Effect of Photoperiod on Growth of Sugar Beet

Sugar beet grown in controlled environments were given similardaily amounts of visible radiation during three different photoperiodtreatments. Plants were given (a) 115 W m^–2 visible irradiancefrom fluorescent and tungsten lamps for 12 h; (b) 88 W m^–2of the same light for 16 h or (c) 115 W m^–2 from fluorescentand tungsten lamps for 12 h extended to 16 h with low intensity(3 W m^–2) incandescent light from the tungsten lamps only.Plant growth was increased similarly in both long day treatments[(b) and (c)] and dry weights were 25 per cent greater thanin the 12 h photoperiod (a) after 6 weeks. Leaf area was increasedby 18 per cent and net assimilation rate by 10 per cent in the16 h photoperiod at 88 W ^m–2 (b). By contrast, extendingthe photoperiod with 4 h of incandescent light (c) triggereda photomorphogenic increase in leaf expansion which increasedleaf area per plant by 47 per cent and leaf-area ratio by 12per cent.     

An Analysis of the Growth of Young Tomato Plants in Water Culture at Different Light Integrals and CO2 Concentrations: I. Physiological Aspects

Young tomato plants were grown from germination in water cultureat light-flux densities from 6 to 110 W m^-2 (400–700 nm),daylengths from 8 to 24 h and CO₂ concentrations from 0.4 to2.2 g CO² m^-3 in controlled environment cabinets. The growth rates and net assimilation rates of 14–17-day-oldplants at the highest light integrals were appreciably greaterthan most values previously recorded for tomato, and diminishedwith time. Plants in the lowest light conditions had leaf arearatios five times larger than those in the highest light, attributablemainly to a difference in leaf dry weight/area. Such flexibilityin leaf area ratio has not previously been associated with ‘sun’plants such as the tomato. Relatively normal growth was obtained in continuous light, incontrast to most other reports. This may have been due to theuse of conditions which would minimise water stress. The efficiency of the conversion of incident light energy tochemical energy by the whole plant ranged from 15 per cent inseedlings in low continuous light to about 6 per cent, tendingto be higher in young plants in long days under CO₂ enrichment.The higher values are probably overestimates because of theexclusion of reflected light from the energy receipt values.     

The Effect of Shading on the Photosynthetic Rate and Longevity of Grass Leaves

The rate at which the net photosynthesis of grass leaves grownin bright light (119 W m^–2) decreased as they aged wasincreased by severe shading (to 21 W m^–2 or less). However,less severe shading (light intensities of 36 W m^–2 ormore) had no effect. The decrease in photosynthesis was unaffectedby whether the whole plant was shaded or only the leaf whosephotosynthesis was measured. In both shaded and unshaded leaves, photosynthesis measuredin bright light fell faster as the leaf aged than did photosynthesisin dim light. Both mesophyll and stomatal diffusion resistancesrose as the leaf aged but the former rose faster. The chlorophyllcontent fell only towards the end of the life of the leaves.     

The Effect of Current Photosynthesis on the Origin of Translocates in Old Tomato Leaves

The rate of carbon transport from an old tomato leaf (54 days),grown at 80 W m^–2, was measured under light flux densitiesbetween 7 and 90 W m^–2. Under low light, the rate of carbontransport over a 6 h period was about 1 mg C dm^–2 h^–1,well in excess of the concurrent photosynthetic rate. The lossfrom these leaves of ¹⁴C-leaf assimilate which was fixed beforethe experimental period amounted to 62 per cent of the totalinitial uptake and was higher than that from leaves with higherconcurrent photosynthetic rates. The higher loss of ¹⁴C fromleaves with low photosynthetic rates was due to a greater contributionof ¹⁴C from the starch and residue fractions. The rate of transportappeared to be determined by the concentration of the labilesucrose, not the total sucrose concentration. In comparisonwith young fully-expanded tomato leaves (Ho, 1976) the sizeof the labile sucrose pool appeared to decrease with age. Thephotosynthesistranslocation coefficient was low (k₁k₂=0•21)for an old tomato leaf. Based on these results a scheme of carbonpartitioning in relation to translocation is proposed. Criteriafor assessing the efficiency of translocation in leaves arediscussed.     

The Effect of Shade During Leaf Expansion on Photosynthesis by White Clover Leaves

In three experiments measurements of photosynthesis were madeon single leaves of white clover (Trifolium repens L.) on threecultivars grown in a controlled environment. Plants which had grown under an irradiance of 30 J m^–2s^–1, or in shade within a simulated mixed sward, producedleaves with photosynthetic capacities some 30 per cent lowerthan did plants grown at 120 J m^–2 s^–1 without shade.There were no differences between treatments either in photosynthesismeasured at 30 J m^–2 s^–1, or in respiration ratesper unit leaf dry weight. Respiration per unit leaf area washigher in the plants grown at 120 J m^–2 s^–1, reflectingthe lower specific leaf area of these leaves. There were nodifferences between the three cultivars examined. Leaves which were removed from the shade of a simulated swardshortly after becoming half expanded achieved photosyntheticcapacities as high as those which were in full light throughouttheir development. It is suggested that it is this characteristicwhich enables clover plants growing in an increasingly densemixed sward to produce a succession of leaves of high photosyntheticcapacity, even though each lamina only reaches the top of thesward at a relatively late stage in its development. Trifolium repens L., white clover, photosynthesis, leaf expansion, shade, specific leaf area, stomatal conductance     

The Effects of Light Intensity and External Potassium Level on Root/Shoot Ratio and Rates of Potassium Uptake in Perennial Ryegrass (Lolium perenne L.)

Loliun perenne L. (cv.S. 23) was grown on vermiculite in winterin a heated greenhouse for 8 weeks under factorial combinationsof two potassium regimes (nominally 6 parts/10⁶ and 156 parts/10⁶in Hewitt's solution) and three densities of artificially supplementedvisible radiation flux (36.1, 7.3, and 2.2 W m^–2). Growthand potassium uptake were studied through the calculation ofvarious growth functions from fitted curves. There was little effect of potassium treatment but the experimentalmaterial responded markedly to light. Leaf-area ratio in thethree treatments showed extreme plasticity in increasing from2–3 x 10^–2 through 6 x 10^–2 to 8–9 x10^–2 m² g^–1 as light intensity decreased. Correspondingdecreases in unit leaf rate, however, caused over-all reductionsin relative growth rate. Specific absorption rates for potassium (A_K, dry-weight basis)were strongly reduced at the lower light intensities but alsodisplayed complex ontogenetic drifts. Values of the allometricconstant, k (the ratio of root and shoot relative growth rates),decreased from c. 0.7 at 36.1 W m^–2 through c. 0.3 at7.3 W m^–2 to a value not significantly different fromzero (P < 0.05) at 2.2 W m^–2. In material grown under the two higher light intensities a constantinverse relationship was found between the mass ratio of rootand shoot and the corresponding activity ratio. The resultsconform to this model: Mass ratio = –0.001+45.0 (1/activityratio) where activity ratio is expressed as specific absorptionrate for potassium (in µg g root^–1 h^–1)/unitshoot rate (rate of increase of whole-plant dry weight per unitshoot dry weight, in mg g shoot^–1 h^–1). The implicationsof this relationship are discussed.     

Growth and Nitrogen Fixation of Phaseolus vulgaris L. at Two Irradiances 1. Growth

Plants of Phaseolus vulgaris grown at 7 and 28 W m^–2 showedno differences in rate of development of leaves or flowers.At 7 W m-Z plants had longer internodes, more succulent stemsand leaves, higher ratios of shoot:root and greater leaf areasthat those at 28 W m^–2. These differences were establishedprior to detectable differences in photosynthesis and couldpartly be attributed to an increased proportion of far-red light. Although the final d. wt, carbon content, and fruit yield werehigher at 28 W m^–2, plants at 7 W m^–2 apparentlyhad similar relative growth rates and greater photosyntheticefficiency. Dry weight differences are most easily interpretedas resulting from the establishment of an earlier net carbongain at 28 W m^–2 than at 7 W m^–2.     

Nitrogen Deficiency in Small Closed Communities of S24 Ryegrass. I. Photosynthesis, Respiration, Dry Matter Production and Partition

Small communities of S24 ryegrass were grown under supplementarylights in a glasshouse at 20°C, and abundantly suppliedwith a complete nutrient solution containing 300 p.p.m. of nitrogen,until they had a leaf area index of 5 and fully interceptedthe light. Half were then given a solution containing only 3p.p.m. of nitrogen (LN) while the rest were kept at 300 p.p.m.(HN). The LN plants had a rate of single leaf photosynthesis lowerthan that of the HN plants at all but the lowest light intensities(33 per cent lower at the saturating irradiance of 170 W m^–2).Similarly, the LN communities had rates of canopy gross photosynthesis(P_sc) markedly lower than those of the HN communities. A comparisonof the observed rates of P_sc with those predicted by a mathematicalmodel of canopy photosynthesis indicated that it was the effectof nitrogen on single leaf photosynthesis, rather than differencesbetween the communities in leaf area, which led to the observeddifferences in P_sc. The superiority of the HN communities in terms of P_sc was partlyoffset by a higher rate of respiration so that they only exceededthe LN communities in terms of canopy net photosynthesis atirradiances in excess of 180 W m_–2, and produced only15 per cent more total dry matter. Nevertheless, the HN plantsdirected less of that dry matter into root and more into topsso that they came to possess twice the weight of live laminae,and the HN communities twice the leaf area, of their nitrogendeficient counterparts. Lolium perenne, S24 ryegrass, photosynthesis, respiration, dry matter production and partition, nitrogen dekieacy     

An Analysis of Growth of Oil Palms under Nursery Conditions: I. Establishment and Growth in the Wet Season

Growth of oil palm seedlings over the period 2–31 weeksafter planting in the nursery was studied using growth-analysistechniques. Curves of the Gompertz type were fitted to the basicdata of plant dry weight and leaf area, and from the equationsof the fitted curves, net assimilation rate (E_A), relative growth-rate(Rw), and relative leaf growth-rate (R_A) were calculated. The low values of E_A (0.16-0–31 g/dm²/week) and Rw (1.4–2.2per cent./per day) confirm earlier work on oil palm seedlings.The time trend of increasing E_A and R_W over the period studiedis associated with steadily increasing solar radiation overthe second half of the period. Leaf-area ratio is markedly affected by transplanting, and asthis unbalance of leaf area/total dry weight has been shownto be associated with low rates of EA in seedlings, it is suggestedthat the low values of E_A and R_W in the first half of the experimentalperiod are due to the effect of transplanting. These findings are discussed in relation to current nurserypractice.     

Acclimation in Seedlings of a Tropical Tree, Bischofia javanica, Following a Stepwise Reduction in Light

Acclimation of fully developed leaves of Bischofia javanicaBlume to shadelight was examined. Seedlings were grown undersimulated daylight (1000 µmol m^–2 s^–1), thentransferred to a simulated shadelight (40 µmol m^–2s^–1). When a high-light leaf was transferred to low light, large negativenet photosynthetic rates (P_m) were recorded. This decrease wasrapid, but within 7 d the rate increased and became equal tothe low-light control leaf. These changes in photosynthesisdid not follow the pattern of changes in stomatal conductance(g_s). Transfer to the low light resulted in a dramatic decreasein leaf weight per unit area (L_w), and most of the decreasesin L_w occurred within 3 d of transfer when the P_m of the transferredleaf was well below that of the low-light control leaf. There was a significant decrease in chlorophyll a in the transferredleaf without an appreciable change in chlorophyll b resultingin a large decrease in the chlorophyll a to chlorophyll b ratio.Leaf chlorophylls per unit area were higher in the transferredleaf than the low-light control leaf. Maximum photosyntheticrate in the transferred leaf was decreased by 40% compared tothat for the high-control leaf, but was almost at the same extenthigher than the low-light control leaf The results are discussedin the context of carbon gain capacity of its seedlings underlight-limiting forest understorey habitats. Bischofia, chlorophylls, light, photosynthesis, shade acclimation, tree seedlings, tropical tree     

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.     

Specific Leaf Weight in Zones of Cell Division, Elongation and Maturation in Tall Fescue Leaf Blades

Patterns of change in specific leaf weight (SLW), water-solublecarbohydrate (WSC) content and leaf width were used to delineatethe region of secondary cell wall accumulation, and determinethe rate of increase in structural material along a developingleaf blade of tall fescue (Festuca arundinacea Schreb.). Structuralspecific leaf weight (SSLW) was determined by subtracting WSCmass from dry weight to emphasize structural material. Becausemeristematic activity, cell elongation, and cellular maturationare arranged successively in the grass leaf, these patternsrepresent a developmental sequence through which each segmentof the leaf blade passes. Patterns were generally similar fortwo genotypes, one selected for high (HYT) and the other forlow (LYT) yield per tiller, for a single genotype grown at 17or 25 C, and for two field-grown populations which differedin leaf area expansion rate (LAER). In all three studies, the elongation zone of the developingleaf had 31 to 39 per cent WSC on a dry weight basis. The LYTgenotype had a higher SLW at all stages of development whengrown at 17 than at 25 C, due to greater WSC accumulation.At 20 C, the HYT genotype had a higher SLW all along the elongatingleaf blade than the LYT genotype. This difference was due toa difference in SSLW, while WSC content was similar. The LERwas 64 per cent higher in the high population than the low,but elongation zones were similar in WSC. In all cases, SSLWwas high in the meristematic region, lowest near the distalend of the cell elongation zone, then increased linearly astissue matured. Rate of increase in SSLW was 8.5 and 5.2 g m^–2d^–1 for the HYT and LYT genotypes, respectively, and 7.6and 6.7 g m^–2 d^–1 for the high and low LAER populations,respectively. Festuca arundinacea Schreb., tall fescue, specific leaf weight, leaf width, water-soluble carbohydrates, leaf elongation rate     

Carbon Translocation in the Tomato: Carbon Import and Fruit Growth

The rates of carbon import by fruits were measured over 48 has the sum of the change in the total organic carbon contentof the fruit and the respiratory loss of carbon. Over a rangeof fruit sizes from 20–90 per cent of the maximum volumethe smaller fruits imported carbon at an absolute rate (mgCfruit^–1 h^–1) nearly twice that of the larger fruits.The imported leaf assimilates, identified as the ¹⁴C-compoundsalong the pathway between a ¹⁴CO₂-fed leaf and a young fruit,comprised 90 per cent sucrose and 10 per cent glutamic acid,aspartic acid and malic acid. Within the fruit the imported¹⁴C-sucrose was hydrolysed into hexoses. The changes in thelevels of starch and insoluble residue in the fruit were positivelycorrelated with the carbon import rates. In the largest fruitswith the lowest import rates, there was breakdown of insolubleresidue and less accumulation of starch, but a significant increasein the level of sucrose. The sink strength of a tomato fruitis dependent more on sink activity than on sink size.     

Effects of Nitrogen Fertilizer on Growth and Yield of Spring Wheat

Nine amounts of nitrogen fertilizer, ranging from 0 to 200 kgN ha^–1, were applied to spring wheat cv. Kleiber in the3 years 1972-1974. In 1972 grain dry weight with 125 kg N ha^–1or more was 100 g m^–2 (23 per cent) greater than withoutnitrogen. Grain yield was unaffected by nitrogen in the otheryears. Leaf area at and after anthesis was increased throughoutthe range of nitrogen tested, most in 1972 and least in 1973.Consequently, the addition of 200 kg N ha^–1 decreasedthe amount of grain produced per unit of leaf area by approximately25 per cent in all years. The dry weight of leaves and stems at anthesis and maturitywas increased by nitrogen in all years, similarly to leaf area.However, the change in stem dry weight between anthesis andmaturity was not affected by nitrogen; stems increased in dryweight for about 20 days after anthesis and then decreased tovalues similar to those at anthesis. The uptake of CO₂ per unit area of flag leaf or second leaf(leaf below the flag leaf) was slightly decreased by nitrogenwhen the increase in leaf area caused by nitrogen appreciablydecreased the light intensity at the surface of these leaves.In spite of such decreases the CO₂ absorbed by flag and secondleaves per unit area of land was always increased by nitrogen,and relatively more than was grain yield. It is suggested that increases in respiratory loss of CO₂ withincreasing nitrogen fertilizer may explain why nitrogen increasedvegetative growth and leaf area relatively more than grain yield.     

The Growth and Survival of Severely-shaded Tillers in Lolium perenne L.

The effect of shading a single tiller to below its compensationpoint for a period of 5 weeks in vegetative plants of Loliumperenne L. cv. S23, was studied in two different experimentseach employing two light regimes, one of which was common toboth experiments. In the first experiment tillers in the axils of the first leafwere shaded three weeks from appearance at both 40 and 70 Wm^–2. None of the shaded tillers died and they continuedto produce new leaves and increase in dry weight but at a reducedrate. In the second experiment, tillers with one emerged leafin any leaf axil position were shaded at 70 W m^–2 andin a treatment in which light was reduced to 13 W m^–2after initial growth at 70 W m^–2. As in the first experimentall shaded tillers survived at 70 W m^–2 but in the 70 13 W m^–2 transfer regime all shaded tillers died. In the second experiment shaded tillers in both light regimeswere supplied with ¹⁴C-assimilate by translocation from theremainder of the plant but in the 70 13 W m^–2 the initialsupport was withdrawn within 5 weeks of shading. The results are discussed in terms of the physiological relationshipsbetween the tillers of the grass plant. Lolium perenne L., growth of tillers, survival of tillers, effect of light     

Growth and Nitrogen Fixation of Phaseolus vulgaris L. at Two Irradiances II. Nitrogen Fixation

Phaseolus vulgaris cv. Glamis plants grown at 7 and 28 W m^–228 W m^–2 in controlled environment cabinets showed copiousnodulation and high levels of acetylene reducing activity. Earlydifferences in nodulation were apparent before differences inphotosynthesis and were attributed to an effect of far-red lighton nodule development. Total plant nitrogen content was greater at 28 W m^–2 thanat 7 W ^–2 but nitrogen content as a percentage of d. wtwas greater at the lower irradiance level. Total acetylene reducing activity (nmol. min^–1 root^–1)was greater at 28 W m^–2 than at 7 W ^–2, but therewas no irradiance effect on specific activity (nmol. min^–1g d. wt of pink nodules^–1 or nmol. min^–1 pink nodule^–1). Transfer of 40-day-old plants from 7 W m^–2 to 28 W m^–2resulted in increased nodulesize(due toincreased size of infectedcells), accompanied by increased total, but not specific, acetylenereducing activity. Transfer of plants from 28 W m^–2 to 7 W m^–2 resultedin a fall of total acetylene reducing activity within 24 h,and senescence of large nodules. Specific acetylene reducingactivity was unaffected The results are interpreted as an effect of light on the productionof nitrogen fixing tissue, rather than on nitrogenase activity.     

Potential Increase in Photosynthetic Efficiency from the Redistribution of Solar Radiation in a Crop

The efficiency of the conversion of photosynthetically activeradiation by C₃ plants falls off with increasing intensity.Hypothesis: an increase in the productivity of direct solarenergy will be achieved if, by redistribution, it is interceptedat a more uniform and lower intensity by a greater proportionof the leaf area of a crop. A model is developed which usesestimates of the proportions of clear and overcast conditionsfrom site records of solar radiation to calculate the resultantphotosynthetic productivity. The amounts of diffuse light anddirect light are estimated for clear conditions. The model predictsthat redistributing direct solar radiation over twice the leafarea at half the intensity would give an increase of 22% inannual productivity. The model gives reasonable values for theproductivity reductions reported for two shading regimes. Tomato plants were grown for 21 d in three cabinets under regimesthat differed from each other only in the distribution of PARenergy over the daily photoperiod: (a) 103 W m^–2 for halfthe photoperiod followed by 13 W m^–2, (b) 13 W m^–2followed by 103 W m^–2 and (c) 58 W m^–2 for the wholephotoperiod. The dry matter increase of plants under the uniformregime was 33% greater than the average of those in the twoasymmetric regimes. It is suggested that, in protected cultivation, screens of partiallyreflective material could be used to redistribute solar radiationfrom leaves exposed to high intensities on to shaded leavesand so raise the photosynthetic efficiency. Assuming an absorptionof direct light by the screens of 0.10, the increase in productivityis estimated to be 17%. Key words: Photosynthesis, C₃, canopy, solar energy, solar redistribution, light interception, partial reflection, growth cabinet, glasshouse, tomato     

Effect of Light Intensity, Carbon Dioxide Concentration, and Leaf Temperature on Gas Exchange of Spray Carnation Plants

The rates of CO₂ assimilation by potted spray carnation plants(cv. Cerise Royalette) were determined over a wide range oflight intensities (45–450 W m^–2 PAR), CO₂ concentrations(200–3100 vpm), and leaf temperatures (5–35 °C).Assimilation rates varied with these factors in a way similarto the response of single leaves of other temperate crops, althoughthe absolute values were lower. The optimal temperature forCO₂ assimilation was between 5 and 10 °C at 45 W m^–2PAR but it increased progressively with increasing light intensityand CO₂ concentration up to 27 °C at 450 W m^–2 PARand 3100 vpm CO₂ as expressed by the equation TOpt = –6.47-h 2.336 In G + 0.031951 where C is CO₂ concentration in vpmand I is photo-synthetically active radiation in W m^–2.CO₂ enrichment also increased stomatal resistance, especiallyat high light intensities. The influence of these results on optimalization of temperaturesand CO₂ concentrations for carnation crops subjected to dailylight variation, and the discrepancy between optimal temperaturesfor growth and net photosynthesis, are discussed briefly     

The Effects of Shade Treatment and Light Intensity on Rihulose-1,5-Diphosphate Carboxylase Activity and Fraction I Protein Level in the First Leaf of Barley

It has been confirmed that shading leaves from day 5 onwardslowers the rate of CO₂ fixation when they are placed in saturatingirradiances. The reduction due to shade treatment is about 46per cent and a similar reduction in maximum chlorophyll contentof the leaf follows shading. Maximum amounts of total solubleprotein and of Fraction I protein are less in shaded leavesthan in control leaves and prolonged treatment leads to a declinein leaf protein content. The relative amounts of different proteinare also affected by treatment; in control leaves Fraction Iprotein accounts for about 45 per cent of the total but in shadedleaves the value is about 30 per cent. Increases and decreasesin leaf protein amount, with concomitant changes in the ratioof Fraction I to total protein can be brought about by removingshades and re-applying them. Such changes can be induced evenin fully expanded leaves in which net protein synthesis is notusually found. Maximal amounts of leaf protein are found in irradiances of60 W m^–2 or more, with lower values at lower light intensities.Where the first leaf is held in a stream of CO₂-free air a lowerlevel of protein is found. This, and the ratio of Fraction Ito total protein, are similar to values for shaded leaves, andsuggest the involvement of photosynthetic carbon fixation indetermining leaf protein amount. A 1:1 linear correlation between amount of Fraction I proteinand RuDP carboxylase activity is shown but the rate of CO₂ incorporationby leaf extracts is 2–3 times greater than that of theintact leaf. The significance of this and the effect of irradianceon leaf protein amount are discussed.     

The Growth and Development of Simulated Swards of Perennial Ryegrass: II. Carbon Assimilation and Respiration in a Seedling Sward

The rates of net photosynthesis (P_n,c) in the light (85 W m^–2visible), and respiration in the dark, of a simulated swardof S24 ryegrass were measured for 12 weeks during its developmentfrom a collection of two-leaved seedlings to a closed canopywith an LAI of 23 (15 of green leaf laminae). By the sixth week light interception was complete (LAI = 10.6)and P_n,c had risen to 24 mg CO₂ dm^–2 h^–1, similarto rates recorded in the field. Photosynthetic functions (lightresponse curves) showed that the swards remained unsaturatedup to energy receipts of almost 400 W m^–2, whereas singleleaves were light saturated at about 130 W m^–2. Earlyin the development of the sward LAI had a greater effect onP_n,c than radiation receipt, later the reverse was true. Thegrowth habit of the sward ranged from moderately erect (an Svalue of 0.72) to moderately prostrate (‘S’ = 0.37),while the ability of the two youngest fully expanded leaveson a tiller to make use of light in photosynthesis declinedas the sward increased in density from values of A max of 20to 5 mg CO₂ dm^–2 h^–1. By varying the values of Sand A max fed into a model of canopy photosynthesis, withinthe above limits, it was demonstrated that, in practice, A maxis a greater determinant of canopy photosynthesis than S, exceptat low LAI where a prostrate sward has a marked advantage overan erect one. The rate of dark respiration rose as the swards increased inweight, although not in proportion to it, until the ninth weekwhen a ceiling yield of live plant tissue was reached. Respiratorylosses from the sward came almost equally from a component associatedwith maintenance (R_m) and one associated with growth (R_g). Therate of Rm was estimated to be about 0.014 g day^–1 pergram of plant tissue, and that of Ra about 0.25 g per gram ofnew tissue produced—both close to theoretical values.The measured dry matter production curve of the swards was comparedwith that estimated from the gas analysis data. Similarly therates of gross photosynthesis estimated from the gas analysisdata were compared with the predictions of the mathematicalmodel. In both cases the fit was reasonably good. A balancesheet was drawn up; of every 100 units of carbon fixed, 45 werelost in respiration and 16 as dead leaf, 5 ended up in the rootand 6 in the stubble; only 28 remained as harvestable live leaftissue.