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The effect of ethephon seed treatment on leaf development and head initiation of wheat
Field Crops Research, 34 ( 1993 ) 113-120
113
Elsevier Science Publishers B.V., Amsterdam
The effect of ethephon seed treatment on leaf development and head initiation of wheat Gary M. Banowetz USDA-ARS, Corvallis, OR, USA (Accepted 24 November 1992 )
ABSTRACT The ability to control the transition from vegetative to reproductive growth could potentially increase wheat yields, particularly in regions subject to midseason drought stress. Little is known about the factors that regulate or affect this transition in wheat and other graminae. Because ethephon is known to accelerate this transition in other plants, the effect of seed treatment on spring wheat growth and development was investigated. Seedlings derived from seeds imbibed in ethephon underwent the transition from vegetative to reproductive growth earlier than seeds imbibed in water. The percentage of 17-day-old plants with four fully-emerged leaves (i.e., plants in which meristem development had proceeded from vegetative apex to double-ridge formation) showed a positive correlation with ethephon concentration. Subsequent yield components were unaffected with the exception of a small increase in the number of kernels/spike in plants derived from seeds imbibed in ethephon solutions for 24 h. Neither root nor shoot dry weights of 21-day-old plants were affected by the treatments. Germination percentages were unaffected by the treatments but 8-day-old plants derived from seeds imbibed for 24 h had reduced height with increasing ethephon concentrations. These results will be useful in further studies of the biochemical basis of the transition to reproductive growth.
INTRODUCTION
The physiological mechanisms involved in photoinduction that regulate the transition from vegetative to reproductive growth remain poorly understood. Because events during this stage of plant development can have profound effects on flowering and subsequent seed production, the ability to regulate this transition may provide a means to enhance yields. One method used to regulate the timing of flowering in pineapple (Ananas comosus L. ) involves treatment of plants with ethylene (Zeevaart, 1978 ) or ethephon, an ethylene-releasing compound (Cooke and Randall, 1968 ). Similarly, treatment of iris (Iris hollandica Hoog) bulbs with ethephon promotes early flowering and reduces flower abortion (Kamerbeek et al., 1980; Cascame and Doss, 1988). The use ofethephon on mango (Mangifera indica L. ) Correspondence to: G.M. Banowetz, USDA-ARS, 3450 S.W. Campus Way, Corvallis, Oregon 97331, USA.
0378-4290/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
114
G.M. BANOWETZ
has been shown to accelerate plant development by reduction of the juvenile phase from 6 to 3.5 years (Chacko et al., 1974). Relatively little is known about the effects of ethylene or ethephon on the transition to reproductive growth in wheat (Triticum aestivum L.), grasses and other crop plants. A number of studies have shown that foliar application of ethephon can be used to control lodging in wheat or barley (Wiersma et al., 1986; Simmons et al., 1988; Taylor et al., 1991 ). Foliar applications of etbephon to barley affected certain yield components, but no effects on flowering were reported (Moes and Stobbe, 199 l a,b,c). Because the foliar applications in each of these studies were made after reproductive growth was initiated, the effect of ethephon on the transition from vegetative growth was unknown. Chrominski and Rozej (1973) found that winter wheat seeds imbibed in either water or solutions ofethephon after vernalization produced plants with a significantly higher number of heads 70 days after sowing. They suggested that this treatment had accelerated the transition of meristems from vegetative to reproductive growth, although no meristems were examined. Woodward and Marshall ( 1987 ) found that treatment of spring barley seeds with Terpal, an ethephon-containing growth regulator, increased tiller bud outgrowth and reduced the growth of shoot leaves. These effects were transient, although the modification of growth at early stages of plant development increased the number of tillers at maturity. Tiller formation also was increased by treatment of wheat seedlings at the two-leaf stage with cthephon (Poovaiah and Leopold, 1973 ). Previous work (Chrominski and Rozej, 1973; Chacko et al., 1974; Cascante and Doss, 1988) suggested that ethephon treatment affected the transition from vegetative to reproductive growth in a variety of plants, including cereals. The objective of the present study was to determine the effects of ethephon seed treatment on subsequent growth and development of spring wheat. Specifically, the effects on germination, time to double-ridge formation, days to heading and flowering, and yield were examined. MATERIALS AND METHODS
lmbibition and ethephon treatment of wheat seeds Wheat seeds (cv. Red Bob ) were placed in cheesecloth and immersed in an aerated solution which contained either 0, 0.25, 0.50, 2.50, or 5.00 g/1 ofethephon in water for 2, 8, or 24 h. Imbibed seeds were planted in pasteurized soil in pots (15 cm diameter), four seeds per pot, five replicate pots per treatment. The pots were arranged randomly in a greenhouse with supplementary lighting to provide 16/8 h day/night photoperiods. This group of plants was grown to maturity. Two separate groups of seeds were imbibed in the identical manner and planted in pasteurized soil, four seeds per container, five con-
EFFECT OF ETHEPHON TREATMENT OF WHEAT SEEDS
1 15
tainers per treatment. These plants were used in replicate trials to confirm the effects of the ethephon treatments on the transition of meristems from vegetative to double-ridge stage. Two additional groups of seeds were planted in pasteurized soil and placed in the greenhouse to provide plants for meristem dissection. One of these groups was imbibed in water, the other in a solution that contained 5 g/1 of ethephon. Meristem examination Seedlings were removed from soil and stripped of leaves before dissection to determine the stage of meristem development. Meristems were removed using fine-tipped forceps and examined with a dissecting microscope. The stage of meristem development, including the appearance of double-ridges, was classified according to Kirby and Appleyard ( 1987 ). Plant measurements Percentage emergence, length of the longest (third) leaf at the three-leaf stage, and days after planting required for full emergence of the fourth leaf were recorded. Time to heading, time to flowering, heads per plant, kernels per spike, and 10-kernel weight were measured in all plants grown to maturity. In the replicated experiment measuring early development, length of the longest (third) leaf at full emergence of the third leaf, time required to full emergence of the fourth leaf, and root and shoot dry weights were measured. RESULTS
Meristem development In order to determine whether leaf development could be correlated with the transition from vegetative apex to double-ridge formation, meristems were examined from plants derived from both water- and ethephon-imbibedseeds at the three-leafand four-leaf stages. The first objective here was to determine whether leaf development, in lieu of meristem dissection, was sufficiently correlated with double-ridge formation to provide a non-destructive indication of the transition of the vegetative apex to reproductive growth. The second objective was to determine whether ethephon seed treatment affected timing of double-ridge formation in relation to leaf development. Virtually 100% of plants examined at the three-leaf stage contained vegetative apices while 100% of plants sampled at the four-leaf stage contained double-ridge meristems. The ethephon seed treatment did not affect the timing of doubleridge formation in relation to leaf development. Untreated plants required approximately 20 days to reach the four-leaf stage under the growth conditions used in this study. In contrast, seedlings derived from seeds treated at the highest level of ethephon used in these studies (5 g/l) required 17 days to reach the same stage of development (80-90% of plants at four-leaf stage).
116
G.M. BANOWETZ
Leaf and seed development Although the ethephon treatments used in these studies did not affect germination, early post-germination development of the plants was affected. The length of the longest (third) leaf, measured at the three-leaf stage of development, was significantly diminished (P= 0.001 ) as ethephon concentration increased (Fig. 1 ). Furthermore, the development of the plants to the fourleaf stage was advanced by the ethephon treatments. This was significant because meristem dissections demonstrated that the full emergence of the fourth leaf indicated the meristem had formed double-ridges and undergone the transition to reproductive growth. The number of 17-day-old seedlings with four fully-emerged leaves was directly correlated to the concentration of ethephon used for seed imbibition (Fig. 2 ). The effect of concentration was highly significant (P=0.001), but the effect of duration of imbibition was not (P=O. 10; Table 1 ). Later development of the wheat plant was less affected by ethephon imbibition. The number of kernels per spike showed some re400 Length= -15.25(dose) + 296.71 R=-0.83, df-23,P=0.001
T
300 A
E E
=. e-
l
200
_e Q ,.I
100
0
0
I
I
l
I
I
1
2
3
4
5
6
Oonoentratlon of e t h e p h o n (g/I)
Fig. 1. The effect of 24 h of ethephon seed treatment on leaf length in spring wheat seedlings. Wheat (Triticum aestivum cv. Red Bob) seeds were soaked, with aeration, in water that contained 0, 0.25, 0.50, 2.50 or 5.00 g/1 of ethephon for 24 h. Seeds were planted in soil and measurements were of the longest leaf were taken 8 days after planting. Data points represent the means for 20 plants. Error bars indicate + s.e.
EFFECT OF ETHEPHON
TREATMENT
l 17
OF WHEAT SEEDS
100
24 h ,#
90 sS os S
80
sS
¢.oS t) m O
/
70
2 h
oS iS
60
0
50
n a
40
N
30 20 10
0
I
I
I
I
I
1
2
3
4
5
6
Concentration of ethaphon (g/I)
Fig. 2. The effect of ethephon seed treatment on leaf emergence in spring wheat seedlings. Wheat (Triticum aestivum cv. Red Bob) seeds were soaked in water which contained 0, 0.25, 0.50, 2.50, or 5.00 g/1 of ethephon for either 2, 8, or 24 h and planted in soil. Determinations of the percentage of plants with four leaves were made 17 days after planting. Plotted values represent means of 20 plants. TABLE 1 ANOVA table for the effects of ethephon concentration and duration of imbibition on the number of 17-day-old wheat plants with four fully emerged leaves Source of variation
d.f.
Mean squares
Significance
Concentration ( C ) Duration (D) CXD Error Total
4 2 8 60 75
13.38 2.68 0.73 1.00
P = 0.001 P=O. 10 N.S.
sponse to ethephon over the concentrations used in this study (Fig. 3 ). No significant differences were found in the average weight of 10 kernels, average weight per seed head, number of reproductive tillers per plant, time to heading or to flowering, or final dry weight of roots or shoots (data not shown).
I 18
G.M. B~dqOWETZ 25 Kernels - 3 . 1 0 ( d o s e ) + 1 9 . 2 6 R-0.576, df-23, P-0.10
23
T O
.~
21
& A &)
+Jl
.= t)
v
19
17
:±
15 0
,
,
t
,
,
1
2
3
4
5
6
Conoentratlon of e t h e p h o n ( g / I )
Fig. 3. The effect of 24 h of ethephon seed treatment on the number of kernels/spike in spring wheat. Wheat (Triticum aestivum cv. Red Bob) seeds were soaked with aeration for 24 h in water that contained 0, 0.25, 0.50, 2.50, or 5.00 g/l of ethephon, planted in soil, and grown to maturity. Data points represent means for 20 plants. Error bars indicate + s.e. DISCUSSION
These studies showed that imbibition of wheat seeds in ethephon-containing solutions altered early seedling development. Based upon the meristem dissections performed in this study, emergence of the fourth leaf served as a nondestructive indication of double-ridge formation, i.e., the transition from vegetative to reproductive growth. Development after the four-leaf stage was largely unaffected. Even though double-ridge formation occurred earlier in plants derived from ethephon-treated seeds, days to heading and flowering were not significantly affected. The ethephon treatment had no affect on tillering in this cultivar. This observation is unlike those of Woodward and Marshall (1987, 1988), who observed increased tiller bud growth after barley (Hordeum distichum L.) seeds had been treated with Terpal, a plant growth regulator that contains mepiquat chloride and ethephon. Their results suggested that seed treatment with Terpal affected early growth of the plants by apparent modification of apical bud dominance. The effects they observed
EFFECTOF ETHEPHONTREATMENTOFWHEATSEEDS
1 19
were transient, but sufficient to modify tiller bud formation and elongation. Poovaiah and Leopold ( 1973 ) found that foliar applications of ethephon to wheat seedlings (cv. Waldron) at the two-leaf stage reduced leaf elongation, but stimulated tiller formation. Allthough enhanced tiller formation was not observed in the present study, it should be noted that the wheat used in this work does not form a high number of tillers. The biochemical mechanism (s) by which exogenous ethephon affects plant development remain unknown. Ievinsh et al. (1990) found that foliar application of ethephon on winter rye (Secale cereale L. ) caused immediate increases in ethylene production by the seedlings. They also demonstrated increases in levels of 1-aminocyclopropane-l-carboxylic acid oxidase and ethylene-forming enzyme after a 15-min lag period. These results suggested that exogenous ethylene had an auto-catalytic role. The use of ethephon to manipulate the transition from vegetative to reproductive growth in a species like wheat may be useful in studies on the biochemical mechanisms of photoinduction. In general, such studies are difficult with a species that does not respond to a single inductive treatment. The lack of synchrony introduced into the population by the need for multiple inductive treatments hinders direct biochemical comparisons of induced and noninduced samples (Bernier, 1989 ). Furthermore, the different light conditions required to produce induced and non-induced plants cause molecular changes unrelated to photoinduction. Wheat and many other graminae require multiple inductive treatments and therefore do not form optimal experimental systems for such investigations. The present work shows that vegetative and reproductive plants of the same age can be produced under identical light conditions by imbibition of one group in water, the other in a solution of ethephon. It is likely that the ethephon treatment also causes changes unrelated to photoinduction that would have to be accounted for by use of appropriate control plants. ACKNOWLEDGEMENTS
I thank Lisa Ouchida and Christine Duggan for excellent technical assistance and Drs. Glen Murray and Robert Doss for advice on statistical analysis. This research was conducted by the Agricultural Research Service, U.S. Dept. of Agriculture, Corvallis, OR, Technical Paper No. 9916 of the Oregon Agricultural Experiment Station. Use of a company or product name by the Department does not imply approval or recommendation of the product to the exclusion of others which may also be suitable. REFERENCES Bernier, G., 1989. Events in the floral transition of meristems. In: E. Lord and G. Bernier (Editors), Plant Reproduction: From Floral Induction to Pollination, Am. Soc. Plant Physiol., Rockville, MD, pp. 42-50.
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Cascante, X.M. and Doss, R.P., 1988. Ethephon dip concentration and heat curing duration influence forcing performance of Dutch Iris. HortScience, 23:1006-1008. Chacko, E.K., Kohli, R.R., Dore Swamy, R. and Randhawa, G.S., 1974. Effect of (2-chloroethyl)phosphonic acid on flower induction in juvenile mango (Mangifera indica) seedlings. Physiol. Plant., 32:188-190. Chrominski, A. and Rozej, B., 1973. Acceleration of vernalization in wheat by 2-chloroethylphosphonic acid-released ethylene. Acta Apron. Acad. Sci. Hungaricae, Tomus, 22: 27-29. Cooke, A.R. and Randall, D.I., 1968.2-haloethanephosphonic acids as ethylene releasing agents for the induction of flowering in pineapples. Nature, 218:974. Ievinsh, G., Iljin, V., Kreicbergs, O. and Romanovskaya, O., 1990. Effect of ethephon on the activity of the ethylene-forming enzyme and the biosynthesis of ethylene in winter rye seedlings. Biochem. Physiol. Pflanz., 186:221-228. Kamerbeek, G.A., Durieux, A.J.B. and Schipper, J.A., 1980. An analysis of the influence of ethrel on flowering of iris 'Ideal': an associated morphogenetic physiological approach. Acta Hortic., 109: 235-240. Kirby, E.J.M. and Appleyard, M., 1987. Cereal Development Guide. Arable Unit, Warwickshire, pp. 32-33. Moes, J. and Stobbe, E.H., 1991 a. Barley treated with Ethephon: I. Yield components and net grain yield. Apron. J., 83: 86-90. Moes, J. and Stobbe, E.H., 199lb. Barley treated with Ethephon: II. Tillering pattern and its impact on yield. Apron. J., 83: 90-94. Moes, J. and Stobbe, E.H., 1991c. Barley treated with Ethephon: III. Kernels per spike and kernel mass. Apron. J., 83: 95-98. Poovaiah, B.W. and Leopold, A.C., 1973. Effects of ethephon on growth of grasses. Crop Sci., 13: 755-758. Simmons, S.R., Oelke, E.A., Wiersma, J.V., Lueschen, W.E. and Warnes, D.D., 1988. Spring wheat and barley responses to Ethephon. Apron. J., 80: 829-834. Taylor, J.S., Foster, K.R. and Caldwell, C.D., 199 I. Ethephon effects on barley in central Alberta. Can. J. Plant Sci., 71: 983-995. Wiersma, D.W., Oplinger, E.S. and Guy, S.O., 1986. Environment and cultivar effects on winter wheat response to Ethephon plant growth regulator. Agron. J., 78:761-764. Woodward, E.J. and Marshall, C., 1987. Effects of seed treatment with a plant growth regulator On growth and tillering in spring barley (Hordeum distichum) cv. Triumph. Ann. Appl. Biol., 110: 629-638. Woodward, E.J. and Marshall, C., 1988. Effects of plant growth regulators and nutrient supply on tiller bud outgrowth in barley (Hordeum distichum L.). Ann. Bot., 6 l: 347-354. Zeevaart, J.A.D., 1978. Phytohormones and flower formation. In: D.S. Letham, P.B. Goodwin and T.J.V. Higgins (Editors), Phytohormones and Related Compounds. A Comprehensive Treatise, Vol. 2. Elsevier, Amsterdam, pp. 298-300.
113
Elsevier Science Publishers B.V., Amsterdam
The effect of ethephon seed treatment on leaf development and head initiation of wheat Gary M. Banowetz USDA-ARS, Corvallis, OR, USA (Accepted 24 November 1992 )
ABSTRACT The ability to control the transition from vegetative to reproductive growth could potentially increase wheat yields, particularly in regions subject to midseason drought stress. Little is known about the factors that regulate or affect this transition in wheat and other graminae. Because ethephon is known to accelerate this transition in other plants, the effect of seed treatment on spring wheat growth and development was investigated. Seedlings derived from seeds imbibed in ethephon underwent the transition from vegetative to reproductive growth earlier than seeds imbibed in water. The percentage of 17-day-old plants with four fully-emerged leaves (i.e., plants in which meristem development had proceeded from vegetative apex to double-ridge formation) showed a positive correlation with ethephon concentration. Subsequent yield components were unaffected with the exception of a small increase in the number of kernels/spike in plants derived from seeds imbibed in ethephon solutions for 24 h. Neither root nor shoot dry weights of 21-day-old plants were affected by the treatments. Germination percentages were unaffected by the treatments but 8-day-old plants derived from seeds imbibed for 24 h had reduced height with increasing ethephon concentrations. These results will be useful in further studies of the biochemical basis of the transition to reproductive growth.
INTRODUCTION
The physiological mechanisms involved in photoinduction that regulate the transition from vegetative to reproductive growth remain poorly understood. Because events during this stage of plant development can have profound effects on flowering and subsequent seed production, the ability to regulate this transition may provide a means to enhance yields. One method used to regulate the timing of flowering in pineapple (Ananas comosus L. ) involves treatment of plants with ethylene (Zeevaart, 1978 ) or ethephon, an ethylene-releasing compound (Cooke and Randall, 1968 ). Similarly, treatment of iris (Iris hollandica Hoog) bulbs with ethephon promotes early flowering and reduces flower abortion (Kamerbeek et al., 1980; Cascame and Doss, 1988). The use ofethephon on mango (Mangifera indica L. ) Correspondence to: G.M. Banowetz, USDA-ARS, 3450 S.W. Campus Way, Corvallis, Oregon 97331, USA.
0378-4290/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
114
G.M. BANOWETZ
has been shown to accelerate plant development by reduction of the juvenile phase from 6 to 3.5 years (Chacko et al., 1974). Relatively little is known about the effects of ethylene or ethephon on the transition to reproductive growth in wheat (Triticum aestivum L.), grasses and other crop plants. A number of studies have shown that foliar application of ethephon can be used to control lodging in wheat or barley (Wiersma et al., 1986; Simmons et al., 1988; Taylor et al., 1991 ). Foliar applications of etbephon to barley affected certain yield components, but no effects on flowering were reported (Moes and Stobbe, 199 l a,b,c). Because the foliar applications in each of these studies were made after reproductive growth was initiated, the effect of ethephon on the transition from vegetative growth was unknown. Chrominski and Rozej (1973) found that winter wheat seeds imbibed in either water or solutions ofethephon after vernalization produced plants with a significantly higher number of heads 70 days after sowing. They suggested that this treatment had accelerated the transition of meristems from vegetative to reproductive growth, although no meristems were examined. Woodward and Marshall ( 1987 ) found that treatment of spring barley seeds with Terpal, an ethephon-containing growth regulator, increased tiller bud outgrowth and reduced the growth of shoot leaves. These effects were transient, although the modification of growth at early stages of plant development increased the number of tillers at maturity. Tiller formation also was increased by treatment of wheat seedlings at the two-leaf stage with cthephon (Poovaiah and Leopold, 1973 ). Previous work (Chrominski and Rozej, 1973; Chacko et al., 1974; Cascante and Doss, 1988) suggested that ethephon treatment affected the transition from vegetative to reproductive growth in a variety of plants, including cereals. The objective of the present study was to determine the effects of ethephon seed treatment on subsequent growth and development of spring wheat. Specifically, the effects on germination, time to double-ridge formation, days to heading and flowering, and yield were examined. MATERIALS AND METHODS
lmbibition and ethephon treatment of wheat seeds Wheat seeds (cv. Red Bob ) were placed in cheesecloth and immersed in an aerated solution which contained either 0, 0.25, 0.50, 2.50, or 5.00 g/1 ofethephon in water for 2, 8, or 24 h. Imbibed seeds were planted in pasteurized soil in pots (15 cm diameter), four seeds per pot, five replicate pots per treatment. The pots were arranged randomly in a greenhouse with supplementary lighting to provide 16/8 h day/night photoperiods. This group of plants was grown to maturity. Two separate groups of seeds were imbibed in the identical manner and planted in pasteurized soil, four seeds per container, five con-
EFFECT OF ETHEPHON TREATMENT OF WHEAT SEEDS
1 15
tainers per treatment. These plants were used in replicate trials to confirm the effects of the ethephon treatments on the transition of meristems from vegetative to double-ridge stage. Two additional groups of seeds were planted in pasteurized soil and placed in the greenhouse to provide plants for meristem dissection. One of these groups was imbibed in water, the other in a solution that contained 5 g/1 of ethephon. Meristem examination Seedlings were removed from soil and stripped of leaves before dissection to determine the stage of meristem development. Meristems were removed using fine-tipped forceps and examined with a dissecting microscope. The stage of meristem development, including the appearance of double-ridges, was classified according to Kirby and Appleyard ( 1987 ). Plant measurements Percentage emergence, length of the longest (third) leaf at the three-leaf stage, and days after planting required for full emergence of the fourth leaf were recorded. Time to heading, time to flowering, heads per plant, kernels per spike, and 10-kernel weight were measured in all plants grown to maturity. In the replicated experiment measuring early development, length of the longest (third) leaf at full emergence of the third leaf, time required to full emergence of the fourth leaf, and root and shoot dry weights were measured. RESULTS
Meristem development In order to determine whether leaf development could be correlated with the transition from vegetative apex to double-ridge formation, meristems were examined from plants derived from both water- and ethephon-imbibedseeds at the three-leafand four-leaf stages. The first objective here was to determine whether leaf development, in lieu of meristem dissection, was sufficiently correlated with double-ridge formation to provide a non-destructive indication of the transition of the vegetative apex to reproductive growth. The second objective was to determine whether ethephon seed treatment affected timing of double-ridge formation in relation to leaf development. Virtually 100% of plants examined at the three-leaf stage contained vegetative apices while 100% of plants sampled at the four-leaf stage contained double-ridge meristems. The ethephon seed treatment did not affect the timing of doubleridge formation in relation to leaf development. Untreated plants required approximately 20 days to reach the four-leaf stage under the growth conditions used in this study. In contrast, seedlings derived from seeds treated at the highest level of ethephon used in these studies (5 g/l) required 17 days to reach the same stage of development (80-90% of plants at four-leaf stage).
116
G.M. BANOWETZ
Leaf and seed development Although the ethephon treatments used in these studies did not affect germination, early post-germination development of the plants was affected. The length of the longest (third) leaf, measured at the three-leaf stage of development, was significantly diminished (P= 0.001 ) as ethephon concentration increased (Fig. 1 ). Furthermore, the development of the plants to the fourleaf stage was advanced by the ethephon treatments. This was significant because meristem dissections demonstrated that the full emergence of the fourth leaf indicated the meristem had formed double-ridges and undergone the transition to reproductive growth. The number of 17-day-old seedlings with four fully-emerged leaves was directly correlated to the concentration of ethephon used for seed imbibition (Fig. 2 ). The effect of concentration was highly significant (P=0.001), but the effect of duration of imbibition was not (P=O. 10; Table 1 ). Later development of the wheat plant was less affected by ethephon imbibition. The number of kernels per spike showed some re400 Length= -15.25(dose) + 296.71 R=-0.83, df-23,P=0.001
T
300 A
E E
=. e-
l
200
_e Q ,.I
100
0
0
I
I
l
I
I
1
2
3
4
5
6
Oonoentratlon of e t h e p h o n (g/I)
Fig. 1. The effect of 24 h of ethephon seed treatment on leaf length in spring wheat seedlings. Wheat (Triticum aestivum cv. Red Bob) seeds were soaked, with aeration, in water that contained 0, 0.25, 0.50, 2.50 or 5.00 g/1 of ethephon for 24 h. Seeds were planted in soil and measurements were of the longest leaf were taken 8 days after planting. Data points represent the means for 20 plants. Error bars indicate + s.e.
EFFECT OF ETHEPHON
TREATMENT
l 17
OF WHEAT SEEDS
100
24 h ,#
90 sS os S
80
sS
¢.oS t) m O
/
70
2 h
oS iS
60
0
50
n a
40
N
30 20 10
0
I
I
I
I
I
1
2
3
4
5
6
Concentration of ethaphon (g/I)
Fig. 2. The effect of ethephon seed treatment on leaf emergence in spring wheat seedlings. Wheat (Triticum aestivum cv. Red Bob) seeds were soaked in water which contained 0, 0.25, 0.50, 2.50, or 5.00 g/1 of ethephon for either 2, 8, or 24 h and planted in soil. Determinations of the percentage of plants with four leaves were made 17 days after planting. Plotted values represent means of 20 plants. TABLE 1 ANOVA table for the effects of ethephon concentration and duration of imbibition on the number of 17-day-old wheat plants with four fully emerged leaves Source of variation
d.f.
Mean squares
Significance
Concentration ( C ) Duration (D) CXD Error Total
4 2 8 60 75
13.38 2.68 0.73 1.00
P = 0.001 P=O. 10 N.S.
sponse to ethephon over the concentrations used in this study (Fig. 3 ). No significant differences were found in the average weight of 10 kernels, average weight per seed head, number of reproductive tillers per plant, time to heading or to flowering, or final dry weight of roots or shoots (data not shown).
I 18
G.M. B~dqOWETZ 25 Kernels - 3 . 1 0 ( d o s e ) + 1 9 . 2 6 R-0.576, df-23, P-0.10
23
T O
.~
21
& A &)
+Jl
.= t)
v
19
17
:±
15 0
,
,
t
,
,
1
2
3
4
5
6
Conoentratlon of e t h e p h o n ( g / I )
Fig. 3. The effect of 24 h of ethephon seed treatment on the number of kernels/spike in spring wheat. Wheat (Triticum aestivum cv. Red Bob) seeds were soaked with aeration for 24 h in water that contained 0, 0.25, 0.50, 2.50, or 5.00 g/l of ethephon, planted in soil, and grown to maturity. Data points represent means for 20 plants. Error bars indicate + s.e. DISCUSSION
These studies showed that imbibition of wheat seeds in ethephon-containing solutions altered early seedling development. Based upon the meristem dissections performed in this study, emergence of the fourth leaf served as a nondestructive indication of double-ridge formation, i.e., the transition from vegetative to reproductive growth. Development after the four-leaf stage was largely unaffected. Even though double-ridge formation occurred earlier in plants derived from ethephon-treated seeds, days to heading and flowering were not significantly affected. The ethephon treatment had no affect on tillering in this cultivar. This observation is unlike those of Woodward and Marshall (1987, 1988), who observed increased tiller bud growth after barley (Hordeum distichum L.) seeds had been treated with Terpal, a plant growth regulator that contains mepiquat chloride and ethephon. Their results suggested that seed treatment with Terpal affected early growth of the plants by apparent modification of apical bud dominance. The effects they observed
EFFECTOF ETHEPHONTREATMENTOFWHEATSEEDS
1 19
were transient, but sufficient to modify tiller bud formation and elongation. Poovaiah and Leopold ( 1973 ) found that foliar applications of ethephon to wheat seedlings (cv. Waldron) at the two-leaf stage reduced leaf elongation, but stimulated tiller formation. Allthough enhanced tiller formation was not observed in the present study, it should be noted that the wheat used in this work does not form a high number of tillers. The biochemical mechanism (s) by which exogenous ethephon affects plant development remain unknown. Ievinsh et al. (1990) found that foliar application of ethephon on winter rye (Secale cereale L. ) caused immediate increases in ethylene production by the seedlings. They also demonstrated increases in levels of 1-aminocyclopropane-l-carboxylic acid oxidase and ethylene-forming enzyme after a 15-min lag period. These results suggested that exogenous ethylene had an auto-catalytic role. The use of ethephon to manipulate the transition from vegetative to reproductive growth in a species like wheat may be useful in studies on the biochemical mechanisms of photoinduction. In general, such studies are difficult with a species that does not respond to a single inductive treatment. The lack of synchrony introduced into the population by the need for multiple inductive treatments hinders direct biochemical comparisons of induced and noninduced samples (Bernier, 1989 ). Furthermore, the different light conditions required to produce induced and non-induced plants cause molecular changes unrelated to photoinduction. Wheat and many other graminae require multiple inductive treatments and therefore do not form optimal experimental systems for such investigations. The present work shows that vegetative and reproductive plants of the same age can be produced under identical light conditions by imbibition of one group in water, the other in a solution of ethephon. It is likely that the ethephon treatment also causes changes unrelated to photoinduction that would have to be accounted for by use of appropriate control plants. ACKNOWLEDGEMENTS
I thank Lisa Ouchida and Christine Duggan for excellent technical assistance and Drs. Glen Murray and Robert Doss for advice on statistical analysis. This research was conducted by the Agricultural Research Service, U.S. Dept. of Agriculture, Corvallis, OR, Technical Paper No. 9916 of the Oregon Agricultural Experiment Station. Use of a company or product name by the Department does not imply approval or recommendation of the product to the exclusion of others which may also be suitable. REFERENCES Bernier, G., 1989. Events in the floral transition of meristems. In: E. Lord and G. Bernier (Editors), Plant Reproduction: From Floral Induction to Pollination, Am. Soc. Plant Physiol., Rockville, MD, pp. 42-50.
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