Microclimate, Canopy Structure and Photosynthesis in Canopies of Three Contrasting Temperate Forage Grasses: I. Canopy Structure and Growth

The crop growth rates and structures of three temperate foragegrasses Lolium perenne cv. S24, L. perenne cv. Reveille andFestuca arundinacea cv. S170, were examined in the field duringa summer growth period. The growth rates of the varieties wereremarkably similar at 7 g DM m^–2 day^–1. The angularstructures of the varieties were different and they varied duringthe experiment. However, these differences did not seem to affectcrop growth rates. Nevertheless, a decrease in the efficiencyof light energy conversion of approximately 24 per cent wasobserved after a change to a more prostrate form of canopy dueto lodging. There appeared to be an inverse relationship betweenthe number of tillers per unit ground area and the weight ofan individual stem. There were large numbers of relatively lighttillers in S24 whereas S1 70 had fewer but heavier tillers.Furthermore, S24 had many small leaves per unit ground areacompared with SI70 which had fewer longer leaves per groundarea and a slower rate of leaf appearance. There were diurnalchanges in the rates of leaf extension for all the varieties.The mean daily extension rates declined as the canopies developed.     

Microclimate, Canopy Structure and Photosynthesis in Canopies of Three Contrasting Temperate Forage Grasses: II. Microclimate and Canopy Structure

Profiles of shortwave radiation, net radiation and temperaturewere measured in swards of three grasses of contrasting structureLolium perenne cv. S24, L. perenne cv. Reveille and Festucaarundinacea cv. S170. Measurements were also made of the reflectionof shortwave radiation, leaf water potential and stomatal resistance.Differences in canopy structure influenced the absorption andreflection of radiation by the varieties. The absorption ofnet radiation and its influence on air temperature inside thecanopy was shown to vary with canopy structure. Calculationsshowed that diurnal changes in the reflection and transmissionof light (400–700 nm) would have little effect on canopyphotosynthesis. No clear relationship between leaf extensionrate, temperature and leaf water potential could be established,although decreases in water potential did appear to reduce thepotential response of leaf extension rate to temperature.     

Canopy Photosynthesis and Crop Growth Rate of Eight Temperate Forage Grasses

The rates of canopy and individual leaf photosynthesis, ratesof growth of shoots and roots, and the extinction coefficientfor light of eight temperate forage grasses were determinedin the field during early autumn. Canopy gross photosynthesiswas calculated as net photosynthesis plus dark respiration adjustedfor temperature using a Q₁₀ = 2. The relationships between canopygross photosynthesis and light intensity were hyperbolic, andthe initial slopes of these curves indicated that light wasbeing utilized efficiently at low light intensities. The initialslope depended on the distribution of light in the canopy andthe quantum efficiency of the individual leaves. The maximumrate of canopy gross photosynthesis reflected the maximum rateof individual leaf photosynthesis. Although the maximum rateof canopy gross photosynthesis was correlated with crop growthrate, there was no significant relationship between daily grossphotosynthesis and crop growth rate. Indeed, daily gross photosynthesisvaried by only 22 per cent, whereas the daily growth of shootsand roots varied by 120 per cent. This poor correlation is influencedby a negative correlation (P < 0.01) between the maximumrate of canopy gross photosynthesis and the initial slope ofthe curve relating canopy gross photosynthesis and light intensity.Difficulties in the interpretation of measurements of dark respirationappeared to confound attempts to relate daily net photosynthesisto crop growth rate, individual leaf photosynthesis, and theextinction coefficient for light.     

Measuring the Canopy Net Photosynthesis of Glasshouse Crops

A null balance method is described for measuring net photosynthesisof mature canopies of cucumber and other protected crops overperiods of 10 min in a single-span glasshouse (c. 9m x 18m inarea). Accuracy of control of the CO₂ concentration in the greenhouseatmosphere is within ±10 vpm of the normal ambient level(c. 350 vpm). The amounts of CO₂ used in canopy net photosynthesisare measured with linear mass flowmeters accurate to within±0.80g. The total errors incurred in measuring canopynet photosynthesis at an ambient CO₂ level are estimated tobe of the order of ± 1·2% in bright light (350W m^–2, PAR)and ±3·6% in dull light (100W m^–2, PAR). Measurements of the rates of net photosynthesis of a maturecanopy of a cucumber crop were made at near-ambient CO₂ concentrationsover a range (0–350 W m^–2) of natural light fluxdensities. A model of light absorption and photosynthesis applicableto row crops was used to obtain a net photosynthesis versuslight response curve for the cucumber crop. At a light fluxdensity of 350 W m^–2 the fitted value of canopy net photosynthesiswas 2.65 mg CO₂ m^–2s^–1 (equivalent to over 95 kgCO₂ ha^–1h^–1). The results are discussed in relationto the need for CO₂ supplements to avoid depletion in both ventilatedand unventilated glasshouses during late spring and summer. Key words: Glasshouse crops, cucumber, measurement, canopy photosynthesis, light, CO₂     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 1. Carbon and Nitrogen Economies of Mixed Swards at Low Temperature

Simulated mixed swards of perennial ryegrass (Lolium perenneL. cv. S23) and white clover (Trifolium repens L. cv. S100)were grown from seed under a constant 10°C day/8°C nighttemperature regime and their growth, and carbon and nitrogeneconomies examined. The swards received a nutrient solution,every second day, which contained either high (220 µgg^–1) or low (40 µg g^–1) nitrate N. The High-N swards had rates of canopy photosynthesis and drymatter production (over the linear phase of growth) similarto those previously shown by mixed swards at high temperature.The Low-N swards grew more slowly; canopy photosynthesis, ata given LAI, was similar to that at High-N but lower LAI's weresustained. Clover increased its contribution to total carbonuptake and total dry weight throughout the period in the Low-Ntreatment and, despite the fact that grass took up most of theavailable nitrate, clover maintained a consistently higher Ncontent by virtue of N₂-fixation. At High-N, grass dominated throughout the measurement period.Earlier, when plants grew as spaced individuals, clover grewless well than grass, but once the canopy was closed it hada similar relative growth rate and thus maintained a steadyproportion of total sward dry weight. It is proposed that earlyin the development of the crop, leaf area production is thelimiting factor for growth, and that in this respect cloveris adversely affected by low temperature relative to grass.Later, as the LAI of the crop builds up, and the canopy becomesfully light intercepting, net canopy photosynthesis plays amore dominant role and here the higher photosynthetic rate perunit leaf area of the clover is crucial. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, low temperature, nitrogen, photosynthesis     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions I. Carbon Assimilation and Utilization

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and their growth and carbon economyexamined. The swards received a nutrient solution daily, whichcontained either High (220 mg l^–1) or Low (10 mg l^–1)nitrate N. Rates of canopy photosynthesis and respiration, and final drymatter yields were similar in the two treatments although theproportions of grass and clover differed greatly. The Low-Nswards were made up largely of clover. The grass plants in theseswards had high root: shoot ratios and low relative photosyntheticrates – both signs of N deficiency – and were clearlyunable to compete with the vigorously growing Low-N clover plants.These had higher relative growth rates and dry matter yieldsthan their High-N counterparts. In the High-N swards clovercontributed around 50 per cent to the sward dry weight throughoutthe measurement period despite having a smaller proportion ofits dry weight in photosynthetic tissue (laminae) than grassover much of it. The latter was compensated for, initially bya higher specific leaf area than grass, and later by a higherphotosynthetic rate per unit leaf weight. The results are discussedin relation to observed declines in the clover content of swardsafter the addition of nitrogen fertilizer in the field. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, nitrogen, photosynthesis, carbon balance     

The Effect of Temperature on the Photosynthesis of Ryegrass Canopies

In bright light, the rate of gross photosynthesis of ryegrasscanopies increased with increasing temperature between 10 and25 °C, in keeping with an increase in the photosynthesisof their constituent leaves. The increase in canopy gross photosynthesisoccurred irrespective of the leaf area index, which would affectthe proportion of the leaves in bright light. Because the responseof gross photosynthesis to temperature was similar to that ofrespiration, net photosynthesis also increased with temperature,irrespective of the relative magnitude of the two processes. However, these increases in photosynthesis were observed whilethe saturation deficit of water vapour was kept small (lessthan 3 g m^–3). The photosynthesis of both leaves and canopieswas reduced when measured at a greater saturation deficit; indicatingthat the increasing saturation deficit, which frequently accompaniesincreasing temperature in the field, may obscure the effectof temperature on photosynthesis. Lack of control of saturationdeficit as measurement temperature is increased may explainwhy some workers have failed to detect a positive effect oftemperature on photosynthesis. Perennial ryegrass, Loliwn perenne L., photosynthesis, temperature response, respiration, irradiance     

Response of Soybean Canopy Photosynthesis to CO2 Concentration, Light, and Temperature

Photosynthetic rates of outdoor-grown soybean (Glycine max L.Merr. cv. Bragg) canopies increased with increasing CO₂ concentrationduring growth, before and after canopy closure (complete lightinterception), when measured over a wide range of solar irradiancevalues. Total canopy leaf area was greater as the CO₂ concentrationduring growth was increased from 160 to 990 mm³ dm^–3.Photosynthetic rates of canopies grown at 330 and 660 mm³ CO₂dm^–3 were similar when measured at the same CO₂ concentrationsand high irradiance. There was no difference in ribulose bisphosphatecarboxylase/oxygenase (rubisco) activity or ribulose 1,5-bisphosphate(RuBP) concentration between plants grown at the two CO₂ concentrations.However, photosynthetic rates averaged 87% greater for the canopiesgrown and measured at 660 mm³ CO₂ dm^–3. A 10°C differencein air temperature during growth resulted in only a 4°Cleaf temperature difference, which was insufficient to changethe photosynthetic rate or rubisco activity in canopies grownand measured at either 330 or 660 mm³ CO₂ dm^–3. RuBP concentrationsdecreased as air temperature during growth was increased atboth CO₂ concentrations. These data indicate that the increasedphotosynthetic rates of soybean canopies at elevated CO₂ aredue to several factors, including: more rapid development ofthe leaf area index; a reduction in substrate CO₂ limitation;and no downward acclimation in photosynthetic capacity, as occurin some other species. Key words: CO₂ concentration, soybean, canopy photosynthesis     

Photosynthesis of White Clover Leaves as Influenced by Canopy Position, Leaf Age, and Temperature

Microswards of white clover (Trifolium repens L.) were grownin controlled environments at 10/7, 18/13 and 26/21 °C day/nighttemperatures. The vertical distribution of leaves of differentages and their rates of ¹⁴CO₂-uptake in situ were studied. Extending petioles carried the laminae of young leaves throughthe existing foliage. A final position was reached within 1/4to 1/3 of the time between unfolding and death. Newly unfoldedleaves had higher rates of ¹⁴CO₂-uptake per leaf area than olderones at the same height in the canopy. At higher temperatures,the decrease with age was faster. However, the light-photosynthesisresponse of leaves which were removed from different heightsin the canopy varied much less with leaf age than did the ratesof ¹⁴CO₂-uptake in situ. The comparison of the rates of ¹⁴CO₂-uptake in situ with thelight-photosynthesis response curves suggests that young leavesreceive more light than older ones at the same height in thecanopy. This would imply that young white clover leaves havethe ability to reach canopy positions having a favourable lightenvironment. This ability may improve the chances of survivalof white clover in competition with other species. Trifolium repens L., white clover, photosynthesis, canopy, leaf age, ¹⁴CO₂-uptake, ecotypes, temperature     

The Sensitivity of Net Photosynthesis in Several Plant Species to Short-term Fumigation with Sulphur Dioxide

The effects of exposure of up to 2 h with sulphur dioxide ona range of plant species was observed by measuring changes inthe rate of net photosynthesis under closely controlled environmentalconditions. Ryegrass, Lolium perenne ‘S23’ was thespecies most sensitive to SO₂; significant inhibition was detectedat 200 nl l^–1. Fumigations at 300 nl l^–1 also inhibitedphotosynthesis in field bean (Vicia faba cv. ‘Three FoldWhite’ and ‘Blaze’) and in barley (Hordeumvulgare cv. ‘Sonja’). No effect was detected inwheat (Triticum aestivum cv. ‘Virtue’) at concentrationsup to 600 nl l^–1 SO₂, or in oil-seed rape (Brassica napuscv. ‘Rafal’) except at 800 nl l^–1 SO₂). Recoverycommenced immediately after the fumigation was terminated andwas complete within 2 h when inhibition had not exceeded 20%during the SO₂ treatment. Key words: Sulphur dioxide, short-term fumigation, photosynthesis     

Effects of Temporary Droughts on Photosynthesis of Alfalfa Plants

The effect of temporary droughts on photosynthesis, total conductanceto water vapour, intercellular CO² concentration, CO² compensationpoint, light-response curves, photorespiration, dark respiration,chlorophyll content, and ribulose 1,5-bisphosphate (RuBP) carboxylase(EC 4.1.1.39 [EC] ) activity has been examined in nodulated alfalfaplants (Medicago saliva cv. Aragón). Plants were subjectedto moderate (S1/RS1) or severe (S2/RS2) cycles of drought (drought/recovery).Photosynthetic light-response curves showed decreased light-saturatedphotosynthetic capacity and decreased apparent quantum yield.Upon rewatering, photosynthesis did not recover whereas conductancedid in moderately stressed plants. Calculated electron transportrate also declined in drought-stressed plants, but upon rewatering,moderately stressed plants exhibited a total recovery. Comparison of photosynthetic intercellular CO² response curvesin well-watered and stressed leaves led to the assertion thateffects in chloroplast metabolism contribute significantly tophotosynthetic inhibition. Although the validity of this entireline of research has been questioned by some recent studiesbecause the occurrence of heterogeneous stomatal closure wouldaffect these curves, in our case, the effect of water stresswas investigated in experimental systems where stomata had beenremoved. Measurements of in vitro RuBP carboxylase activityand protein content showed a strong decline during drought treatmentsand upon rewatering no recovery was observed. Therefore, ourresults suggest the major implication of non-stomatal factorsin the decline of photosynthesis in alfalfa plants under cyclicdrought conditions. Key words: Alfalfa, water deficits, photosynthesis, ribulose, 1,5-bisphosphate carboxylase activity, stomatal limitation     

Validation of a Photosynthesis Model through the Use of the CO2Balance of a Greenhouse Tomato Canopy

Several leaf photosynthesis models were developed from wellcontrolled experiments in growth chambers. However, only a fewhave been validated under greenhouse conditions for their quantitativeand qualitative adequacy. In this paper, rates of net photosynthesisfor a tomato crop (Lycopersicon esculentum Mill) were measuredin a semi-commercial greenhouse (615 m³) for a significant timeperiod. Concomitant measurements of climatic conditions andLAI were used for simulation of net photosynthesis using theTOMGRO model which integrates Acock's model for photosynthesiscalculations. From simulations and from sensitivity analysis,the prediction of net photosynthesis appeared to be very sensitiveto the quantum use efficiency. The Acock model with originalparameters underestimated the net photosynthesis rate, but anincrease in the quantum use efficiency by 10% gave a good fit.In an effort to generalize the validity of the model, a residualanalysis was performed and showed a systematic bias relatedto light intensity intercepted by the canopy. The Marquardtalgorithm was used to adjust our data to the model but did noteliminate residual heterogeneity of variance with new parametervalues. On the basis of collected data, the criteria of goodnessof fit used showed that the photosynthesis model is inadequatein describing the CO₂-balance of the greenhouse agrosystem.However, it was determined that it could be used as a submodelwithin a more complex model for predicting growth and development.Copyright 1999 Annals of Botany Company Greenhouse, CO₂-balance, photosynthesis, TOMGRO model, Acock's model, residuals, tomato Lycopersicon esculentum Mill.     

Seasonal Changes in the Physiology of S24 Perennial Ryegrass (Lolium perenne L.). 2. Potential Leaf and Canopy Photosynthesis during the Transition from Vegetative to Reproductive growth

The photosynthetic potential of successive youngest fully-expandedleaves of S24 L. perenne, grown as simulated swards under naturalenvironmental conditions, was measured during establishmentin autumn, over winter and during the transition from vegetativeto reproductive growth the following spring. Measurements weremade at a standard light energy receipt of 250 J m^–2 s^–1(400–700 nm) and at 15 °C. The photosynthetic potential of the leaves decreased in autumnas the swards increased in density under worsening environmentalconditions. During the spring, photosynthetic rates rose fromlow over-winter values so that by March, before stem elongationbegan, they were equal to the rates in the previous autumn.Following stem elongation there was a further increase in leafpotential. Reasons for these changes in leaf potential are discussed. During spring, the photosynthetic potential of the canopy alsorose - both as measured, and as predicted by the Monteith modelof canopy photosynthesis. Use of the model suggested that increasingleaf potential made the greatest contribution to the rise inthe potential of the canopy, although, following stem elongation,changes in LAI and canopy structure had a further significanteffect.     

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     

Photosynthesis, Water Relations, Temperature and Canopy Structure as Factors Influencing the Growth of Sainfoin (Onobrychis viciifolia Scop.) and Lucerne (Medicago sativa L.)

Measurements of the growth of sainfoin and lucerne were madein the field after cutting on 31 May 1977. Sainfoin reacheda total above-ground dry weight of 408 g m^–2 over thegrowing period of 48 days compared with 598 g m^–2 in lucerne.Final leaf area indices (LAIs) were 2.8 in sainfoin and 6.1in lucerne. The specific leaf areas (SLAs) for sainfoin wereapproximately half those of lucerne throughout the regrowthperiod. The maximum rates of leaf appearance were 0.12 leavesper day in sainfoin and 0.85 leaves per day in lucerne. Themaximum mean rate of plant extension growth for lucerne of 2.12mm h^–1 occurred during the night, whereas, in sainfointhe maximum rate of 1.72 mm h^–1 occurred during the day. Measurements of extinction coefficients for PAR ranged from0.45 to 0.89 in sainfoin and from 0 42 to 0.57 in lucerne. Asthe lucerne crop increased in size leaf water potentials andsolute potentials became more negative. In sainfoin leaf waterpotentials remained remarkably high throughout the growth period,solute potentials decreased and turgor potentials increased.The stomatal conductances of the two species were similar. The photosynthetic capacities and rates of dark respirationper unit leaf area in both species were similar. The rate ofcanopy ‘gross’ photosynthesis at 295 W m^–2was always greater in lucerne than in sainfoin. This was largelya matter of differences between the species in LAI, althoughat higher LAIs the more erect structure of lucerne leads toa better utilization of photosynthetically active radiation. Onobrychis vicifolia Scop, sainfoin, Medicago sativa L., lucerne, photosynthesis, water relations, temperature, canopy structure     

The Effect of Clipping on Net Photosynthesis and Dark Respiration Rates of Plants from an Upland Grassland, with Reference to Carbon Partitioning in Festuca ovina

Four co-existing species (Deschampsia flexuosa, Festuca ovina,Juncus squarrosus and Nardus stricta) were subjected to clippingand the net photosynthetic and dark respiration rates were followedafter this treatment for 50 d. Concurrently carbon partitioningin F. ovina plants clipped initially and again at 50 and 100d was examined. An expansion of new leaf lamina was observed with F. ovina,which had a greater net photsynthetic rate per unit leaf areathan unclipped lamina. The remaining leaf lamina (stubble) afterclipping also showed net photosynthetic and dark respirationrates greater than unclipped lamina; these responses were uniqueto F. ovina plants. N. stricta was the only other species toattain a pre-clipping photosynthetic rate within 6 d. Clipped F. ovina plants showed a change in carbon allocationpattern, with a reduction in carbon allocated to roots. ¹⁴Caccumulated in roots and stubble was shown to have a role inregrowth, as was current assimilate via the production of newleaf lamina. Plants initially clipped before exposure to ¹⁴C,redistributed less ¹⁴C to new shoot growth and, therefore, lostless when subsequently clipped. Further redistribution of ¹⁴Ccame from leaf stubble tissue and not at the expense of roots.The variation between species in clipping response are discussedin terms of the implications for coexistence. Carbon partitioning, clipping, gas exchange, grasses     

Photosynthesis and Photorespiration in Lolium multiflorum and Lolium perenne

The response of net photosynthesis to changing light-flux densityby leaves of Lolium multiflorum (S. 22) and L. perenne (S. 321)is more adequately described by current models when a term allowingfor photorespiration is included. The magnitude of this termwas determined from the changes in the slope of the light-responsecurves for net photosynthesis. A pseudo first-order rate-constantfor photorespiration, and a pseudo second-order rate-constantfor photosynthesis calculated by this technique for L. multiflorumwere found to be similar to corresponding parameters calculatedfrom light-compensation-point measurements using a simple modeldescribed by Brown (1969). The relative magnitudes of respirationand photosynthesis at light saturation for both Lolium specieswere similar to reported values for other temperate species(Lake, 1967). Two selection lines of L. perenne (S. 321) with contrastingdry-matter yields were found to have the same parameters forrespiration and photosynthesis.     

The Influence of Water Stress on the Photosynthesis of a Simulated Sward of Perennial Ryegrass

The rate of canopy photosynthesis, single leaf photosynthesis,leaf resistance to gaseous exchange, and leaf water potentialof simulated swards of perennial ryegrass (Lolium perenne cv.S24) in a controlled environment, were determined during a periodof increasing water stress and recovery from that stress. Canopyphotosynthesis did not decline immediately water was withheldbut continued at an undiminished rate for several days; thereafterit fell rapidly, particularly at first. As water stress increasedsuccessive relationships between canopy photosynthesis and irradiancebecame more curved, indicating that the effect of water stressincreased with increasing irradiance. After the swards werere-watered canopy photosynthesis rose, most rapidly during thefirst 24 h. In general, the pattern of change of leaf waterpotential was similar to that of canopy photosynthesis, althougha more detailed examination of this relationship showed it tobe hysteresial; in particular, the fall in leaf water potentialpreceded that of canopy photosynthesis. Single leaf photosynthesisappeared to be the main agent through which water stress influencedcanopy photosynthesis although in the more severely stressedswards (leaf water potentials of about—15 bars) some leaftissue died and so limited the recovery of canopy photosynthesis.The leaf resistance to gaseous diffusion increased with increasingwater stress, as did the CO₂ compensation point, thereby influencingsingle-leaf photosynthesis and through it canopy photosynthesis.     

Studies on the Relation Between Net Photosynthesis and Nitrogenase-linked Respiration in Subterranean Clover

The relation between the rate of nitrogenase-linked respirationand net photosynthesis, and the effect of defoliation on thisrelation, was studied in plants of subterranean clover (Trifoliumsubterraneum L. cv. Seaton Park). Nitrogenase-linked respirationwas estimated as the difference between the rate of nodulatedroot respiration at 21% O₂ and at 3% O₂. The level to which the rate of nitrogenase-linked respirationfell several hours after defoliation was directly proportionalto the decline in the rate of net photosynthesis. Approximately9% of net photosynthesis was always expended in nitrogenaseactivity, irrespective of whether or not the plants were defoliated.This proportion was maintained during the first 3 d of regrowth. To determine whether the decline in nitrogenase-linked respirationafter defoliation was due solely to the decline in the rateof photosynthesis, a further experiment was conducted in whichthe pre-defoliation rate of net photosynthesis was restoredimmediately (with supplementary light) or within 5 min (supplementarylight and CO₂) after defoliation. Restoring the rate of netphotosynthesis did not prevent the post-defoliation declinein nitrogenase-linked respiration. However, when photosynthesiswas reduced to zero by the imposition of darkness, and the rateof nitrogenase-linked respiration allowed to decline to a steadyrate after 3 h, a rapid recovery in the rate of nodulated rootrespiration began within 2 h of returning the plants to thelight. It was hypothesized that a ‘shoot factor’,which was affected by defoliation, could override the apparentrelation between nitrogenase-linked respiration and the rateof current photosynthesis. Key words: Defoliation, N₂ fixation, photosynthesis, nitrogenase-linked respiration, subterranean clover     

Optimal Nitrogen Content and Photosynthesis in Cauliflower (Brassica oleracea L. botrytis). Scaling up from a Leaf to the Whole Plant

A simple model of photosynthesis is described which is dependenton leaf area, organic nitrogen content and distribution withinthe canopy as well as on the light and temperature environments.The model is parameterized using a cauliflower crop as an example.The optimized protein-N profile within the canopy is calculatedwith respect to daily growth rate. By comparison with measuredprotein-N contents, the amount of super-optimal N-uptake, i.e.the N-uptake which does not increase productivity, is assessedfor two different nitrogen and light treatments. The amountof super-optimal N accumulated in cauliflower depends on N-supplyand can exceed 80 kg N ha^-1. Copyright 2000 Annals of BotanyCompany Brassica oleracea L. botrytis, cauliflower, nitrogen, photosynthesis, respiration, model, optimization