Pacific Science (1980), vol. 34, no. 2© 1981 by The University Press of Hawaii. All rights reserved

Effect of Different Photon Flux Densities (PAR) on Seedling Growth andMorphology of Metrosideros collina (Forst.) Gray 1

DOUGLAS J. FRIEND2

ABSTRACT: Seedlings of Metrosideros were capable of net accumulation ofdry matter at photon flux densities as low as 13 /lmol m- 2

S-I PAR (about0.6 percent sunlight), when grown under a 12-hour day1ength at 20° or 25°Cfor over 4 months. Seedlings became adapted to shade at low levels of PARby an increased leafiness of the plant (expressed as the leaf area ratio). Thisincreased leafiness was brought about by a marked reduction in leaf thicknessrather than by an increase in the proportion of assimilates distributed toleaves.

'OHI'A (Metrosideros sp., Myrtaceae) ANDKOA (Acacia Koa Gray) are the two majorendemic tall trees found in the Hawaiian rainforests. Metrosideros collina (Forst.) Gray isa highly variable species, with seedlings capa­ble of colonizing the high-radiation environ­ment of a lava flow exposed to full sunlightor the highly shaded environment of a tree­fern stem (Cibotium sp.) deep within the rainforest (Corn 1972, Smathers and Mueller­Dombois 1974). As part of studies on theimportance of radiation in controlling theregeneration of Metrosideros in the forestenvironment, the following experiments werecarried out to determine the lower limit ofphotosynthetically active radiation (PAR)over the wavelength range 400 to 700 nmneeded to support early growth of seedlings.The methods of growth analysis (reviewed byKvet et al. 1971) were used to further examinethe adaptive response of seedlings to the levelof PAR at which they were grown.

MATERIALS AND METHODS

Mature seed ofM etrosideros collina (Forst.)Gray subsp. polymorpha (Gaud.) Rock was

1 Contribution no. 74, Island Ecosystems IRP/IBPHawaii, NSF grant GB-23230. Manuscript accepted 13July 1979.

2 University of Hawaii, Botany Department, 3190Maile Way, Honolulu, Hawaii 96822.

obtained from the Kilauea rain forest on theisland of Hawaii. All seed was from one seedcluster, and was sown on the surface of piecesof shredded hiipu'u (tree-fern root) in 4-inchplastic pots. The pots were watered frombelow and the tops of the pots were coveredwith plastic Petri dish lids.

Germination took place in about one weekin a growth room maintained at a constanttemperature of 25°C and a daylength of 12hours provided by white fluorescent lamps.The photon flux density of wavelengths in thephotosynthetically active part of the spec­trum from 400 to 700 nm (PAR level) wasmeasured with a Lambda Instruments quan­tum sensor LI-185. For white fluorescentlamps, 1 /lmol m- 2s- 1 PAR was equivalentto 48 lux. Seeds were germinated at PARvalues of 40 /lmol m- 2s-1 and then trans­ferred to a series of PAR levels ranging from8 to 235 /lmol m- 2s- 1 in growth rooms main­tained at a 12-hour daylength at either 20°or 25°C. A second series of plants was trans­ferred to a similar series of PAR levels butunder a 24-hour daylength at 20°C. Thedifferent levels of PAR were obtained byvarying the distance of the plants from thelight.

Plants were watered with one-quarterstrength Hoaglands solution (Hoagland andArnon 1939), at 2- to 3-day intervals, andharvested after 126 days growth at the 12­hour daylength and after 262 days at the 24­hour daylength. The number of leaves and

93

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total plant dry weights were measured forplants grown under 12-hour daylengths. Inaddition, the number of nodes and leaf stemand root weights were recorded for the 24~hour series. Six replicate pots of 3 to 5 plantswere grown at each different treatment com­bination of temperature and PAR levelgiving mean values for growth measurement~based on 20 to 30 replicate plants per treat­ment.

Measurements of leaf area were taken forplants grown in the 24-hour daylength so thatsome of the methods of growth analysiscould be used to determine the nature of theadaptive response to shading. Leaves were~attened by a glass plate and photoduplicatedm a dry copy machine, and leaf areas weredetermined by cutting out the leaf outlinesweighing the paper, and calculating the leafarea from the known area per unit weight ofpaper. Measurements of leaf area (A) andtotal plant dry weight (W) were used tocalculate the leafarea ratio (A W- 1

, dm2 g-1) ameasure of "Ieafiness." This ratio is the pro­duct of two other ratios, one morphological,an assessment of leaf thickness, and the otherph~siological, a measure of the way in whichassImIlates are distributed to developingleaves, stem, or root. The assessment of leafthickness is given by the specific leaf areaAWL-1, dm2g-1, the ratio of leaf area to leafdry weight-the higher the ratio the thinnerthe leaf. This relationship has recently beenconfirmed for Metrosideros (Corn 1979). Ameasure of assimilate distribution to thelea~es is given by the leaf weight ratio, theratIO of the dry weight of leaves to that ofthe whole plant (WLW- I

). The relationshipbetween the leaf area ratio and its two com­po~ents is AW- 1 = AWL- 1 (WLW- 1

). TheratIOs of the dry weights of stems (Ws) androots (WR) to that of the whole plant (WsW- 1

and WR W- 1) similarly measure the distribu­

tion of assimilates to plant parts other thanleav~s. These. three ratios provide more pre­cIse mformatlOn on the nature of assimilatepartitioning than the shoot/root ratio (It': +IV 1 ' sYYL) WR- • The other components of growthanalySIS, the relative growth rate and netassimilation rate, were not measured in theseexperiments.

PACIFIC SCIENCE, Volume 34, April 1980

RESULTS

At both 24- and 12-hour daylengthsseedlings survived at the lowest PAR value~used (8 to 13 ,umol m- 2s-1), a level about0.6 percent that of full sunlight. Plants wereactually accumulating carbon at this PARlevel as the mean total plant dry weight wasover 0.1 mg even at the lowest PAR levelused at 25°C; this plant dry weight wasgreater than the dry weight of the very smallseed (about 0.07 mg per seed). With increasedPAR levels at both daylengths, total plant dryweight, number of leaves, and number ofnodes increased (Figures I, 2, and 3).

The nature of the growth response to PARlevel was analyzed more fully for the 24-hourdaylength series. Plants became morpholo­gically modified at low PAR values by areduction in leaf thickness, as measured bythe increased specific leaf area (A WL -1 inFigure 4). The PAR level had less effect onthe proportion of assimilates distributed toleaves, measured as the leaf weight ratio(WL W- 1 in Figure 4) which was lowest atlow PAR values. Because of the large effectof PAR on AWL -1 the net result was anincreased "Ieafiness" at low PAR levels,"leafiness" being measured as the leaf arearatio (A W- 1 in Figure 4). The proportion ofroots was little affected by PAR level (WR W- 1

in Figure 5), so that the decreased WL W- 1 atlow PAR levels was mainly attributable to theincreased diversion of assimilates into stem(increased W., W- 1 in Figure 5).

DISCUSSION

The ability of seedlings of Metrosideros togrow at PAR levels as low as 0.6 percentsunlight was achieved mainly by the adaptivechanges that occurred in leaf thickness(increase in AWL -1 with decreasing PARlevels in Figure 4). This photomorphogeneticeffect was a direct consequence of the alteredlevel of PAR. The ratio of energies in the redand far-red regions of the spectrum wasmaintained constant, unlike the field situa­tion where shading by leaves of other plantslowers the ratio of energy at 600 nm (red) to

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Effect of Different Photon Flux Densities on Metrosideros-FRIEND 95

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FIGURE I. Effect of PAR level during growth on leaf number and total plant dry weight of Me/rosideros seedlingsgrown for 126 days under a 12-hour daylength at either 20° or 25°C constant temperature. Points are mean valuesfrom about 30 plants.

96 PACIFIC SCIENCE, Volume 34, April 1980

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FIGURE 2. Effect of PAR level during growth on leaf number, node number, plant height, and total plant dryweight of Metrosideros seedlings grown for 126 days under a 24-hour daylength at 20°C. Points are mean valuesfrom about 30 plants .

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Effect of Different Photon Flux Densities on Metrosideros-FRIEND 97

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13 44 135 238 PARFIGURE 3. Morphology of Melrosideros seedlings grown for 262 days under a 24-hour daylength at 20°C at PAR

values of 13, 44,135, and 235 jlmol m- 2 s-'.

that at 730 nm (far red), with profoundconsequences on the action of the photomor­phogenetic pigment phytochrome (Holmesand McCartney 1976).

Leaf thickness increases from the earliest­to the later-formed leaves in seedlings ofMetrosideros, so that the increased leafinessof plants grown at low levels of PAR maybe in part a result of their more juvenile stageof development. Such ontogenetic changesare well documented in other plants (e.g.,Friend, Helson, and Fisher 1962).

Another morphological consequence oflow levels of PAR was the increased propor­tion ofassimilates distributed to stems, shownby the increase in Ws W- 1 (Figure 5). Thelength of stem for each unit dry weight ofstem was also increased under shade. Valuesof stem height per milligram were 1.7, 2.3,

7.4, 16.6, and 23.6 cm over the range of PARvalues of 235, 135, 44, 21, and 13 {Lmolm- 2s- 1 respectively. Both these adaptations(etiolation) would optimize photon capturein photosynthesis by carrying the leavesabove the shade of competing seedlings. Asin the case of leaf adaptations, ontogeneticchanges in stem morphology cannot beseparated from direct effects of the environ­ment in these studies.

The PAR level necessary for early seedlinggrowth of Metrosideros seedlings with ade­quate mineral and water supply is probablyclose to 10 {Lmol m- 2s- 1 under a 12-hourdaylength at 20°C (Figure 1). The lowerlimits of PAR level are at present not knownfor successful establishment of Metrosiderosseedlings within the deep shade of the rainforest. However, an examination of the size

98 PACIFIC SCIENCE, Volume 34, April 1980

24 HRL3

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FIGURE 4. Effect of PAR level on the leaf area ratio (A W- 1) and its two components, the leaf weight ratio (WLW- 1

)

and the specific leaf area (A WL -I). Seedlings of Melrosideros grown at 20°C under a 24-hour daylength for 262 days.Points are mean values from 20 plants.

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Effect of Different Photon Flux Densities on Metrosideros-FRIEND 99

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FIGURE 5. Effect of PAR level on the distribution of assimilates to leaves (WLW- I), stems (WSW-I) and roots

(WR W- 1). Seedlings of Metrosideros grown at 200 e under a 24-hour daylength for 262 days. Points are mean values

from 20 plants.

100

classes present in young plants of M etro­sideros in a healthy rain forest showed about104 plants per hectare less than 10 cm inheight but only 10 between 50 and 100 cmand none between I and 5 m (Mueller­Dombois 1977). A similar rapid decline indensities between size classes below andabove 50 cm was found by Cooray (1974).Further work is necessary to determine thelowest PAR level at which long-term seedlingestablishment can succeed and the physiolo­gically basis of shade tolerance in young trees.

ACKNOWLEDGMENTS

I wish to thank Dr. D. Mueller-Dombois,Dr. R. Becker, and other members of theIsland Ecosystems IRP for their assistance.

LITERATURE CITED

COORAY, R. G. 1974. Stand structure of amontane rain forest on Mauna Loa, Ha­waii. M.S. Thesis. University of Hawaii,Honolulu. x + 167 pp.

CORN, C. A. 1972. Seed dispersal methods inHawaiian Metrosideros. Pages 442-435 inJ. A. Behnke, ed. Challenging biologicalproblems: directions toward their solution.Oxford University Press, New York.

---. 1979. Variation in Hawaiian Metro-

PACIFIC SCIENCE, Volume 34, April 1980

sideros. Ph.D. Thesis. University of Ha­waii, Honolulu. xvi + 295 pp.

FRIEND, D. J. c., V. A. HELSON, and J. E.FISHER. 1962. Leaf growth in Marquiswheat, as regulated by temperature, lightintensity, and daylength. Can. J. Bot.40:1299-1311.

HOAGLAND, D. R., and D. I. ARNON. 1939.The water culture method for growingplants without soil. Circ. Calif. Agric. Exp.Stn. 347: 1-39.

HOLMES, M. G., and H. A. MCCARTNEY.1976. Spectral energy distribution in thenatural environment and its implicationsfor phytochrome function. Pages 467-476in H. Smith, ed. Light and plant develop­ment. Butterworths, London.

KVET, J., J. P. ONDOK, J. NECAS, and P. G.JARVIS. 1971. Methods of growth analysis.Pages 343-391 in Z. Sestak, J. Catsky, andP. G. Jarvis, eds. Plant photosyntheticproduction. Manual of methods. Dr. W.Junk, The Hague. xxxii + 800 pp.

MUELLER-DoMBOIS, D. 1977. Ohia rainforeststudy. Tech. Rep. 20. CPSU/UH 010/6.Botany Dept. University of Hawaii, Hono­lulu. 117 pp.

SMATHERS, G. A., and D. MUELLER-DoMBOIS.1974. Invasion and recovery of vegetationafter a volcanic eruption in Hawaii. Na­tional Park Service Sci. Man. Ser. 5. Pub.No. NPS 118. 129 pp.

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