DETERMINATION OF ZONES OF DIFFERENT PLUM GROWING PERIOD LENGTH IN SERBIA

Abstract: Zoning of Serbia territory was performed according to the length of active period of photosynthesis for plum cv. Požegača. Relations between thermal indicators of location and plum growing period length are examined. It was found that the length of growing period is of great extent, determined by the length of frost-free period, as well as by the mean annual air temperature and the mean air temperature for the growing period. The study was carried out using data from 55 phenological and 26 climatological stations in Serbia for the period from 1961 to 1995.


I n t r o d u c t i o n
Despite the decline of total tree number in last few decades, aggravation of tree age composition, unfavorable assortment, poor health condition caused by virus sharka (Plum Pox) and smaller income competitiveness compared to other fruit species, plum is still, according to its participation in total fruit production, among the most important fruits in Serbia. Almost all programs and projects of revitalization of plum growing in Serbia include zoning as one of the basic presuppositions for more successful production of this fruit.
Spatial distribution of the growing period is very important for plum zoning, because macrophenological analysis with spatial and temporal distribution of the most important phenophases is, in methodological sense, a necessary element of land-use planning and crop zoning. Since there is no data for other varieties of this fruit, spatial and temporal distribution of cv. Požegača may serve as a marker and indicator of regional influence on phenology of other varieties as well.

Material and Methods
Determination of zones of different growing period length for plum cv. Požegača is based on phenological observations from 55 phenological stations in Serbia for the period from 1961 to 1995. Požegača is the only plum cultivar which has a representative data set (long enough for sufficient number of stations). Phenological data are shown in two tables. In Table 1a are data for locations where both phenological and climatological observations were made, while data for locations with only phenological data avaliable are shown in Table  1b.
Since the monitoring program of Republic Hydrometeorological Service of Serbia includes only some phenological stages of cv. Požegača, growing period is defined in a narrower sense as an active period of photosynthesis as it was done, for example, by M e n z e l, (2003) in her studies. This period begins with leaf unfolding and ends with general leaf colouring.
Mapping of phenological zones was done by izophens. The degree of correlativity between thermal and phenological indicators is determined using standard statistical methods.

Results and Discusion
The leaf unfolding and the general leaf colouring of plum cv. Požegača in Serbia occur at different times at different locations (Tab. 1a i Tab. 1b). On analyzed locations, the time span between the earliest (April 6 -Minićevo) and the latest mean date of leaf unfolding (May 2 -Zlatibor) is 26 days. The difference between the earliest (September 21-Ljubovija) and the latest mean date of general leaf colouring (October 24 -Knjaževac, Predejanje) is 33 days. Zonal distributions of different times of leaf unfolding and general leaf colouring are presented in Figs 1 and 2.
The beginning of Požegača leaf unfolding is strongly influenced by air temperature in March, the month that precedes this phenological phase. Correlation between mean date of leaf unfolding and mean air temperature in March is very strong and negative, r = -0.80 (Tab. 2). Warmer March causes earlier start of plum growing period, so its leafing begins earlier. Higher temperatures in March usually shift dates of late frost to earlier occurrence. Consequently, correlation between the mean date of leaf unfolding and the mean date of last frost is very high and positive, r = 0.79 (Tab. 2). Therefore, these two meteorological parametars may serve as indicators for determination of potential locations suitable for production of fast ripening plum varieties (i.e. Obrenovačka Posavina).  Compared to leaf unfolding, correlation between the mean date of general leaf colouring and the mean air temperature in September is weaker (Tab. 2). The same findings for influence of thermal conditions on the beginning and the end of growing period were reported by other autors (M e n z e l and F a b i a n, 1999; The length of active period of photosynthesis for cv. Požegača varies from 160 to 196 days. The leaf unfolding is largely determined by thermal conditions of the environment, but not the general leaf colouring. Long plum growing period is characteristic of locations with high mean annual and mean temperature for the period April -October, and with a long frost-free period.
Locations with high mean air temperature in March and mean date of last frost are potentially suitable for growing of fast riping plum varieties.