The zone in which peaches can be economically grown is limited to both north and south by the temperature. In the northern hemisphere, the winters become too mild for peach as we go south and too cold as we go north. Hungary is located on the northern borders of the peach-growing zone, so low temperatures in the dormant season are the critical factors for the survival of the trees and for the reliability of production. The various overwintering organs of the trees have very different levels of frost resistance, which also vary greatly over time. The experiments carried out in apricot and peach chiefly examined changes in the frost resistance of the flower buds, but there is information on the frost tolerance of the vegetative buds and the twigs as well (Guerriero, 1982; Hatch and Walker, 1969; Hewett, 1976; Layne and Gadsby, 1995; Miranda et al., 2005; Proebsting and Mills, 1978; Quamme, 1974; Szabó, 1992; Werner et al., 1993). Farm experience shows that during dormancy, it is the flower buds that most frequently suffer frost damage (Nyéki and Szabó, 1989; Szabó, 2002; Szabó and Nyéki, 1988, 1991, 1998; Timon, 2000). The vegetative buds are usually damaged at lower temperatures (4 to 5 °C) than the flower buds, although it has been reported that for some genotypes, the vegetative buds are more sensitive to frost than the flower buds in the first half of the winter (Szalay, 2001). This can be explained by the diverse development rates of the two types of buds and by their differing responses to changes in environmental conditions. The flower buds play a key role in yield stability, because damage to these organs leads directly to yield losses. If the vegetative buds on the nodes are destroyed by frost, they can be partially replaced by the latent buds on older branches. From a practical point of view, the frost resistance and winter-hardiness of the flower buds thus deserves more detailed study.
In preparation for the winter, the flower buds gradually develop frost resistance as a result of the joint effects of inherited genotypic traits and environmental factors. This process begins long before frost is actually experienced, probably in response to the shortening of the days and the gradual decline in the temperature (Ashworth et al., 1983; Ashworth and Wisniewski, 1991; Flore, 1994; Westwood, 1993). Investigations on the vegetative organs of apple trees and forest trees indicated that the frost tolerance of overwintering organs developed in several stages (Howell and Weiser, 1970; Tromp, 2005). Studies on the frost tolerance of the overwintering organs of woody plants demonstrated that the hardening processes taking place during the first half of dormancy could be divided into several stages (Tromp, 2005). Two stages could be distinguished for fruit trees in the temperate zone and three for trees in the northern boreal zone. In the case of apple trees, the first stage of hardening was found to start well before the trees shed their leaves in fall and continued until the external temperature dropped consistently below 0 °C. Temperatures below 0 °C were required if the frost tolerance of the tissues was to increase. If this condition was not satisfied, the second stage of hardening failed to take place (Howell and Weiser, 1970).
In addition to genetically determined traits and ecological factors, other factors also influence the frost resistance and winter-hardiness of overwintering organs. These include the training system, the cultural practices, and the yield in the previous year (Byers and Marini, 1994; Flore et al., 1983, 1987; Timon, 2000). Frost resistance also depends on the health status of the trees, because many pathogens, particularly bacteria, may cause severe reduction in frost resistance (Gross, 1984; Lindow et al., 1982).
Because considerable differences have been observed between the frost resistance levels of various peach accessions, one of the most important tasks facing breeders at the northern limit of the production zone is to obtain precise knowledge on the frost resistance and winter-hardiness of the genotypes and to breed resistant cultivars (Layne, 1989, 1996). Thus, the development rate of the overwintering organs of peach and changes in their frost resistance during the dormancy period have been regularly investigated in recent years in the gene bank collection of Corvinus University of Budapest.
The present article provides a detailed analysis of the hardening and dehardening of flower buds, of the effects of environmental factors, and of the genotypic differences in two consecutive winters with very diverse weather conditions.
Ashworth, E.N., Rowse, D.J. & Billmyer, L.A. 1983 The freezing of water in woody tissues of apricot and peach and the relationship to freezing injury J. Amer. Soc. Hort. Sci. 108 299 303
Bartholy, J., Pongrácz, R., Barcza, Z., Haszpra, L., Gelybó, G., Kern, A., Hidy, D., Torma, C., Hunyady, A. & Kardos, P. 2007 The regional effects of climate change: Present state and the expected tendencies Földrajzi Közlemények 55 257 270
Byers, R.E. & Marini, R.P. 1994 Influence of blossom and fruit thinning on peach flower bud tolerance to an early spring freeze HortScience 29 136 148
Dunkel, Z. 2005 Characteristic features of climate of Hungary from 1901 to today National Meteorological Service, Paletta Press Budapest, Hungary
Flore, J.A. 1994 Stone fruit 233 270 Schaffer B. & Anderson P.C. Handbook of environmental physiology of fruit crops. Vol. 1. Temperate crops CRC Press Boca Raton, FL
Flore, J.A., Howell G.S. Jr, Gucci, R. & Perry, R.L. 1987 High density peach production related to cold hardiness Compact Fruit Tree 20 60 65
Flore, J.A., Howell, G.S. & Sams, C.E. 1983 The effect of artificial shading on cold hardiness of peach and sour cherry HortScience 18 321 322
Gross, D.C. 1984 The effect of ice nucleation–active bacteria on temperatures of ice nucleation and freeze injury of Prunus flower buds at various stages of development J. Amer. Soc. Hort. Sci. 109 375 380
Hatch, A.H. & Walker, D.R. 1969 Rest intensity of dormant peach and apricot leaf buds as influenced by temperature, cold hardiness and respiration J. Amer. Soc. Hort. Sci. 94 304 307
Layne, R.E.C. & Gadsby, M.F. 1995 Determination of cold hardiness and estimation of potential breeding value of apricot germplasm Fruit Varieties Journal 49 242 248
Mersich, I., Prager, T., Ambrozy, P., Hunkar, M. & Dunkel, Z. 2002 Atlas of climate of Hungary National Meteorological Service Budapest, Hungary
Miranda, C., Santesteban, L.G. & Royo, J.B. 2005 Variability in the relationship between frost temperature and injury level for some cultivated Prunus species HortScience 40 357 361
Proebsting E.L. Jr 1970 Relation of fall and winter temperatures to flower bud behavior and wood hardiness of deciduous fruit trees HortScience 5 422 424
Proebsting E.L. Jr & Mills, H.H. 1966 A standardized temperature-survival curve for dormant Elberta peach fruit buds Proc. Amer. Soc. Hort. Sci. 103 842 845
Proebsting E.L. Jr & Mills, H.H. 1978 A synoptic analysis of peach and cherry flower bud hardiness J. Amer. Soc. Hort. Sci. 103 842 845
Quamme, H.A. 1974 An exothermic process involved in the freezing injury to flower buds of several Prunus species J. Amer. Soc. Hort. Sci. 99 315 318
Richardson, E.A., Seeley, S.D. & Walker, D.R. 1974 A model for estimating the completion of rest for ‘Redhaven’ and ‘Elberta’ peach trees HortScience 9 331 332
Szabó, Z., Nyéki, J., Szél, I., Pedryc, A. & Szalay, L. 1998 Low temperature injury in peach and nectarine cultivars Acta Hort. 465 399 404
Szalay, L. 2001 Frost resistance and winter hardiness of apricot and peach varieties PhD thesis, Faculty of Horticulture, Szent István University Budapest, Hungary
Szalay, L., Papp, J. & Szabó, Z. 2000 Evaluation of frost tolerance of peach varieties in artificial freezing test Acta Hort. 538 407 410
Tromp, J. 2005 Frost and plant hardiness 74 83 Tromp J., Webster A.D. & Wertheim S.J. Fundamentals of temperate zone tree fruit production Backhuys Publishers Leiden, The Netherlands
Werner, C.M., Crassweller, R.M. & Clark, T.E. 1993 Cold hardiness of peach stem tissue over two dormant seasons Fruit Var. Journal 47 72 79
Westwood, M.N. 1993 Dormancy and plant hardiness 382 419 Westwood M.N. Temperate-zone pomology: Physiology and culture 3rd Ed Timber Press Portland, OR