Edited, commented upon and adapted by Carlos José Tapia T. Technical Director for Avium and Content Director for SmartCherry. 2019.
Adapted from: Flower Induction and flower bud development in Apple and Sweetcherry. Nikolaos Koutinas1, Guepo Pepelyankov2, Valentin Lichev2.
1Alexander Technological Education Institute of Thessaloniki, Plant Production Department, Thessaloniki, Greece. 2Agricultural University, Plovdiv, Bulgaria. Published in Biotechnol & Biotechnol. Eq. 2010, 24 (1), 1549-1558.
As a result of research performed in the past 20-30 years, we have obtained new information on the formation of flower buds in some fruit species, especially in cherry and apple trees, including some data on the presence of genes that determine reproductive organs. Some problems with floral induction, the apical meristem’s histological transformation, morphological differentiation in flower buds, factors and conditions that influence the formation of flower buds and the quality of reproductive organs will be discussed in this report. We have paid some attention to the pruning process, especially summer pruning; and the use of growth regulators through flower bud formation processes that may be regulated.
Keywords: Cherry trees, apple trees, floral induction, histological transformation, morphological differentiation, agricultural practices.
To this day a large portion of the most important features in flower bud formation is shrouded in mystery, not having been sufficiently cleared up by theoretical biology and applied horticulture (Buban, 1996). Deserving of special attention are the clarification of morphogenetic changes that take place in the buds’ vegetative state and the initiation of organs within the flower (Foster et.al.,2003).
The dormancy period allows buds to complete their development cycle within two successive calendar years. This provides the opportunity to diminish nutrient competition in flower and vegetative development, thus presenting a substantial advantage for fruit trees, especially in cherries.
According to Zimmerman (1972), the duration of the youthful period, a unique feature of plants that propagate through seeds, is genetically controlled. The appearance time for the first flowers is given by genotypes that can be accelerated applying different methods. It seems that the most effective route is the stimulation of vegetative growth, so that new plants can reach a large size as soon as possible. The beginning of the formation of flower buds in new apple and cherry plants can be accelerated using less-vigorous rootstocks.
Transition from youthful phase to mature phase can be caused by several mechanisms, including hormonal control after absorption distribution is completed in the apical meristem zone (Hacket, 1976).
In order to reduce the duration of an unproductive period in young trees, dwarfing or semi-dwarfing rootstocks are used – nowadays these are applied at a massive scale. However, growth weakening is not always a desirable trait, particularly in the first and second year after planting (Costes et.al, 2004).
The formation of mature flower buds is fundamentally determined by hormone presence (Buban, 1996). The lack of initiation in flower buds within alternate-bearing crops is directly related to a past with an exhaustion effect in tree reserve accumulation from one season to the next, especially in cherries; and to a hormonal effect in other species such as apple trees, which can be attributed to the action of hormones and particularly to gibberellins produced in young fruit trees known as “yearly” (Luckwill and Silva, 1979).
2. Flower bud formation sites.
Most apple and cherry cultivars produce fruit mainly in “spurs” (Forshey and Efving, 1989). In general, it is during this period of preliminary induction in flower bud formation that “spurs” are produced, especially in cherries where we can also see current-year twigs (Buban and Faust, 1982). Leaves in these “darts” can constitute more than 60% of the tree’s total leaf area.
In both cases, buds are unique flower buds (not mixed), however, for the case of “spurs” there is always (at least) one vegetative bud, which ensures renovation for this fruit spot. This is not the case for year-old twigs, which after blooming or bearing fruit result in a “blind” spot.
2.1 Flower bud development.
The formation of flower buds is a complex phenomenon. It must be considered that genes for the formation of flower buds are only a basic factor in their appearance, and the formation of gametes at the moment of meiosis is the final event. Wellensiek (1977) formulated that the initial reasons for the formation of flower buds can be defined as “something that takes place between the flower’s gene formation and meiosis.” This declaration remains valid today (Buban, 1996). For the initiation of the flower it is necessary to halt the retention of genes that are responsible for flower bud formation; in that sense inducing the formation of flower buds can have a similar significance in unblocking the process.
Identifying and characterizing a number of genes connected to the formation of flower buds in some angiosperm species models such as Antirrhinum majus L and Arabidopsis thaliana (L) Heynh, has, in the past ten-year period, allowed us to search for their homology in fruit species.
The formation of flower buds takes place in the following succession: induction to the formation of flower buds, histologic transformation and morphologic differentiation (Buban y Faust,1982).
2.2 Induction of flower buds.
According to Dolega et al., 1997, induction gives a sufficiently developed and “susceptible” impulse to the transition of vegetative buds toward a generative or floral phase, but it is still not clear which the fundamental reasons for induction are (Buban, 1996, Link 1992). It could be said that it is a qualitative change; the final result can be programmed by the parts that are strategically placed from the meristem to the point of flower formation (Buban and Faust, 1982). Induction can also be seen as a process where the information that is undergoing transformation is being repressed in order to form a new structure, that is, the flower bud. In the case of apple trees, AFL-type genes (Apple Floricaula Leafy) are supposed to be included in the induction process (Kotoda et. al., 2000). According to Luckwill (1974) induction is connected to changes in hormonal balance, and according to Sachs (1977) it is connected to changes in distribution in the apical meristem’s absorption. Link (1992) shares the point of view that induction of flower buds, as well as all fruits and the fall of fruits or primordia in different organs within the plant, depend on the interaction in space and time of their own substances that promote or stunt growth. Even in apple trees, it is possible to use GA3 spraying within the supposed induction period, thus counteracting the effect of flower bud formation (Li et.al., 1994).
2.3 Histologic Transformation.
When the appendix of the still-vegetative bud receives the signal to differentiate into a flower bud (approximately 70 Days From Full Bloom-DFFB in cherry trees), a sequence of events takes place. Mitotic activity becomes complete for the histologic structure’s whole apex. The central meristem is further unfurled but without any morphological changes in the appendix, histologic transformation takes place, and from the moment that blooming begins it becomes irreversible. This is from the point of approximately 90 DFFB, the halfway point for the flower induction stage.
2.4 Morphologic differentiation.
The appearance of flower primordia in cherry trees, within the scope of the same bud, is almost simultaneous (Zeller, 1955). Certain differences in the phases of initial flower development in a bud can be observed up to the point of anther and carpel initiation; after that, all flowers look similar (Díaz et.al.,1981).
The start of bud morphologic differentiation in the month of January (for the southern hemisphere) is followed by the formation of flower parts, which takes place up to the transition toward a winter dormancy state (Kolomietz, 1976). Toward the end of autumn all flower primordia, with the exception of sepals, are in an undifferentiated state. Carpels usually assemble into seed cells, but they are still not present in pistil shape.
Flower bud development rate is not constant across different phases, particularly under unfavorable weather conditions (Huang, 1996). Development can be delayed for a certain period of time. Consequently, it is hard to establish how long a specific phase lasts.
3. Factors and conditions for the formation of flower buds.
The development of flower buds, from induction up to anthesis (full bloom), is subject to the influence of several factors and conditions, which at a certain point are responsible for the formation of flower buds.
3.1 Crops and cultivars.
The beginning stages of histologic (Buban, 1967) and morphologic processes (Bulatovic, 1978, Zeller, 1954) may depend on crop traits. Crops and cultivars can also be influential toward developing more flower buds (Mihailov y Topchiiski, 1975).
Rootstocks, as components of fruit trees with scions, can influence the initial appearance of flower buds in time. According to Elek (1974) some cherry rootstocks exercise this influence only in specific crops. Apparently, as soon as an interaction between both tree components takes place, it causes a change in the beginning of morphologic differentiation. In some studies the differences between observed rootstocks were not significant. Some authors found that rootstocks do not influence the final growth time for buds, even though its length depends on the rootstock’s vigor. They also do not influence the “critical number” of appendixes in the transition from vegetative buds to those in reproductive state or time for flower initiation. The “critical number” of appendixes was found in 20 years’ worth of research, regardless of differences in climate conditions. However, the amount of spurs with flower buds increases under the influence of less vigorous rootstocks. Rootstocks showing different vigor levels do not significantly influence the beginning of the development of flower buds in cherry tree crops (Kuhn and Callensen, 2001).
3.3 Fruit branches.
Differences in timing for the initiation of flower buds in different fruit branches have commonly been noted. Bud differentiation usually begins early in spurs, somewhat later for young and vigorous ones, and later in buds (Abdulkadyrov et.al.,1972, Kolomietz, 1976).
Sometimes the beginning of flower formation in branches with identical positioning (same age) can take place suddenly and almost simultaneously in all of them (Luckwill and Silva, 1979), and in other cases in a much longer time span (Keremidarska, 1968, McArtney and Li, 1998), which means that branch position, whether it be vertical or horizontal in plants, appears not to influence the induction and flower differentiation process.
3.4 Sprout growth.
Flower bud differentiation is often associated with the growth of sprouts. The determination or detention of growth is considered a prerequisite for flower initiation (Abdulkadyrov et.al.,1972, Hirst and Ferree, 1995).
Vegetative growth and flower bud formation are considered antagonizing processes (Davis, 1957, Tromp, 1976). In other words, once the season’s vegetative growth stops (approximately 70 DFFB), the flower induction process begins.
3.5 Detrimental influence of fruit.
The detrimental influence of fruit over the formation of flower buds is a common phenomenon in fruit tree species such as apples, and research shows it is mainly caused by a high hormonal load (especially gibberellins), which is responsible for intervening against flower induction. This can be explained: the flower induction process coincides temporally with fruit development and seed maturation (Davis, 1957, Monselise and Goldshmidt, 1982).
However, in cherries, hormonal load influence is not linked or considered a factor that results in less induction, but rather in high potential that generates exhaustion in plants, agitating the reserve system for the following season. This results in a low response to productive potential. The latter is also affected by other factors.
Five types of clearly defined plant hormones (gibberellins, auxins, cytokinins, ethylene and growth inhibitors) have been recognized. Each of them performs numerous functions, which act simultaneously in order to regulate the fruit plant’s behavior (Luckwill, 1980). In research related to the formation of flower buds, special attention has been placed on gibberellins, which have been named the fundamental factor for the lack of flower bud formation in apples and pears (Luckwill, 1974, Luckwill, 1980). They act against the formation of flower buds with the goal of overcoming alternating delay, which is also seen in the application of exogenous gibberellins.
In summary, the opinions of different authors on the way in which auxins and gibberellins act in the formation of flower buds point toward auxins having an indirect yet favorable effect in the initiation of flower buds in the beginning of the growth season. Auxins located in seeds attract more nutrients to the spurs. This is important early in the season, for quick development of the leaf primordium and of young leaves within the season, and is a prerequisite for the initiation of flower buds. Gibberellins, translocated from seeds, would seem to be counteracting the favorable effects of auxin and decreasing the formation of flower buds. Through the improvement of branch growth, they indirectly decrease the formation of flower buds.
3.7 Ecologic conditions.
Initiation and development of flower buds can be influenced by ecologic conditions. High temperatures can indirectly inhibit the formation of buds in some fruit crops, and daytime or nighttime temperature variations in terms of amplitude can also have a depressive influence in the formation of flower buds (Abbott et.al.,1974). Cold weather before the beginning of morphologic differentiation, as was seen a few years ago, can direct an initial decrease (Abbott, 1977, Mihailov, 1988). Variations due to heat accumulation in the period between full bloom and the beginning of flower initiation, however, are not quantified until the inter-season period (McArtney et.al., 2001). Climate conditions that are favorable to photosynthesis with a large after-harvest period, even in late crops in countries like New Zealand, can lead to the formation of “more vigorous flowers” and that is a favorable effect for crop productivity (Ferree et.al., 2001).
Subtropical climate conditions, where low temperatures are necessary to satisfy cold requirements are insufficient: part of the flower buds in some fruit species – those that had already begun their development – became vegetative.
4. Reproductive organ quality.
The flowers’ quality in cherry trees can be connected to blooming time, and these lapses depend on the different positions of flowers regarding the fruit tree’s wood (Patten et.al., 1986).
Thus a late bloom within the same plant might mean lower fresh weight in its flowers, pistils, ovaries, fruits and soluble solids in fruit contents. Flower buds within spurs usually have a later bloom than those that are located at the base of the year’s branches.
Formation of double fruits is the result of the formation of two pistils at the time of previous-year flower bud differentiation (January in the southern hemisphere), and represents a significant problem in some regions with warmer summers (Micke et.al.,1983). A critical factor in the formation of double-pistil flowers in cherry crops is a daily air temperature over 30ºC (Beppu and Kataoka, 1999). Buds are more sensitive to high temperatures when they are in the transitional phase between sepal differentiation and petal differentiation (Beppu et.al., 2001). It is possible to reduce the formation of double pistils with the use of artificial shade, particularly during unusually warm summers, with the objective of decreasing air temperature (Beppu and Kataoka, 2000). Forcing cherry trees to accelerate maturation can decrease the formation of double pistils through the acceleration of bud differentiation dates, thus avoiding their exposition to high temperatures when they are still sensitive to them (Beppu et.al., 2001). The quality of flower buds and flowers in cherry trees will determine their sensitivity to frost in late winter and early spring (Kuhn and Callesen, 2001).
Pruning during dormancy and its influence on growth and fruition in fruit trees has been the subject of several studies (Forshey and Efving, 1989, Mika, 1986). Pruning research in the summer is comparatively limited.
Summer pruning done to young, vigorous trees, can be favorable for the formation of flower buds (Miller, 1982). It is desirable for this type of pruning to be done during the first half of the summer (Lord et.al., 1979) so that it will have actual repercussions in the formation of buds.
However, in some cases summer pruning has no positive influence in the initiation of flower buds (Greene and Lord, 1983) and its effects depend on crop features (Webster and Shepherd, 1984). Summer pruning can reduce the number of flower buds per tree, but can increment the number of flowers per inflorescence; therefore the total number of flowers per tree might not finally change (Myers and Ferree, 1983). All of the above is related to the amount of luminosity in plants at the moment of pruning. Without a doubt, vigorous crops with low lighting levels will always benefit from summer pruning.
When it comes to dormant pruning, the cutting of year-old branches can be done in horizontal growth crops once flower buds are initiated in year-old branches, without a negative effect in the formation of flower buds, but not allowing for the development and formation of spurs a year after in this type of wood.
In cherries, trimming sprouts from the current season up to 40-50 days from full bloom increases the number of flower buds at the bases, accelerates flower initiation and favors the formation of larger flower buds in late March in the southern hemisphere (Guimond et.al., 1998a, Guimond et.al.,1998b). The authors recommend summer pruning for young trees that are very densely planted, reporting that inclination in extended sprouts by the third year after planting increases the number of flower buds in basic sprouts. However, the total number of flower buds per tree can decrease (Webster and Shepherd, 1984). The aforementioned is based on branch orthopedic tasks that are widely done during the formation stage.
5.2 Fertilizer use.
Fertilization in fruit trees is a treatment that can considerably affect the formation of flower buds and fruits in general. It must be performed accordingly with the soil’s nutritional condition, cultivar features, rootstocks, climate conditions, fruit load and desired fruit quality (Jonkers, 1979). Applying high doses of phosphorus in tree crops can increase the number of flower bud initiation instances, only if it’s necessary depending on the soil’s condition (Neilsen et.at., 1990, Williams y Thompson, 1979). Phosphorus is supposed to directly influence the formation of flower buds by changing the level of synthetized cytokinins within the roots (Hirst y Ferree, 1995c).
Fertilization done with nitrogen after extension root growth has finished can stimulate the formation of flower buds (William and Renninson, 1963). Early application of N stimulates growth and this is not desirable for the potential formation of flower buds. Fertilization in the summer done with nitrogen, added to an application in spring, increases ovule and stigma vitality in some cultivars (William, 1965). However, in an alternate bearing situation, fertilization with nitrogen during the springtime must be limited, while autumnal fertilization as well as fertilization performed the following spring must be abundant (Werth, 1976).
The influence of optimal irrigation programming is favorable for the formation of flower buds in fruit trees (Keremidarska, 1968). Special attention must be paid to fertigation; in young trees more flower buds are initiated regardless of growth acceleration (Dencker and Hansen, 1990, Dencker and Hansen, 1994a, Dencker and Hansen, 1994b). Competition between growth and fruition may be aggravated as a result of unbalanced influence due to a rootstock, irrigation or other factors. This situation may be changed, however, when growth is stimulated by simultaneous supplies of water and nutrients. This could affect metabolism in a more balanced manner, possibly avoiding the lack of specific metabolites, which are important for flower initiation at critical stages. Through fertigation, root activity, growth and development of sprouts and formation of base buds could improve to a degree that would result in a large portion of these buds becoming flower buds.
In cherries, it is important to pay special attention to providing adequate pollination at its due time, keeping in mind the short vitality of ovules and the embryo sac (Stosser and Anvari, 1983).
5.5 Growth regulators.
Some of the changes that occur in the process of flower bud formation in fruit trees, as a result of the application of growth regulators, were discussed in detail by Mitov et al. (1977). There, some points of view, expressed in the past three decades, were presented. Changes are mainly oriented in two directions: induction of the formation of flower buds via growth suppression using retardant treatment (retardants are substances that suppress growth) (Hansen and Grauslund, 1980, Luckwill, 1974, Luckwill, 1980) or cytokinins (Ramírez et.al., 2000) and the inhibition of the formation of flower buds through applications of gibberellic acid (Hansen and Grauslund, 1980, McLaughlin y Greene, 1991b, Werth, 1976). According to Luckwill and Silva (1979) treatments with retardants can weaken the competitive vigor of yearly sprouts, resulting in increased initiation of flower buds. In the case of gibberellic acid, an opposite mechanism can be expected. Sometimes retardants reduce growth in the extensions of sprouts without ever influencing floral initiation (Greene and Lord, 1983). Treatments with Paclobutrazol usually increase the number of flower buds, but some of the leaves may fall early, resulting in a diminished amount of fruits and performance (Bargioni et.al.1986).
It is assumed that spraying with GA3 gibberellic acid blocks the first step in the process of flower bud formation before sprout growth has stopped (Tromp, 1972). This growth regulator can suppress the formation of flower buds without an influence on growth (Hansen and Grauslund, 1980). Diverse effects can be obtained with growth regulators, shown in different studies, which all stress the special care that is required in their application. Growth regulators must be used as an additional measure to traditional agricultural practices. Their restrictive application is also necessary considering the preservation of the natural environment.
The formation of flower buds in fruit trees in warm climate zones, including apples and cherries, is a complicated biological phenomenon, which has been the object of many studies, the latest of which have included explanations on their genetic essence. However, some of the most characteristic moments in the transition from vegetative bud to one in reproductive state and its further development are still not sufficiently clarified. There is the existence of supposed versatile relations between genetic control, hormonal balance and the presence of enough assimilates in the plant as a whole – and more precisely in the formation of flower buds. The development of flower buds is related with the fruit species’ features and its farming, ecological conditions and agricultural practices. The quality of reproductive organs depends on factors and conditions for the formation of flower buds, which in turn have an influence on fruit production quantity and quality.
The initiation and development of flower buds in apples and cherries can be successfully regulated through well-founded scientific methods and agro-technical practices such as pruning, fertilization, irrigation and treatments using growth regulators.
- Abbott D.L. (1977) Rep. Long Ash. Res. Station for 1976, 167-176.
- Abbott D.L., Bull V., Bishop S.N. (1974) Rep. Long Ash. Res. Station for 1973, 31-33.
- Abdulkadyrov S.K, Batyrkhnov Sh. G., Dzhabaev B.R. (1972) Trudy Dagestanskogo Sel´skokhozyaistvennogo Instituta 22, 58-71. (in Russian) (Hort. Abstr.,43, 4999).
- Bargioni G., Madinelli C., Ramina A., Tonutti P. (1986) Acta Horticulturae, 179(2), 581-582.
- Beppu K., Ikeda T., Kataoka I. (2001) Scient Horticulturae, 87(1-2), 77-84.
- Beppu K. and Kataoka I. (1999) Scient Horticulturae, 81(2), 125-134.
- Beppu K. and Kataoka I. (2000) Scient Horticulturae, 83(3-4), 241-247.
- Buban T. (1967) Arch. Gartenbau, 15, 129-148.
- Buban T. (1996) In: Floral biology of temperature zone fruit trees and small fruits (J.Nieki, M. Soltesz, Eds.) Akademiai Kiado, Budapest, 3-54.
- Buban T. and Faust M. (1982) Hort. Revewes, 4, 174-203.
- Bulatovic M. (1978) Archiv za Poljoprivredne Nauke, 114, 159-164.
- Costes E., Belouin A., Brouard L., Lelezec M., (2004) J.Hort. Sci. Biot., 79(1), 67-74.
- Davis L.H. (1957) Proc. Am. Soc.Hortic. Sci., 70, 545-556.
- Dencker I. and Hansen P. (1990) Gartenbauwissenschaft, 55(4), 145-148.
- Dencker I. and Hansen P. (1994a) Gartenbauwissenschaft, 59(4), 145-149.
- Dencker I. and Hansen P. (1994b) J. Hort. Sci., 69, 869-876.
- Diaz D.H., Rasmussen H.P., Dennis F.G.Jr. (1981) J. Amer. Soc. Hort. Sci., 106(4), 513-515.
- Dolega E., Bertschinger L., Fankhauser F., Stadler W. (1997) Obst-Weinbau, 133(25), 633-635.
- Elek E., (1974) Kerteszeti Egyetem Kozlemeyei, 38(6), 161-174.
- Ferree D.C., Bishop B.L., Schupp J.K., Tustin D.S., Cashmore W.M. (2001) J. Hort. Sci.,76(1),1-8.
- Forshey C.G. and Efving D.C. (1989) Horticultural Reviews, 11, 229-287.
- Foster T., Jonston R., Seleznyova E. (2003) Ann Botany, 92,199-206.
- Greene D.W. and Lord W.J. (1983) J. Amer. Soc. Hort. Sci., 108(4), 590-595.
- Guimond P.K., Andrews P.K., Lang G.A. (1998a) J. Amer. Soc. Hort. Sci, 123(4), 509-512.
- Guimond P.K., Andrews P.K., Lang G.A. (1998b) HortScience, 33(4), 647-649.
- Hackett W.P. (1976) Acta Horticulturae, 56, 143-154.
- Hansen P. and Grauslund Og.J., (1980) Tidskr. Planteavl, 84(3), 215-227.
- Hirst P.M. and Ferree D.C. (1995a) J. Amer. Soc. Hort. Sci., 120(4), 622-634.
- Hirst P.M. and Ferree D.C. (1995c) J. Amer. Soc. Hort. Sci., 120(6), 1018-1024.
- Huang H. (1996) Journal of Fruit and Ornamental Plant Research, 4(3), 95-107.
- Jonkers H. (1979) Scientia Horticulturae, 11, 303-317.
- Keremidarska S. (1968) Gradinar. Lozar. Nauka, 5(3), 3-17. (in Bulgarian).
- Kolomietz I.A. (1976) Surmounting of periodicity of bearing in the apple, Kiev, Urozaj, 17-50. (in Russian).
- Kotoda N., Wada M., Komori S., Kidou S., Abe K., Masuda T., Soejima J. (2000) J. Am. Soc. Hort. Sci., 125, 398-403.
- Kuhn B.F and Callensen O. (2001). Gartenbauwissenschaft, 66(1), 39-45.
- Li Sh., Meng Sh., Li T., Lin H., Tu Y. (1994) Advances in Horticulture, 338-341. (in Chinese).
- Link H. (1992) In: Die Blute (F. Winter et.al., Eds). Lucas Anleitung zum Obstbau, Verlag Eugen Ulmer, Stuttgart, 26-30.
- Lord W.J., Greene D.W., Damon R. Jr. (1979) J. Amer. Soc. Hort. Sci.,104, 540-544.
- Luckwill L.C. (1974) Proc. 19th Intern. Hort. Congr., Warsaw, 3, 237-245.
- Luckwill L.C. (1980) Scientific Horticulture, 31, 60-68.
- Luckwill L.C. and Silva J.M. (1979) J.Hort Sci.,54(3), 217-223.
- McArtney S.J. and Li S.H., (1998) HortScience, 33, 699-700.
- McArtney S.J., Hoover E.M., Hirst P.M., Brooking I.R. (2001) J.Hort Sci. and Biotechn., 76(5), 536-540.
- McLaughlin J.M. and Greene D.W. (1991b) J. Amer. Soc. Hort. Sci., 116(3), 450-453.
- Mihailov Ts. (1988) Rasteniev. Nauki, 25(1), 85-91 (in Bulgarian)
- Mihailov Ts. and Topchiiski SI., (1975) Gradinar. Lozar. Nauka, 12(5), 13-20 (in Bulgarian).
- Mika A. (1986) Hort Reviews, 8, 337-378.
- Micke W., Doyle J.E., Yeager J.T. (1983) California Agriculture, 37(3/4) 24-25.
- Miller S.S (1982) J. Amer. Soc. Hort. Sci.,107(6), 975-978.
- Mitov P., Pepelyankov G., Diakov D. (1977) Centre nautch-techn. Ikonom., Minist. Zemed. Hranit. Prom., Sofia, 15-24. (in Bulgarian).
- Monselise S.P. and Goldshmidt E.E. (1982) Hort. Reviews, 4, 142-173.
- Myers S.C. and Ferree D.C. (1983) J. Amer. Soc. Hort. Sci.,108(4), 634-638.
- Neilsen G.H., Hogue E.J., Parchomchuk P. (2000) HortScience, 25(10), 1247-1250.
- Patten K.D., Patterson M.E., Proebsting E.X. (1986) J. Amer. Soc. Hort. Sci., 111(3), 356-360.
- Ramírez H., Bodson M., Verhoyen M.N.J. (2000) Acta Horticulturae, 514, 245-248.
- Sachs R.M. (1977) HortScience, 12(3), 220-222.
- Stosser R. and Anvari S.F. (1983) Acta Horticulturae, 139, 13-22.
- Tromp J. (1972) J. Hort. Sci., 47, 525-533.
- Tromp J. (1976) Scientia Horticulturae, 5, 331-338.
- Wellensiek S.J. (1977) Acta Horticulturae, 68, 17-27.
- Webster A.D. and Shepherd U.M. (1984) J.Hort. Sci. 59(2), 175-182.
- Werth K (1976) Obstbau und Weinbau, 13, 48-51.
- William R.R. (1965) J.Hort Sci., 40, 31-41.
- William R.R. and Thompson A. H. (1979) HortScience, 14(6), 703-704.
- William R.R. and Renninson R.W. (1963) Horticulturae, 9, 34-38.
- Zeller O. (1954) Angew. Botanik, 28, 178-191.
- Zeller O. (1955) Angew. Botanik, 29, 69-89.
- Zimmerman R.H. (1972) Hort. Science, 7(5), 447-455.