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Seed germination in Citrullus lanatus. Part 2. The involvement of phytochrome and ethylene in controlling light sensitivity
Seed germination in ·citrullus lanatus. Part 2. The involvement of phytochrome and ethylene in controlling light sensitivity F.C. Botha, J.G.C. Small and N. Grobbelaar Margaretha Mes Institute for Seed Research, Department of Botany, University of Pretoria
The inhibitory effect of light on the germination of Citrul/us lanatus seed shows the typical red - far-red photo· reversibility o.t a phytochrome mediated system. During the dark period following an inhibitory far-red light treatment a secondary skotodormancy develops. The inhibiting effect of light on germination can be effectively reversed by ethylene. S. Afr. J. Bot. 1982, ·1: 131-133
Die remmende invloed van lig op die saadkieming van Citrul/us lanatus toon die tipiese rooi - verrooilig· omkeerhaarheid van 'n fitochroom-beheerde sisteem. Gedurende die donkerperiode na 'n remmende verrooi ligbehandeling ontwikkel daar 'n sekondere rustoestand by die sade. Die remmende invloed van lig kan doeltreffend deur etileen opgehef word. S.·Afr. Tydskr. Plantk. 1982, 1: 131 -133
Keywords: Citrul/us lanatus , germination, dormancy , phytochrome , ethylene
Introduction The germination behaviour at 27 oc of the wild form of Citrul/us /anatus is typically negatively photoblastic (Botha eta/. 1982). From work on a cultivated form of C. lanatus (Loy & Evensen 1979) it could be speculated that also in the wild form, phytochrome could be involved in controlling the light sensitivity of the seeds. However, it appeared important to ascertain this experimentally as was done in the work now being reported on . In addition, the effect of ethylene gas on the light sensitivity of the seed was investigated. In previous studies on C. /anatus (Botha eta/. 1982) and on Cucumis anguria (Felippe & Litjens 1979) the compound ' Ethrel' (2-chloroethanephosphonic acid) was found to be most effective in reversing the inhibitory effect of light. In both cases it was speculated that the 'Ethrel' effect could be ascribed to the ethylene which ethrel is known to liberate under the experimental conditions employed. Materials and Methods
F.C . Botha*, J .G .C. Small and N. Grobbelaar Margaretha Mes Institute for Seed Research, Department of Botany, Uni versity o f Pretoria, Pretoria 0002, Republic of South Afri ca *To whom corres pondence should be addressed
Accepted 19 May 1982
Seeds of Citrullus lanatus (Thunb .) Matsumura & Nakai were obtained as described by Botha et a!. (1982). Seeds were air dried at room temperature and stored in air-tight glass containers in the dark at 0- 5 oc. Experiments with ethylene were conducted in 500 cm 3 glass bottles capped with air-tight lids which were fitted with serum septums. Seeds were incubated in these glass bottles on a single 7,0 em diameter sheet of Whatman no. I filter paper which completely lined the base of each bottle. The filter paper was moistened with 2,5 cm 3 distilled water. Ethylene was injected into the flasks through the serum septums to achieve the desired concentration. In all other experiments seeds were germinated in 9 em Petri dishes containing a single 7 ,0-cm sheet of Whatman no. 1 filter paper moistened with 5 cm 3 of test solution. In all cases 30 seeds were used per replicate. Germination tests were conducted at 27 oc in the light and in the dark. White light was obtained from 'cool white' fluorescent tubes which delivered a quantum flux of 16,6 flE em - 2 s- 1 at the position of the seeds. For the demonstration of red - far-red photoreversibility of germination, actinic light was produced as follows (Smith 1975) . Red light was obtained from 'cool white' fluorescent tubes in combination with one layer each of no. 1 and 14 Cinemoid filters which delivered a total quantum flux of 0, 75 flE m - 2 s - 1 at the position of the seeds.
S.-Afr. Tydskr. Plantk., 1982, 1(4)
132 Far-red light was obtained by filtering incandescent light through a 2-cm layer of water held in a perspex tray and one layer each of no. 5a and 20 Cinemoid filters. This delivered a total quantum flux of 0, 95 ~-tE m - 2 s- 1 at the position of the seeds. Germinated seeds were counted under a dim green safe light (Smith 1975). A seed was considered germinated when the radicle had emerged. At least six replicates per treatment were used throughout. Least significant differences (LSD) were calculated by using the Tukey procedure (Nie eta/. 1975).
Table 1 The effect of a light interruption after 22 h of incubation in the dark on the germination of Citrul/us /anatus seeds at 27 °C. (Percentage germination given after a total incubation time of 48 h and 96 h) Treatment for the first 22 hours Dark
between 22 and 25 hours•
48 h
96 h
7 ± 3
12 ± 3
I h Far-red light 2 h Dark
Results and Discussion The germination of Citrullus lanatus seeds is completely inhibited by continuous far-red light (data not shown). To ascertain the stage of germination during which the seeds are most sensitive to light inhibition the seeds were put out to germinate in the dark and exposed to far-red light for 1 h at various times after the onset of the experiment. From the results obtained it is evident that the seeds are most sensitive to far-red light at 27 oc 18 to 22 h after being moistened (Figure 1). Similar results were obtained by Loy & Evensen (1979) with the WB-2 dwarf strain of C. /anatus with continuous far-red light after different initial dark incubation periods.
O?o Germination
Dark
I h Far-red light I h Red light I h Dark
89 ± 2
100 ± 0
Dark
I h Far-red light l h Red light I h Far-red light
8 ± 2
II ± 3
83 ± 4
96 ± 3
Dark
I h Red light 2 h Dark
Dark
I h Red light I h Far-red light I h Dark
19 ± 8
24 ± 5
Dark
Dark
81 ± 4
97 ± 2
±
= Standard -deviation
=
After this treatment seeds were held in constant dark until the termination of the experiment a
100
~
§50
~
.5
E ... Ql
<.!)
0
10 20 30 Time during the incubation period at which the 1h far-red light treatment commenced
Figure 1 The effect of a I h far-red light treatment at different times during dark incubation on germination of Citrullus lanatus seeds at 27 The germination percentage was recorded 96 h after the experiment was started.
oc.
The inhibitory effect of light on the germination of C.
tanatus seeds shows the typical red- far-red photoreversibility of a phytochrome system (Table 1). A single exposure of the seed to 1 h of far-red light after 22 h of dark incubation inhibited germination by more than 8007o (Figure 1 and Table 1). The inhibitory effect of far-red light can be nullified by a red light exposure of 1 h immediately after the far-red light treatment.
From these results and also previous reported results (Botha et at. 1982) it is evident that the reaction of C. /anatus seeds to light is typical of a low-energy phytochrome system (LER) (Kendrick 1976; Smith 1975). Seeds of several negatively photoblastic species will germinate in the dark even after prolonged exposure to continuous or intermittent far-red light (Loy & Evensen 1979; Boisard et at. 1968; Kendrick et at. 1969; Spruit & Mancinelli 1969). A pre-treatment of 48 h continuous far-red light is insufficient to keep the seeds of Cucumis anguria dormant in the dark (Noronha et at. 1978). Some seeds of the WB-2 strain of C. tanatus will germinate in the dark even after a 72 h continuous far-red light treatment. This need for continuous or intermittent far-red light to inhibit germination was initially ascribed to the 'inverse dark reversion' of phytochrome (Boisard et a/. 1968). This phenomenon can also be explained on the basis of a slow transformation of phytochrome intermediates (probably Meta-Rb) to Pr, (Kendrick 1976; Loy & Evensen 1979). The transformation of Meta-Rb to Pr, is the slowest dark reaction in the conversion of P, to Pr, (Kendrick & Spruit 1975). The inhibition of germination in the wild form of C. tanatus after 22 h of dark incubation by a 1 h far-red light treatment (Table 1) indicates that in this species the transformation of Meta-Rb to Pr, is completed in 22 h after the start of imbibition. The obtained results also contradict the idea of 'inverse dark reversion' of phytochrome (Boisard et at. 1968; Spruit & Mancinelli 1969). After about 22 h of dark incubation complete transformation of MetaRb to Prr has probably occurred and the far-red light treatment results in an almost complete shift from Prr to P,. This P, apparently cannot revert to Prr in the succeeding dark period and the seeds therefore remain dormant.
S. Afr. J. Bot., 1982, 1(4)
133 100
e-eWhitelight control
0
o-o Dark control .....-... 4 x 10-7 mol ethylene dm-3 <'>-64 x 106mol ethylene dm- 3 •-•l,lx I0- 5mol ethylene dm- 3 o-o4,5x I0- 5mol ethylene dm-3
·--:
0~ c 50
T
I
l
I
8
~c
I
1~1-I
~/ l
0
·e...
I
-
I LSD PO,OS :
0
I
50
100
Cll
(,!)
~-300
24
Length of dark period (h) between far-red-
56
88
Incubation time (h)
and red light treatments Figure 2 The effect of the length of the dark period between a l h farred and a l h red light treatment on the germination of Citrullus lana/us seeds at 27 oc. The far red light treatment was applied after an initial 22 h dark period. The percentage germination was determined 72 h after the red light treatment.
Figure 3 The effect of different ethylene concentrations on the germ in ation of Cirrul/us lanatus seeds in white li ght at 27 oc (LSD = 11 ,90?o).
References The inhibitory effect of far-red light is reversed by a red light treatment if applied immediately after the exposure to far-red light. It appears that there is a gradual loss in the photoreversibility of germination with an increase in the length of the dark period following the far-red light treatment (Figure 2). When the seeds are left in the dark for 300 h after a far-red light treatment there appears to be a complete loss in photoreversibility. The development of a secondary skotodormancy in positively photoblastic seeds is well documented (Taylorson & Hendricks 1973; I kuma & Thimann 1964). According to Duke eta/. (1977) the loss in light sensitivity is probably due to the changing receptivity or level of a hypothetic component x with which Prr interacts to initiate germination. The inhibitory effect of white light on germination can be partially reversed by GA 3 and kinetin and can be completely reversed by 'Ethrel' (Both a et a/. 1982). It was speculated that the effect of 'Ethrel' might be the result of the ethylene which is released by this compound when absorbed by biological material (Wareing & Phillips 1978). From the results presented in Figure 3 it is evident that ethylene is very effective in reversing the inhibitory effect of white light. The optimal ethylene concentration was found to be 4 X 10 - 6 mol dm - 3 . Although it is well documented that ethylene can stimulate the germination of light-requiring seeds in the absence of light there appears to be a lack of information on the role of ethylene as such on the germination of negatively photo blastic seeds.
Acknowledgements The authors wish to thank the University of Pretoria and the CSIR for grants received for this project.
BOISARD, J., SPRUIT, C.J.P. & ROLLIN , P. 1968. Phytochrome in seeds and an apparent dark reversion of P, to Pr,. Meded. Landbouwhogeschool Wageningen 68: l -5 . BOTHA, F .C., GROBBELAAR, N. & SMALL, J.G.C. 1982. Seed germination in Citrullus lanarus. I. Effect of white light and growth substances on germination. S. Afr. J. Bor. I: I 0 - 13. DUKE, S.O., EGLEY, G.H. & REGER, B.J. 1977. Model for variable light sensitivity in imbibed dark-dormant seeds. Planr Physiol. 59: 244 - 249. FELIPPE, G.M. & LITJENS , J.H.M. 1979. Effect of growth regulators on overcoming the light inhibition on germi nation of Cucumis anguria L. Bioi. Plant. 21: 407 - 411. !K UMA, H. & THIMANN , K.V. 1964. Analysis of germination processes of lettuce seeds by means of temperature and anaerobis. Plant Physiol. 39: 756 -7 67. KENDRICK, R.E. 1976. Photocontrol of seed germination. Sci. Prog. 63: 347 - 367. KENDRICK , R.E., FRANKLAND, B. & SPRUIT, C.J .P. 1969. Phytochrome in seeds of Amaranthus caudatus. Plan/a 88: 293 - 302. LOY, J.B . & EVENSEN, K.B. 1979. Phytochrome regulation of seed germination in a dwarf strain of watermelon. J. Am. Soc. Hort. Sci. 104: 496 - 499. NIE, N.H., HULL, C.H., JENKINS, J.G., STEINBRENNER, K. & BENT, D.H . 1975 . Statistical package for social sciences. 2nd edn, Ch. 21: 422-433. McGraw-Hill, New York . NORONHA, A., VINCENTE, M . & FELIPPE, G.M. 1978. Photocontrol of germination of Cucumis anguria L. Bioi. Plant. 20: 281-286. SMITH, H. 1975. Phytochrome and photomorphogenesis. McGrawHill, London. SPRUIT, C.J.P. & MANCINELLI, A.D. 1969. Phytochrome in cucumber seeds. Plan/a 88: 303 - 310 . TA YLORSON, R.B. & HENDRICKS, S.B. 1973. Phytochrome transformation and action in seeds of Rumex crispus L. during secondary dormancy. Plant Physiol. 52: 475 - 479 . WAREING, P.F. & PHILLIPS, I.D.J. 1978. The control of growth and differentiation in plants, 2nd Edn, Pergamon Press, New York.
The inhibitory effect of light on the germination of Citrul/us lanatus seed shows the typical red - far-red photo· reversibility o.t a phytochrome mediated system. During the dark period following an inhibitory far-red light treatment a secondary skotodormancy develops. The inhibiting effect of light on germination can be effectively reversed by ethylene. S. Afr. J. Bot. 1982, ·1: 131-133
Die remmende invloed van lig op die saadkieming van Citrul/us lanatus toon die tipiese rooi - verrooilig· omkeerhaarheid van 'n fitochroom-beheerde sisteem. Gedurende die donkerperiode na 'n remmende verrooi ligbehandeling ontwikkel daar 'n sekondere rustoestand by die sade. Die remmende invloed van lig kan doeltreffend deur etileen opgehef word. S.·Afr. Tydskr. Plantk. 1982, 1: 131 -133
Keywords: Citrul/us lanatus , germination, dormancy , phytochrome , ethylene
Introduction The germination behaviour at 27 oc of the wild form of Citrul/us /anatus is typically negatively photoblastic (Botha eta/. 1982). From work on a cultivated form of C. lanatus (Loy & Evensen 1979) it could be speculated that also in the wild form, phytochrome could be involved in controlling the light sensitivity of the seeds. However, it appeared important to ascertain this experimentally as was done in the work now being reported on . In addition, the effect of ethylene gas on the light sensitivity of the seed was investigated. In previous studies on C. /anatus (Botha eta/. 1982) and on Cucumis anguria (Felippe & Litjens 1979) the compound ' Ethrel' (2-chloroethanephosphonic acid) was found to be most effective in reversing the inhibitory effect of light. In both cases it was speculated that the 'Ethrel' effect could be ascribed to the ethylene which ethrel is known to liberate under the experimental conditions employed. Materials and Methods
F.C . Botha*, J .G .C. Small and N. Grobbelaar Margaretha Mes Institute for Seed Research, Department of Botany, Uni versity o f Pretoria, Pretoria 0002, Republic of South Afri ca *To whom corres pondence should be addressed
Accepted 19 May 1982
Seeds of Citrullus lanatus (Thunb .) Matsumura & Nakai were obtained as described by Botha et a!. (1982). Seeds were air dried at room temperature and stored in air-tight glass containers in the dark at 0- 5 oc. Experiments with ethylene were conducted in 500 cm 3 glass bottles capped with air-tight lids which were fitted with serum septums. Seeds were incubated in these glass bottles on a single 7,0 em diameter sheet of Whatman no. I filter paper which completely lined the base of each bottle. The filter paper was moistened with 2,5 cm 3 distilled water. Ethylene was injected into the flasks through the serum septums to achieve the desired concentration. In all other experiments seeds were germinated in 9 em Petri dishes containing a single 7 ,0-cm sheet of Whatman no. 1 filter paper moistened with 5 cm 3 of test solution. In all cases 30 seeds were used per replicate. Germination tests were conducted at 27 oc in the light and in the dark. White light was obtained from 'cool white' fluorescent tubes which delivered a quantum flux of 16,6 flE em - 2 s- 1 at the position of the seeds. For the demonstration of red - far-red photoreversibility of germination, actinic light was produced as follows (Smith 1975) . Red light was obtained from 'cool white' fluorescent tubes in combination with one layer each of no. 1 and 14 Cinemoid filters which delivered a total quantum flux of 0, 75 flE m - 2 s - 1 at the position of the seeds.
S.-Afr. Tydskr. Plantk., 1982, 1(4)
132 Far-red light was obtained by filtering incandescent light through a 2-cm layer of water held in a perspex tray and one layer each of no. 5a and 20 Cinemoid filters. This delivered a total quantum flux of 0, 95 ~-tE m - 2 s- 1 at the position of the seeds. Germinated seeds were counted under a dim green safe light (Smith 1975). A seed was considered germinated when the radicle had emerged. At least six replicates per treatment were used throughout. Least significant differences (LSD) were calculated by using the Tukey procedure (Nie eta/. 1975).
Table 1 The effect of a light interruption after 22 h of incubation in the dark on the germination of Citrul/us /anatus seeds at 27 °C. (Percentage germination given after a total incubation time of 48 h and 96 h) Treatment for the first 22 hours Dark
between 22 and 25 hours•
48 h
96 h
7 ± 3
12 ± 3
I h Far-red light 2 h Dark
Results and Discussion The germination of Citrullus lanatus seeds is completely inhibited by continuous far-red light (data not shown). To ascertain the stage of germination during which the seeds are most sensitive to light inhibition the seeds were put out to germinate in the dark and exposed to far-red light for 1 h at various times after the onset of the experiment. From the results obtained it is evident that the seeds are most sensitive to far-red light at 27 oc 18 to 22 h after being moistened (Figure 1). Similar results were obtained by Loy & Evensen (1979) with the WB-2 dwarf strain of C. /anatus with continuous far-red light after different initial dark incubation periods.
O?o Germination
Dark
I h Far-red light I h Red light I h Dark
89 ± 2
100 ± 0
Dark
I h Far-red light l h Red light I h Far-red light
8 ± 2
II ± 3
83 ± 4
96 ± 3
Dark
I h Red light 2 h Dark
Dark
I h Red light I h Far-red light I h Dark
19 ± 8
24 ± 5
Dark
Dark
81 ± 4
97 ± 2
±
= Standard -deviation
=
After this treatment seeds were held in constant dark until the termination of the experiment a
100
~
§50
~
.5
E ... Ql
<.!)
0
10 20 30 Time during the incubation period at which the 1h far-red light treatment commenced
Figure 1 The effect of a I h far-red light treatment at different times during dark incubation on germination of Citrullus lanatus seeds at 27 The germination percentage was recorded 96 h after the experiment was started.
oc.
The inhibitory effect of light on the germination of C.
tanatus seeds shows the typical red- far-red photoreversibility of a phytochrome system (Table 1). A single exposure of the seed to 1 h of far-red light after 22 h of dark incubation inhibited germination by more than 8007o (Figure 1 and Table 1). The inhibitory effect of far-red light can be nullified by a red light exposure of 1 h immediately after the far-red light treatment.
From these results and also previous reported results (Botha et at. 1982) it is evident that the reaction of C. /anatus seeds to light is typical of a low-energy phytochrome system (LER) (Kendrick 1976; Smith 1975). Seeds of several negatively photoblastic species will germinate in the dark even after prolonged exposure to continuous or intermittent far-red light (Loy & Evensen 1979; Boisard et at. 1968; Kendrick et at. 1969; Spruit & Mancinelli 1969). A pre-treatment of 48 h continuous far-red light is insufficient to keep the seeds of Cucumis anguria dormant in the dark (Noronha et at. 1978). Some seeds of the WB-2 strain of C. tanatus will germinate in the dark even after a 72 h continuous far-red light treatment. This need for continuous or intermittent far-red light to inhibit germination was initially ascribed to the 'inverse dark reversion' of phytochrome (Boisard et a/. 1968). This phenomenon can also be explained on the basis of a slow transformation of phytochrome intermediates (probably Meta-Rb) to Pr, (Kendrick 1976; Loy & Evensen 1979). The transformation of Meta-Rb to Pr, is the slowest dark reaction in the conversion of P, to Pr, (Kendrick & Spruit 1975). The inhibition of germination in the wild form of C. tanatus after 22 h of dark incubation by a 1 h far-red light treatment (Table 1) indicates that in this species the transformation of Meta-Rb to Pr, is completed in 22 h after the start of imbibition. The obtained results also contradict the idea of 'inverse dark reversion' of phytochrome (Boisard et at. 1968; Spruit & Mancinelli 1969). After about 22 h of dark incubation complete transformation of MetaRb to Prr has probably occurred and the far-red light treatment results in an almost complete shift from Prr to P,. This P, apparently cannot revert to Prr in the succeeding dark period and the seeds therefore remain dormant.
S. Afr. J. Bot., 1982, 1(4)
133 100
e-eWhitelight control
0
o-o Dark control .....-... 4 x 10-7 mol ethylene dm-3 <'>-64 x 106mol ethylene dm- 3 •-•l,lx I0- 5mol ethylene dm- 3 o-o4,5x I0- 5mol ethylene dm-3
·--:
0~ c 50
T
I
l
I
8
~c
I
1~1-I
~/ l
0
·e...
I
-
I LSD PO,OS :
0
I
50
100
Cll
(,!)
~-300
24
Length of dark period (h) between far-red-
56
88
Incubation time (h)
and red light treatments Figure 2 The effect of the length of the dark period between a l h farred and a l h red light treatment on the germination of Citrullus lana/us seeds at 27 oc. The far red light treatment was applied after an initial 22 h dark period. The percentage germination was determined 72 h after the red light treatment.
Figure 3 The effect of different ethylene concentrations on the germ in ation of Cirrul/us lanatus seeds in white li ght at 27 oc (LSD = 11 ,90?o).
References The inhibitory effect of far-red light is reversed by a red light treatment if applied immediately after the exposure to far-red light. It appears that there is a gradual loss in the photoreversibility of germination with an increase in the length of the dark period following the far-red light treatment (Figure 2). When the seeds are left in the dark for 300 h after a far-red light treatment there appears to be a complete loss in photoreversibility. The development of a secondary skotodormancy in positively photoblastic seeds is well documented (Taylorson & Hendricks 1973; I kuma & Thimann 1964). According to Duke eta/. (1977) the loss in light sensitivity is probably due to the changing receptivity or level of a hypothetic component x with which Prr interacts to initiate germination. The inhibitory effect of white light on germination can be partially reversed by GA 3 and kinetin and can be completely reversed by 'Ethrel' (Both a et a/. 1982). It was speculated that the effect of 'Ethrel' might be the result of the ethylene which is released by this compound when absorbed by biological material (Wareing & Phillips 1978). From the results presented in Figure 3 it is evident that ethylene is very effective in reversing the inhibitory effect of white light. The optimal ethylene concentration was found to be 4 X 10 - 6 mol dm - 3 . Although it is well documented that ethylene can stimulate the germination of light-requiring seeds in the absence of light there appears to be a lack of information on the role of ethylene as such on the germination of negatively photo blastic seeds.
Acknowledgements The authors wish to thank the University of Pretoria and the CSIR for grants received for this project.
BOISARD, J., SPRUIT, C.J.P. & ROLLIN , P. 1968. Phytochrome in seeds and an apparent dark reversion of P, to Pr,. Meded. Landbouwhogeschool Wageningen 68: l -5 . BOTHA, F .C., GROBBELAAR, N. & SMALL, J.G.C. 1982. Seed germination in Citrullus lanarus. I. Effect of white light and growth substances on germination. S. Afr. J. Bor. I: I 0 - 13. DUKE, S.O., EGLEY, G.H. & REGER, B.J. 1977. Model for variable light sensitivity in imbibed dark-dormant seeds. Planr Physiol. 59: 244 - 249. FELIPPE, G.M. & LITJENS , J.H.M. 1979. Effect of growth regulators on overcoming the light inhibition on germi nation of Cucumis anguria L. Bioi. Plant. 21: 407 - 411. !K UMA, H. & THIMANN , K.V. 1964. Analysis of germination processes of lettuce seeds by means of temperature and anaerobis. Plant Physiol. 39: 756 -7 67. KENDRICK, R.E. 1976. Photocontrol of seed germination. Sci. Prog. 63: 347 - 367. KENDRICK , R.E., FRANKLAND, B. & SPRUIT, C.J .P. 1969. Phytochrome in seeds of Amaranthus caudatus. Plan/a 88: 293 - 302. LOY, J.B . & EVENSEN, K.B. 1979. Phytochrome regulation of seed germination in a dwarf strain of watermelon. J. Am. Soc. Hort. Sci. 104: 496 - 499. NIE, N.H., HULL, C.H., JENKINS, J.G., STEINBRENNER, K. & BENT, D.H . 1975 . Statistical package for social sciences. 2nd edn, Ch. 21: 422-433. McGraw-Hill, New York . NORONHA, A., VINCENTE, M . & FELIPPE, G.M. 1978. Photocontrol of germination of Cucumis anguria L. Bioi. Plant. 20: 281-286. SMITH, H. 1975. Phytochrome and photomorphogenesis. McGrawHill, London. SPRUIT, C.J.P. & MANCINELLI, A.D. 1969. Phytochrome in cucumber seeds. Plan/a 88: 303 - 310 . TA YLORSON, R.B. & HENDRICKS, S.B. 1973. Phytochrome transformation and action in seeds of Rumex crispus L. during secondary dormancy. Plant Physiol. 52: 475 - 479 . WAREING, P.F. & PHILLIPS, I.D.J. 1978. The control of growth and differentiation in plants, 2nd Edn, Pergamon Press, New York.