Morphology and growth of stolons and rhizomes in three clonal grasses, as affected by different light supply

Vegetatio 116: 25-32, 1995. 25 @ 1995 KluwerAcademic Publishers. Printed in Belgium.

Morphology and growth of stolons and rhizomes in three clonal grasses, as affected by different light supply

M i n g D o n g & M a r i a G r a z i a P i e rdomin ic i* Department of plant Ecology and Evolutionary Biology, University of Utrecht, P.O. Box 800.84 3508 TB Utrecht, The Netherlands; *Present address: Department of Botany and Ecology, University of Camerino, Italy

Accepted 23 June 1994

Key words: Bud bank, Clonality, Foraging, Light intensity, Rhizome, Stolon

Abstract

In this paper, the hypothesis is tested that, in clonal grasses producing stolons and/or rhizomes, stolons always show a higher morphological plasticity than rhizomes in response to variation in light availability. Agrostis stolonifera (a stoloniferous grass), Holcus mollis (a rhizomatous grass) and Cynodon dactylon (a grass forming both stolons and rhizomes), were grown in pots and subjected to three levels of light intensities. Both stolons and rhizomes branched more intensively under higher light levels. Irrespective of species, stolons consisted of longer internodes under lower light levels, while rhizome morphology did not respond significantly. Biomass partitioning to rhizomes was lower under lower light intensities while partitioning to stolons was not affected. Rhizomes usually had more dormant buds than did stolons. Our results suggest that stolons serve primarily as foraging organs for light, whereas the main function of rhizomes is storage of meristem and carbohydrates, irrespective of whether the grass species involved produces both rhizomes and stolons or only one type of spacer.

Introduction

Rhizomatous and stoloniferous clonal herbs are gen- erally able to respond to variation in resource availability by plastic adjustment in branching intensity (the propensity of lateral meristems of primary spacer to grow out as secondary spacer) as well as length and weight of the spacer organs (stolons, rhizomes, Bell 1984). Morphological plasticity of some species, particularly plasticity in spacer length and branching intensity, is considered to be adaptive in heterogeneous environments ('foraging', Hutchings & Slade 1988; Sutherland & Stillman 1988; Evans 1992, Hutchings & de Kroon 1994). In a previous study on Cynodon dactylon, a grass species forming stolons and rhizomes, Dong & de Kroon (1994) found that stolons are more plastic in morphology (branching intensity, internode length), but less plastic in biomass partitioning, as compared to rhizomes, in response to variation in light and nutrient availability. These results were interpreted as an indication that stolons enable the plant to forage

for light, while rhizomes serve as storage organs for meristems and carbohydrates.

The question is asked whether the distinct responses of stolons and rhizomes were a function of intraclonal competition within clones of Cynodon which form both types of spacers. For grass species that produce only a single type of spacer, a similar differentiation in responses of stolons and rhizomes would be expect- ed if the responses were adaptive in clonal grasses. Alternatively, a grass species that forms only one type of spacer would possess plasticity in both spacer morphology an biomass partitioning to spacers because of reduced intraclonal competition in such plants.

To test this hypothesis, we compared the responses in spacer morphology and biomass partitioning to spacers of Cynodon with those of a stoloniferous grass (Agrostis stolonifera) and a rhizomatous grass (Hol- cus mollis). The comparison is based on results from a garden experiment in which these three species were subjected to three levels of light intensities.

26

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Fig. 1. Total weight per plant (mean ± SE) in Agrostis stoloni[era (As), Cynodon dactylon (Cd) and Holcus mollis (Hm), in response to low (L), medium (M) and high (H) light intensity. For each species, the bars sharing the same letter are not significantly different at p=0.05 (Tukey grouping). Two-way ANOVA of weights on treatment and species indicated significant effects of treatment, while the effects of species and the interaction effect between species and treatment were not significant.

Methods

The species

Cynodon dactylon (L.) Pers. is a perennial grass producing three types of modules (products of an apical meristem, White 1979; Cook 1985), i.e. stolons, rhizomes and orthotropic shoots. The apical meristems of rhizomes usually develop into orthotropic shoots and the apical meristems of stolons may form orthotropic inflorescences. The apical meristems of rhizomes and stolons may grow out into above-ground stolons and below-ground rhizomes, respectively. Macroscopical- ly recognisable nodes of rhizomes and stolons (they

are also referred as 'compound nodes') usually consist of three actual metamers (sensu White 1979). These compound nodes have one axillary bud which is able to grow out as a new rhizome, stolon or orthotropic shoot (Rawal & Harlan 1971 ; Hanna 1992; Dong & de Kroon 1994).

Agrostis stolonifera L. is a stoloniferous perennial grass. Nodes of monopodial stolons bear one leaf and one axillary bud and usually form adventitious roots (Grime etal. 1988; Kik etal. 1990). Axillary buds may grow out as new stolons, and no orthotropic shoots were formed in the present experiment.

Holcus mollis L. is a perennial grass that produces sympodial rhizomes that usually grow out into orthotropic shoots. Nodes of rhizomes bear a single scale leaf and one axillary bud, and usually form adventitious roots. The axillary buds may give rise to new rhizomes or orthotropic shoots (Ovington & Scurfield 1965; Grime et aI. 1988). The species are further referred to with their genus names only.

Experimental design

Plant material of Agrostis and Holcus were collected at the Uithof Botanical Garden in Utrecht and in a forest near Gronsveld in the province of South Limburg, the Netherlands, respectively. Material of Cynodon was obtained from Igueste de San Andres, Tenerife, Spain. Experimental plants of each of the three species were cloned from a single original plant for six months. The experiment was started with small plants that consisted of a rooted and leafy stolon node for Agrostis and Cynodon, and a single rooted and leafy orthotropic shoot for Holcus. The plants were grown in plastic pots (25 cm in diameter and 30 cm high) filled with river sand. The pots were watered as needed and pro- vided with a nutrient solution (4.3730 g 1-1 NH4NO3, 2.0630 g 1-1 Na2HPO4.2H20 and 2.8760 g 1-1 KC1; 64 ml pot- i wk- 1). The experiment was conducted in De UithofBotanical Garden (Utrecht, the Netherlands) from 15 May to 20 July 1992. Three levels of photo- synthetic photon flux density (PPFD), measured by a means of LI-1800 Spectro Radiometer with remote cosine receptor (LI-COR), were applied using neutral shading cloth. The high light level (H) was unshaded daylight, the medium level (M) was 40% of daylight and the low level (L) was 20% of daylight. There were six replicates for each treatment and species.

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T r e a t m e n t s Fig. 2. Number (mean ± SE) of stolons, rhizomes and orthotropic shoots per plants in Agrostis (As), Cynodon (Cd) and Holcus (Hm), in response to low (L), medium (M) and high (H) light intensity./For each species and parameter, the bars sharing the same letter are not significantly different at p = 0.05 (Tukey grouping). Two-way ANOVA for total number of modules (stolons + rhizomes + orthotropic shoots) per plant on treatment and species indicated significant effects of species and treatment as well as their interaction.

Harvesting and analyses

The plants were harvested after 65 days of growth. We measured (1) numbers of rhizomes, shoots and stolons per plant, (2) the fate of buds of primary rhizomes and stolons, i.e. new rhizomes, shoots or stolons, and (3) lengths of primary and secondary rhizomes (only including those having formed or starting to form shoots) and stolons and their internodes. The material was separated into leaves, roots, stolons, rhizomes, stems and inflorescences, and dried for at least 48 hours at 75 °C before weighing.

Stolon and rhizome branching intensities were cal- culated by dividing the number of secondary structures grown out from a primary stolon or rhizome by the number of nodes of the primary stolon or rhizome, respectively. Internode lengths along a stolon or rhizome are not independent and, thus, were ana-

lyzed by means of Repeated Measures ANOVA with Huynh-Feldt-corrected p-values (SAS 1985; Potvin etal. 1990; Dong 1993).

Biomass partitioning to a certain structure of a plant may respond to environmental change and may also be simply a function of plant size. Therefore, it is crit- ical to disentangle size effect from treatment effect on partitioning. To do so, a 'graphical size-regression approach' (Samson & Werk 1986; Weiner 1988; Geber 1989; Hartnett 1990; de Kroon & Schieving 1991) was applied. In this approach, absolute parti t ioning to a certain structure was regressed on plant size. Plant size is given as the total weight of plant structures that are directly involved in resource acquisition, i.e. the leaves and roots. Analyses were conducted in two main steps. The first one is an analysis of covariance (ANCOVA, Sokal & Rohlf 1981; procedure GLM, SAS 1985). A significant effect of treatment in addition to effects

28

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Fig. 3. Stolon branching intensity (mean ! SE) of Cynodon (Cd) and Agrostis (As), and rhizome branching intensity of Cynodon (Cd) and Holcus (Hm), in response to low (L), medium (M) and high (H) light intensity. For each species and parameter, the bars sharing the same letter are not significantly different at p = 0.05 (Tukey grouping). Open, solid and dotted portions of bars indicate whether the buds grew out to form stolons, rhizomes and shoots, respectively.

of size indicates that partitioning differs significantly between treatments. Second, when ANCOVA shows a significant effect of treatment on partitioning, partitioning was subsequently analyzed by linear regression and by a graphical inspection of the relationship between the weights. Partitioning in one treatment is higher when the slope and/or the y-intercept are larger than in another treatment.

Results

Overall plant performance. Total plant dry weight was higher under higher photon flux densities and plants growing under the high light level were 8- to 9-fold heavier than plants growing under the low light level. For each species, the difference in plant weight between plants growing under successive light levels was highly significant. In a given treatment, the dry weights were not significantly different between any of the species (Fig. 1).

Similarly, plants produced more modules (stolons, rhizomes, orthotropic shoots) under higher light levels, regardless of species. In both Cynodon and HoIcus, the response level in the number of modules was lower than that in total plant weight. In Agrostis, the response level in the number of modules was similar to that in

total plant weight (Fig. 2). This difference between the species was apparent as a significant interaction effect between species and treatment. Cynodon plants failed to develop rhizomes under the low light level and failed to form orthotropic shoots under the high light level (Fig. 2). Under lower light levels, all species developed longer and larger leaves.

Branching intensities of stolons and rhizomes. In all three species, branching intensities of rhizomes and/or stolons increased with increasing light intensity and, for a given treatment, stolons usually branched more intensively than rhizomes (Fig. 3). All activated secondary meristems of Agrostis grew out as stolons. In Cynodon, most activated buds of primary stolons developed into stolons and only a very small percentage of the stolon buds grew out as rhizomes or shoots. In all treatments, a large percentage of the primary rhizome axillary buds in Cynodon and Holcus remained dormant. Under the most favourable conditions, no more than 45% of the rhizome buds were activated, and most of them developed into rhizomes.

Lengths of stolons and rhizomes and their internodes. The lengths of most stolons and their internodes significantly responded to light treatments, but the responses were rather limited (Fig. 4). In Cynodon, primary

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Fig. 4. Length (mean ~ SE) per stolon and length (mean :t: SE) per stolon internode in Cynodon (Cd) and Agrostis (As), and length (mean :i: SE) per rhizome and length (mean ± SE) per rhizome internode in Cynodon (Cd) and Holcus (Hm), in response to low (L), medium (M) and high (H) light intensity. Solid and hatched bars denote primary and secondary structures, respectively. Lengths of stolons and rhizomes m'e tested by means of ANOVA (Tukey-grouping). The internode lengths were tested by means of Repeated Measures ANOVA (CONTRAST option, GLM procedure, SAS 1985). Effects of treatment ('between subjects') are given; 'within subjects' effects were not significant, except for the internodes of Agrostis. For each species and parameter, the bars sharing the same letter are not significant at p = 0.05. N.A. = not available.

s tolons tended to be longer under lower l ight levels

but the d i f fe rence was not significant. Pr imary stolon

internodes were s ignif icant ly longer under low than

under ei ther m e d i u m or high l ight levels. Secondary

stolons and their in ternodes were longer under medi -

um than under h igh l ight levels and were in termedia te

under low l ight condit ions. In Agrostis, primary and

secondary stolons were shorter at the low than under

ei ther m e d i u m or h igh l ight levels , whi le their internodes were shortest under high light. Cynodon respond-

ed to t rea tment m u c h more strongly than Agrostis in

terms of pr imary stolon internode length. In Cynodon, primary stolon internodes o f plants g rowing under low

light were approximate ly 100% longer than those of

plants g rowing under high light, whi le in Agrostis this d i f ference was about 15% only.

In contrast to the stolons, none o f the responses o f

the lengths of rh izomes or their in ternodes was statis-

t ically significant. This was true both for Cynodon and Holcus.

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Leaf+root mass (g) Fig. 5. Stolon mass and rhizome mass as functions of plant mass (leaf + root) in Agrostis (As), Cynodon (Cd) and Holcus (Hm). Note that the scales of the y- and x-axes are the same for all four panels. Symbols of 0, o and A represent low, medium and high light intensities, respectively. Statistical tests are given in Table 1.

Biomass partitioning to stolons and rhizomes. ANCO- VA results did not show significant effects of treatment (F2,14 = 3.34, p>0 .05 for Agrostis stolonifera; F2j4=0.22, p>0 .10 for Cynodon) on stolon mass in addition to effects of plant size in terms of leaf+ root mass (F1,14 = 142.43, p<0.001 for Agrostis; Fl,14 =20.90, p<0.001 for Cynodon), indicating that biomass partitioning to stolons did not respond to light treatments (Fig. 5).

ANCOVA results showed significant effects of treatment (F1,8=5.850, p<0 .05 for Cynodon; F2,14 = 17.27, p<0 .05 for Holcus) on rhizome mass in addition to effects of plant size (Fl,s = 0.002, p>0 .10 for Cynodon; FL]4=0.01, p>0 .10 for HoIcus). Test for homogeneity of slopes showed no difference in slope among treatments at p = 0.05, indicating that the significant effect of treatment in ANCOVA was the result of the difference in the y-intercept. For Cyn- odon, the y-intercept was significantly larger under

higher light levels, indicating higher biomass partitioning to rhizomes when plants grew under higher light levels (Fig. 5; Table 1). For Itolcus, the y-intercept was smallest under low light level, largest under medium light level. At the size trajectory relevant in the present experiment, partitioning to rhizomes was higher under higher light levels (Fig. 5).

In accordance with these results on size-dependent partitioning, proportional partitioning to stolons remained unresponsive, but proportional partitioning to rhizomes was larger under higher light levels.

Discussion

Plants of Cynodon had longer stolons and stolon internodes, lower branching intensity, larger leaves and more orthotropic shoots under lower light levels, similar to our previous study (Dong & de Kroon 1994). As a

31

Table 1. Linear regression analyses between the weight of stolons (M, to ) and rhizomes (M~t~I) and the weight o f leaves + roo t s (Ml + ~) in three clonal grass species. Regression parameters with asterisk are significantly different f rom

zero, Proport ional allocation is the ratio of the weight of these structures to the weight of leaves +roots . For each row and species, data shar ing the same lower-case letter are not significantly different at p = 0 . 0 5 (parameters were tested using A N C O V A and slope heterogeneous analysis, Sokal & Rohl f 1981; procedure GLM, SAS 1985; see text for further explanat ion). Codes for species and treatments are as in Fig. 1. - = not applicable.

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Regress ion parameters

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Slope (m) 0.39 ~ 0.79 *= 0.86 **~ 0.97 *a

Intercept (b) 0.29 *~ 0.69 b 0.88 b - 0 . 1 2 a

Mrhl = m M I + v + b

Slope (m) . . . .

Intercept (b) . . . .

Proport ional allocation

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(0.2) (0.05) (0.03) (0.08)

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0.11 a - 0 . 0 9 a 0.06 a - 0 . 3 7 a 0.20 '~

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0.7 a 0.6 a _ _ _

(o.1) (o.i) 0.2 a 0.4 b 0. I a 0.7 ~b 1.0 b

(0.06) (0.1) (0.06) (0.2) (0.2)

result of this high level of plasticity in stolon morphology (c. 100% elongation ofinternodes and c. 50% reduction in branching under lower light levels), Cynodon may be able to selectively place ramets in favourable patches in the grassland habitat (Sutherland & Still- man 1988; Dong & de Kroon 1994). Plants of Agrostis showed similar responses in stolon morphology, but with a much smaller magnitude (c. 10% elongation of internodes and c. 15% reduction in branching). These minor plastic responses may not enable Agrostis to realize selective placement of ramets in a patchy habitat, as may be true for many other stoloniferous species (Hutchings & de Kroon 1994; de Kroon & Hutchings in press). Shade-induced shortening of stolons in Agrostis may have been the result of the very low photon flux density under the lowest light level (see also Thompson 1993).

In contrast to the stolons, none of the responses in the lengths of rhizomes and their internodes in both Cynodon and Holcus was statistically significant. This lack of morphological response makes it impossible for rhizomes to place ramets selectively in light patches in the habitat.

The usually high percentage of dormant buds of rhizomes (60-80% for Cynodon; 65-90% for Hol- cus) relative to those of stolons (15-35% for Agrostis;

31-66% for Cynodon) indicates that rhizomes but not stolons are able to effectively store meristems for future regeneration of the plants. Higher biomass partitioning to rhizomes under higher light levels in Cynodon and Holcus implies that relatively more carbohydrates are being stored under higher light levels. This suggests that the storage function of the rhizomes is compro- mised when light is in short supply, which is in accordance with the contention that the storage may not be essential for present growth and survival, but for future regeneration of the plants (Grime 1979; White 1979; Leakey 1981).

In summary, stolons exhibited morphological responses to light quantity while biomass partitioning to stolons was unresponsive, and the opposite was true for rhizomes. Thus, a given type of spacer showed similar responses, irrespective of whether it was formed by a species with two (Cynodon) or only one (Agrostis, Holcus) type of spacers. Our results suggest that rhizomes and stolons serve unique functional roles in clonal grass species.

Data taken from literature also show markedly different responses between stoloniferous and rhizomatous species in morphology, especially in spacer length (de Kroon & Hutchings in press). Of six stoloniferous species for which we have been able to find data,

32

five species, Glechoma hederacea (Slade & Hutchings 1987), Lamiastrum galeobdolon (Mitchell & Wood- ward 1988; Dong 1993), Potentilla anserina and P. rep- tans (J. Stuefer, unpublished) and Trifolium repens (Thompson 1993), exhibit shorter stolon internodes and higher branching intensity under higher light levels. By contrast, the rhizomes of five out of six rhizomatous species, Agropyron repens (Williams 1971), Agrostis gigantea (Wiliams 1971 ), Brachypodium p in- nature (de Kroon & Knops 1990), Carex flacca (de Kroon & Knops 1990) and Glaux maritima (Jerling 1988), show higher branching intensities, but are not shorter under higher light intensity. Therefore, the contention that rhizomes serve mainly as storage organs of meristems and carbohydrates, and stolons primarily as foraging organs for light, seems to have more general validity.

Acknowledgments

We would like to thank Hans de Kroon, Heinjo Dur- ing, Marinus Werger and two anonymous referees for valuable comments on the manuscript, and L. Brandi for assistance during experiment.

References

Bell, A. D. 1984. Dynamic morphology: a contribution to plant population ecology, pp. 48-65. In: Dirzo, R. & Sarukhan, J. (eds), Perspectives on plant population ecology. Sinauer, Sunderland.

Cook, R. E. 1985. Growth and development in clonal plant population. pp. 259-296. In: Jackson, J. B. C., Buss, L. W. &Cook, R. E. (eds), Population biology and evolution of clonal organisms. Yale University Press, New Haven.

de Kroon, H. & Hutchings, M. J. (in press) Morphological plasticity in clonal plants: The foraging concept reconcidered. Journal of Ecology.

de Kroon, H. & Knops, J. 1990. Habitat exploration through morphological plasticity in two chalk grassland perennials. Oikos 59: 39-49.

de Kroon, H. & Schieving, F. 1991. Resource allocation patterns as a function of clonal morphology: a general model applied to foraging clonal plant. Journal of Ecology 79:519-530.

Dong, M. 1993. Morphological plasticity of the clonal herb Lami- astrum galeobdolon (L.) Ehrend. & Polatschek in response to partial shading. New Phytol. 124: 291-300.

Dong, M. & de Kroon, H. 1994. Plasticity in morphology and biomass allocation in Cynodon dactyIon, a grass species forming stolons and rhizomes. Oikos 70: 99-106.

Evans, J. R 1992. The effect of local resource availability and clonal integration on ramet functional morphology in Hydrocotyle bonariensis. Oecologia 89: 265-276.

Geber, M. 1989. Interplay of morphology and development on size inequality: A Polygonum greenhouse study. Ecological Mono- graphs 59: 267-288.

Grime, J. R 1979. Plant strategies and vegetation processes. John Wiley & Sons, Chichester.

Grime, J. E, Hodgson, J. G. & Hunt, R. 1988. Comparative Plant Ecology. Unwin Hyman, London.

Hanna, W. W. 1992. Cynodon dactylon, pp. 100-102. In: 't Mannetje, L. & Jones, R. M. (eds), Plant resources of south-east asia. No 4: Forages. Pudoc Scientific Publishers, Wageningen.

Hartnett, D. C. 1990. Size-dependent allocation to sexual and veg- etative reproduction in four clonal composites. Oecologia 84: 254-259.

Hutchings, M. J. & de Kroon, H. 1994. Foraging in plants: the role of morphological plasticity in resource acquisition. Advance in Ecological Research 25: 159-238.

Hutchings, M. J. & Slade, A. J. 1988. Morphological plasticity, foraging and integration in clonal perennial herbs, pp. 83-109. In: Davy, A. J., Hutchings, M. J. & Watkinson, A. R. (eds). Plant population ecology. Blackwell Scientific Publications, Oxford.

Jerling, L. 1988. Clonal dynamics, population dynamics and vegetation pattern of Glaux maritima on a Baltic sea shore meadow. Vegetatio 74: 171-185.

Kik, C,. van Andel, J., Joenie, W. & Bijlsma, R. 1990. Colonization and differentiation in the clonal perennial Agrostis stolonifera. Journal of Ecology 78:949-961.

Leakey, R. R. B. 1981. Adaptive biology of vegetatively regenerating weeds. Advances in Applied Biology 6: 57-90.

Mitchell, R L. & Woodward, F. I. 1988. Responses of three woodland herbs to reduced photosynthetically active radiation and low red to far-red ratio in shade. Journal of Ecology 76: 807-825.

Ovington, J. D. & Scurfield, G. 1965. Biological flora of the British Isles: Holcus molIis L. Journal of Ecology 44: 272-280.

Potvin, C., Lechowicz, M. J. & Tardif, S. 1990. The statistic analysis of ecophysiological response curves obtained from experiments involving repeated measure. Ecology 71: 1389-1400.

Rawal, K. M. & Harlan, J. R. 1971. The evolution of growth habit in Cynodon L. C. Rich (Gramineae). Illinois State Academy Science 64: 110-118.

Samson, D. A. & Werk, K. S. 1986. Size-dependent effects in the analysis of reproductive effort in plants. The American Naturalist 127: 66%680.

SAS. 1985. SAS/STAT guide for personal computers, version 6 Edition. SAS Institute Inc., Cary, NC.

Slade, A. J. & Hutchings, M. J. 1987. The effect of light intensity on foraging the clonal herb Glechoma hederacea. Journal of Ecology 75: 639-650.

Sokal, R. R. & Rohlf, E J. 1981. Biometry. W. H. Freeman and Company.

Sutherland, W. J. & Stillman, R. A. 1988. The foraging tactics of plants. Oikos 52: 239-244.

Thompson, L. 1993. The influence of natural canopy density on the growth of white clover, Trifidium repens. Oikos 67: 321-324.

Weiner, J. 1988. The influence of competition on plant reproduction. pp. 228-245. In: Lovett Doust, J. & Lovett Doust, L. (eds), Plant reproductive ecology - pattern and strategies. Oxford University Press, Oxford.

White, J. 1979. The plant as a metapopulation. Annual Review of Ecology and Systematics 10: 109-45.

Williams, E. D. 1971. Effects of light intensity, photoperiod and nitrogen on the growth of seedlings of Agropyron repens (L.) Beauv. and Agrostis gigantea Roth. Weed Research 11 : 159-170.

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Morphology and growth of stolons and rhizomes in three clonal grasses, as affected by different light supply