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Germination and Root Gravitropism of flacca, the tomato Mutant Deficient in Abscisic Acid
Short Communication
Germination and Root Gravitropism of flacca, the tomato Mutant Deficient in Abscisic Acid JONATHAN
D. B.
WEYERS
Department of Biological Sciences, University of Dundee, Dundee DDl 4HN, Tayside U.K. Received April 11, 1985· Accepted June 18, 1985
Summary Seeds offiacca, the wilty tomato mutant deficient in abscisic acid (ABA), had a shorter mean germination time than those of the control cultivar, Rheinlands Ruhm. Applied ABA delayed germination of both control and mutant seeds, the latter appearing to be more sensitive to the hormone. Transfer from ABA to water allowed germination levels to reach those of controls within two or three days. It is speculated that low endogenous ABA levels in fiacca seeds might result in the observed precocious germination. Viviparous germination was observed within ripeflacca fruit which may also have resulted from the hormone deficiency. Flacca seedlings exhibited normal gravitropic behaviour. This finding is discussed in regard to current theories of hormonal control of gravitropism.
Key words: L ycopersicon esculentum, tomato, Solanaceae, jlacca, abscisic acid, geotropism, gravi· tropism, germination.
Introduction
Tal (1966) described the tomato mutant jlaeea (fie) which Stubbe obtained from the cultivar Rheinlands Ruhm (RR). This single-gene mutant had a permanently wilty appearance which Tal showed to be due to high rates of transpiration caused by stomata which never closed. Subsequent investigations by Tal and his coworkers into the reasons for this excessive stomatal opening revealed an inability of the mutant to synthesize the plant hormone abscisic acid (ABA). Although the early bioassay estimates ofjle hormone levels must be considered unreliable, Tal and Nevo (1973) confirmed that ABA levels injlc were roughly 17% of RR by measuring with physico-chemical methods. Nevo and Tal (1973) demonstrated by metabolic studies that jle had a low rate of ABA synthesis. The genetic lesion was assumed to be in the anabolic pathway between farnesyl pyrophosphate and ABA. Furthermore, lack of the
Abbreviations: ABA: abscisic acid; fie: fiacca; G so: mean germination time; RR: Rheinlands Ruhm.
J. Plant Physiol.
Vol. 121. pp. 475 -480 (1985)
476
JONATHAN
D. B.
WEYERS
hormone led to a number of pleiotropic effects, and foliar sprays of ABA caused phenotypic reversion for the majority of these (Imber and Tal, 1970). Abscisic acid has been implicated in the control of plant water relations (Walton, 1980) and research on fie has concentrated on this aspect, the mutant syndrome being widely cited in support of the hypothesis that ABA is involved in stress-induced stomatal closure. However, the hormone has also been postulated to have a role in seed dormancy and root gravitropism, amongst other phenomena (Walton, 1980). The availability of an ABA-deficient mutant would appear to be an excellent opportunity to investigate such possibilities. During our studies on the water relations of fie, it was observed that mutant seeds consistently germinated before those of RR and that the seedlings were normal with respect to gravitropic behaviour. This communication reports and quantifies these observations. Materials and Methods Seeds of Lycopersicon esculentum Mill. cv Rheinlands Ruhm and its mutant flacca were supplied by Dr. M. Tal, Ben Gurion University of the Negev, Beer-Sheva, Israel. Plants grown at Dundee from this source were used to provide seeds for the experiments reported here. The plants were grown in Levington's Universal compost (Fisons Ltd, Ipswich, U.K.), kept well watered, and allowed to self-pollinate (Rick, 1978). Fruits were harvested at the «red ripe» stage (Rick, 1978) and the (unwashed) seeds dried on absorbent paper. Thereafter, the seeds were stored before use in air-dry conditions at room temperature (20-25 °C). Germination experiments were carried out in 50 mm diameter plastic Petri dishes on filter paper discs (Whatman No.1), each dish having a filter paper bridge to a reservoir of distilled water. 20 seeds were placed in each dish and there were 6 dishes per treatment. The dishes were placed in randomised order under white fluorescent lamps (16h day, PAR = ca. 7W m- 2) in an incubator at 24±2°C. If used, (±)-ABA (Sigma Ltd, Poole, U.K.) was added to dishes as 2x 10- 6 m 3 of 1O- 2 molm- 3 solution. The criterion for germination was radicle emergence from the testa, and the mean time for germination (G50) was determined from graphs of cumulative germination as the time at which 50 % of the total germination in controls had occurred. The gravitropism experiment was carried out on seedlings germinated as above with straight radicles of 5 -15 mm in length. 33 seedlings per genotype were placed on the surface of 1 % purified agar (Oxoid Ltd, Basingstoke, U.K.; type L28) in 90 mm diameter plastic Petri dishes. The dishes were then placed in a vertical position such that the radicle was initially horizontal, in light at 24±2 °C as above.
Results
The difference in G 50 between fie and RR seed was 36 h (Figs. 1 a and b). In three replicate experiments the mean germination time for fie was 186 h and for RR 225 h (t = 6.5, P<0.01). (±)-ABA at 1O- 2 molm- 3 retarded germination in both fie and RR, the mutant being more sensitive to the compound: the fie G 50 was delayed 173 h by ABA treatment (Fig. 1 a), whereas the RR G 50 was delayed by 55 h (Fig. 1 b). Transfer of ABA-treated seed to water after washing accelerated germination, which reached levels for controls within 48 -72 h (Figs. 1 a and b).
J Plant Physiol. Vol. 121. pp. 475-480 (1985)
Flacca germination and gravitropism
477
100
50
0
0
. ,!! 0
c
0
0
:;:; 0
c
'E...
100
~
50
o
o
5
10
15
time/days Fig. 1: Germination of jlacca and Rheinlands Ruhm seeds. Graph 1 a: flacca, 1 b: Rheinlands Ruhm. Open circles are water controls, closed circles ABA treatment. Closed triangles and dashed lines show germination of ABA-treated seed which was washed and then incubated in water after the point marked with an upward arrow. The downward pointing dashed arrows represent the G so times.
A further observation regarding fruit of the mutant and control plants was that viviparous seed germination was often seen in ripe fie fruit, but never in those of RR. It was found that fie seedlings in the latter stages of the above experiments exhibited normal gravitropic behaviour in both root and shoot growth, as did mature plants. This was further tested by placing seedlings on agar plates and observing the growth pattern which developed when the plates were placed vertically. Little difference was found in the positive orthogravitropic behaviour of the radicle in mutant and control seedlings. After 24 h of 1 g gravistimulation the mean downward root growth direction was 55° from horizontal infie and 61 ° in RR (t = 1.81, dJ. = 64, NS). After 48 h this angle was 72° for fie and 76° for RR (t = 0.72, dJ. = 64, NS). No significant difference in the angular growth of the genotypes was found in several similar replicate experiments or when the experiment was carried out in the dark. The hypocotyl of both types of seedlings showed normal negative orthogravitropism.
J. Plant Physiol.
Vol. 121. pp. 475-480 (1985)
478
JONATHAN
D. B.
WEYERS
Discussion The use of single-gene mutants has great potential in the study of plant hormones. Plant tissues lack the localised centres of hormone production that animals have, so «excision» of the hormone supply (sensu Jacobs, 1959) is greatly facilitated by mutants with lesions in hormone anabolism pathways. Observations of mutant physiology can thus be used in application of such criteria as the «PESIGS» rules Gacobs, 1959} to examine the possibility of chemical control of physiological processes. Apart from studies on fiaeea, examples of useful contributions to the literature include the study of gibberellin-deficient dwarf mutants of maiz~ and rice and the diagravitropic ethene requiring tomato mutant diageotropiea Gones, 1973; Zobel, 1973}. Though problems regarding pleiotropy and hormone localisation undoubtedly exist, the potential of many of the mutants has not been fully realised and it should be borne in mind that negative findings with mutants may be as valuable as descriptions of the primary effects of genetic lesions. In this study, two phenomena related to the physiology offiaeea seed and seedlings have been investigated. Firstly, differences are described between the germination rates of the ABA-deficient mutant and those of Rheinlands Ruhm, the control cultivar, which support the contention that ABA is involved in the control of tomato seed germination (Fig.1}.1t was not clear, however, to what extent the precocious fie germination was due to faster imbibition, accelerated cell division, or more rapid radicle elongation. As with the phenotypic reversion caused by the compound for other features of fiaeea's physiology, application of ABA retarded germination of the mutant seed. At the concentration of ABA applied, the fie G so time was increased well beyond that of the RR control, so fie appeared to be more sensitive to the hormone than RR. Another possibility is that the uptake of ABA was different in the two genotypes. The ABA effect was easily reversible, in the sense that removal of ABA resulted in very rapid germination, indicating that some process had been delayed or suspended by the hormone. Such a phenomenon has been reported for tomato after hydration/ dehydration treatments to seeds (Berrie and Drennan, 1971). Correlations between germination rates and inhibitors in seeds have been found in a number of studies (Walton, 1980), e.g. in precocious maize mutants (Brenner et aI., 1977) and during precocious French Bean germination (Morris, 1978), and the hypothesis that the effects noted here arose from differences in the ABA levels of fie and RR seed appears worthy of investigation. It is generally accepted that inhibitors produced by the maternal tissues of many fruits prevent viviparous germination during ripening (Mayer and Poljakoff-Mayber, 1982). On the other hand, vivipary is observed naturally in certain species (e.g. mango) and will occur in many fruits after harvest (Gindel, 1960). The extensive literature on ABA and seed development, dormancy and germination (Milborrow, 1974; Walton, 1980; Mayer and Poljakoff-Mayber, 1982) includes a long-running
J. Plant Physiol.
Vol. 121. pp. 475 -480 (1985)
Flacca germination and gravitropism
479
controversy as to the role of pH and solute potential relative to hormonal influences in maternal inhibition of tomato seed germination (K6ckemann, 1934; Konis, 1940; Evanari, 1949). Each may playa part in the phenomenon (D6rffling, 1970). In this study we report that seed germinated inside «red ripe» fic fruit, a phenomenon never observed in similar RR fruits. Cultivar differences in viviparous germination of tomato have previously been described, and they may have arisen from differences in the levels of endogenous inhibitors (Dos Santos and Yamaguchi, 1979). Furthermore, the droopy mutant of potato also exhibits pronounced viviparity (Simmonds, 1966). This mutant syndrome, like that of fiacca, is associated with lack of ABA (Quarrie, 1982). It thus seems that an endogenous inhibitor, possibly ABA, can influence germination within fruits of members of the Solanaceae. In the second physiological area studied, no significant differences in fic and RR root gravitropism were found, as might be expected if ABA were intimately involved in this response. This result adds to the controversy regarding the identity of the inhibitor produced in the root cap which controls root gravitropism. There is evidence both for and against the theory that this inhibitor is ABA aackson and Barlow, 1981; Wilkins, 1984). The data presented here are essentially similar to those of Moore and Smith (1984), who showed that Zea seedlings treated with fluridone, an inhibitor of ABA synthesis, exhibited normal gravitropic behaviour. In marked contrast to these results showing a lack of effect of removal of the source of ABA, the ethene-deficient mutant diageotropica does exhibit abnormal gravitropic responses (Zobel, 1973). Although it is tempting to make firm conclusions about ABA involvement in seed dormancy and gravitropism from the above, caution is necessary with any speculations. The prevailing ABA level at the relevant sites in mutant tissues is critical, and even though Tal and Nevo's (1973) measurements indicate that there is likely to be only one major pathway for ABA synthesis in tomato, the fact that ABA production is low in the mutant does not mean that levels of the hormone at a particular site of action are necessarily low: transport and accumulation of the compound might occur. Also, the relative ABA level at different sites might be more important than absolute concentrations. It is clear that painstaking measurement of endogenous ABA levels in fic and RR tissues is required, and the observations reported here give clear indications as to the direction of this future research. Another problem concerns pleiotropic effects and the possible carry-over of mutant effects between generations. Thus, maternal preconditioning inside fic fruit may influence seed germination properties (d. Walton, 1980, on stress/ABA effects). However, pleiotropic effects might not be expected to be significant in studies of very young seedlings. These complications notwithstanding, it can be stated that the evidence presented here supports the possibility of ABA involvement in seed dormancy in tomato, but runs contrary to certain theories regarding the control of gravitropism.
J Plant Physiol.
Vol. 121. pp. 475-480 (1985)
480
JONATHAN D. B. WEYERS
Acknowledgements I thank Moshe Tal for the kind gift of jlacca and RR seed, C.S. Baxter, A. Berkley, S. Bowra, and A. Hardy for observations supporting this work, Dr. A. M. M. Berrie for helpful discussions, J. johnston for technical assistance, and J. R. Weyers for materials.
References BERRIE, A. M. M. and D. S. H. DRENNAN: New Phyto!', 70, 135-142 (1971). BRENNER, M. L., BURR, B., and F. BURR: Plant Physio!. Supps., 59, 76 (1977). DORFFLING, K.: Planta, 93, 233-242 (1970). Dos SANTOS, D. and M. YAMAGUCHI: Sci. Hortic., 11, 131-140 (1979). EVANARI, M.: Bot. Rev., 15, 153 -194 (1949). GINDEL, I.: Nature, 187, 42-44 (1960). IMBER, D. and M. TAL: Science, 169, 592-593 (1970). JACKSON, M. B. and P. W. BARLOW: Plant, Cell & Env., 4, 107 -123 (1981). JACOBS, W. P.: Dev. Bio!., 1, 527-533 (1959). JONES, R. L.: Ann. Rev. Plant Physio!., 24, 571-598 (1973). KOCKEMANN, A.: Ber. Deut. Bot. Ges., 52, 523-526 (1934). KONIS, E.: Pa!' j. Bot., 2, 6-27 (1940). MAYER, A. M. and A. POLJAKOFF-MAYBER: The Germination of Seeds, 3rd Ed. Pergamon Press, Oxford, U.K. (1982). MILBORROW, B. V.: Ann. Rev. Plant Physio!. 25, 259-307 (1974). MOORE, R. and J. D. SMITH: Planta, 162, 342-344 (1984). MORRIS, D. A.: Z. Pflanzenphysio!., 86, 433 -441 (1978). QUARRIE, S. A.: Plant, Cell & Env., 5, 23-26 (1982). RICK, C. M.: Sci. Amer., 239,76-87 (1978). SIMMONDS, N. W.: Heredity, 21, 473-479 (1966). TAL, M.: Plant Physio!', 41, 1387 (1966). TAL, M. and Y. NEVO: Biochem. Genet., 8, 291-299 (1973). WALTON, D. Ann. Rev. Plant Physio!., 31,453-489 (1980). WILKINS, M. B.: Gravitropism. In: WILKINS, M. B. (ed.): Advanced Plant Physiology, pp. 163165, Pitman, London, U.K. (1984). ZOBEL, R. W.: Plant Physio!., 52, 385-389 (1973).
c.:
J. Plant Physiol.
Vol. 121. pp. 475 -480 (1985)
Germination and Root Gravitropism of flacca, the tomato Mutant Deficient in Abscisic Acid JONATHAN
D. B.
WEYERS
Department of Biological Sciences, University of Dundee, Dundee DDl 4HN, Tayside U.K. Received April 11, 1985· Accepted June 18, 1985
Summary Seeds offiacca, the wilty tomato mutant deficient in abscisic acid (ABA), had a shorter mean germination time than those of the control cultivar, Rheinlands Ruhm. Applied ABA delayed germination of both control and mutant seeds, the latter appearing to be more sensitive to the hormone. Transfer from ABA to water allowed germination levels to reach those of controls within two or three days. It is speculated that low endogenous ABA levels in fiacca seeds might result in the observed precocious germination. Viviparous germination was observed within ripeflacca fruit which may also have resulted from the hormone deficiency. Flacca seedlings exhibited normal gravitropic behaviour. This finding is discussed in regard to current theories of hormonal control of gravitropism.
Key words: L ycopersicon esculentum, tomato, Solanaceae, jlacca, abscisic acid, geotropism, gravi· tropism, germination.
Introduction
Tal (1966) described the tomato mutant jlaeea (fie) which Stubbe obtained from the cultivar Rheinlands Ruhm (RR). This single-gene mutant had a permanently wilty appearance which Tal showed to be due to high rates of transpiration caused by stomata which never closed. Subsequent investigations by Tal and his coworkers into the reasons for this excessive stomatal opening revealed an inability of the mutant to synthesize the plant hormone abscisic acid (ABA). Although the early bioassay estimates ofjle hormone levels must be considered unreliable, Tal and Nevo (1973) confirmed that ABA levels injlc were roughly 17% of RR by measuring with physico-chemical methods. Nevo and Tal (1973) demonstrated by metabolic studies that jle had a low rate of ABA synthesis. The genetic lesion was assumed to be in the anabolic pathway between farnesyl pyrophosphate and ABA. Furthermore, lack of the
Abbreviations: ABA: abscisic acid; fie: fiacca; G so: mean germination time; RR: Rheinlands Ruhm.
J. Plant Physiol.
Vol. 121. pp. 475 -480 (1985)
476
JONATHAN
D. B.
WEYERS
hormone led to a number of pleiotropic effects, and foliar sprays of ABA caused phenotypic reversion for the majority of these (Imber and Tal, 1970). Abscisic acid has been implicated in the control of plant water relations (Walton, 1980) and research on fie has concentrated on this aspect, the mutant syndrome being widely cited in support of the hypothesis that ABA is involved in stress-induced stomatal closure. However, the hormone has also been postulated to have a role in seed dormancy and root gravitropism, amongst other phenomena (Walton, 1980). The availability of an ABA-deficient mutant would appear to be an excellent opportunity to investigate such possibilities. During our studies on the water relations of fie, it was observed that mutant seeds consistently germinated before those of RR and that the seedlings were normal with respect to gravitropic behaviour. This communication reports and quantifies these observations. Materials and Methods Seeds of Lycopersicon esculentum Mill. cv Rheinlands Ruhm and its mutant flacca were supplied by Dr. M. Tal, Ben Gurion University of the Negev, Beer-Sheva, Israel. Plants grown at Dundee from this source were used to provide seeds for the experiments reported here. The plants were grown in Levington's Universal compost (Fisons Ltd, Ipswich, U.K.), kept well watered, and allowed to self-pollinate (Rick, 1978). Fruits were harvested at the «red ripe» stage (Rick, 1978) and the (unwashed) seeds dried on absorbent paper. Thereafter, the seeds were stored before use in air-dry conditions at room temperature (20-25 °C). Germination experiments were carried out in 50 mm diameter plastic Petri dishes on filter paper discs (Whatman No.1), each dish having a filter paper bridge to a reservoir of distilled water. 20 seeds were placed in each dish and there were 6 dishes per treatment. The dishes were placed in randomised order under white fluorescent lamps (16h day, PAR = ca. 7W m- 2) in an incubator at 24±2°C. If used, (±)-ABA (Sigma Ltd, Poole, U.K.) was added to dishes as 2x 10- 6 m 3 of 1O- 2 molm- 3 solution. The criterion for germination was radicle emergence from the testa, and the mean time for germination (G50) was determined from graphs of cumulative germination as the time at which 50 % of the total germination in controls had occurred. The gravitropism experiment was carried out on seedlings germinated as above with straight radicles of 5 -15 mm in length. 33 seedlings per genotype were placed on the surface of 1 % purified agar (Oxoid Ltd, Basingstoke, U.K.; type L28) in 90 mm diameter plastic Petri dishes. The dishes were then placed in a vertical position such that the radicle was initially horizontal, in light at 24±2 °C as above.
Results
The difference in G 50 between fie and RR seed was 36 h (Figs. 1 a and b). In three replicate experiments the mean germination time for fie was 186 h and for RR 225 h (t = 6.5, P<0.01). (±)-ABA at 1O- 2 molm- 3 retarded germination in both fie and RR, the mutant being more sensitive to the compound: the fie G 50 was delayed 173 h by ABA treatment (Fig. 1 a), whereas the RR G 50 was delayed by 55 h (Fig. 1 b). Transfer of ABA-treated seed to water after washing accelerated germination, which reached levels for controls within 48 -72 h (Figs. 1 a and b).
J Plant Physiol. Vol. 121. pp. 475-480 (1985)
Flacca germination and gravitropism
477
100
50
0
0
. ,!! 0
c
0
0
:;:; 0
c
'E...
100
~
50
o
o
5
10
15
time/days Fig. 1: Germination of jlacca and Rheinlands Ruhm seeds. Graph 1 a: flacca, 1 b: Rheinlands Ruhm. Open circles are water controls, closed circles ABA treatment. Closed triangles and dashed lines show germination of ABA-treated seed which was washed and then incubated in water after the point marked with an upward arrow. The downward pointing dashed arrows represent the G so times.
A further observation regarding fruit of the mutant and control plants was that viviparous seed germination was often seen in ripe fie fruit, but never in those of RR. It was found that fie seedlings in the latter stages of the above experiments exhibited normal gravitropic behaviour in both root and shoot growth, as did mature plants. This was further tested by placing seedlings on agar plates and observing the growth pattern which developed when the plates were placed vertically. Little difference was found in the positive orthogravitropic behaviour of the radicle in mutant and control seedlings. After 24 h of 1 g gravistimulation the mean downward root growth direction was 55° from horizontal infie and 61 ° in RR (t = 1.81, dJ. = 64, NS). After 48 h this angle was 72° for fie and 76° for RR (t = 0.72, dJ. = 64, NS). No significant difference in the angular growth of the genotypes was found in several similar replicate experiments or when the experiment was carried out in the dark. The hypocotyl of both types of seedlings showed normal negative orthogravitropism.
J. Plant Physiol.
Vol. 121. pp. 475-480 (1985)
478
JONATHAN
D. B.
WEYERS
Discussion The use of single-gene mutants has great potential in the study of plant hormones. Plant tissues lack the localised centres of hormone production that animals have, so «excision» of the hormone supply (sensu Jacobs, 1959) is greatly facilitated by mutants with lesions in hormone anabolism pathways. Observations of mutant physiology can thus be used in application of such criteria as the «PESIGS» rules Gacobs, 1959} to examine the possibility of chemical control of physiological processes. Apart from studies on fiaeea, examples of useful contributions to the literature include the study of gibberellin-deficient dwarf mutants of maiz~ and rice and the diagravitropic ethene requiring tomato mutant diageotropiea Gones, 1973; Zobel, 1973}. Though problems regarding pleiotropy and hormone localisation undoubtedly exist, the potential of many of the mutants has not been fully realised and it should be borne in mind that negative findings with mutants may be as valuable as descriptions of the primary effects of genetic lesions. In this study, two phenomena related to the physiology offiaeea seed and seedlings have been investigated. Firstly, differences are described between the germination rates of the ABA-deficient mutant and those of Rheinlands Ruhm, the control cultivar, which support the contention that ABA is involved in the control of tomato seed germination (Fig.1}.1t was not clear, however, to what extent the precocious fie germination was due to faster imbibition, accelerated cell division, or more rapid radicle elongation. As with the phenotypic reversion caused by the compound for other features of fiaeea's physiology, application of ABA retarded germination of the mutant seed. At the concentration of ABA applied, the fie G so time was increased well beyond that of the RR control, so fie appeared to be more sensitive to the hormone than RR. Another possibility is that the uptake of ABA was different in the two genotypes. The ABA effect was easily reversible, in the sense that removal of ABA resulted in very rapid germination, indicating that some process had been delayed or suspended by the hormone. Such a phenomenon has been reported for tomato after hydration/ dehydration treatments to seeds (Berrie and Drennan, 1971). Correlations between germination rates and inhibitors in seeds have been found in a number of studies (Walton, 1980), e.g. in precocious maize mutants (Brenner et aI., 1977) and during precocious French Bean germination (Morris, 1978), and the hypothesis that the effects noted here arose from differences in the ABA levels of fie and RR seed appears worthy of investigation. It is generally accepted that inhibitors produced by the maternal tissues of many fruits prevent viviparous germination during ripening (Mayer and Poljakoff-Mayber, 1982). On the other hand, vivipary is observed naturally in certain species (e.g. mango) and will occur in many fruits after harvest (Gindel, 1960). The extensive literature on ABA and seed development, dormancy and germination (Milborrow, 1974; Walton, 1980; Mayer and Poljakoff-Mayber, 1982) includes a long-running
J. Plant Physiol.
Vol. 121. pp. 475 -480 (1985)
Flacca germination and gravitropism
479
controversy as to the role of pH and solute potential relative to hormonal influences in maternal inhibition of tomato seed germination (K6ckemann, 1934; Konis, 1940; Evanari, 1949). Each may playa part in the phenomenon (D6rffling, 1970). In this study we report that seed germinated inside «red ripe» fic fruit, a phenomenon never observed in similar RR fruits. Cultivar differences in viviparous germination of tomato have previously been described, and they may have arisen from differences in the levels of endogenous inhibitors (Dos Santos and Yamaguchi, 1979). Furthermore, the droopy mutant of potato also exhibits pronounced viviparity (Simmonds, 1966). This mutant syndrome, like that of fiacca, is associated with lack of ABA (Quarrie, 1982). It thus seems that an endogenous inhibitor, possibly ABA, can influence germination within fruits of members of the Solanaceae. In the second physiological area studied, no significant differences in fic and RR root gravitropism were found, as might be expected if ABA were intimately involved in this response. This result adds to the controversy regarding the identity of the inhibitor produced in the root cap which controls root gravitropism. There is evidence both for and against the theory that this inhibitor is ABA aackson and Barlow, 1981; Wilkins, 1984). The data presented here are essentially similar to those of Moore and Smith (1984), who showed that Zea seedlings treated with fluridone, an inhibitor of ABA synthesis, exhibited normal gravitropic behaviour. In marked contrast to these results showing a lack of effect of removal of the source of ABA, the ethene-deficient mutant diageotropica does exhibit abnormal gravitropic responses (Zobel, 1973). Although it is tempting to make firm conclusions about ABA involvement in seed dormancy and gravitropism from the above, caution is necessary with any speculations. The prevailing ABA level at the relevant sites in mutant tissues is critical, and even though Tal and Nevo's (1973) measurements indicate that there is likely to be only one major pathway for ABA synthesis in tomato, the fact that ABA production is low in the mutant does not mean that levels of the hormone at a particular site of action are necessarily low: transport and accumulation of the compound might occur. Also, the relative ABA level at different sites might be more important than absolute concentrations. It is clear that painstaking measurement of endogenous ABA levels in fic and RR tissues is required, and the observations reported here give clear indications as to the direction of this future research. Another problem concerns pleiotropic effects and the possible carry-over of mutant effects between generations. Thus, maternal preconditioning inside fic fruit may influence seed germination properties (d. Walton, 1980, on stress/ABA effects). However, pleiotropic effects might not be expected to be significant in studies of very young seedlings. These complications notwithstanding, it can be stated that the evidence presented here supports the possibility of ABA involvement in seed dormancy in tomato, but runs contrary to certain theories regarding the control of gravitropism.
J Plant Physiol.
Vol. 121. pp. 475-480 (1985)
480
JONATHAN D. B. WEYERS
Acknowledgements I thank Moshe Tal for the kind gift of jlacca and RR seed, C.S. Baxter, A. Berkley, S. Bowra, and A. Hardy for observations supporting this work, Dr. A. M. M. Berrie for helpful discussions, J. johnston for technical assistance, and J. R. Weyers for materials.
References BERRIE, A. M. M. and D. S. H. DRENNAN: New Phyto!', 70, 135-142 (1971). BRENNER, M. L., BURR, B., and F. BURR: Plant Physio!. Supps., 59, 76 (1977). DORFFLING, K.: Planta, 93, 233-242 (1970). Dos SANTOS, D. and M. YAMAGUCHI: Sci. Hortic., 11, 131-140 (1979). EVANARI, M.: Bot. Rev., 15, 153 -194 (1949). GINDEL, I.: Nature, 187, 42-44 (1960). IMBER, D. and M. TAL: Science, 169, 592-593 (1970). JACKSON, M. B. and P. W. BARLOW: Plant, Cell & Env., 4, 107 -123 (1981). JACOBS, W. P.: Dev. Bio!., 1, 527-533 (1959). JONES, R. L.: Ann. Rev. Plant Physio!., 24, 571-598 (1973). KOCKEMANN, A.: Ber. Deut. Bot. Ges., 52, 523-526 (1934). KONIS, E.: Pa!' j. Bot., 2, 6-27 (1940). MAYER, A. M. and A. POLJAKOFF-MAYBER: The Germination of Seeds, 3rd Ed. Pergamon Press, Oxford, U.K. (1982). MILBORROW, B. V.: Ann. Rev. Plant Physio!. 25, 259-307 (1974). MOORE, R. and J. D. SMITH: Planta, 162, 342-344 (1984). MORRIS, D. A.: Z. Pflanzenphysio!., 86, 433 -441 (1978). QUARRIE, S. A.: Plant, Cell & Env., 5, 23-26 (1982). RICK, C. M.: Sci. Amer., 239,76-87 (1978). SIMMONDS, N. W.: Heredity, 21, 473-479 (1966). TAL, M.: Plant Physio!', 41, 1387 (1966). TAL, M. and Y. NEVO: Biochem. Genet., 8, 291-299 (1973). WALTON, D. Ann. Rev. Plant Physio!., 31,453-489 (1980). WILKINS, M. B.: Gravitropism. In: WILKINS, M. B. (ed.): Advanced Plant Physiology, pp. 163165, Pitman, London, U.K. (1984). ZOBEL, R. W.: Plant Physio!., 52, 385-389 (1973).
c.:
J. Plant Physiol.
Vol. 121. pp. 475 -480 (1985)