PLANTPlant Physiol. (1969) 44, 623-630 Vol. 44 No. 5 Changes in Photosensitive Stem Growth in Intact Peas Following Irradiation' William M. Elliott2 and John H. Miller Department of

Plant Physiol. (1969) 44, 623-630

Vol. 44 No. 5

Changes in Photosensitive Stem Growth in IntactPeas Following Irradiation'

William M. Elliott2 and John H. MillerDepartment of Bacteriology and Botany, Syracuse University, Syracuse, New York 13210

Received October 21, 1968.

Abstract. Etiolated pea seedlings given a short red-light pretreatment followed by 30 hrof darkness no longer showed a typical red-light inhibition of internode elongation. Theinduction of phytochrome-insensitive growth was itself mediated by phytochrome, since far-redlight reversed the effect of the short red-light pretreatment. Peas grown in white light showeda similar insensitivity to red light. However, in this instance the phytoohrome system exertedsome control over internode elongation since far-red light promoted growth slightly, and thiseffect was red-reversible.

The loss of sensitivity to red light was correlated with a decrease in the amount of spectro-photometrically assayable phytochrome. However, the loss of phytochrome occurred in arelatively short time oompared to the period necessary to attain maximal insensitive growth(2 hr versus about 30 hr). Also, after the red-light and dark pretreatment, although 40 % ofthe original amount of phytochrome remained, red light had no effect on elongation. Neitherloss of phytochrome nor loss of red-light sensitive growth was observed at 0 to 10.

Inhibition of internode elongation is one of severaldevelopmental responses controlled by the phyto-chrome system in etiolated peas (11,17). Thomsonand Miller (20) found that the phytochrome-mediatedinhibition was no longer observed after an extendedperiod of red-light exposure. Similarly Lockhart(14) and Russell (18) determined, by measuringgrowth of either whole etiolated pea seedlings or thethird internode of intact peas, that under continuousexposure to red light the initial inhibition of stemgrowth declined after about 1 or 2 days, and theplants then grew at a rate equall to or greater thanthat' of dark-grown seedlings. Even red-light ex-posures of only a few min daily result in a subsequentincrease in growth of pea plants\ after the initialinhibitory period (21).

Exposure to red light also causes a decrease inthe amount of spectrophotometrically detectable phy-tochrome. In etiolated maize seedlings the totaldetectable phyto,chrome decreases in darkness aftera saturating dose of red light. This decrease inmeasurable phytochrome was due to a loss in most ofthe far-red absorbing form of phytochrome (PFR)with little or no dark reversion to the red absorbingform (PR). The loss was almost complete in about4 hr (2,3). A similar decrease in PFR is found in

1 Supported in part by National Science Foundationgrant GB-7043.'NDEA Title IV Fellow.

etiolated pea seedlings (9). The PFR form ofphvtochrom.e is generally regarded as the physio-logically active form which produces the initial in-hibition of internode elongation immediately follow-ing a red light exposure (10). The question thusarises, to what extent are changes in phytochromelevel following red irradiation causal,ly related tochanges in photosensitive stem growth. The experi-ments described in the present paper were carriedout during a study of the changes in photosensitivegrowth in intact peas following red irradiation andthe correlated changes in spectrophotometrically as-savable phytochrome in the tissue.

Materials and Methods

Seeds of Pisumwl sativum L. cultivar "Alaska"(Asgrow Seed Company, New Haven, Connecticut)were soaked for at least 5 hr in a solution of 1.1 g/lHv,ponex (Hydroponics Chemical Corporation, Cop-ley, Ohio). Twenty seeds each were planted in9X9X8 cm plastic containers filled with vermiculite.The seeds were irrigated with tap water when plantedand on the third day after planting.

The dark-grown seedlings developed in a darkroom at 25 + 30 for about 5 and one-half davs atwhich time the third internode was generally 5 to10 mm long. Light-grown plants were raised for6 and one-half day s in a Percival PGC-78 growthchamber at 25 ± 0 under 450 ft-c of continuous

623

PLANTPHYNIOLOGY May 1969

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PLANT PHYSIOLOGY

white fluorescent light. At this tiiie the third inter-nod,e was about 10 mm long. Growth of both light-and dark-grown plants was measured by marking a5 mm section o,n the third internode as close aspossible to the apex with 2 fine thlreads held parallelin a wood clamp and moisten-ed with India ink. Theincrease in length of the marked segment was meas-ured after an additional 24 hr o,f growth. Approxi-mately 10 plants were measured per treatment.

The standard red light source consisted of asingle 15 w red fluorescent tube wrapped with 4layers of red cellophane. The light energy at thelevel of the seedlings Nas approximately 20 pAW/cm2.For a higher-intensity red light source ( 200 1uW/cm2)the 4 layers of cellophanle were omitted. The far-redsource was a 100 xv incandescenit bulb behind aCorning glass far-red filter (No. 2600). The plantsreceived abouit 170 pAV/cml2 o,ver a wavelength rangeof 700 to 800 nm1. Light inteinsities wvere mleasuredwitlh an Isco spectroracliometer SR. For white-lighttreatmenits the plalnts wx-ere placed in the Percivalgrowth chamber.

The spectro,photonietric assay for plhytochromecontent wAas conducted wxith a dual beami spectro-photomieter (Ratiospect mlodel R-2, AgriculturalSpecialty Company, HyattsNville, -Maryland) generallyfollowing the mletlhods of Furtuva anid Hillman (9).The sample of pea stem tisssue con,sisted of twentv-txo5-mm apical sections (totaliing 0.18 g) each cut into3 equal pieces and packed into a 6-nmnm diameterbore imietal cutvette. The difference in absorhancy(A OD) of the sample at 660 imuarn(n 730 nm wasmeasuired after far-red and(I re(I illumination of thesample and the clhancge in A OD betwxeen illunlina-tion1s [A(A OD)] was assumiiied to be lproportionalto the phy-tochrome contenit ( 3). A correction forthe sliglht conv-ersion of phytochrome to the activeformi (PI ) by the far-red souirce wcas imiade follow-iibgIglillmiiain (12).

Results

Growzeth .Stitdies. Six anid a lhalf day0o0l peaseedlings were grown under 3 differenit liglht colidi-tions: A) total darkness, B1 ) continutotus whitefluiorescenit light and C) darkniess for 5 aln(d one-halfdays follo,wed by a 20-hr red pretreatmenit and 10 hrof darkness jutst before nmarkin,g. A 5-nmm sectionw-as marked on the thil-d interniode, alndl elongationof this section was measured after 24 hr in eithercontinuous white, red, or far-red liglht or darkness(table 1). Unless otherwise stated, valuies giveen inthis table and( later figures and( tables repl)resent themeans and standard deviation!s of 4 sel)arate experi-ments. Ilants grown uinder these various conditionsdiffere(d in tlheir subsequent response to light.Growtlh of the completely dark-growni plants wasgreatly inhibited 1b white and( red light and, to alesser extent, 1v far-red liglht. In contrast the light-grown aind re(l-light l)retreated plants, while still

Table I. Responise of Pea Plantts Grown UnderDifferentt Light Conditions to Varions

Light TreatmlenttsPea seedlings were grown for 6 and one-half days

in complete darkness, in darkness with a 20 hr redlight and 10 hr dark pretreatment, or in white light.Then growth of a 5-mm marked section on the thirdinternode was measured after 24 hr in darkness orcontinuous red, far-red or white light. Al = change inlength of the 5-mm nmarked section. D = dark, R -red, and FR - far-red.

AI

Completely dark-gro-wn24 hr light24 hr D24 hr R24 hr FR

light-grow-n24 hr light24 hr D24 hr R24 hr FR

Dark-grown, 20 hlr Rpretreatmiienit + 10 hr D

2 hr light2A4 hr D24 hr R24 hr FR

Iililt

0.57 + 0.1215.3 ± 0.601.8 ± 0.176.8 + 0.17

6.0 ± 0.1212.4 + 1.0212.0 ± 0.6014.8 ± 1.15

2.8 + 0.5216.3 + 1.601.61 ± 1.2716.6 + 1.66

inhibited by wNhite light, were iio longer inllibited byred or far-red light.

It xxvas found that red-liglht pretreatments of muichshorter dutration xere ftlly- capable of causing suib-se({uelt stemii growth to he insensitive to inhibitionibv red light (Fig. 1'). Dark-grown plants x-eregiven red pretreatmnents of fronm 0 to 20 mimi followedhv a 30-lhr dark period. Tlle plants were then placeduinder contintuouis red lighlt, and the groxth of amarked 5-mm segmilent xvas mne.asured dturing 24 hrof irradiationi. Under tllese conditions the fouirthiinterio(le of about 20 % of the plants had begun toelongate by the end of the 24 lhr ried light period,but the groxvth of these pllants xas not uised indetermininig the average amIonint of elong-ationl. Bothshort anld conltilinuouis red light exposures are knoxwnto hasteni internode developiment (13, 21'). Thegroxvth of dark-groxwn plants, as expected. wasstrongly inhlibite(d uinder continu0ous redl irradiation.For those pllants receiving a red-light piretr-eatnieiietthe growtlh rate increased abrtuptly during the stubse-tllient 24 hr periodl of red irradiation. Interestinglv,

regardless of the length of the red light pretreatmenit,the ultimate growth rate for the pre-irradiated plants\was the same. Hoxwever, the dturation of the red-liglht pr-etr-eatmeilts affected the lengtlh of the lagperiod before the groxx th rate increased. This re-duiction in the lag period xxwas proportional to thelogarithm of total liglit energy uip to 20 niin (24mJ/cn'2) of red light. Red light pretreatments ofover 20 min produiced ni-o additional effect In an-

624



ELLIOTT AND MILLER--CHANGES IN PHOTOSENSITIVE STEM GROWTH

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HOURS GROWTH IN RED LIGHTFIG. 1. (top) Growth rates in red light of pea plants

given varying red-light pretreatments. Five and a halfday old dark-grown pea seedlings were given red-lightpretreatments from 0 to 30 min and placed in darknessfor 30 hr. Then the growth rate of a 5-mm marked sec-tion was measured over a 24-hr period in continuousred light. Each point represents an average of 10 mea-surements. Duration of red pretreatment (min) indi-cated by figures at right of the curves.

FIG. 2. (bottom) Growth rates in red light of pea

plants given a 20-min red-light pretreatment followedby varying periods of darkness (0 to 30 hr indicated byfigures at riglht curves). Other conditions as in Fig. 1.

other experiment the inten,sity of red light wvas in-creased 10 times without any further effect. Themaximal response required 1 to 5 min from thislight source.

The lag period before increased growth rate inred liglht was also affected by the length of the darkperiod following a saturating 20 min red light pre-

treatment (Fig. 2). Between 14 and 22 hr of dark-ness, each 4 hr increase in the length of the darkperiod caused a corresponding 4 hr reduction in thelength of the lag period before increased growth raitein red light. If each 4-hr increase in the darkperiod continued to result in a 4-hr decrease in thelag period, then after about 26 hr no lag periodshould have been observed. However, between the22- and 30-hr dark periods the reduction in lag

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FIG. 3. Growth rates of light-grown peas over a

24 hr period in white, red, or far-red light or darkness.A 5-mm section was marked on 6 and one-half dayold light-grown peas. Growth was measured over a

24 hr period under the 4 light conditions (Squares =

white; open circles = darkness; closed circles = red;triangles = far-red). Each point represents an averageof 10 measurements.

FIG. 4. Fate of detectable phytochrome after a red-light pretreatment. Dark-grown pea plants were givena 20-min red-light pretreatment and placed in darkness.Loss of detectable phytochrome was followed over a 30-hr dark period.

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625



PLANT PHYSIOLOGY

period was only abotut 2 hr. These effects of a20 min red-light pretreatment were reversed bv a30 min far-red treatment (see Fig. 6 and the follow-ing section of Results).

Although most of the growth of red-light pre-treated or white-light grown plants seems to escapefrom phytochrome control, an apparent small effectof phytochrome is detectible in white-light grownplants. Following far-red treatment the light-grownplants showed a small increase in growth comparedto plants receiving red irradiation or darkness. Thegrowth promotion was significant at the 99.5 % con-fidence level with the t-test. Fig. 3 indicates that thefar-red promotion is not due to a higher growth ratecompared to plants transferred to red light or dark-ness, but rather to a decrease in the lag periodbetween the time the plants were transferred fromwhite light and the time when the increased growthrate was observed. For red- or dark-treated plants,a definite lag period of 4 hr was required before cheonset of the increased growth rate, while far-redtreated plants required a much shorter period. Fur-ther, this far-red reduction in lag period was redreversible (table II). The relative effect of redand far-red light was observed consistently in 4separate experiments. The results of a single ex-periment are given in table II. M\Ieasurements weremade after only a 5-hr growtlh period because pro-portionately the difference between growth in redand far-red light was greatest (cf. Fig. 3). Light-grownv plants given a 10-hr dark period before ex-posure to red and far-red light showed the samerelative response as plants transferred immediatelyfrom white light to red or far-red light.

Correlation Betzween Loss of Phitochrorne andRed Light Inhibition. The time course of the de-crease in detectable phytochrome content during the30 hr dark period following a 20-min red pretreat-ment is shown in Fig. 4. After a 20-min red-lightpretreatment 43 % of the phytochrome was convertedto the PFR form. Phytochrome content decreasedinitially at the rate of about 22 % per *hr so that

Table II. Reversibility of Far-Red Promootion ofof Growzvth bi, Red Light

Growth of a 5-mm marked section of the third in-ternode of light-grown plants was measured after 5 hrunder various light and dark conditions. In the secondexperiment a 10 hr dar-k period preceded the 5 hrtreatment.

No dark period 10 Hr dark periodConditions of before treatmenit before treatment5 hr growth A1 A

5 hr D 1.0 ± 0.19 1.2 ± 0.135 hr R 1.0 0.14 1.2 0.055 hr FR 1.5 ± 0.19 1.6 ± 0.101 hr R - 4 hr D 1.0 ± 0.201 hr FR - 4 hr D ... 1.5 ± 0.081 hr R - 4 hr FR 1.6 ± 0.06 1.5 ± 0.081 hr FR- 4 hr R 0.9 ± 0.07 1.2 ± 0.06

after 2 hr 44 % of the initial detectable phytochromevas no loniger present. The decrease in total phvto-chrome after 2 hr apparently represents an almostcomplete loss of PFR formed by the initial 20-mirired irradiation. PFR was no longer detectable afterabout 2 hr, and the remainin,g phytochrome existedas PR. Between 2 and 8 hr into the dark period,there was an apparent synthesis of phytochrome,similar to that observed by Clarkson and Hillman(4, 6). The phytochrome concentration then de-creased to about half the value initially present inetiolated peas.

The correlation between loss of red-light sensi-tivity after a red-light pretreatment an,d decrease inthe level of measutrable phvtochrome at 2 tempera-tures is shown in Fig. 5. The phytochrome contentat the beginning of the growth period is comparedto the subsequent red-light-sensitivity of the plants.Zero- to 20-min red-light pretreatments were fol-lowed by a 30-hr dark period at QO or 250. Growthwas then measured in 5-nmm marked stem segmentsafter anl additional 24 hr in red light at 250. Thefate of detectable phytochrome during this 24 hrred-light period was determined in a separate experi-ment (Fig. 7). The level of phytochrome wasnmeasured immediately after the 30-hr dark period.At the higher temperature the phytochrome contentafter 30 hr declined with increasin,g red-light pre-treatment, and the inhibition of growth under con-tinuous red light decreased correspondingly. How-ever, plants which had become completely insensitiveto red light as the result of a 20-min red-light pre-treatment still contained substantial amotunts ofphvtochrome. When the pea seedlings were incu-bated at 0 to 10 for the 30 hr dark period, there Nva'sno measurable loss in red light sensitivity and phvto-chromie content decreased only slightly (13 %). Inanother experiment a red-light source 10 times asintense (converting 80 % of the detectable phyto-chrome to the PFR state in 1 to 5 min) did not pro-duce a loss in red-light sensitivity or a decrease in thelevel of detectable ;phytoclhrome quantitatively greaterthan that shown in Fig. 5.

The loss of phytochrome and the decrease inred-light sensitivity followin,g a red-light pretreat-ment could be reduced by subsequent irradiation withfar-red light (Fig. 6). A 30-min far-red light treat-ment immediately following the red-light pretreat-ment reduced the loss of detectable phytochrome andred light inhibition to the level of far-red treatmentgiven alone. As the far-red treatment was givenlater into the 30 hr dark period, the reversal wasless. However even after 4 or 8 hr of darknessbetweeni red and far-red treatments there was stillsome reversal, althouigh it was shown previouslythat after 2 hr of darkness PFR was no longerdetectable. Apparently far-red reversal is presentafter none of the PFR is detectable.

Certain properties of the measurable phvtochromepresent in dark-grown seedlings and of that remain-

626



ELLIOTT AND MILLER-CHANGES IN PHOTOSENSITIVE STEM GROWTH

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FIG. 5. Comparison between the loss of detectable phytochrome and loss of red-light sensitivity for plants keptat 25° or 0 to 10 during the 30 hr dark period. Dark-grown pea plants were given red-light pretreatments ofdifferent durations before the 30-hr dark period. Phytochrome content was measured following the 30 hr of darkness.Growth of a 5-mm marked section of the third internode was measured after an additional 24 hr in red light. Opencircles = A(AOD) ; closed circles = A length.

FIG. 6. Far-red reversal of the red-light pretreatment. Dark-grown pea plants were given a 20-min red-lightpretreatment followed by a 30-hr dark period. A 30-min far-red treatment was given at 0 to 8 hr into the dark period.A(,&OD) (open circles) and A length (closed circles) measured as in Fig. 5. The insert in the lower right handcorner contains the control values for plants given red (R) or far-red (FR) pretreatments alone, or no preirradi-ation (D).

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PLANT PHYSIOLOGY

Discussion

An initial red-light treatment inhibits the elonga--T tion of etiolated pea stems (21). Such inhibitionII-"rT is under the control of the phvtoclhronme system (11,

l\ 17). However, after an extended exposure to redX\T light, this initial inhibition is followed by a growth

rate actually greater than the corresponding darkcontrol (14). The present study indicates that the

D induction of this red-light insensitive growtlh is itself

T\ phvtoclhrome mediated. Short exposures to red light\I arle fully capable of inducing growth whiclh is insen-

sitiv-e to later red light exposures (Fig. 1). Alsofar-red light following an initial short pretreatment

I1 of red light reverses the effect of the pretreatment

R to the level of the far-red control, and growtlh isagain senisitive to later red-light treatmiients (Fig. 6).

The (duiration of this short pretreatmelnt with red, light has nIo effect oIn the magnitude of the growth

rate in the red light period that follows. but deter-mines the time at wlhiclh the increased growth ratein red light is initiated (Fig. 1). The onset of thisred-light insensitive growth is well defined and isdirectly dependent upon the logarithlm of the energy

of the red-light pretreatment.The growth rates of white-light-growvn peas are

similar when transferred either to red light or dark-o 2 3 4 ness (Fig. 3). However, far-red light, which pro-

motes growth slightly, acts by advalncing the time at

HOURS RED LI GH T which the plants begin to growv at an increased

Comparison of the apparent loss of phyto- growth rate similar to plants placed in darkness orontinuous red light between completely dark- red light (Fig. 3). Since the reduction in lag

and red-light pretreated (R) plants. The period is red reversible, a control by phvtochromeetreatment was described in Fig. 5. is indicated for part of the elongation at least of

w\hite-light grown plants. However, most of theelongation seems to be insensitive to red light (tableII). Lockhart >(9, 16) observed in light-grown

red-pretreatment and 30 hr of darkness Pinto bean seedlings a similar thouglh more pro-ared. The phytochrome from both types nounced response, probably because measurenment wasiow the slight conversion to the PFR form made after a longer growth period. This sensitivitylight (table III). The kinetics of phvto- of light-grown peas to red an,d far-red light was notss were apparently somewhat different. altered by a 10-hr dark period betwteen removal fromcontrols show a simple log-linear first fluorescent light and exposure to red and far-redtics. Phytochrome from red-pretreated light (table II). A change in sensitivity might beever shows an initial rapid phase followed expected if the PFR present when the peas werer rate. initially removed from fluorescent light reverted to

PR during the 10-hr dark periodl. as is observedspectrophotometrically in light grown catiliflowerhead,s (3). Apparently there is no dark reversal

Samtple Assay of Phytochromize Fromit Dark of PFR to PR in light grown peas, at least as meas-zun anid Red Light Pretreated Peas ured bv physiological response. In the red-light-ome levels were measured in entirely dark- pretreated peas grown in darkness no similar effector dark-grown peas given a 20-min red-light of red or far-red light was observed within experi-

t plus 30 hr darkness. mental variability (table I).

There are certain parallels in the present workbetween loss of phytochrome and induction of red-

Initial FR R A(AOD) insentsitive growth. A decrease in phytochrome con-

0.200 0.193 0.080 0.120centration after red-light pretreatmlelnt is correlated

Led 0.200 0.196 0.145 0.055 with a corresponding decrease in red-light inhibitiond200000 5a0.(Fig. 5). If the dark inculbationl after the red-

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Table III.Gro7

Phytochrgrown peaspretreatment

Dark-grownRed-pretreat



ELLIOTT AND 'MILLER--CHANGES IN PHOTOSENSITIVE STE'M GROWTH

pretreatment occurred in the cold the effect of thered-light pretreatment was not observed (Fig. 5).Probably the low temperature prevented the tem-perature dependent loss in detectability of phyto-chrome (cf. 3).

There are exceptions, however, to a simple cor-relation of phytochrome loss and induction of red-light insensitive growth. By 2 hr into the darkperiod, almost 45 % of the phytochrome was nolonger detectable. Yet a dark period of 30 hr wasrequired for optimum loss of sensitivity at whichtime only a 55 % loss in detectable phvtochrome wasobservable. Apparently more than a simple loss indetectable phytochrome is required for the darkperiod to have a significant influence on subsequentred light insensitivity.

Another apparent inconsistency between physio-logical response and the properties of phvtochromewas observed. Far-red light was still effective inreversing the red-light pretreatment when there wasno detectaible PFRP present (Fig. 6). This effect issimilar to Hillman's observation of the far-red re-versal of reed light inhibition of etiolated pea stemsections after PFR was no longer detectable (12).

The residual phvtochrome remaining in peas thatare in the red-light insensitive state had no apparentinfluence on inhibition of internode elongation.After the 30-hr dark period, the detectable phyto-chrome concentration was still over 40 % of theinitial concentration. Even after 2 hr of the 24 hrred light period had elapsed. at which time the abruptincrease in 'growth rate was observed. about 20 %was still present (Figs. 2 and /7).

This residual phytochrome however differs fromthat of dark-grown peas in its spectrophotometricproperties. It shows red, far-red reversibility (tableIII), but its rate of apparent disappearance seemsto showv a higher order kinetics than phvtochromefrom the dark control (Fig. /7). Fox and Hillman(8) found distinct differences in the nonphoto-chemical transformation curves of phytochrome frometiolated and red, far-red pretreated plants. PFRreversion to PR in etiolated tissue was about twicea,s great as that in pretreated tissue.

The phytochrome from both etiolated and red-light pretreated plants decreased below the detectionlimit of the ratiospect after a sufficient period inred light (Fig. 7). However Clarkson and Hillman(5) observed than an initial continuous red-lightexposure produced a stationarv concentration ofdetectable phytochrome in Pisum sections. If 2,4-Dwas added to the medium the initial red-light treat-ment also reduced the phytochrome content belowthe detection limit.

Two explanations for this lack of response toresidual phytochrome might be advanced. Oncephytochrome falls below some critical level possiblyit no longer is effective in inhibiting internodeelongation. For example, Clarkson and Hillman (4)found that phytochrome concentration must fall below

a sharply defined critical level of 16 to 22 % of theinitial concentration before additional phytochromewould be synthesized. This explanation gives nofunction to the residual phytochrome. Two forms ofphytochrome have been postulated: a physiologicallyactive and a bulk form (2, 12). There is indicationthat the 2 forms might be physiologically separateand their properties differ somewhat (1). Possiblynone of the physiologically active phytochrome isfunctioiial at the initiation of red-light insensitivegrowth and only bulk phytochrome remains.

Alternately, after a certain period of time theinternode may become physiologically unresponsiveto phvtochrome. Active phytochrome produces awide range of structural, phvsiological and bio-chemical changes, and loss of red-light sensitivitymight be one such change. Red and white lightaccelerate maturation of tissue in Pisurtn (20) andthis apparent loss of red light sensitivity nmight be anaspect of the effect on tissue maturation. Fox andHillman (8) correlate a similar decrease in responseto PFR in Pisumt sections with the degree of de-etiolation of the tissue rather than with phvtochromeamounts. It is not apparent whether this lack ofsensitivity can be explained in terms of differencesin phytochrome or differences in the plant's responseto phvtochrome.

The nature of the photoreceptor for tlle inhibitionof light grown and red-light pretreated peas in whitelight is undetermined. Lockhart (15) and Sale andVince (19) attribute a similar response in peas tothe "high energy reaction" (HER). An actionspectrum consistent with the HER found for inhibi-tion of etiolated hypocotyls of lettuce and petunia(7). In this instance activation of both the HERand the low energy phytochrome reaction was re-quired for any inhibition.

Literature Cited

1. BRIGGS, W. R. AND H. P. CHON. 1966. The phy-siological versus the spectrophotometric status ofphytochrome in corn coleoptiles. Plant Physiol.41: 1159-66.

2. BUTLER, W. L. AND H. C. LANE. 1965. Darktransformations of phytochrome in vivo. II. PlantPhysiol. 40: 13-17.

3. BUTLER, W. L., H. C. LANE, AND H. W. SIEGEL-MAN. 1963. Nonphotochemical transformations ofphytochrome in vivo. Plant Physiol. 38: 514-19.

4. CLARKSON, D. T. AND W. S. HILLMAN. 1967.Apparent phytochrome synthesis in Piswtin tissue.Nature 213: 468-70.

5. CLARKSON, D. T. AND W. S. HILLMAN. 1967.Stability of phytochrome concentrations in dico-tyledonous tissue under continuous far-red light.Planta 75: 286-90.

6. CLARKSON, D. T. AND W. S. HILLMAN. 1968.Stable concentrations of phytochrome in Piswn7zunder continuous illumination with red light. PlantPhysiol. 43: 88-92

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7. EVANS, L. T., S. B. HENDRICKS, AND H. A. BORTH-WICK. 1965. The role of light in suppressinghypocotyl elongation in lettuce and petunia. Planta64: 201-18.

8. Fox, L. R. AND W. S. HILLMAN. 1968. Dif-ferences in photoresponse and phytochrome spec-trophometry between etiolated and de-etiolated peastem tissue. Plant Physiol. 43: 1799-1804.

9. FURUYA, M. AND W. S. HILLMAN. 1964. Obser-vations on spectrophotometrically assayable phyto-chrome int vivo in etiolated Pisumiit seedlings. Planta63: 31-42.

10. HENDRICKS, S. B. AND H. A. BORTHWICK. 1965.The physiological functions of phytochrome. In:Chemistry and Biochemistry of Plant Pigments.T. W. Goodwin, ed. Academic Press. p 405-36.

11. HILLMAN, W. S. 1959. Interaction of growth sub-stances and photoperiodically active radiation on

the growth of pea internode sections. In: Photo-periodism and Related Phenomenon. R. B. With-row, ed. A.A.A.S. p 181-96.

12. HILLMAN, W. S. 1965. Phytochrome conversionby brief illumination and the subsequent elonga-tion of etiolated Pisunt stem segments. PlantPhysiol. 38: 346-58.

13. LOCKHART, J. A. 1956. Reversal of light inhibitionof pea stenm growth by the gibberellins. Proc.Natl. Acad. Sci. U.S. 42: 841-48.

14. LOCKHART, J. A. 1959. Studies on the mechanismof stem growth inhibition by visible radiation.Plant Physiol. 34: 457-60.

15. LOCKHART, J. A. 1961. Photoinhibition of stemelongation by full solar radiation. Am. J. Botany48: 387-92.

16. LOCKHART, J. A. 1961. Interactions between gib-berellin and various environmental factors on stemgrowth. Am. J. Botany 48: 516-25.

17. PARKER, M. W., S. B. HENDRICKS, H. A. BORTH-WICK, AND F. W. WENT. 1949. Spectral sensi-tivities for leaf and stem growth of etiolated peaseedlings and their similarity to action spectra forphotoperiodism. Am. J. Botany 36: 194-204.

18. RUSSELL, D. W. 1966. Studies on the biochemistryof photomorphogenesis in the pea plant. DoctoralDissertation. Yale University, New Haven, Con-necticut.

19. SALE, P. J. M. AND D. VINCE. 1959. Effects ofwavelength and time of irradiation on internodelength in Pisum sativum and Tropaeolumii inajuts.Nature 183: 1174-75.

20. THOMSON, B. F. AND P. M. MILLER. 1961.Growth patterns of seedlings in darkness and inred and white light. Am. J. Botany 48: 256-61.

21. WENT, F. W. 1941. Effects of light on stem andleaf growth. Am. J. Botany 28: 83-95.

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PLANTPlant Physiol. (1969) 44, 623-630 Vol. 44 No. 5 Changes in Photosensitive Stem Growth in Intact Peas Following Irradiation' William M. Elliott2 and John H. Miller Department of