Breaking seed dormancy in almond (Prunus dulcis (Mill.) D.A. Webb)
Marta García-Gusano, Pedro Martínez-Gómez and Federico Dicenta ,
Departamento de Mejora y Patología Vegetal, Centro de Edafología y Biología Aplicada del Segura (CSIC), P.O. Box 4195, E-30080, Murcia, Spain
Accepted 21 July 2003. ; Available online 17 September 2003.
Abstract
Seed dormancy affects germination and subsequent seedling growth in many plant species. In this work the effect of temperature (cold and heat treatments), flowering time of genotypes, and hardness of endocarp on the break of seed dormancy in four almond cultivars, including a broad range of flowering dates, was studied. Mature seeds, with and without endocarp, were stratified at 7 °C for 1–10 weeks, followed by 5 weeks at 22 °C. The number of germinated seeds were recorded weekly for each cultivar, and cold and heat treatments. Despite the great variability observed within cultivars, a relationship between chilling requirements of cultivars for flowering and stratification requirements of seeds for germination was observed. With endocarp, stratification time required to overcome seed dormancy ranged between 6 (early flowering cultivars Desmayo Largueta and Ramillete) and 8 weeks (late-flowering cultivars Mono and Wawona) on average. Elimination of the endocarp reduced the period of stratification needed for germination in hard-shelled cultivars by 3 weeks. This effect was less important in soft-shelled almonds. No important differences between cultivars were observed for heat requirements. Two weeks at 22 °C was enough to reach the higher germination percentage after each cold treatment.
Author Keywords: Almond; Prunus dulcis; Germination; Seed dormancy; Endocarp; Stratification requirements; Heat requirements
Article Outline
1. Introduction
2. Materials and methods
2.1. Almond cultivars
2.2. Experimental design
2.3. Statistical analysis
3. Results
3.1. Variability of stratification requirement for germination
3.2. Effect of flowering time on chill and heat requirements for seed germination
3.3. Influence of shell hardness on germination
4. Discussion
4.1. Variability of stratification requirements for seed germination
4.2. Effect of flowering time on chill and heat requirements for seed germination
4.3. Effect of endocarp hardness on stratification requirements for seed germination
5. Conclusions
Acknowledgements
References
1. Introduction
Seed dormancy is an adaptative mechanism in Prunus species to protect temperate fruit trees from freeze damage during the winter. This mechanism consists of the inhibition of immediate seed germination even under appropriate oxygen and temperature conditions, and affects both seed germination and subsequent seedling growth ([Du Toit et al., 1979, Mehanna et al., 1985 and Frisby and Seeley, 1993]). [Seeley et al., 1998] and [Martinez-Gomez and Dicenta, 2001] characterised the phenomenon in peach (P. persica L.) as two independent mechanisms: seed coat dormancy (external), with a hormonal nature manifested in the inhibition of germination, and embryo dormancy (internal) with a genetic nature expressed mainly in later plant growth. [Grigorian, 1972] and [Kester et al., 1977] indicated the influence of embryo dormancy in seed germination in almond.
Stratification has been the method traditionally used to break seed dormancy in Prunus species ([Zigas and Coombe, 1977a, Mehanna et al., 1985, Frisby and Seeley, 1993 and Seeley et al., 1998]). In order to accelerate this process, the application of hormones ( [Diaz and Martin, 1972, Zigas and Coombe, 1977b and Mehanna and Martin, 1985]) and the removal of the seed coat ( [Zigas and Coombe, 1977a and Du Toit et al., 1979]) have been used.
In almond, studies about seed dormancy are very scare. In agreement with the results observed in other Prunus species, [Gonzalez-Cepeda, 1975] described the hormonal nature of the mechanism of dormancy in almond seeds. In addition, [Grigorian, 1972] and [Kester et al., 1977] indicated an optimum period for stratification of 8–10 weeks in the almond cultivars studied.
In this work the effect of temperature (cold and heat treatments), flowering time of genotypes, and the hardness of endocarp on the break of seed dormancy in four almond cultivars having a broad range of flowering times was studied.
2. Materials and methods
2.1. Almond cultivars
Almond (Prunus dulcis (Mill.) D.A. Webb) cultivars assayed were the Spanish cultivars Desmayo Largueta and Ramillete (early flowering and hard-shelled) and the American cultivars Mono and Wawona (late-flowering and soft-shelled). These cultivars included a broad range of flowering dates in almond species (Table 1).
Table 1. Flowering time and shell hardness of almond cultivars
2.2. Experimental design
Mature nuts from open-pollinated almond cultivars were treated in a 2% TMTD® (tetramethylthiuram disulphide) fungicide solution for 30 min. Ten samples of 30 replications with endocarp (nuts) and another 10 without endocarp (seeds) were immersed in water for 48 h, and then placed in plastic mesh bags in humid vermiculite (wet to field capacity) in a cold chamber at 7±0.5 °C and darkness, for 1–10 weeks.
After each cold treatment (1–10 weeks), two samples (one with and other without endocarp) were placed in humid vermiculite (wet to field capacity) in a growth chamber at 22±1 °C and darkness for 5 weeks. Seed germination, defined as the emergence of the radicle, was observed weekly. The initial percentage of germination was recorded at the end of each stratification period before introducing seeds in the growth chamber. The final percentage of germination was recorded after 5 weeks in the growth chamber at 22±1 °C.
2.3. Statistical analysis
The data were statistically analysed by means of the test of comparison of proportions. The variability in seed germination was quantified statistically as the range between the first week where the seeds in each treatment started to germinate and the week where the percentage of seed germinated reached 100%.
3. Results
3.1. Variability of stratification requirement for germination
Seeds showed a broad range of chilling requirement for germination, even within cultivars. Ranges of weeks needed for germination (initial and final) for each cultivar (with and without shell) were higher in the late-flowering cultivars Mono and Wawona (till 10 weeks) in comparison with the earlier Desmayo Largueta and Ramillete (Table 2).
Table 2. Range of weeks between the first week where the seeds started to germinate (Min) and the week where the percentage of seed germinated reached 100% (Max) in each treatmenta
In addition, the variability observed was higher in the case of final germination due to seeds needing less weeks at 7 °C to germinate, after they were exposed to warm temperature. The same effect was observed when endocarp was removed. Ramillete showed the narrowest range of germination (3 weeks for initial germination) and Wawona the broadest (more than 10 weeks).
3.2. Effect of flowering time on chill and heat requirements for seed germination
In spite of the great variability observed in the assays, a relationship between chilling requirements of genotypes for flowering and stratification requirements of seeds for germination was observed.
Considering seeds without endocarp (to eliminate the possible effect of differences in endocarp hardness), the seeds of the early flowering cultivars need 6–7 weeks to achieve 90% initial germination, while Mono needed 10 weeks, which was not even enough to reach this value in Wawona (Table 3).
Table 3. Percentage of initial seed germination of four almond cultivars after 1–10 weeks at 7 °C with and without endocarp
This relationship was better observed when we considered the final germination (Table 4). In the case of Desmayo Largueta and Ramillete, seeds (without endocarp) needed 3 weeks to reach 90%, while the later Mono and Wawona needed 7–8 weeks to reach these values. Again, Wawona did not reach 100% germination, even after 10 weeks at 7 °C followed by 5 weeks at 22 °C.
Table 4. Percentage of final seed germination of four almond cultivars after 1–10 weeks at 7 °C followed by 5 weeks at 22 °C, with and without endocarp
Comparison of results for initial and final germination (Table 3 and Table 4) showed the influence of heat requirements for seed germination. For each stratification treatment, percentages of final germination (after 5 weeks at 22 °C) were higher than those observed for initial germination (just after the stratification period). In general, the heat application reduced (in around 3 weeks) the number of weeks at 7 °C required to begin the germination of each cultivar. Differences between cultivars regarding heat requirements were not observed, being around two weeks at 22 °C enough to reach the maximum percentages of germination after each cold period (data not shown).
3.3. Influence of shell hardness on germination
Our results showed the clear effect of endocarp hardness on the stratification period needed for seed germination. Considering initial germination, the hard-shelled cultivars germinated 2 weeks faster when the shell was removed. This effect was not as great in the seeds of soft-shelled cultivars (Table 3).
Similar results were observed in the final germination. Seeds of cultivars with harder shells needed around 3 weeks less to germinate when the endocarp was removed. In the case of soft-shelled cultivars Mono and Wawona this reduction was less (neither or 1 week) (Table 4).
4. Discussion
4.1. Variability of stratification requirements for seed germination
Seeds of each cultivar showed a broad range of stratification requirements for germination, mainly in late-flowering cultivars. This large variability observed within cultivars, could be due to the different hardness and sealing of each nut that could affect the emergence of the radicle. In fact, [Gradziel and Martinez-Gomez, 2002] described great differences in the seal of nuts and the lignin density at the suture line in different samples from the same almond cultivar. However, this cannot be the reason for seeds without endocarp, where other factors (content of inhibitors in seed coat, state of the embryo) must be involved. In this regard, [Grigorian, 1972] established that germination was controlled both by the external seed dormancy (due to the seed coat) and the internal seed dormancy (due to the embryo).
The broader range observed in the final germination was due to the application of heat that promoted the germination of the seeds that had satisfied their chill requirements to germinate, but did not promote the germination of the seeds that had not satisfied the chill requirements.
Our results for initial and final germination showed a curve as a function of the stratification period and the genotypes. This curve of germination has been observed also in almond by [Grigorian, 1972] and by other authors in several Prunus species ([Diaz and Martin, 1972, Mehanna et al., 1985 and Frisby and Seeley, 1993]). Percentages of germination were higher for longer stratification periods.
4.2. Effect of flowering time on chill and heat requirements for seed germination
A relationship between chilling requirements of cultivars for flowering and stratification requirements of seeds for germination was observed. The optimum periods of stratification to overcome seed dormancy in the early flowering cultivars (Desmayo Largueta and Ramillete) were shorter than those of late-flowering cultivars (Mono and Wawona). This relationship was confirmed also in other almond cultivars by [Grigorian, 1972] and [Kester et al., 1977]. These results show a relationship between the mechanisms of dormancy breakage for vegetative or floral buds and seed germination.
The results for initial and final germination showed the positive effect of heat treatment to accelerate the germination of seeds that had satisfied their chill requirements. Stratification requirements for final germination after 5 weeks at 22 °C were consequently less than those observed for initial germination, just after the stratification period. These results could indicate a mechanism to break the seed dormancy similar to the mechanism for completing flower and leaf bud dormancy ([Erez and Couvillon, 1987]), based on a cold/heat balance. In this sense, [Powell, 1987] indicated a similar hormonal mechanism related to bud and seed dormancy. These results agree with those obtained in peach by [Seeley et al., 1998], who indicated an interaction between cold and heat requirements for seed germination and concluded that temperature cycling could accelerate the germination of seeds.
However, when chilling requirements were not satisfied, heat treatment was not able to induce the germination. This was probably the reason that some seeds of Wawona did not germinate even after 10 weeks at 7 °C followed by 5 weeks at 22 °C.
4.3. Effect of endocarp hardness on stratification requirements for seed germination
Results indicated a clear effect of the hardness of endocarp on seed germination. Endocarp elimination reduced the stratification requirements, mainly in cultivars with harder shells (Desmayo Largueta and Ramillete), and to a lesser degree in the softer shelled cultivars (Mono and Wawona). On the other hand, this influence seems to be greater in the case of shorter stratification periods, disappearing after longer stratification periods (more than 6 weeks).
This effect of endocarp removal on seed germination was previously described in other almond cultivars by [Grigorian, 1972]. The endocarp could have a mechanical effect, an imbibition effect or leaching of the seed coat hormones, thus modifying the stratification period required for germination ( [Du Toit et al., 1979]) and contributing to the great variability observed within cultivars.
Endocarp removal could have an interesting application in seed stratification in almond breeding programmes, allowing a faster stratification and increasing the uniformity of seed germination.
5. Conclusions
Results showed a great variability within cultivars with respect to the stratification requirements of seeds for germination. In all the cases, for a given stratification treatment, percentages of final germination were higher than those observed for initial germination. Results then showed the complementarity between chill and heat requirements of seeds for germination. Despite the great variability observed, a relationship between chilling requirements of cultivars for flowering and stratification requirements of seeds for germination was observed. Early flowering cultivars needed shorter stratification periods to overcome seed dormancy than did late-flowering cultivars. Elimination of the endocarp reduced the periods necessary in stratification, mainly during the first weeks and in the case of cultivars with harder shells.
Acknowledgements
This work has been financed with the project “Mejora Genética del Almendro” (AGL2001-1054-C03-01) from the “Plan Nacional de I+D” of Spanish Ministry of Science and Technology.
References
Diaz and Martin, 1972. D.H. Diaz and G.C. Martin, Peach seed dormancy in relation to endogenous inhibitors and applied growth substances. J. Am. Soc. Hort. Sci. 97 (1972), pp. 652–654.
Du Toit et al., 1979. H.G. Du Toit, G. Jacobs and D.K. Strydom, Role of various seed parts in peach seed dormancy and initial seedling growth. J. Am. Soc. Hort. Sci. 104 (1979), pp. 490–492.
Erez and Couvillon, 1987. A. Erez and G.A. Couvillon, Characterization of the influence of moderate temperatures on rest in peach. J. Am. Soc. Hort. Sci. 112 (1987), pp. 677–680.
Frisby and Seeley, 1993. J.W. Frisby and S.D. Seeley, Chilling of endodormant peach propagules: seed germination and emergence. J. Am. Soc. Hort. Sci. 118 (1993), pp. 248–252.
Gonzalez-Cepeda, 1975. González-Cepeda, I.A., 1975. Dormancy in almond seeds: a study in relation to stratification temperature and growth regulator levels. M.Sc. Dissertation. University of California, Davis, CA, 40 pp.
Gradziel and Martinez-Gomez, 2002. T.M. Gradziel and P. Martínez-Gómez, Shell seal breakdown in almond is associated with the site of secondary ovule abortion. J. Am. Soc. Hort. Sci. 127 (2002), pp. 69–74. View Record in Scopus | Cited By in Scopus (3)
Grigorian, 1972. Grigorian, V., 1972. L’embryogenèse chez l’Amandier (Prunus amygdalus Batsch) étude comparé de la dormances des graines et de la dormances des bourgerons végétatifs. Ph.D. Dissertation. University of Bourdeaux, Bourdeaux, France, 144 pp.
Kester et al., 1977. D.E. Kester, P. Raddi and R.N. Assay, Correlation of chilling requirements for germination, blooming and leafing within and among seedling populations of almond. J. Am. Soc. Hort. Sci. 102 (1977), pp. 145–148.
Martinez-Gomez and Dicenta, 2001. P. Martínez-Gómez and F. Dicenta, Mechanisms of dormancy in seeds of peach (Prunus persica (L.) Batsch) cv. GF305. Sci. Hort. 91 (2001), pp. 51–58. SummaryPlus | Full Text + Links | PDF (70 K) | View Record in Scopus | Cited By in Scopus (6)
Mehanna and Martin, 1985. H.T. Mehanna and G.C. Martin, Effect of seed coat on peach seed germination. Sci. Hort. 25 (1985), pp. 247–254. Abstract | Full Text + Links | PDF (422 K)
Mehanna et al., 1985. H.T. Mehanna, G.C. Martin and C. Nishijuma, Effects of temperature, chemical treatments and endogenous hormone content on peach seed germination and subsequent seedling growth. Sci. Hort. 27 (1985), pp. 63–73. Abstract | Full Text + Links | PDF (605 K)
Powell, 1987. L.E. Powell, Hormonal aspects of bud and seed dormancy in temperate-zone woody plants. HortScience 22 (1987), pp. 845–850.
Seeley et al., 1998. S.D. Seeley, H. Ayanoglu and J.W. Frisby, Peach seedling emergence and growth in response to isothermal and cycled stratification treatments reveal two dormancy components. J. Am. Soc. Hort. Sci. 123 (1998), pp. 776–780. View Record in Scopus | Cited By in Scopus (4)
Zigas and Coombe, 1977a. R.P. Zigas and B.G. Coombe, Seedling development in peach, Prunus persica (L.) Batsch. I. Effects of testa and temperature. Aust. J. Plant Physiol. 4 (1977), pp. 349–358. Full Text via CrossRef
Zigas and Coombe, 1977b. R.P. Zigas and B.G. Coombe, Seedling development in peach, Prunus persica (L.) Batsch. II. Effects of plant growth regulators and their possible role. Aust. J. Plant Physiol. 4 (1977), pp. 359–362. Full Text via CrossRef
Corresponding author. Tel.: +34-968-396-309; fax: +34-968-396-213 |
收藏
点赞!
反对!