in 1966 the plants were kept for the first 11 days in a controlledenvironment and then hardened off in the open. Between 21 and28 days when the second sample was taken half the plants weresubjected to light shade (0.65–0.70 daylight). Multiple regression analysis showed that the relative growthrate of the whole plant (RGR), the rate of increase in leafarea (RLGR), and the net assimilation rate (NAR) were positivelydependent on both radiation and mean air temperature. In 1965there were negative effects of minimum temperature on RGR andNAR and a positive response of RLGR to leaf number. In 1966an increase in leaf number led to a higher RGR and LAR but depressedRLGR, while minimal temperature had no significant effects.In all these regressions the variation accounted for was high,ranging from 80 to 89 per cent. At the second sampling occasion the leaf-area ratio (LAR_f) wasinversely related to radiation, negatively dependent on daytemperature, but positively linked with night temperature. Theorder of the initial LAR exerted no influence. The RGRs of theshoot and the root were positively associated with both radiationand mean temperature. In 1965 there were small negative responsesof the shoot to both minimum temperature and leaf number andfor the root only leaf number. The variation accounted for wasleast for LAR_f (59–62 per cent) and intermediate for RGR_aand RGR_r (77–89 per cent). For RGR, NAR, and RLGR the calculated partial regression coefficientsfor mean temperature in 1965 were larger than those for radiationbut in 1966, apart from RLGR, they were equalled by radiation.The discrepancies between years can be ascribed to a highercorrelation coefficient between radiation and temperature in1965 (0.53) as against 1966 (0.33). The value of multiple regression analysis in the evaluationof the environment by carefully designed field experiments isemphasized in relation to other investigations of light andtemperature undertaken under controlled conditions.     

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 Effect of Temperature on Vegetative Growth in Climatic Races of Dactylis glomerata in Controlled Environments

The growth patterns in two natural populations of Dactylis glomeratafrom contrasting climatic regions, Norway and Portugal, werestudied at four constant temperatures (5, 10, 20, and 30°C) in a 16-h photoperiod. Marked changes in relative growth-rateat different temperatures were positively correlated with changesin both net assimilation rate and leaf-area ratio, whereas differencesbetween the populations in the relative growth-rate were theresult of differences in net assimilation rate, and were negativelycorrelated with differences in leaf-area ratio. The changesin leaf-area ratio at different temperatures were correlatedwith changes in leaf morphology and distribution of assimilateswithin the plant. The possible adaptive advantage of these vegetativegrowth patterns is discussed in relation to the survival ofthe plants in the original environments.     

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.     

The Interrelated Effects of Stage of Development and Seasonal Changes in Light and Temperature on the Components of Growth in Zea mays

Seeds of Orla 266, a double cross hybrid of Zea mays, were sownin pots in the open 3, 6, and 9 weeks before the individualplants were sampled at consecutive weekly intervals from theend of August. On each occasion the weights of the whole plantand its parts plus leaf area were recorded, and the proceduresof growth analysis applied to the data. Between the youngest and oldest plants over all occasions thegreatest reductions with age were for the rate of leaf expansionand the ratio of leaf area to leaf weight. The net assimilationrate (NAR) and the leaf weight ratio (LWR) were depressed leastwhile the relative growth-rate (RGR) and leaf area ratio (LAR)occupied an intermediate position. The age effects were mostpronounced for RGR, NAR and the ratio of leaf area to leaf weightat the beginning of the experiment, whereas for LAR and LWRthe divergencies were largest at the end. On the basis of the recorded changes in solar radiation andtemperature and prior studies of the interacting effects oflight and temperature on the vegetative growth of Zea it waspossible to predict for the several growth components the patternsof change which should be expected. For the youngest and intermediatepopulations there was close agreement between the expected andobserved values for RGR, NAR, and LAR and it was concluded thatthe time courses were primarily dependent on environmental factors.On the other hand, for the oldest plants from the third intervalonwards the observed values for NAR and less so for RGR werematerially above expectation. It was noted that this divergencecoincided with the rapid extension of the shoot which was confinedto the oldest plants. It is advanced that the higher NAR isassociated with the vertical separation of the leaves.     

Physiological and Ecological Studies in the Analysis of Plant Environment: XI. A Further Assessment of the Influence of Shading on the Growth of Different Species in the Vegetative Phase

In a further analysis of the effects of varying light intensityon growth and development in the vegetative phase the reactionsof thirteen herbaceous species have been recorded. In some experimentsthe degree of shading has been extended to 0.055 daylight, alevel near or below the compensation point. For Lathyrus maritimus, Trifolium pratense, and Vicia faba,the net assimilation rate is directly related to the logarithmof the light intensity, but for Helianthus annuus, T. repent,T. hybridum, Medicago sativa, Phaseolus multiflorus, and Loliummultiflorum the relationship, though curvilinear, is not logarithmic.It is concluded that for all species the assimilation rate ofunshaded plants was limited by light even though in high summerthe recorded light energy between 4,000–7,000 A averaged1,900–2,200 foot-candles. For all these species between daylight and 0.12 daylight theleaf-area ratio rises as the intensity decreases and in generalthe trend is logarithmic. When the degree of shading is increasedto 0.055 daylight the logarithmic relationship still holds forL. maritimus and V. faba though this level is below the compensationpoint. For other species, such as P. multiflorus andH. annuusthe trend may be reversed below 0.12 daylight and the ratiothen falls. When the light intensity is reduced from daylight to 0.5 daylight,then for the species already cited and for Lolium perenne, Phleumpratcnse, and Festuca pratensis the relative growth-rate isinvariably depressed. At 0.055 daylight the relative growth-ratenever exceeded 1 per cent. per day. For L. perennet, P. pratense,and Dactylis glomerata the reactions to shading of ‘hay’and ‘grazing’ strains were different. The ecological and physiological implications of these findingsare discussed.     

Effect of Temperature on Growth of Crotalaria juncea L. and Crotalaria sericea Retz.

Growth performances of Crotalaria juncea L. and C. sericea Retz.have been compared at two controlled temperatures, 16–20°C, and 28–32 °C, with respect to increase ind. wt and leaf area, relative growth rate, leaf area ratio,specific leaf area, leaf weight ratio, net assimilation rate,the ratio of mean relative growth rate to mean relative rateof leaf area increase () and shoot/root ratios. Both the speciesgrew better at the higher temperature; however the relativegrowth rate was more affected by temperature in C. sericea thanin C. juncea. Further, it was observed to be more dependenton net assimilation rate than on the leaf area ratio. Crotalaria juncea L., Crotalaria sericea Retz., relative growth rate, leaf area ratio, specific leaf area, leaf weight ratio, leaf area increase, net assimilation rate, shoot/root ratio     

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.     

Assimilation Rate and Growth of Lolium Populations in the Glasshouse in Contrasting Light Intensities

Using growth-analysis techniques, the variation in relativegrowth-rate (RGR) and its components, net assimilation rate(NAR), and leaf-area ratio (LAR), was examined in 18 populationsof L. perenne, six of L. multiflorum, and two hybrid cultivarsfrom contrasting climatic and agronomic origins, grown at lowand high light intensities in the glasshouse. Significant differences between populations were found for RGR,NAR, and LAR at both light intensities. At both intensitiesthe annual or biennial multiflorum group had a greater LAR anda lower specific leaf weight and chlorophyll content than theperennial perenne group. At the low intensity this was compensatedby a greater NAR in the perenne group, with no resultant differencein RGR. At the high intensity there was no difference betweenthe groups in NAR, and hence a greater RGR in the multiflorumgroup. Within the perenne and multiflorum groups, at both light intensities,the variation between populations in RGR was based on differencesin NAR rather than in LAR. There was no regular correlationof NAR with either specific leaf weight, or chlorophyll contentat either light intensity, though at low light intensity itwas significantly correlated with shoot-root ratio.     

Variation in Photosynthesis and Enzyme Activity in Cenchrus ciliaris L.

Populations of Cenchrus ciliaris differ significantly in relativegrowth-rate (R), net assimilation rate (E), and leaf-area ratio(F). Differences in R were related to differences in E ratherthan in F. The populations also differed in the rate of apparentphotosynthesis of individual leaves as measured by infra-redgas analysis. Warburg and ^l4CO₂ techniques. Temperature andlight-response measurements revealed optimum temperature forphotosynthesis of 35 °C and the rate in most populationscontinued to increase up to the highest levels of irradianceemployed (200 W m²). The relative order of the populations wassimilar in all methods of analysis, but there was no relationshipevident between leaf photosynthesis and E derived from growthanalysis. The activity of the enzyme PEP carboxylase, expressedin terms of unit leaf area, was correlated with photosyntheticrate of the leaf. Activities of PEP carboxylase and of proteinsynthesis in vitro expressed on a basis of soluble protein contentwere correlated with E, but no such relationship was found forthe enzymes acid phosphatase and alanine--ketoglutarate aminotransferase.     

Changes in the Growth of Salvinia natans Induced by Cycles of Light and Darkness of widely Different Duration

The growth of Salvinia natans has been examined when clonalmaterial, maintained at a constant temperature (30 ^°C) receivesthe same amount of light energy per day but where the light-darkcycle of equal intervals varies in eight steps from 1 min to12 h. The area per leaf, the rate of leaf production, the netassimilation rate, and the relative growth-rate increase withthe lengthening of the cycle, but the leaf-area ratio is reduced.The magnitude of the changes differs between the criteria whilethe order of the response may be disparate between consecutiveintervals. In supporting experiments the same measurements weremade on plants subjected to a wide range of light intensitieswith a common photoperiod of 12 h. Here decreasing light intensityproduces similar trends to those recorded for increasing thelength of the cycle but the patterns of response may diverge.It was also established that the size of the stomatal pore isthe same in the light and in the dark. It is postulated thatvanations in the light-dark cycle may influence both the levelof photosynthetic activity and the pathways.     

Responses of Net Photosynthesis and Conductance to Independent Changes in the Humidity Environments of the Upper and Lower Surfaces of Leaves of Sunflower and Soybean

A dual-surface leaf chamber was used to investigate the responsesof net photosynthesis and leaf conductance to independent changesin the humidity environments of the upper and lower surfacesof leaves of sunflower and soybean. In sunflower decreasingthe humidity around the upper leaf surface while maintainingthat of the lower surface constant and high reduced both thephotosynthetic rate and the conductance of the lower surface.These reductions could not be attributed to changes in bulkleaf water potential since the transpiration rate of the wholeleaf remained constant. Similarly, the reductions were not relatedto localized water deficits in the lower epidermis or lowermesophyll since the transpiration rate of the lower surfacewas reduced. Possible mechanisms whereby the gas exchange characteristicsof the lower leaf surface of sunflower respond to the humidityenvironment of the upper surface are discussed. In contrastto sunflower, the photosynthetic rate of the lower surface ofsoybean was insensitive to the humidity environment of the uppersurface. In leaves of sunflower grown under a moderate temperature anda medium light level, simultaneous decreases of humidity atboth leaf surfaces reduced the photosynthetic rate of the wholeleaf without affecting the substomatal partial pressure of CO₂.In contrast, with leaves developed under a cool temperatureand a high light level, both the photosynthetic rate and thesubstomatal partial pressure of CO₂ were reduced. Evidently,the occurrence in sunflower of the response pattern suggestinga non-stomatal inhibition of photosynthesis by low humiditydepends upon the environment during growth. The possibilitythat this non-stomatal inhibition may be an artifact due toan error in the assumption of water vapour saturation withinthe leaf airspace is considered. Key words: Vapour pressure deficit, photosynthesis, conductance, non-stomatal inhibition, Helianthus annuus, Glycine max     

Plant Growth Analysis: A Re-examination of the Methods of Calculation of Relative Growth and Net Assimilation Rates Without Using Fitted Functions

A method is proposed for estimating expected values of meanrelative growth and net assimilation rates, and their variances,over an interval of time, from replicated plant weight and leafarea data at each end of the time interval. The advantage ofthe method is that it avoids the necessity of pairing replicateplants at each of the two harvests, and gives exact resultsfor relative growth rate. The results for net assimilation rateare approximate, but the method proposed still avoids the pairingprocess which is regarded as an artificial necessity inherentin the conventional method. net assimilation rate, relative growth rate, mathematical analysis, Helianthus annuusL, sunflower, Triticum aestiuumL, wheat     

A Comparison of the Growth of the C4 Grass Spartina anglica with the C3 Grass Lolium perenne at Different Temperatures

Dunn, R., Thomas, S. M., Keys, A. J. and Long, S. P. 1987. Acomparison of the growth of the C₄ grass Spartina anglica withthe C₃ grass Lolium perenne at different temperatures.—J.exp. Bot. 38: 433–441. S. anglica is one of the few C₄ species which occurs naturallyin cool temperate zones. It is known to attain photosyntheticrates which equal or exceed those of C₃ grasses over the temperaturerange typical of the spring and summer in cool temperate climates.This study examines whether S. anglica can also attain comparablegrowth rates at these temperatures. Seedlings of S. anglicaand L. perenne cv. S23 were grown in controlled environmentsat 10,15,20 and 25 °C. Quantitative growth analysis wasconducted by taking frequent harvests to determine the progressionsof leaf area and plant weight of individual plants with time.Quadratic regressions were found to describe these progressionswell. Instantaneous derived growth parameters were calculatedfrom the fitted regressions. Both absolute and relative growthrates of S. anglica were significantly lower than for L. perenne,this being largely attributable to a lower ratio of leaf areaproduction per unit of plant dry weight. Although the amountof dry matter invested into leaves was similar, the leaf areaper unit of leaf dry weight was lower in S. anglica. In comparisonto L. perenne, the rate of dry matter accumulation per unitof leaf area (ULR) was higher in S. anglica at 25 °C andinitially equal at 10 °C. Prolonged exposure to 10 °Csteadily reduced ULR in S. anglica which approached zero at80 d. Although growth in S. anglica is reduced more by low temperaturethan it is in L. perenne, by comparison to other C₄ speciesthe assimilatory capacity of S. anglica is more tolerant oflow temperature exposure. Key words: C4 photosynthesis, temperature, quantitative growth analysis     

Characterisation of Non-Uniform Photosynthesis Induced by Abscisic Acid in Leaves Having Different Mesophyll Anatomies

The effects of abscisic acid (ABA) on photosynthesis in leavesof Helianthus annuus L. were compared with those in leaves ofVicia faba L. After the ABA treatment, the response of photosyntheticCO₂ assimilation rate, A, to calculated intercellular partialpressure of CO₂, P_i, (A(p_i) relationship) was markedly depressedin H. annuus. A less marked depression was also observed inV.faba. However, when the abaxial epidermes were removed fromthese leaves, neither the maximum rate nor the CO₂ responseof photosynthetic oxygen evolution was affected by the applicationof ABA. Starch-iodine tests revealed that photosynthesis was not uniformover the leaves of H. annuus treated with ABA. The starch contentwas diffferent in each bundle sheath extension compartment (thesmallest subdivision of mesophyll by veins with bundle sheathextensions, having an area of ca. 0.25 mm² and ca. 50 stomata).In some compartments, no starch was detected. The distributionof open stomata, examined using the silicone rubber impressiontechniques, was similar to the pattern of starch accumulation.In V.faba leaves, which lack bundle sheath extensions, distributionof starch was more homogeneous. These results indicate that the apparent non-stomatal inhibitionof photosynthesis by ABA deduced from the depression of A(pⁱ)relationship is an artifact which can be attributed to the non-uniformdistribution of transpiration and photosynthesis over the leaf.Intercellular gaseous environment in the ABA-treated leavesis discussed in relation to mesophyll anatomy. ¹ Present address: Department of Botany, Duke University, Durham,NC 27706, U.S.A. (Received September 30, 1987; Accepted January 13, 1988)     

Growth Analysis of Soybean Seedlings During the Lifespan of the Cotyledons

Patterns of seedling growth of Glycine max in light and darknesswere compared during the period from germination to cotyledonabscission. Fitted growth curves and the derived functions,relative growth rate, unit leaf rate, leaf area ratio and specificleaf area, were used to assess the relative importance in seedlinggrowth of cotyledon storage reserves, cotyledon photosynthesisand leaf photosynthesis. The cotyledons are of an intermediatetype with a predominant storage and a minimal photosyntheticfunction. Cotyledon reserves support seedling growth until theprimary leaves expand, after which growth depends on leaf photosynthesis. Glycine max (L.) Merr., soybean, cotyledons, growth analysis, seedling development