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Effect of length and thermal time of the growing season on blueberry production

. Highbush blueberry is one of the most popular fruit-bearing culture in the world. But its cultivation is associated with a number of peculiarities, which must be considered when setting up plantations in a given region. To ensure large stable harvests cultivars require sufficient heat sum and time for successful formation of flower buds, which provide the next year’s harvest of berries. Initiation and formation of flower buds begins during crop maturation and ends with the growing season. According to our observation data, blueberry cultivars require no less than 100 days and a heat sum of no less than 900⁰C (of temperatures above 0⁰C) during this period to successfully initiate flower buds. Failing to meet this mark leads to a significant (2,0-2,5 times) drop in yield. When setting up industrial plantations it is important to consider these circumstances and to select the cultivars, maturation times of which allow the plants to successfully form the number of flower buds, which provide a consistent high yield and profitability of the grown culture in the given region.


Introduction
The most important economically viable trait of any type of fruit bearing crop is its productivity -a complex trait, which characterizes the genotype's resilience to hostile climate conditions and diseases. Productivity is the deciding factor of a successful introduction, and thus an important goal in this process is to determine the ability of plant cultivars to produce high quality harvest.
As the result of many years of monitoring the productivity of different cultivars of highbush blueberry [1,2] it has been determined that cultivars are characterized by genetically determined fruiting periodicity [3]. But, in addition to genetic traits, harvest size is affected by a wide variety of factors, in particular by the climate conditions of the region where the plant is grown. Some of these factors, such as, for example, the amount of moisture the plants receive, are controllable and can be adjusted in accordance with the needs of a plant. Others, such as the beginning and the end of the growing season, its length, thermal time, are impossible to manage artificially when growing the plant in open soil. Therefore they often provide the key influence on the productivity of the cultivated plants. Blueberry productivity is largely characterized by the plants' ability to initiate flower buds, inflorescences in the year, preceding the fruiting. flowering strength and mass yield of the fruits [4]. And, as productivity of blueberry cultivars depends on how favorable the circumstances of for initiation and formation of flower buds were during the previous season, the conditions of the previous year play the chief role, rather than those of the current year. And they may affect cultivars of different maturation times in different fashions.
The aim of our research was to determine the effect of the length and thermal time of a growing season, as well as the length and thermal time of the period from mass crop maturity until the end of the growing season on formation of harvest and productivity of blueberry cultivars of different ripening periods.

Materials and methods
The research was conducted form years 2011 to 2019 on Gantsevichi experimental base of the Central Botanical Garden NAS of Belarus, sutiated in Gantsevichi district, Brest region. The object of the research were 8 cultivars of highbush blueberry of different ripening periods. early ripening -Bluetta, Duke, Patriot; mid-ripening: Bluecrop, Denise Blue; late ripening: Coville, Darrow, Elizabeth, and 2 cultivars of half-high blueberry: Northcounthry and Northblue.
Climate characteristics of a given research year were calculated based on the monitoring data provided by Gantsevichi weather station. Phenological monitoring was conducted according to the methodology of I.D. Yurkevich et al. [5]. The research of fruiting peculiarities was conducted according to the methodology of S.Ye. Korovin et al. [6]. Harvest evaluation was done by multiple weightings as the fruits ripened, by weighing and summing up the masses of the harvested fruit on each experimental bush, after which average values were calculated for each cultivar.
Statistical processing of the data was conducted on a personal computer using the «Microsoft Excel 2019» application.

Results and discussion
As the research data has shown, the dates of the beginning and end of the frost-free period noticeably varied during the monitoring years (table 1). At the same time, variation of such metrics as the length of the frost-free period and the number of days with temperatures above +10⁰C was not notable (table 1) and did not affect the formation of harvest and productivity of cultivated blueberry in any significant way. The length of the growing season during the monitoring years was within normal values at 230-260 days, and the number of days with temperature above +10⁰C was 135-168 days. A similar situation could be seen with thermal time, both for individual months as well as for the entire growing season (table 2). In general, the thermal time of a growing season also was within normal values at 3100-3460⁰C. Nonetheless, the yield of the blueberry cultivars during the research period greatly varied, and, at the same time, depended neither on the length of the growing season of the year preceding the fruiting, nor on its thermal time. Frosts during the flowering period of blueberry, as well as extreme winter conditions a long thaw followed by an abrupt temperature drop made some adjustments in certain years of the research (2011, 2014), but in other years these factors did not affect anything.
Considering that all researched cultivars grow on the same plot, in the same environment, it follows that all biotic and abiotic regional growth factors affect them in the exact same way. Nonetheless, the results provided in table 3 show that early-and midripening cultivars of highbush blueberry and early ripening cultivars of half-high blueberry display the highest and most consistent productivity in our environment, unlike the late ripening cultivars.
Annual productivity of late ripening cultivars of blueberry was 2,0-2,5 times lower, than in early-and mid-ripening cultivars (table 3). Because of that it has been decided to analyze the effect of the length and thermal time of the period from mass crop maturity until the end of the growing season on blueberry productivity, as initiation of flower buds of the next year's harvest takes place precisely at this time.   Mass crop maturity in early-and mid-ripening cultivars of highbush blueberry, as well as cultivars of half-high blueberry in our environment happens, as a rule, in July and the first half of August, and these months are the warmest ones of the entire growing season (table 4). Mass crop maturity in late ripening cultivars happens in September, in rare cases it occurs in в August. Therefore, in late cultivars the warmest months of the season take place during fruit formation period and the beginning of fruit ripening. As a result, late cultivars have a shorter period to initiate flower buds for the next year's harvest and receive much less heat to undergo this process. As calculation of the length of the time period from mass crop maturity to the end of the growing season has shown, along with the calculation of the heat sum of positive temperatures during this period, the difference in these values in early and late cultivars is 1.5-2.0 times (tables 5, 6). In particular, the length of the period from mass crop maturity until the end of the growing season in early ripening cultivars during the monitoring years was 99 to 133 days, in mid-ripening ones -95 to 127 days, and in late ripening ones -62-102 days (table 5). Thermal time of this period also varied significantly was 1166⁰-1640⁰C for early ripening cultivars, 977-1664⁰C for mid-ripening ones and only 816-1146⁰C for late ripening ones (table 6). Table 4. Maximum temperatures by ten day periods of the second half of a growing season.

Monitoring year
Maximum temperatures by ten day periods of the second half of the season July August September October November  I  II  III  I  II  III  I  II  III  I  II  III  I  II III  2011 27  Table 5. The length of the period from mass crop maturity until the end of the vegetation during the research.