PLANTVol. 37 No. 6

PHlYSIOLOGY November 1962

Effects of Temperature & Light Intensity on CO2 Uptake,Respiration, & Growth of White Clover"2

George Beinhart

Crops Research Division, Agricultural Research Service, USDA, Clemson, S.C.

Net photosynthesis is defined (13) as the differ-ence between the CO gained in photosynthesis andthat lost in respiration. As pointed out by TaIling(17), the carbon increase indicated by plant growthrepresents the integration of net photosynthesis withtime. Therefore, rate measurements of these twoprocesses should contribute to a better understandingof environmental effects on growth. In the studiesreported in this paper, rates of CO, uptake and res-piration were measured in white clover (Trifoliumrepens L.) and compared with the growth of plantssubjected to different temperature and light intensityregimes. '

Mitchell and coworkers (14, 15,16) have studiedtemperature effects on the growth of pasture species.White clover grew best at 24 C, but grew nearly aswell at 18 or at 30 (14). Stolon growth and therate of appearance of new stolons were the most use-ful indicators of responses to be expected under fieldconditions (15); these characteristics varied greatlywith temperature (15, 16).

Blackman (2) and Blackman and Templeman(4, 5) investigated light intensity effects on growthin clover-grass associations and concluded that thelegume-grass balance within a sward depends largelyon competition for light, particularly with respectto effects on root growth. Black (1) dealt with theinfluence of light intensity on the growth of herbageplants and stated that pasture legumes are intolerantof even moderate shade. Black concluded, as didBrown and Munsell (8), that competition for lightis a major factor in the disappearance of clover fromfield stands.

At low light intensities and high temperatures,respiration may limit the rate of dry matter gain ofa plant community. Thus, the net photosynthesis ofa forage community decreases when foliage densitybecomes too high, due to respiration by the deeply

1 Manuscript received Feb. 1, 1961.2 Technical Contribution No. 352, South Carolina

Agricultural Experiment Station.

shaded lower leaves (12). Several recent studieshave related growth rates to the amount of leaf areapresent (3,6,11,23). An important result of theseand similar studies has been the development of theleaf area index (LAI) concept. The LAI of a plantcommunity is defined as the ratio of leaf area tosoil surface area. Net assimilation rates (i.e., drymatter gain per unit of photosynthetic surface) andcrop growth, rates vary with LAI, and differentspecies show different responses (3) ; for a concisereview, see (17, p 149).

The LAI concept is, for the most part, based onrelationships within a plant community. The im-plications of a dependency of growth rate upon leafarea, however, are very broad; Watson has statedthat leaf area may be more important than net as-similation rate as a factor limiting growth rate (24).In studying white clover plants under controlled en-vironments, therefore, I have attempted to assessthe relative importance of assimilation rates and leafarea as factors limiting growth rates.

Materials & Methods- Apparent Photosynthesis Measurements. CO2

uptake was measured in a closed system in whichthree to five white clover leaves were sealed insidea Lucite chamber. A diaphragm pump drew airfrom this space through a silica gel drying tube andthence to a Liston-Becker infra-red gas analyzer.The gas analyzer was connected to an Angus-Esterline recorder, allowing continuous monitoring of theCO, content of the air stream. After passing throughthe gas analyzer, the air reentered the photosynthesischamber.

The chamber enclosing the leaves was clamped toa base plate made of two pieces of Lucite fitted to-gether by a tongue and groove joint. Petioles en-tered through a hole in the center of the base plate,and the assembly was made gas tight by sealingaround the petioles and joints with a mixture ofpetrolatum and paraffin. Two chamber assemblies

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PLANT PHYSIOLOGY

wvere used; while measurements were made in one,the other was prepared for the next reading.

Water from a refrigerated reservoir circulatingbetween the double walls of the chamber effectivelycontrolled air temperature inside the photosynthesischamber. All plants were measured at 10 C, then at20 C, and finally at 30 C.

The light source was a 150w reflector spot lightapproximately 18 inches above the chamber. Re-duced light intensities were obtained by placing cheesecloth filters on top of the chambers. The measure-ment sequence for each plant was: A, high intensity(4,200 ft-c) ; B, medium intensity (1,200 ft-c); C,low intensity (800 ft-c). Plants were equilibratedapproxinmately 30 minutes before measurement at highintensity; between high and medium, and medium andlow intensities, they were equilibrated at least 20minutes.

Six white clover plants (Trifoliumt repens L.)were used for this study; all were of the same cloneused in the growth chamber experiments describedbelow. For 4 weeks prior to the photosynthesismeasurements, these plants were maintained in agrowth chamber at a constant temperature of 21 C,2,000 ft-c light intensity, and a 14.5 hour daylength.

CO2 uptake was measured by a procedure de-scribed in detail by Bourdeau (7) ; the time requiredto reduce CO., content from 10 % above normal to10 % below normal concentration (0.03 %) was de-termined from the distance moved by the recorderchart during the passage from the upper to the lowerconcentration limits. CO, uptake was calculated asmilligrams per hour per square centimeter of sto-mated leaf surface; both upper and lower leaf surfacespossess stomates (10). Two or more separate meas-urements were made for each-plant at each light in-tensity and temperature and averaged for use in fur-ther calculations. The data were statistically an-alyzed as a factorial experiment with three levels ofeach of two factors (9).

- Respiration Measurements. Respiration, as 02uptake, was measured in a Warburg apparatus. Leafdiscs (6 mrm diameter) were cut with a cork borerand transferred unwashed into Warburg flasks.Stolon respiration was measured using 1 cm longinternode sections. Distilled water (0.2 mul) wasadded to the sample compartment of each Warburgflask; 0.2 ml of 20 % KOH was placed in the centerwell with a fluted filter paper wick to increase theCO2-absorbing surface. Readings were taken at 10or 15 minute intervals over a period of 5 hours afterwhich the samples were dried overnight at 85 C andthen weighed. 02 uptake was calculated by themethod of Umbreit (20).

- Growth Chamber Experiments. Two factorialexperiments were carried out concurrently in growthchambers; each tested the influence of two factors onthe growth of clover plants. Both experiments em-ployed 14.5 hour light, 9.5 hour dark photoperiodcycles and lasted 6 weeks.

In one experiment, day temperature (DT) and

night temperature (NT) were the variables, eachat four levels: 10, 17, 23, and 30 C. All 16 possi-ble DT/NT combinations were tested. Plants inthis experiment received 600 ft-c light intensity.

The variables in the second experiment were lightintensity and NT. 600, 900 and 2,000 ft-c light in-tensities were tested in all nine combinations with10, 17, and 30 C NT; all plants were subjected to30 C DT.

The growth chambers used in the DT/NT ex-periment were converted walkin refrigerators, ap-proximately 8 X 8 X 8 feet in outside dimensions,equipped with fluorescent and incandescent lights andthermostatic controls (19). Four chambers wereused, each maintained at a different constant tempera-ture. DT/NT combinations were obtained by mov-ing the plants among the chambers at the end of eachlight and dark period.

The light intensity/NT experiment was carriedout in a separate chamber providing 2,000 ft-c lightintensity at plant height (18). Reduced light in-tensity was obtained by shading individual cans withcheese cloth circles placed on wire screen discs andsupported by dowels placed in each plant container.For the dark periods the 10 and 17 C NT plants weremoved to rooms that had temperature controls butno lights; transfers were made twice daily as de-scribed above.

All plants were rooted cuttings from a singleclone. One to two inch long stolon segments wereplaced in moist sand and maintained at 17 C underartificial illumination for 2 weeks, then carefully re-moved from the sand and randomly distributed amongthe containers. Five plants were placed in each con-tainer. The containers were cans, 6 inches in di-ameter and 7 inches high, filled with soil. Sixreplicate cans (30 plants) were included in eachtreatment.

All leaves in excess of two per plant were re-moved at the start of the experiment. Living anddead leaves were counted, and the dead leaves re-moved, at the end of the 2nd, 3rd, 4th, and 5th weeks.

Leaf area measurements were based on the ob-servation that laminal area varies directly with thediameter of a circle that just circumscribes a trifoliateleaf. The so-called diameter of a leaf was measuredby pressing it against a piece of clear plastic havinga series of concentric circles marked on its surface.The circles were so spaced that when their centerwas superimposed on the center of the trifoliate leafthe leaf diameter was measured to the nearest 0.1 cm

by each of the three leaflets. Photographic contactprints were made of various-sized leaves, their di-ameters measured, and the prints cut out and weighed.The area of each print was obtained from a calibra-tion curve of area vs. weight of the paper. A re-

gression of leaf area on leaf diameter was then cal-culated for the prints and this equation was used to

estimate leaf area for the experimental plants. Oneplant in each can was randomly selected and all ofits fully expanded leaves were measured.

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BEINHART-TEMPERATURE & LIGHT EFFECTS ON CO2 UPTAKE

Half of the plants in each treatment were har-vested at the end of a 14.5 hour light period and theother half after the succeeding dark period. At eachharvest the leaves of all plants in a treatment werecombined as a single sample in a tared cloth bag.Stolons were similarly combined, and the roots werewashed free of soil and combined. The samples were

dried rapidly in a force-draft oven at 95 C and thenin an 85 C convection oven for 18 hours. The driedsamples were weighed and total plant weights wereobtained for each treatment by adding the leaf, stolon,and root fractions.

The methods of Cochran and Cox (9) were usedfor statistical analyses.

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hour period at temperatures - 35 C; time scale is expanded in upper portion of figure to show rate differences be-

tween temperatures.Fig. 3 (Upper right). Cumulative 02 uptake by internode sections of white clover stolons.Fig. 4 (Lower right). Regression of mean total dry weight on average leaf area present for white clover plants

grown under 16 DT/NT regimes with 600 ft-c light intensity.

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PLANT PHYSIOLOGY

Results- CO2 Uptake. The CO2 uptake at different tem-

peratures and light intensities is summarized by fig-ure 1. The main effects and interactions of tempera-ture and light intensity were statistically highly sig-nificant (p <.01).

At 10 C, variation in light intensity caused no sig-nificant effect; the small rate differences between800, 1,200, and 4,200 ft-c were well within the rangeof experimental error. At 20 C, photosynthetic ratesexceeded those at 10 C at all light intensities; CO,uptake rose sharply as light increased from 800 to1,200 ft-c, but only slightly as light intensity increasedfurther to 4,200 ft-c. At a temperature of 30 C, CO2uptake rates exceeded those at 20 C at each light in-tensity; also, the response to the increase from 1,200to 4,200 ft-c was greater than 20 C.0 Respiration. In leaf tissue, respiration ratesvaried directly with temperature from 20 to 35 C(fig 2); within this range, rates remained nearlyconstant during the measurement period. Above35 C, respiration rates decreased with time. Thedecline was sharp at 45 C, while at 50 C there waspractically no 02 exchange after the 2nd hour.

The initial rates of stolon internode respirationincreased with each increase in temperature (fig 3).The stolon sections at 50 C were still respiring after5 hours, although their rate steadily declined afterthe 1st hour. At 45 and 40 C, respiration rates de-clined after approximately two hours to that shownfor 35 C. During the last 2 hours of observationsthere was little difference in respiration rate overthe 15 C temperature range tested.

- Growth Chamber Experiments.I. Day TemperaturelNight Temnperature Ex-

periment. Leaf production was greatly influencedby DT (table I). Total production rose sharply asDT increased from 10 to 17 C, but decreased as DTincreased to 23 and to 30 C. All differences betweenDT means were statistically highly significant.

Responses to NT were less regular. At 10 C andat 30 C DT, leaf production varied directly with NTand was maximum at 30 C NT. At 17 and 23 CDTs, production increased as NT rose from 10 to17 C, but (lecreased at 30 C NT.

Table I

Total Leaf Production By Plants Under DifferentDay/Night Temperature Regimes*

Day Night temp. (C)temp. Mean(C) 10 17 23 30

10 62 115 122 146 11117 144 228 208 196 19423 164 181 203 180 18230 119 125 135 142 130

Mean 122 162 167 166 154LSD Individual cells 13.5 (5 %); 18.2 (1 %)LSD DT & NT means 6.8 (5 %); 9.1 (1 %)* Data are 4-week totals for 30 plants in each treatment.

Table II

Number of Leaves Dying Under DifferentDay/Night Temperature Regimes*

Day Night temp. (C)temp. Mean(C) 10 17 23 30

10 69 76 98 48 7317 73 81 89 66 7723 69 70 65 69 6830 53 53 67 63 59

Mean 66 70 80 62 69LSD Individual cells : 9.3 (5 %); 12.5 (1 %)LSD DT & NT means : 4.6 (5 %); 6.2 (1 %)* Data are 4-week totals for 30 plants in each treatment.

No simple response to temperature is evident fromthese data. Maximum leaf production occurred at17 C/17 C, but at this DT a drop in NT of only 7 Cdrastically depressed production. At 23 C DT, onthe other hand, production was much less affectedby low NT. Therefore, leaf production under fieldconditions should be greater at an average DT of23 C than at 17 C, since night temperatures willnormally be at least 5 to 10 C lower than day tem-peratures.

Temperature also affected the rate at which leavesdied (table II). Leaf mortality at 23 C DT wassignificantly less than at the two lower temperatures,and was least at 30 C DT. The effect of NT wasagain greatest at 10 C DT. Leaf mortality increasedwith incremental changes in NT from 10 to 23 C,but fell to the lowest level of the experiment at 10 C/30 C. A similar trend was evident at 17 C DT, whileNT had no large effect on leaf mortality at 23 or30 C DT.

Table III

Estimated Total Leaf Area Produced During4 Week Period (as cm2)

Day Night temp. (C)temp. Ma(C) 10 17 23 30

10172330

384 644 8911,310 2,075 2,1011,460 1,629 1,868833 800 891

1,241 7991,627 1,7851,224 1,529611 794

Mean 976 1,231 1,403 1,179 1,140

Total leaf area production was estimated by mul-tiplying the mean area of single leaves by the numberof leaves produced in each treatment (table III).The significance of leaf area production to the growthof plants in this experiment is discussed below.

Plant weights were significantly influenced by

712

BEINHART-TEMPERATURE & LIGHT EFFECTS ON CO2 UPTAKE

Table IVHarvested Weights of Plants Grown Under Different

Day/Night Temperature Regimes*Day Night temp. (C)temp. Mean(C) 10 17 23 30

10 0.178 0.252 0.223 0.335 0.24717 0.412 0.495 0.478 0.437 0.45623 0.477 0.481 0.523 0.395 0.46930 0.327 0.310 0.312 0.290 0.310

Mean 0.349 0.384 0.384 0.364 0.370LSD Between cells 0.036 (5 %); 0.049 (1 %)LSD DT & NT means 0.018 (5 %); 0.025 (1 %)* Data as grams total dry weight per plant.

DT (table IV). The moderate DTs of 17 and 23 Cwere more favorable than either extreme. The ef-fects of NT on plant dry weights were great at 10 CDT, but were much less at higher DTs.

II. Light Intensity/Night Temperatuire Experi-ment. Leaf production was stimulated by each in-

Table VTotal Leaf Production Under Different Light

Intensity/Night Temperature Regimes*

Light Night temp. (C)intensity Mean(ft-c) 10 17 30

600 119 125 142 129900 287 305 250 281

2,000 350 419 333 367

Mean 252 283 242 259* Data are 4-week totals for 30 plants in each treatment.

crease in light intensity (table V). The greatestdifference was between 600 and 900 ft-c. At 600ft-c, leaf production increased with each rise in NT;at 900 and at 2,000 ft-c light intensity, leaf productionwas maximal at 17 C NT and minimal at 30 C NT.

Leaf mortality rates were not significantly affect-ed by NT in this experiment, but the means for lightintensity groups did differ significantly. The cumu-lative responses are summarized on a per plant basisin table VI. Since no plant had more than twoleaves at the beginning of the experiment, it is as-sumed here that each bore two leaves at the start.

Plants at 600 ft-c lost an average of one-half ofthe original leaves before the end of the 3rd week;at the end of the 5th week a few of their original

Table VIAverage Cumulative Leaf Mortality* Under Different

Light Intensity/Night Temperature Regimes

Light Weekintensity (ft-c) 2nd 3rd 4th 5th

600 0.88 1.27 1.57 1.85900 1.06 1.70 1.91 2.92

2,000 1.51 2.29 2.66 3.48* Number of dead leaves per plant.

leaves remained. At 900 ft-c light intensity, half ofthe original leaves were dead by the end of the 2ndweek and after 4 weeks almost all of these leaveswere gone. Plants at 2,000 ft-c lost 75 % of theiroriginal leaves by the end of the 2nd week; 1 weeklater, these leaves were all gone and leaves less than3 weeks old were dying. During the first 5 weeksat 2,000 ft-c, an average of 1.5 leaves had emerged,matured, and died on each plant. The correspond-ing figure for the 900 ft-c light intensity group was0.9 and at 600 ft-c it was zero.

More leaves were produced at higher light in-tensities and their average sizes exceeded those at600 ft-c. The net effect of these factors on esti-mated total leaf area production is indicated bytable VII.

Table VIIEstimated Total Leaf Area Produced Under Different

Light Intensity/Night TemperatureRegimes (as cm2)

Light Night temp. (C)intensity Mean(ft-c) 10 17 30

600 833 800 611 748900 2,675 2,861 2,018 2,506

2,000 3,112 3,872 2,084 2,984

Mean . 2,122 2,349 1,508 1,976

Differences in plant weight were greatest be-tween 600 and 900 ft-c light intensity (table VIII).The further increase in weight as light intensity in-creased to 2,000 ft-c was smaller but statisticallyhighly significant. The weight responses to lightintensity and NT paralleled total leaf area productionshown in table VII and suggested that plant weightswere proportional to average leaf area at each lightintensity.

Table VIIIHarvested Dry Weights of Plants Grown Under

Different Light Intensity/NightTemperature Regimes*

Light Night temp. (C)intensity Mean(ft-c) 10 17 30

600 0.328 0.310 0.290 0.309900 1.086 1.1-20 0.826 1.011

2,000 1.324 1.345 1.051 1.240

Mean 0.908 0.925 0.722 0.853LSD Individual cells : 0.227 (5 %); 0.326 (1 %)LSD Light intensity & : 0.131 (5 %); 0.188 (1 %)

NT means* Data as grams total dry weight per plant.

Discussion

In the DT/NT experiment, total growth at 30 CDT was considerably less than at 17 and 23 C DTs(table IV). A modest increase in light intensity

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PLANT PHYSIOLOGY

from 600 to 900 ft-c offset the high temperature effect(table VIII). This response, as well as the CO2uptake measurements (fig 1), appear to contradictBlack's conclusion (1) that no important pasturespecies is shade tolerant. The plants in my experi-ments received full benefit of incident light, how-ever, while competition under field conditions im-plies that photosynthesis in most leaves of a cloverplant may be light-limited even in full sunlight (3).

In general, the data on CO, uptake agreed withresults of the growth chamber experiments. At 800ft-c light intensity, temperature effects on photo-synthesis were modest (fig 1). At 30C, CO2 up-take increased sharply as light increased from 800to 1,200 ft-c, agreeing with the pattern of plantweights in the light intensity/NT experiment (tableVIII).

Neither photosynthesis nor respiration (figs 2,3) was adversely affected by temperature within the10 to 30 C range included in the growth chamberexperiments. Poor growth of clover under condi-tions of low light intensity and high temperature,therefore, does not imply damage to either of thesefundamental processes.

According to Watson (24), cultural practices thataffect yield do so by affecting leaf growth, especiallythe photosynthetic area. He questions whether thephotosynthetic efficiency of a crop species can bemuch improved by breeding, since the increased pro-duction of improved varieties results from greaterleaf area rather than higher assimilation rates. Be-cause of the important implications of Watson's hy-pothesis, the relationship between total dry weightand leaf area of clover is now considered.

Dry weight variation in the DT/NT experiment(table IV) closely paralleled the estimated leaf area(table III). Therefore, a regression of total dryweight on average leaf area present was calculated(fig 4). The differences in plant weights were al-most entirely accounted for in terms of correspondingvariations in leaf area. Weight gain per incrementof leaf area was essentially constant over a widerange of temperatures.

In the light intensity/NT experiment, on the otherhand, changes in light intensity presumably affectedthe rate of photosynthesis as well as leaf area pro-duction (fig 1, table VII) and it was concluded thatthe rate of gain per unit of leaf surface present in-creased with each increment of light. Thus a linearregression of dry weight on average leaf area presentwas not calculated. When total dry weights wererelated to the total leaf area produced, instead ofthe average area present, however, a strikingly dif-ferent result was obtained. The mean dry weightsof plants at each light intensity (from table VIII)divided by the appropriate total leaf area produced(derived from table VII), gave the following valuesfor net dry matter gain:

600 ft-c:(0.309/22.4) = 0.0138 g/cm2900 ft-c:(1.011/75.2) = 0.0134 g/cm2

2,000 ft-c:(1.240/89.5) = 0.0138 g/cm2A linear regression of total dry weight on total leafarea produced by these plants fitted the data very

closely with a correlation coefficient, r, of +0.973.Regardless of light intensity, therefore, each unit ofleaf area contributed the same amount of net drymatter gain.

This finding supports Watson's hypothesis, butseemingly contradicts the conclusion that the weightgain per unit of area varied directly with light in-tensity. The apparent contradiction is due to thetwo different measures of leaf area production. Thetotal leaf area produced, used in the present discus-sion, is simply the best available estimate of the totalphotosynthetic area produced dluring the experiment.The average leaf area present accounted for both theproduction of new leaves and the death of old onesand was calculated from the number of live leavespresent at each counting.

In addition to differences in photosynthesis, leafmortality rates differed significantly between thethree light intensities. Therefore, the apparent con-tradictions with respect to assimilation rates are ex-plained as follows: A, dry weight gain was the netresult of photosynthesis by all of the leaves producedduring the experiment, and total weight divided bytotal leaf area equals the net gain per unit of leafarea which was constant; B, since the leaves athigher light intensities had shorter life spans, thegain per unit of area per unit of time must have in-creased with increments of light intensity; C, thusat 600 ft-c leaves produced dry matter at lower rates,but for longer periods of time, than at 900 and 2,000ft-c; higher photosynthetic rates were offset by short-er life spans; D, the overall result was that at 30 CDT, dry matter production per unit of leaf area pro-duced was nearly constant over a wide range of lightintensity.

The photosynthetic area depended upon the num-ber of leaves produced, their size, and their longevity;therefore, measuring these parameters on plantsgrown under controlled conditions may prove usefulas a screening procedure in a breeding program.Clonal variation in growth at low light intensity hasbeen studied in white clover (21). Among 135clones tested, only 5 survived for 100 days at 150ft-c light intensity; half survived less than 50 days.Further experiments showed that survival at lowlight was usually greater at low or moderate tem-peratures than at high temperature, but clonal dif-ferences were found at all temperatures tested (22).These differences indicate that the physiological traitsneeded to endure high temperatures and low light in-tensities probably exist within the species.

SummaryCO2 uptake by white clover leaves was measured

at all combinations of 10, 20, and 30 C temperaturesand 800, 1,200 and 4,200 ft-c light intensity. Maxi-mum rates occurred at 30 C for all light intensities.At 10 C, CO2 uptake did not change significantly aslight increased from the lowest to the highest inten-sity. At 20 and 30 C, significant increases in CO2uptake occurred as light intensity was increased.

Temperature effects on respiration rates of leafdiscs and stolon internode sections were measured

714

BEINHART-TEMPERATURE & LIGHT EFFECTS ON CO2 UPTAKE

in a Warburg apparatus. Rates in leaf discs in-creased as temperature increased from 20 to 35 Cbut declined at higher temperatures; there was almostno O2 exchange after the second hour at 50 C. Stolontissue was more tolerant of high temperature, andcumulative 02 uptake for a 5 hour period was in thisorder: 50C >45C >40C >35C.

Two factorial growth chamber experiments meas-ured growth and development under different daytemperature (DT), night temperature (NT), andlight intensity regimes. One experiment employed600 ft-c light intensity and tested 10, 17, 23, and30 C in all combinations as DT and NT. Plant re-sponses to DT were generally greater than to NT.Dry weight, leaf production, and leaf size were maxi-mum at 17 and 23 C DTs. A regression of dryweight on average leaf area present during the 6-week experiment accounted for 90 % of the differ-ences in plant weights.

The second growth chamber experiment testedall combinations of three levels of light intensity(600, 900, & 2,000 ft-c) and NT (10, 17, & 30 C)on plants at a DT of 30 C. The higher light intensi-ties increased leaf production, leaf size, and plantweights, but decreased the average life span of leaves.In this experiment each unit of leaf area yieldednearly the same amount of dry matter regardless oflight intensity; shorter life spans of leaves apparent-ly offset higher photosynthetic rates at 900 and 2,000ft-c light intensity.

It was concluded that both temperature and lightintensity influence growth rates of white clover plantsby affecting the rate of leaf area production.

AcknowledgmentsThe counsel of Dr. P. J. Kramer is gratefully acknowl-

edged. The photosynthesis measurements were made inlaboratories of the Botany Department, Duke University;thanks are due Mr. Boyd Post for instructions in thenecessary techniques. Mrs. Betty King rendered valuableassistance in several phases of this study.

Literature Cited1. BLACK, J. N. 1957. The influence of varying light

intensity on the growth of herbage plants. Her-bage Abstr. 27: 89-98.

2. BLACKMAN, G. E. 1938. The interaction of lightintensity & nitrogen supply in the growth & me-tabolism of grasses & clover (Trifoliuim repens).I. Effects of light intensity & nitrogen supply onthe clover content of a sward. Ann. Botany NS2: 257-280.

3. BLACKMAN, G. E. & J. N. BLACK. 1959. Physio-logical & ecological studies in the analysis of plantenvironment. XII. The role of the light factorin limiting growth. Ann. Botany NS 23: 131-145.

4. BLACKMAN, G. E. & W. G. TEMPLEMAN. 1938.The interaction of light intensity & nitrogen supplyin the growth & metabolism of grasses & clover(T. repens). II. Influence of light intensity &nitrogen supply on the leaf production of frequent-ly defoliated plants. Ann. Botany NS 2: 765-791.

5. BLACKMAN, G. E. & W. G. TEMPLEMAN. 1940.The interaction of light intensity & n;trogen supplyin the growth of metabolism of grasses & clover(T. repens). IV. The relation of light intensity

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6. BLACKMAN, G. E. & G. L. WILSON. 1951. Physio-logical & ecological studies in the analysis of plantenvironment. VI. The constancy for differentspecies of a logarithmic relationship between netassimilation rate & light intensity & its ecologicalsignificance. Ann. Botany NS 15: 63-94.

7. BOURDEAU, P. 1954. Oak seedling ecology deter-mining segregation of species in Piedmont oak-hickory forests. Ecol. Monographs 24: 297-320.

8. BROWN, B. A. & R. I. MUNSELL. 1956. Effectsof cutting systems on ladino clover. Storrs (Con-necticut) Agric. Exp. Sta. Bull. 313.

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12. MCCLOUD, D. E. & C. W. ALEXANDER. 1961. Theleaf area index (LAI) & light interception con-cept. Agron. Abstr. 53: 62.

13. MEYER, B. S. & D. B. ANDERSON. 1952. PlantPhysiology. 2nd ed. D. Van Nostrand Co., Inc.,New York. 784 p; p 316.

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16. MITCHELL, K. J. & R. LuCANUS. 1960. Growth ofpasture species in controlled environment. II.Growth at low temperatures. New Zealand J.Agric. Res. 3: 647-655.

17. TALLING, J. F. 1961. Photosynthesis under naturalconditions. Ann. Rev. Plant Physiol. 12: 133-149.

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