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.     

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.     

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     

Effect of CO2 Concentration on Growth of Sugar-beet, Barley, Kale, and Maize

Increasing the concentration of CO₂ in the air from the usual300 ppm to 1, 000 ppm in growth rooms with temperatures of 20°C during the 16-h light period and 15° C during the 8-hdark period increased the total dry weight of sugar-beet, barley,and kale by about 5o per cent. A further increase in CO, concentrationto 3, 300 ppm increased dry weight slightly more. These effectsoccurred with light intensities ranging from 3.7 to II.6 caldm–² min–¹ of visible radiation supplied by a mixtureof fluorescent and tungsten lamps, and were only slightly greaterwith the brighter light. Extra CO₂ also increased leaf area,though relatively less than dry weight, and the number of barleyshoots but not of sugar-beet or kale leaves; it decreased leaf-arearatio, specific leaf area, and the ratio of tops to roots. Maizewas taller with extra CO₂. Net assimilation rates in 1, 000 and 3, 300 ppm CO₂ were about20 and 30 per cent respectively greater than in 300 ppm. Uptakeof CO₂ in the light by complete tops and single leaves alsoincreased with increase in CO₂ concentration. Photosynthesisof leaves of plants recently transferred to a new CO₂ concentrationdepended only on that concentration and not on the originalone. Doubling the light intensity from 3.7 to 7.7 cal dm–²min–¹ affected dry weight, leaf area, net assimilationrate, etc., similarly to a tenfold increase in CO₂ concentration.     

太白山红桦种子的萌发特性

 红桦(Betula albo-sinensis)广布于我国太白山中高海拔地区, 普遍存在着更新障碍。为了给红桦林的自然更新提供新的解释与证据, 我们对2 400粒红桦种子进行了去果皮观察并计数发芽率, 观察了红桦种子在不同光合有效辐射(Photosynthetically active radiation, PAR) 和不同光照时间(强光照: PAR为 464.7 μmol photons·m^-2·s^-1, 12 h·d^-1; 中光照: PAR为233.8 μmol photons·m^-2·s^-1, 12 h·d^-1; 弱光 照: PAR为233.8 μmol photons·m^-2·s^-1, 0.5 h·d^-1)、不同昼夜温差(25/20 ℃、20/15 ℃、15/10 ℃)和去果皮处理下的发芽率, 以及无光 照处理下的发芽率。此外, 还观察了种子在不同覆盖物(阔叶、针叶、针阔混合)和不同基质(土、沙)中以及在落叶(阔叶、针阔混合)中的发芽 率。红桦种子成熟时, 大约54.29%为饱满具活力的种子。种子在25/20 ℃的昼夜温差下发芽率最高, 而在15/10 ℃的昼夜温差下极少萌发。中 光照下, 种子发芽率略高于其它两种光照, 无光照则完全抑制了种子萌发。去果皮处理可促进其萌发, 各种覆盖物会导致种子发芽率下降。不 同的萌发基质对萌发无影响。以上结果暗示, 自然条件下太白山红桦种子的萌发将依赖于自然的扰动。     

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     

Photosynthesis of Lamina and Sheath of Barley Leaves

Apparent photosynthesis, in mg. CO₂ absorbed per dm.² per hour,of the sheath and enclosed stem of a barley leaf was about 50per cent. of that of the lamina of the same leaf, when the photosynthesizingarea was measured as one side of the lamina and the outer exposedsurface of the sheath. Apparent photosynthesis of a particularlamina or sheath was about 70 per cent. of that of the one aboveon the same stem. Respiration per dm.², though not per g. dry weight, of sheathwith enclosed stem was greater than of lamina in one experimentdone with low-intensity illumination so that true rates of photosynthesisof lamina and sheath were similar. Differences in respirationrates per unit area of laminae and sheaths probably accountedfor most of the greater apparent photosynthesis of the formerin other experiments done with higher intensity illumination. It is suggested that for growth-analysis studies the size ofthe photosynthetic system of cereals should be measured as thatof one side of the leaf laminae plus the outer surface of thecombined leaf sheaths. In the later stages of growth the surfacearea of exposed stem and peduncle should also be included.     

Some Effects of Temperature and Irradiance on Growth of the First Four Leaves of Wheat, Triticum aestivum

Plants of Heron wheat were grown at 20 and 15 °C and inquantum flux densities of 400 and 200 µmol m^–2 s^–1.At completion of expansion of the first or second leaf, plantswere transferred between temperatures and quantum flux densities.Final size and cell number were measured for each of the firstfour main-stem leaves. Leaf area was affected only slightlyby treatment and effects on leaf length and width were alsosmall. It was concluded that leaf extension rate, which waslower at the lower temperature and in the lower light regime,is inversely related to the duration of leaf expansion. Leafdry wt was higher for plants grown in high light and for plantsgrown at 15 °C; transfer treatments led to readjustmentswhereby dry wts of leaves expanded after transfer resembledthose of leaves on plants kept throughout in the post-transferconditions. Leaf cell number was not affected by treatment but mean drywt per cell was significantly greater in high light, and forthe first two leaves, at 15 °C. There was a major and highlysignificant effect of treatment on the ratio of dry: fresh wtper cell, this being larger for leaves in high light. Transfertreatments between light regimes led to rapid changes in expandingleaves as was found for leaf dry wt. It was concluded that theexpanding grass leaf is much less dependent on older leavesto provide the necessary materials for cell division and expansionthan is the dicotyledon leaf. It is suggested that the increasein cell dry wt in high light is associated with an increasein cell wall material which is under photomorphogenic control. Triticum aestivum, wheat, leaf growth, cell division, cell expansion, cell size     

Effects of Chemical Antitranspirants on Transpiration and Growth of Grass

The effects of foliar sprays of the metabolic inhibitors dodecenylsuccinicacid (DSA) and phenylmercuric acetate (PMA), as antitranspirants,were tested on grass grown outdoors (in lysimeters), but moreaccurate tests were made with PMA in growth rooms, using smallweighable transpirometers. Concentrations of PMA which wereweaker than 10^-3.8 M resulted in only slight reductions in transpiration,whereas concentrations stronger than 10^-3.2 M were phytotoxic,though water losses were reduced by about 30 per cent. PMA at10^-3.5 M gave the greatest decrease in transpiration (about20 per cent) without reducing growth, but its effectivenessdepended on the amount applied per unit area of vegetation.The effects of PMA also differed with plant species and withenvironment, being greatest under conditions of low soil moisturestress and temperature. The antitranspirant reduced stomatalapertures and increased leaf temperatures.     

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     

Partitioning of 14C Labelled Assimilates in Arctium tomentosum

Plants of the biennial Arctium tomentosum were grown from seedto seed-set in an open field under three different treatments:control plants receiving full light intensity, plants with aleaf area reduced by 45 per cent, and shaded plants receivingonly 20 per cent of natural illumination. At various stagesof development the youngest fully expanded leaf of one plantin each treatment was exposed to ¹⁴CO₂ for half an hour. Subsequentdistribution of labelled assimilates in various plant partswas determined after eight hours. In the first year, the mostdominant sink was the tap root irrespective of variation inassimilate supply. During the production of new vegetative growthin the second season, a larger amount of radioactive photosynthatewas recovered from above ground parts, especially during formationof lateral branches. Seed filling consumed 80–90 per centof labelled carbon exported from the exposed leaf. In the secondyear, the most pronounced difference between treatments wasin the degree of apical dominance, being highest in shaded plantsand lowest in the plants with cut leaves. Results from ¹⁴C experimentsagreed fairly well with a ‘partitioning coefficient’derived from a growth analysis of plants grown independentlyunder the same experimental conditions. Reasons for discrepanciesbetween the ¹⁴C results and the partitioning coefficient arediscussed. Arctium tomentosum, burdock, variation in assimilate supply, assimilate distribution, ¹⁴CO₂, labelling, growth analysis     

Effect of Light Intensity on Growth of Natural Populations of Dactylis glomerata L.

The growth of two natural populations of cocksfoot from contrastingclimatic regions, Norway and Portugal, was studied in two photoperiodsat three temperatures with three levels of light energy (48,144, and 240 W m^–2 in the wavelength interval 400–700nm). There was a consistent increase in relative growth-rate(RGR) in response to increased light energy up to 144 W m^–2,but above this energy level there was either no change, or,in some treatments, a decline. Net assimilation rate (NAR) increased,whilst leaf area ratio decreased from the lowest to the highestenergy level in most treatments. The decrease of LAR with increasedlight energy could be attributed to a decrease of both leafweight ratio (LWR) and specific leaf area (SLA), a greater proportionof dry matter being distributed to plant parts other than leaf.This effect occurred although there was a positive relationshipbetween light energy and relative leaf growth-rate (RLGR). Populationdifferences in these growth attributes were most marked in thetreatments with low-temperature and short-day conditions. Theefficiency of energy conversion of visible radiation declinedfrom 3–4 per cent at the lowest energy level to 1–2per cent at the highest energy level.     

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.     

Kinetics of Leaf Growth in Lolium temulentum at Optimal and Chilling Temperatures

Lolium temulentum plants were grown at 20 °C, under an 8-hdaylength, in a controlled-environment chamber, and the kineticsof leaf expansion were observed by measuring the movement ofan optical grid attached to the fourth leaf. The leaf emerged23–24 d after sowing and was fully expanded 9–10d later. Extension rate was maximal between the second and fifthdays after emergence and declined markedly thereafter. Duringthe rapid growth phase the rate of elongation exhibited a distinctdiurnal rhythm, fluctuating between 1.9 to 2.3 mm h^–1in the light period, and 1.3 to 1.7 mm h^–1 in the dark.A circadian oscillation with a period of about 27 h was observedin leaves elongating in continuous darkness. When plants weretransferred to 5 °C soon after emergence of the fourth leafthere was an immediate reduction in rate of growth to about22 per cent of the rate at 20 °C: the Q₁₀ for the mean elongationrate in the range 20–5 °C was 3.7. When plants weretransferred from 20 to 2 °C at fourth leaf emergence, meanextension rate declined to less than 5 per cent, correspondingto a Q₁₀ in the range 5–2 °C of more than 300. Furthermore,growth at 2 °C was confined almost entirely to the darkphase of the photoperiod cycle. The responsive tissue was shownto be a small area of expanding leafless than 1.5 cm above theshoot apex and the possible mechanisms underlying low temperatureeffects in this region are discussed. Lolium temulentum L., leaf growth, auxanometer, low temperature, diurnal rhythm     

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.     

Metabolism of Barley Leaves Inoculated with Erysiphe graminis Merat

Inoculating Proctor barley leaves with Erysiphe graminis decreasedrates of photosynthesis, after an initial lag period, and increasedrespiration. Increasing the area inoculated progressively decreased ratesof photosynthesis, but the effects cannot be attributed to asimple loss of leaf area. When less than 30 per cent of a leafwas inoculated, decreases were equivalent to area losses greaterthan those inoculated; when more than 30 per cent was inoculatedthe photo-synthetic losses were equivalent to area losses lessthan those inoculated. Although the relative effects of E. graminis on photosynthesisand respiration were of the same order, the absolute effectson photosynthesis were greater than those on respiration. Inoculating30 per cent of a leaf decreased photosynthesis by 5–6mg CO₂/dm²/hr from 12.9 in the uninoculated controls to 7.3.Respiration increased by 0.6 mg CO₂/dm/hr, from 1.7 to 2.3-     

The Stability of the Growth Pattern of Young Apple-trees under Four Levels of Illumination: With four Figures in the Text

As part of a programme on the characterization of the growthpattern of the apple-tree, uniform I-year-old plants carryingtwo new shoots were grown for a season under 100, 78, 41, or24 per cent, natural light. Weights of leaf, new stem, old stem,and root were determined by sampling on 6 occasions. The dryweightincrements over the whole season ranged from 17 to 151 g. ofwhich, in full light, 22 per cent, was leaf and 22 per cent,was root; in deep shade, 32 percent, was leaf and 8 per cent,was root; and under all conditions 56–60 per cent, wasstem. These percentages were the cumulative result of currentrates of growth of leaf, stem, and root over the whole growingperiod. The rates were not constant relative to one another,but the much greater quantity of growth made over the period100–200 days from bud-break swamped earlier differences.All treatments showed the same basic pattern of growth withminor differences increasing with the intensity of shading.These differences are discussed in relation to utilization ofreserves and the demands of the various regions of the plant.The response to shading was analysed into the effects of reducedlight interception, increased leaf dispersion, increased leafproportion, reduced length of growing season, and lighter leafweight at the beginning of the 20-leaf stage. The result ofthese factors was a doubling of the growth expected in the absenceof response. It was concluded that the pattern of growth isstable, and the conditions for the perpetuation of such stablepatterns as a necessary qualification for tree growth are brieflydiscussed.     

不同光照处理下青藏高原克隆植物黄帚橐吾(Ligulariavirgaurea)种子大小对其幼苗生长的影响

以高寒草甸克隆植物黄帚橐吾为实验材料,通过遮荫网模拟植被遮荫,研究种子大小与萌发及幼苗生长能力的关系和幼苗对光照条件的反应。结果表明：（1）在自然光照下,黄帚橐吾种子大小对种子萌发的影响显著,大种子的萌发率高于小种子。遮荫生境下,大、小种子萌发率有所降低,但遮荫对小种子萌发的影响比大种子显著。小种子的萌发率下降了近1/8,而大种子的萌发率仅下降了1/11。（2）黄帚橐吾种子大小对幼苗生物量积累影响显著,大种子幼苗总生物量（TB）大于小种子幼苗的。但生物量的分配与播种时间相关,播种后60 d,在自然光照条件下,大种子幼苗对根生物量的分配大于小种子幼苗,而对叶生物量的分配则正好相反。在遮荫环境中,大、小种子幼苗普遍对根的生物量分配增加,大种子幼苗根冠比（R/S）大于小种子幼苗。（3）黄帚橐吾种子大小对幼苗的生长也有明显影响。在自然光照下,小种子幼苗的相对生长速率（RGR）较大于大种子幼苗,但叶面积比率（LAR）、叶面积干质量比（SLA）、叶干质量（LWR）差别不明显。在遮荫条件下,幼苗的LAR、SLA、LWR显著增加,但大、小种子幼苗间差异不显著,幼苗的RGR减小,小种子幼苗的减小趋势大于大种子幼苗。     

Effect of growth irradiance on the maximum photosynthetic capacity of water hyacinth [Eichhornia crassipes (Mart.) Solms]

We grew water hyacinth [Eichhornia crassipes (Mart.) Solms]for 60 days in a greenhouse under natural light and in a controlledenvironment room at 31/25?C day/night temperatures and 90, 320and 750/µEm^–2sec^–1. We then determined maximumphotosynthetic rates in 21% and 1% oxygen, stomatal diffusionresistances, contents of chlorophyll and soluble protein, andthe size and density of the photosynthetic units (PSU) in representativeleaves from the four treatments. In air containing 21% oxygen,maximum photosynthetic rates were 14, 27 and 29 mg CO₂ dm^–2hr^–1for plants grown in artificial light at 90, 320 and 750µEm^–2sec^–1,respectively. Plants grown in natural light (maximum of 2000µEm^–2sec^–1) had maximum photosynthetic ratesof 34 mg CO₂ dm^–2hr^–1. In all treatments, photosyntheticrates in 1% oxygen were about 50% greater than rates in normalair, indicating the presence of photorespiration in water hyacinth.There was no apparent relationship between maximum photosyntheticrate per unit leaf area and stomatal conductance, chlorophyllcontent per unit area, or PSU density per unit area. However,the higher maximum photosynthetic rates were associated withgreater mesophyll conductances, specific leaf weights and proteincontents per unit area. When plants grown at 90µEm^–2sec^–1for 120 days were transferred to 750µEm^–2sec^–1for 5 days, only young leaves that were just beginning to expandat the time of transfer exhibited adaptation to the higher irradiance.The 40% increase in light-saturated photosynthetic rate in theseyoung leaves was associated with increases in mesophyll conductance,soluble protein content per unit area, and specific leaf weight. ¹ Mississippi Agricultural and Forestry Experiment Station cooperating. (Received July 19, 1978; )