Seed germination in Citrullus lanatus. Part 4. The inhibition of germination within the fruits by light

F.C. Botha, J.G.C. Small, N. Grobbelaar and B.M. Eller Margaretha Mes Institute for Seed Research, Department of Botany, University of Pretoria

The germination of Citrullus lanatus and Lactuca sativa cv. Grand Rapids seeds is inhibited in the light when covered with fruit shells of C. lanatus. This inhibition appears to be due to the light which penetrates the fruit shells.

Studies on lettuce seed showed that the penetrating light is far-red rich. Although covering with fruit shells inhibits the ger­mination of C. lanatus seeds in the light, high germination is obtained when such covered seeds are transferred to the dark. This is taken to indicate that the light penetrating the fruit shells is not monochromatic far-red light. This allows for the formation of phytochrome intermediates within the seeds, which in time could cause subsequent dark germination of C. lanatus seeds.

The results obtained support the hypothesis that light con­tributes to the inhibition of germination of C. lanatus seeds within the fleshy fruits. S. Afr. J. Bot. 1983, 2: 181- 183

Die kieming van Citrullus lanatus en Lactuca sativa cv. Grand Rapids-sade word in lig gerem wanneer die sade met vrugdoppe van C. lanatus bedek word. Hierdie remming van saadkieming is waarskynlik slegs aan die lig wat die vrugdoppe deurdring toe te skryf.

Proewe met slaaisaad het aangetoon dat die lig wat die vrugdoppe deurdring ryk aan verrooilig is. Alhoewel die kie­ming van C. lanatus-sade gerem word wanneer die sade met vrugdoppe in die lig bedek word , kiem sulke behandelde sade goed indien dit na die danker oorgeplaas word. Hierdie waarneming dui daarop dat die lig wat die vrugdoppe deur­dring nie monochromatiese verrooilig is nie en gevolglik aanleiding tot die vorming van fitochroomintermediere verbin­dings in die sade sal gee. Dit sal die sade van C. lanatus in staat stel om gedurende 'n opvolg-donkerbehandeling te kiem.

Die resultate wat verkry is ondersteun die hipotese dat lig 'n belangrike bydraende faktor is om die kieming van C. lanatus­sade in die vlesige vrugte te rem. S.·Afr. Tydskr. Plantk. 1983, 2: 181- 183

Keywords: Germination, Citrullus lanatus, Lactuca sativa, light, fruits

F.C. Botha*, J.G.C. Small and N. Grobbelaar Margaretha Mes Institute for Seed Research, Department of Botany, University of Pretoria, Pretoria 0002, Republic of South Africa

B.M. Eller Present address: Institut fiir Pflanzenbiologie, Zollikerstrasse 107, CH-8008 Ziirich

*To whom correspondence should be addressed

Accepted 25 April 1983

Introduction

Different reasons have been given for the failure of mature seeds to germinate inside fleshy fruits. Some authors ascribe the inhibition to the presence of metabolic inhibitors in the fruits (Akkerman & Veldstra 1947; Luckwill1952; Bennet­Ciarck & Kefford 1953) while other authors relate inhibi­tion to a low water potential of the fruit tissue surrounding the seeds (Fukaki 1930; Litvinow 1938; Lerner eta/. 1959) . In some cases a combination of germination inhibitors and low water potential of the fruit tissue has been favoured as the causative factor (Dorffling 1970; Mayer & Poljakoff­Mayber 1975).

Based on light measurement studies and a theoretical calculation of phytochrome shifts, we postulated for the first time that light which penetrates the fruits of Citrullus lanatus might be involved in inhibiting seed germination inside the fruits (Eller et a/. 1982). This possibility was further in­vestigated by using the fruit shells of C. lanatus as light filters in germination tests and further evidence is presented to support our postulate.

Materials and Methods

Seeds of Citrullus lanatus (Thunb.) Matsumura & Nakai and Lactuca sativa L. cv. Grand Rapids were obtained as previously described (Both a et a/. 1982a; Eller et a/. 1982) .

Seeds were germinated in 7,0 em Petri dishes each con­taining a single sheet of 7,0 em Whatman no. 1 filter paper and moistened with 4 cm3 of distilled water. All germina­tion tests were conducted at 27°C ( ± 3 oq in a temperature controlled greenhouse. In experiments with C. lanatus, 30 seeds, and with L. sativa, 50 seeds were used per replicate. At least four replicates per treatment were used throughout.

White light was obtained from full sunlight for approxi­mately 12 h in every 24-h period and light from a combina­tion of 'cool white' fluorescent tubes and tungsten bulbs was used for the rest of the 24-h period. Red light was ob­tained as described by Botha eta/. (1982b) .

Fruit shells of C. lanatus were used as light filters in ger­mination tests. The fruit shells were prepared by cutting each fruit transversely into two approximately equal halves. The core and seed bearing tissue, consisting largely of placental tissue, was removed with a spatula to approximately 9 mm from the outer fruit surface. Hereafter the inner surfaces of the fruit shells were washed under running tap water for 10 min. The fruit shells were then used to cover seeds as shown in Figure 1. The temperature fluctuation beneath the

182 S.-Afr. Tydskr. Plantk., 1983, 2(3)

·t shell

lg~~~~~~r~~~~~~====~~::,:; to prevent drying of the fruit shell

'-----------Double layer black plastic sheet

'---------------What man no 1 filter paper

Figure 1 Diagram of the experimental system used to investigate the light filtering capacity of the fruit shells of Citrullus /anatus.

fruit shells in white light in the greenhouse over a 24-h period was approximately 3°C.

Germinated seeds were counted under a dim green safe light (Smith 1975). A seed was considered to have ger­minated when the radicle had emerged. Least significant dif­ferences (LSD) were calculated by using the Tukey pro­cedure (Steel & Torrie 1980).

Results and Discussion The germination of Citrullus tanatus and Lactuca sativa seeds is inhibited when covered with fruit shells during ex­posure to white light (Table 1). This inhibition appears to be due to light which penetrates the fruit shell as C. tanatus

Table 1 Effect of various light and Citrullus lanatus fruit shell covering treatments on the germination of Citrullus /anatus and Lactuca sativa cv. Grand Rapids seeds

Treatment

I . Citrul/us /anatus

1.1 Covered with shells in white light

1.2 Uncovered in white light

1.3 Covered with shells in the dark

1.4 Uncovered in the dark

2. Lactuca sativa

2.1 Covered with shells in white light

2 .2 Uncovered in white light

2.3 Covered with shells in the dark

2.4 Uncovered in the dark

2.5 Covered with shells for 24 h in white light, 2 h uncovered in white light and thereafter covered with shells in white light

2.6 Covered with shells for 24 h in white light 2 h uncovered in white light and thereafter covered with shells in the dark

2.7 Covered with shells for 24 h in white light, 2 h uncovered in red light and thereafter covered with shells in the dark

2.8 Covered with shells for 24 h in white light, 2 h uncovered in red light and thereafter covered with shells in white light.

Percentage germination• at 27°C after :

24 h 48 h 72h

0 0 2

0 0 0

0 65 88

0 60 90

0 0 0

58 70 75

3 5 5

0 6 8

0 0 0

2 50 52

0 48 88

0 7 10

• A difference of more than 9,2 between any two figures is significant at

the 507o level of probability.

seeds do germinate under the shell in continuous darkness (treatment 1.3. in Table 1). From the results obtained with lettuce seed, it is evident that the light which penetrates the fruit shell must be rich in far-red light. The inhibition of germination obtained by covering lettuce seeds with fruit shells is not due to an absence of light. The stimulatory ef­fect of red light on the germination of lettuce seed is reversed when the seeds, covered with fruit shells, are placed in white light (compare treatments 2. 7 and 2.8 in Table 1).

These results support to a large extent the conclusions drawn from theoretical calculations of phytochrome shifts inside the fruits of C. tanatus after exposure to different light regimes (Eller et at. 1982). Owing to the light filtering capacity of the fruit, mainly far-red rich light appears to penetrate the fruit. Should this be the case a shift from Pfr to Pr inside the seeds would occur. According to Gutter­man & Porath (1975) the germination of the seeds of two Cucumis spp. is influenced by the light quality under which the fruits are stored. These results and also those obtained by Fritts & Loy (1981) indicate that light penetrates the fruits of certain species. This, however, does not imply that light of certain wavelengths is filtered out by these fruits. Although the quality of the light falling on the fruit could play a role, the light filtering properties of the fruit will primarily determine the form in which phytochrome will predominantly prevail in seeds when fruits are exposed to light.

The seeds of C. tanatus germinate well in the dark after being exposed to white light when covered with fruit shells (Table 2). Since it has already been shown with isolated seeds of C. tanatus that the inhibitory effect of far-red light is not alleviated by a follow-up dark treatment (Botha et at. 1982b), the present results indicate that the light penetrating the fruits is not monochromatic far-red light. This is in agreement with the measured optical properties of the fruits of this species (Eller et at. 1982). The quality of the light

Table 2 The effect of different light treatments at 27°C on the germination of Citrullus lanatus seeds when covered with fruit shells

Percentage germination after:

Light treatment 48 h 72h 96 h

Dark 58±7 84±6 93±4

White light 0 2±1 8±4

White light for 48 h, thereafter in the dark 0 40±8 70±6

± = standard deviation

S. Afr. J . Bot., 1983, 2(3)

which reaches the seeds inside the fruits is such that it ap­parently causes a Pfr concentration lower than the threshold value required for germination. However, as the penetrating light is not monochromatic far-red light, it will result in a fair amount of pigment cycling between Pr and Pfr (Ken­drick & Spruit 1972). During such conditions of pigment cycling, substantial amounts of Meta-Rb will form which can revert to Pfr in the dark (Kendrick 1976).

Based on these results and on previous work (Eller eta/. 1982), we believe that sufficient evidence exists to postulate that light which penetrates the fruits of C. lanatus is an im­portant inhibitory factor for seed germination within the fruit.

Acknowledgements

We are grateful to the University of Pretoria and the CSIR for grants received for this project.

References

AKKERMAN, A.M. & VELDSTRA, H. 1947. The chemical nature of Kiickemann's blastocholine for Lycopersicon esculentum. Mill. Rec. Trow. Chim. 66: 411-412.

BENNET-CLARCK, T.A. & KEFFORD, H.P. 1953 . Chromato­graphy of the growth substances in plant extracts. Nature, Land. 171: 645.

BOTHA, F.C., GROBBELAAR, N. & SMALL, J .G.C. 1982a. Seed germination in Citrullus lanatus I. Effect of white light and growth substances on germination. S. Afr. J. Bot. I: 10- 13.

BOTHA, F.C., SMALL, J .G.C. & GROBBELAAR, N. 1982b. Seed germination in Citrullus /anatus 2. The involvement of phytochrome and ethylene in controlling light sensitivity. S. Afr. J. Bot. 1: 131-133.

DORFFLING, K. 1970. Abscisinsiiure und Kiemingshemmung in der

183

Tomatenfrucht. Planta (Berl .) 93 : 243-256. ELLER, B.M., BOTHA, F.C., GROBBELAAR, N. & SMALL,

J.G.C. 1982. Seed germination in Citrullus lanatus 3. The possibility of light as an inhibitory factor for germination of seeds within the fruits based on light measurement studies. S. Ajr. J. Bot. I: 134- 138.

FRITTS, S.K. & LOY, J .B. 1981. Influence of light quality during development and drying on germination in watermelon. J. Am. Soc. Hart. Sci. 106(3): 262 - 266.

FUKAKI, S. 1930. Dber die Frage der Beeinflussung des eigenen Fruchtsaftes auf die Samenkeimung. Bull. Sci. Fac. Terku/t. Kjusu Imp. Univ. 4(2): 119-133.

GUTTERMAN, Y. & PORATH, D. 1975. Influences of photoperiodism and light treatments during fruit storage on the phytochrome and on seed germination of Cucumis prophetarum L. and Cucumis sativus L. seeds. Oeco/ogia (Berl.) 18: 37-43 .

KENDRICK, R.E. 1976. Photocontrol of seed germination. Sci. Prog. 63 : 347- 367.

KENDRICK, R.E. & SPRUIT, C .J .P . 1972. Light maintains high levels of phytochrome intermediates . Nature New. Bioi. 237 (78): 281-282.

LERNER, H.R., MAYER, A.M. & EVENARI, M. 1959. The nature of the germination inhibitors present in the dispersal units of Zygophyllum dumosum and Trigonella arabica. Physiol. Plant. 12: 245-250.

LITVINOW, L.S. 1938. On the causes of the inhibiting effect of tomato sap on the germination of tomato seeds. Perm . Bioi. Nauch. !soled. Inst. Bull. II : 163 - 171.

LUCKWILL, L.C. 1952. Growth promoting and growth inhibiting substances in relation to dormancy of apple seeds. J. Hart. Sci. 27: 325 - 330.

MAYER, A.M. & POLJAKOFF-MAYBER, A. 1975. The germina­tion of seeds. 2nd edn . Pergamon Press, New York.

SMITH, H. 1975 . Phytochrome and photomorphogenesis. McGraw­Hill, London .

STEEL, R.G.D. & TORRIE, J .H. 1980. Principles and procedures of statistics. 2nd edn. McGraw-Hill, Johannesburg.