Comparison of Photosynthetic Pigment Contents of the Resurrection Plants <i>Ramonda serbica</i> and <i>Ramonda nathaliae</i> of Some Different Populations from Kosovo, Albania and Macedonia

doi:10.4236/ajps.2012.311192

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American Journal of Plant Sciences, 2012, 3, 1588-1593

http://dx.doi.org/10.4236/ajps.2012.311192 Published Online November 2012 (http://www.SciRP.org/journal/ajps)

Comparison of Photosynthetic Pigment Contents of the

Resurrection Plants Ramonda serbica and Ramonda

nathaliae of Some Different Populations from Kosovo,

Albania and Macedonia

Bekim Gashi1,2, Kasamedin Abdullai1, Efigjeni Kongji k a3

1Department of Biology, Faculty of Mathematics and Natural Sciences, University of Pristina, Pristina, Republic of Kosovo;

2Department of Biotechnology, Faculty of Natural Sciences, University of Tirana, Tirana, Albania; 3Section of Natural and Technical

Sciences, Academy of Sciences of Albania, Tirana, Albania

Email: bekimgashi[email protected]

Received August 17th, 2012; revised September 19th, 2012; accepted October 9th, 2012

ABSTRACT

The resurrection plants Ramonda serbica and Ramonda nathaliae are the physiological model plant and endemic spe-

cies from Balkan Peninsula. Study was carried out to assess the impact of different populations on photosynthetic pig-

ment contents and their effect on physiological activity of these species. The experiment was conducted with leaves of

R. serbica collected from nine populations in Kosovo, three in Albania and two in Macedonia, while for R. nathaliae

four populations in Macedonia. For this purpose plants after collection from their natural populations were kept for 24

hours at constant temperature and humidity and in natural photoperiod. According to our data the pigment contents

(Total chl + Carot) of R. serbica, measured as mg per gram dry weight (DW), were higher (7.06 mg·g−1 DW) in Radaci

populations (Kosovo) and lower (4.63 mg·g−1 DW) in Jukniu Mountain-Kruja (Albania). On the other hand, the higher

(5.28 mg·g−1 DW) of pigment contents (Total chl + Carot) of R. nathaliae were observed in Vorca populations and the

lower (4.24 mg·g−1 DW) in Kaparllëk populations. Ratio chlorophyll a/b of R. serbica in Kruja Castle populations from

Albania was the higher (3.68) comparing with Zhlebi populations from Kosovo (1.68). Similarly in case of R. nathaliae

the higher (3.36) value was in Kaparllëk populations and the lower (3.12) in Matka populations. The data obtained were

further analyzed using one-way ANOVA and a significant change was recorded in the different populations. These

studies clearly indicate that the Ramonda plants from different ecological habitats there have been changes of photo-

synthetic pigment contents.

Keywords: Resurrection Plants; Physiological Model; Populations; Photosynthetic Pigments

1. Introduction

A small number of higher plant species, not closely re-

lated, have adapted to environments with rapidly devel-

oping and often extended periods of extreme dryness,

followed by sudden water availability. These are called

desiccation-tolerant or resurrection plants. Examples are

Ramonda serbica and Ramonda nathaliae, a rare resur-

rection plant of the Balkan Peninsula, an endemic relicts

of the tertiary period.

The species of family Gesneriaceae are good exam-

ples of ecophysiological divergence regarding the plant

water relations and the adaptations to the conditions of

water regime in the habitat. Under the conditions of wa-

ter deficit in the habitat, these plants gradually wilt and

pass to anabiosis. Resurrection plants are usually subdi-

vided into homoiochlorophyllous plants retaining their

chlorophyll (Chl) during desiccation and poikilochloro-

phyllous plants where desiccation results in the loss of

Chl which must be re-synthesized following rehydration

[1]. However, pigment loss and destruction of the other

thylakoid pigments are highly organised responses to

desiccation, realised via a well-defined metabolic path-

way [1]. From an ecological point of view, R. serbica is a

perennial, herbaceous, shade-adapted species belonging

to the group of homoiochlorophyllous poikilohydric

plants which preserve more than 80% of the chlorophyll

content during dehydration [2,3].

Its ability to survive in harsh environmental conditions

has so far been studied in terms of morphologic, physic-

ologic, and biochemical as well as the propagation

through in vitro cultivation. This research has brought the

Comparison of Photosynthetic Pigment Contents of the Resurrection Plants Ramonda serbica and

Ramonda nathaliae of Some Different Populations from Kosovo, Albania and Macedonia

1589

evidence of adaptive features necessary to preserve cell

membrane integrity [4], antioxidative capacity [5,6],

photosynthetic activity [1], CO2 fixation and chlorophyll

a fluorescence [7], genome size variation and polyploidy

[8], osmotic adjustment [9], seed germination [10] and in

vitro cultivation from seeds of Ramonda plants [11-13].

Nevertheless, up to now there is only a few data from

other authors for photosynthetic pigments content of

Ramonda serbica or similar species from Geseneriaceae

in different populations. Right now still are missing the

study belonging to Ramonda species on concentration of

photosynthetic pigments.

We are incited to find the concentration of chlorophyll

a, b, total chlorophyll, carotenoids and their ratio from

different ecological habitats conditions and their impact

on ecophysiological activity for these plants.

2. Materials and Methods

The resurrection plants Ramonda serbica and Ramonda

nathaliae were collected from their natural habitat. R.

serbica in nine populations in Kosovo, three in Albania

and two in Macedonia, while for R. nathaliae four popu-

lations in Macedonia (Table 1). Plants of the same age

were harvested together with the layer of soil on which

they grew.

For the photosynthetic pigments content study, three

leaves of similar ages, comparable in size, were collected

from five plants from the middle of the rosette.

2.1. Chlorophyll (a, b and Total) and

Carotenoids Analysis

Pigments were extracted by grinding 60 - 80 mg freshly

sampled leaves in 80% (v/v) acetone/water containing

MgCO3 (0.5% w/v) at room temperature for 24 h in the

dark. Photosynthetic pigments of all the samples were

extracted in triplicate to minimize experimental errors.

Concentration of chlorophyll and carotenoid contents

were measured by using absorbance recorded at 663 nm,

644 nm and 452.5 nm for maximum absorption of chlo-

rophyll “a” (Chl a), chlorophyll “b” (Chl b) and Carote-

noids, respectively. The extinction coefficients were de-

termined by a UV-Vis spectrophotometer. Pigment con-

tents were calculated in mg·g−1 dry leaf weight (DW) by

applying the absorption coefficient equations described

Table 1. Geographic origin and descr iption of Ramonda plants.

Code of populations Species Locality of populations Directions Altitude (m) Country

1 Ramonda nathaliae Gorge of river Matka N 348 Macedonia

2 Ramonda nathaliae Kaparllëk N and NE 448 Macedonia

3 Ramonda nathaliae Cerovë N and NE 591 Macedonia

4 Ramonda nathaliae Vorcë N and NE 407 Macedonia

5 Ramonda serbica Gorge of River Radika N 920 Macedonia

6 Ramonda serbica Llukovo N 630 Macedonia

7 Ramonda serbica Gorge of Zhlebi N and NE 1250 Kosovo

8 Ramonda serbica Radac N 545 Kosovo

9 Ramonda serbica Gorge river of Sushica N 676 Kosovo

10 Ramonda serbica Canyon of Rugova N 800 Kosovo

11 Ramonda serbica Gorge of Koprivnik N 750 Kosovo

12 Ramonda serbica Gorge of River Prizren N 530 Kosovo

13 Ramonda serbica Gorge of Rusenica N 1340 Kosovo

14 Ramonda serbica Gorge of Matosi N 910 Kosovo

15 Ramonda serbica Shkëmbi i përgjakur N 1170 Kosovo

16 Ramonda serbica Kruja Castle N and NE 565 Albania

17 Ramonda serbica Jukniu Mountain N and NE 827 Albania

18 Ramonda serbica Petrela Castle N 340 Albania

Comparison of Photosynthetic Pigment Contents of the Resurrection Plants Ramonda serbica and

Ramonda nathaliae of Some Different Populations from Kosovo, Albania and Macedonia

1590

by Lichtenthaler (1986) [14]:



 

Chl mggDW

10.3OD6630.918OD644V100FWDW.

a



 











Chl mggDW

19.7OD6443.87OD663V100 FWDW.

b

















Carotenoids mg gDW

4.75 OD452.50.226 ChlChl

V100FW DW.











 





where

DW = Dry leaf weight.

FW = Fresh leaf weight.

OD = Optical density.

V = Volume of sample.

2.2. Data Analysis

The experiment was performed in a randomized design

with five replicates. Differences among parameters and

between the populations were tested using SPSS 17 sta-

tistical program. Statistical variance analysis of the all

data was performed using one-way ANOVA and com-

pared with least significant difference (LSD) at the 5%

and 1% level. Mean comparison was performed with

Duncan’s test at the 5% level of significance.

3. Results and Discussions

Based on the obtained results and presented on the Ta-

bles 2 and 3, high significant differences at P < 0.01 have

been ascertained between populations, for all researched

parameters of Ramondas plants on different populations.

The effect of different ecogeographic factors combine on

highest differences on photosynthetic pigment concentra-

tion and their ratio between different populations.

At R. nathaliae, the highest content of Chl a, Carete-

noids and Total Chlorophyll + Carotenoids has been as-

certained at Vorca population, whereas the content of Chl

b and Total Chl was higher at Gorge of river Matka

population. The lowest content of all photosynthetic

pigments was obtained at Kaparllëk population (Table 2).

At Gorge of river Matka population, the Ratio Chl a/b

(Figure 1) was the lowest (3.12), whereas the Ratio Total

Chl/Carot was the highest (4.53) compared to other

populations. Based on the results collected in the field,

the Gorge of River Matka population had less sunlight

due to the hindrance by the forest trees. For this reason

the Chl a/b ratio has been frequently used as an indicator

of plant response to shading [15].

Based on the gained and analysed results with One-

way ANOVA and compared with Duncan test, at R. ser-

bica leaves, the highest photosynthetic pigments content

(Table 3) was ascertained at Radac and Llukovo popula-

tions. The similar results were gained for the Gorge of

Matosi population, too. Out of all investigated popula-

tions, it is worth mentioning that these three populations

have been with less sunlight compared with other popu-

lations. This increase of the photosynthetic pigments

content is justified by the fact which for many plant spe-

cies that grow under environmental shade conditions will

result in growth of pigment content. In addition, Gon-

çalves et al. (2001) [16] at Mahogany and Tonka bean,

leaf chlorophyll concentrations (Chl a, Chl b, Chl tot) on

a fresh mass basis were higher in shade leaves than in

sun leaves. According to Tan et al. (2000) [17] there is

widespread interest in chlorophylls and their degradation

reactions. Chlorophyll is synthesized and degraded

(photo oxidation) under irradiation. At high irradiance,

however, the degradation rate overtakes the rate of syn-

thesis, therefore, a lower chlorophyll concentration is

observed. Due to this fact, shade leaves in comparison

with sun leaves tend to show higher chlorophyll concen-

trations per unit leaf weight [18].

Lower photosynthetic pigments content was observed

at R. serbica population from Albania, especially in

Jukniu Mountain and Kruja Castle populations. These

populations are with more intensity of the sunlight com-

pared with other populations from Albania, Kosovo and

Macedonia (Table 3). Based on statistical analysis re-

sults with Duncan test, the populations’ alignment with

approximate content of photosynthetic pigments clearly

indicates the similar ecological conditions of these popu-

lations.

The highest content of Chl a, Total Chl and Total Chl/

Carot (Table 3) for all the investigated population of R.

serbica was ascertained at Radac population (4.47, 5.90,

and 7.06 mg·g−1 DW, respectively), whereas the lowest

at Jukniu Mountain (2.86, 3.70, and 4.63 mg·g−1 DW,

respectively). For the Chl b the highest content was at

Gorge of Zhlebi population (1.76 mg·g−1 DW), whereas

the lowest at Shkembi i pergjakur population (1.42

mg· g −1 DW). Our results of higher Chl b in the shade

environment are in accordance with other authors results.

Gonçalves et al. (2001) [16] showed that Chl b concen-

tration on a mass basis of tonka bean and mahogany was

higher in the shade environment.

The highest Carotenoids concentrations was at Gorge

of Matosi population (1.26 mg·g−1 DW), whereas the

lowest at Gorge of Zhlebi population (0.81 mg·g−1 DW).

Another very important indicator for the intensity of

sunlight is also Ratio Chl a/b, where the highest values

for this parameters (Figure 1) were shown at Kruja Cas-

tle and Jukniu Mountain populations (3.68 and 3.42, re-

Comparison of Photosynthetic Pigment Contents of the Resurrection Plants Ramonda serbica and

Ramonda nathaliae of Some Different Populations from Kosovo, Albania and Macedonia

1591

Table 2. The effects of different populations of R. nathaliae from Macedonia on photosynthetic pigment content in mg per

gram dry weight (DW).

Chl a Chl b Total Chl Carotenoids Total Chl + Carot

Code of populations

Mean ± SX Mean ± SX Mean ± SX Mean ± SX Mean ± SX

1 3.27 ± 0.16 a 1.06 ± 0.06 a 4.33 ± 0.22 a 0.95 ± 0.04 b 5.28 ± 0.25 a

2 2.58 ± 0.10 b 0.80 ± 0.05 bc 3.38 ± 0.14 b 0.86 ± 0.03 b 4.24 ± 0.17 b

3 2.82 ± 0.15 b 0.90 ± 0.06 ab 3.72 ± 0.20 b 0.94 ± 0.04 b 4.66 ± 0.24 b

4 3.32 ± 0.08 a 1.01 ± 0.03 b 4.33 ± 0.11 a 1.06 ± 0.02 a 5.39 ± 0.13 a

P ≤ 0.05 0.388 0.135 0.478 0.091 0.566

LSD

P ≤ 0.01 0.588** 0.204** 0.727** 0.138** 0.858**

Chl—Chlorophyll; Carot—Carotenoides; *Significant at 0.05 probability level; **Significant at 0.01 probability level. Columns with different letters differ sig-

nificantly at P < 0.05 by one-way ANOVA with Duncan’s multiple range test.

Table 3. The effects of different populations of R. serbica from Kosovo, Albania and Macedonia on photosynthetic pigment

content in mg per gram dry weight (DW).

Chl a Chl b Total Chl Carotenoids Total Chl + Carot

Code of populations

Mean ± SX Mean ± SX Mean ± SX Mean ± SX Mean ± SX

5 3.43 ± 0.09 de 1.14 ± 0.04 d 4.57 ± 0.13 c 1.11 ± 0.03 bcd 5.68 ± 0.15 cd

6 4.38 ± 0.08 a 1.43 ± 0.05 c 5.81 ± 0.12 a 1.19 ± 0.05 ab 7.00 ± 0.15 a

7 2.86 ± 0.08 g 1.76 ± 0.06 a 4.61 ± 0.13 c 0.81 ± 0.02 f 5.43 ± 0.15 cd

8 4.47 ± 0.13 a 1.43 ± 0.04 c 5.90 ± 0.16 a 1.16 ± 0.05 bc 7.06 ± 0.20 a

9 3.70 ± 0.09 c 1.34 ± 0.03 c 5.04 ± 0.12 b 1.07 ± 0.02 cd 6.12 ± 0.14 b

10 3.49 ± 0.07 cde 1.10 ± 0.03 d 4.58 ± 0.08 c 1.08 ± 0.02 cd 5.66 ± 0.10 cd

11 3.42 ± 0.11 de 1.19 ± 0.04 d 4.62 ± 0.10 c 1.04 ± 0.05 d 5.66 ± 0.11 cd

12 3.37 ± 0.07 e 1.10 ± 0.02 d 4.46 ± 0.09 c 1.08 ± 0.02 cd 5.55 ± 0.11 cd

13 3.13 ± 0.07 f 1.20 ± 0.02 d 4.33 ± 0.09 c 1.05 ± 0.02 d 5.37 ± 0.12 d

14 4.02 ± 0.08 b 1.58 ± 0.06 b 5.61 ± 0.13 a 1.26 ± 0.03 a 6.86 ± 0.14 a

15 3.67 ± 0.08 cd 1.42 ± 0.03 c 5.09 ± 0.11 b 1.11 ± 0.03 bcd 6.20 ± 0.13 b

16 2.96 ± 0.06 fg 0.81 ± 0.02 e 3.76 ± 0.07 d 0.93 ± 0.02 e 4.69 ± 0.09 e

17 2.86 ± 0.06 g 0.84 ± 0.03 e 3.70 ± 0.09 d 0.93 ± 0.02 e 4.63 ± 0.11 e

18 3.59 ± 0.09 cde 1.10 ± 0.03 d 4.68 ± 0.12 c 1.13 ± 0.03 bcd 5.81 ± 0.15 bc

P ≤ 0.05 0.339 0.149 0.431 0.098 0.492

LSD

P ≤ 0.01 0.458** 0.201** 0.582** 0.132** 0.470**

Chl—Chlorophyll; Carot—Carotenoides; *Significant at 0.05 probability level; **Significant at 0.01 probability level. Columns with different letters differ sig-

nificantly at P < 0.05 by one-way ANOVA with Duncan’s multiple range test.

spectively).

At the Gorge of Zhlebi population was observed the

lowest value of Ratio Chl a/b and the highest the Ratio of

Total Chl/Carot (5.67). Gonçalves et al. (2001) [16] con-

firm a slight differences in Chl a/b ratio and chlorophyll/

carotenoid ratio between species and environments. In

the open site, the Chl a/b ratio was higher and chloro-

phyll/carotenoid ratio was lower in tonka bean and ma-

Comparison of Photosynthetic Pigment Contents of the Resurrection Plants Ramonda serbica and

Ramonda nathaliae of Some Different Populations from Kosovo, Albania and Macedonia

1592

Figure 1. Ratio chlorophyll a/b and Total chlorophyll/Carotenoids of Ramonda plants from different populations.

hogany.

4. Conclusions

The populations with north-east direction and the highest

exposition to the sunlight have lowest photosynthetic

pigment content compared to populations with less sun-

light. The lowest Chl a/b ratio is at populations with less

sunlight whereas the highest Chl/Carot ratio.

According to statistical analysis results with Duncan

test, the populations’ alignment with approximate content

of photosynthetic pigments clearly indicates the similar

ecological conditions of these populations.

There are no significant differences of photosynthetic

pigments content between populations and their eleva-

tion.

5. Acknowledgements

We would like to thank Prof. Fadil Millaku for literature

and professional collaboration.

REFERENCES

[1] Z. Tuba, M. C. F. Proctor and Z. Csintalan, “Ecophysi-

ological Responses of Homoichlorophyllous and Poikilo-

chlorophyllous Desiccation-Tolerant Plant Xerophyta sca-

brica at Present Day CO2 Concentration,” Journal of

Plant Physiology and Biochemistry, Vol. 35, 1998, pp.

381-386.

[2] Y. K. Markovska, T. D. Tsonev, G. P. Kimenov and A. A.

Tutekova, “Physiological Changes in Higher Poikilo-

hydric Plants—Haberlea rhodopensis Friv. and Ramonda

Serbica Panc. during Drought and Rewatering at Differ-

ent Light Regimes,” Journal of Plant Physiology, Vol.

144, No. 1, 1994, pp. 100-108.

doi:10.1016/S0176-1617(11)81000-X

[3] A. Augusti, A. Scartazza, F. Navari-Izzo, C. L. M.

Sgherri, B. Stevanovic and E. Brugnoli, “Photosystem II

Photochemical Efficiency, Zeaxanthin and Antioxidant

Contents in the Poikilohydric Ramonda serbica during

Dehydration and Rehydration,” Photosynthesis Research,

Vol. 67, No. 1-2, 2001, pp. 79-88.

doi:10.1023/A:1010692632408

[4] M. F. Quartacci, O. Glisic, B. Stevanovic and F. Navari-

Izzo, “Plasma Membrane Lipids in the Resurrection Plant

Ramonda serbica Following Dehydration and Rehydra-

tion,” Journal of Experimental Botany, Vol. 53, No. 378,

2002, pp. 2159-2166. doi:10.1093/jxb/erf076

[5] C. Sgherri, B. Stevanovic and F. Navari-Izzo, “Role of

Phenolics in the Antioxidative Status of the Resurrection

Plant Ramonda serbica during Dehydration and Rehydra-

tion,” Physiologia Plantarum, Vol. 122, No. 4, 2004, pp.

478-485. doi:10.1111/j.1399-3054.2004.00428.x

[6] Z. Jovanovic, T. Rakic, B. Stevanovic and S. Radovic,

“Characterization of Oxidative and Antioxidative Events

during Dehydration and Rehydration of Resurrection

Plant Ramonda nathaliae,” Journal of Plant Growth

Regulator, Vol. 64, No. 3, 2011, pp. 231-240.

doi:10.1007/s10725-011-9563-4

[7] E. Degl’Innocenti, L. Guidi, B. Stevanovic and F. Navari,

“CO2 Fixation and Chlorophyll a Fluorescence in Leaves

of Ramonda serbica during a Dehydration-Rehydration

Cycle,” Journal of Plant Physiology, Vol. 165, No. 7,

2008, pp. 723-733. doi:10.1016/j.jplph.2007.06.009

[8] S. Yakovlev-Siljak, V. Stevanovic, M. Tomasevic, C. S.

Brown and B. Stevanovic, “Genome Size Variation and

Polyploidy in the Resurrection Plant Genus Ramonda:

Cytogeography of Living Fossils,” Environmental and

Experimental Botany, Vol. 62, No. 2, 2008, pp. 101-112.

doi:10.1016/j.envexpbot.2007.07.017

[9] T. Zivkovic, M. F. Quartacci, B. Stevanovic, F. Marinone

and F. Navari-Izzo, “Low Molecular Weight Substances

in the Poikilohydric Plant Ramonda serbica during De-

hydrationand Rehydration,” Plant Science, Vol. 168, No.

1, 2005, pp. 105-111. doi: 10.1016/j.plantsci.2004.07.018

[10] B. Gashi, K. Abdullai, V. Mata and E. Kongjika, “Effect

of Gibberellic Acid and Potassium Nitrate on Seed Ger-

mination of the Resurrection Plants Ramonda serbica and

Ramonda nathaliae,” African Journal of Biotechnology,

Vol. 11, No. 20, 2012, pp. 4537-4542.

Comparison of Photosynthetic Pigment Contents of the Resurrection Plants Ramonda serbica and

Ramonda nathaliae of Some Different Populations from Kosovo, Albania and Macedonia

1593

[11] E. Kongjika, Zh. Zekaj, E. Çaushi and I. Stamo, “Plant

Biotechnology—In Vitro Culture,” Academy of Science,

Institute of Biological Researchers, Tirana, 2002, pp.

119-140.

[12] S. Dontcheva, E. Daskalova, G. Yahubyan, I. Denev, I.

Minkov and V. Toneva, “Conservation of the Protected

Resurrection Species Ramonda serbica Panc.—Habitat

Montana District, Bulgaria as in Vitro Plants through a

Modified Micropropagation System,” Biotechnology and

Biotechnological Equipment, Vol. 23, No. 2, 2009, pp.

369-372.

[13] B. Gashi, K. Abdullai, V. Mata, S. Misimi, M. Osmani

and E. Kongjika, “In Vitro Culture—A Tool to Overcome

the Poor in Vivo Development of Genus Ramonda

Plants,” Bulletin of Natural Science, Special Edition,

2011, pp. 536-543.

[14] H. Lichtenthaler, “Laser-Induced Chlorophyll Fluores-

cence of Living Plants,” Proceedings of the Remote Sens-

ing Symposium, Band III, ESA Publication Division,

Nordwijk, 1986, pp. 1571-1579.

[15] G. A. F. Hendry and A. H. Price, “Stress Indicators:

Chlorophylls and Carotenoids,” In: G. A. F. Hendry, Ed.,

Methods in Comparative Plant Ecology, Chapman & Hall,

London, 1993, pp. 148-152.

doi:10.1016/j.plantsci.2004.07.018

[16] J. F. C. Gonçalves, R. A. Marenco and G. Vieira, “Con-

cetration of Photosynthetic Pigments and Chlorophyll

Flourescence of Mahogany and Tonka Bean under Two

Light Environments,” Revista Brasileira de Fisiologia

Vegetal, Vol. 13, No. 2, 2001, pp. 149-157.

doi:10.1590/S0103-31312001000200004

[17] Y. Tan, J. H. Jiang, H. L. Wu, H. Cui and R. Q. Yu,

“Resolution of Kinetic System of Simultaneous Degrada-

tion of Chlorophyll a and b by PARAFAC,” Analytica

Chimica Acta, Vol. 412, 2000, pp. 195-202.

doi:10.1016/S0003-2670(99)00813-2

[18] K. N. Boardman, “Comparative Photosynthesis of Sun

and Shade Plants,” Annual Review of Plant Physiology,

Vol. 28, 1977, pp. 355-377.

doi:10.1146/annurev.pp.28.060177.002035