Performance of ‘ Emerald ’ and ‘ Jewel ’ blueberry cultivars under no-chill incidence 1

Due mainly to its nutraceutical properties and gastronomic versatility, the growing demand for blueberry (Vaccinium spp.) has driven a significant increase in the world production, in recent years, from approximately 144,000 tons in 1998 to 552,000 tons in 2016, led by countries such as the United States, Canada, Chile, Mexico, Poland and Germany (Brazelton 2015, FAO 2018). The United States and Canada are also the largest consumers, with the production being inadequate to keep up with the consumers demands, particularly in the off-season. This creates an ABSTRACT RESUMO

Due to the requirement of cold weather to induce blooming (Olmstead et al. 2015), the growing of blueberry plants in Brazil has been restricted to the three states of the southern region and to some high-altitude regions in the states of São Paulo and Minas Gerais, at elevations of 1,600 m and 1,200-1,500 m, respectively.
There are two varietal groups that are grown commercially in Brazil: the Rabbiteye group, which currently includes the most common cultivars, requiring 300-650 chill hours; and the Southern Highbush group, including cultivars such as 'Misty' and 'O'Neal', which were introduced to Brazil in 2000 and are planted in frost-free southern regions of the country, requiring at least 250 chill hours to induce fruit production, and are not well adapted to hot climates (Cantuarias-Avilés et al. 2014).
The gradual increases in air temperature recorded in the last decades around the world have restricted the areas suitable for temperate fruit crops.This emphasizes the need for programs involving cold weather stone fruit trees and berries that seek to develop less cold-dependent cultivars (Byrne 2005, Lyrene 2008).Such programs have already reached some success in areas of low cold incidence, such as southern California, Hawaii, Mexico, Spain and Portugal (Hummer et al. 2007).
Blueberry cultivars of the Southern Highbush group, developed by the University of Florida, that have low to very low cold weather dependence (100-400 h) (Lyrene 2008), were introduced into Brazil in 2010.The introduction of these cultivars aims to facilitate the expansion of the blueberry cultivation and diversify the fruit production, especially among small producers with family labor.
This study aimed to evaluate the performance of 'Emerald' and 'Jewel' blueberry cultivars planted under subtropical climate, with no cold winter period.

MATERIAL AND METHODS
The study was conducted between 2014 and 2015, in a commercial plantation with 'Jewel' and 'Emerald' blueberry cultivars, in Piracicaba, São Paulo state, Brazil (22º43'30"S, 47º38'51"W and Cwa climate, by the Köppen and Geiger classification).The plants were disposed in a semi-protected environment, covered with lowdensity polyethylene and with the sides open.The monthly averages of minimum, mean and maximum temperatures (ºC) (Table 1) were obtained from the meteorological station of the Escola Superior de Agricultura Luiz de Queiroz, situated 7 km from the study site.Air temperature data were measured every 15 min and compiled to calculate the number of chill hours below 7.2 ºC, 10 ºC, 12 ºC and 15 ºC (Table 1).
Thirty 3-year-old plants of each cultivar ('Jewel' and 'Emerald') were placed at a spacing of 2.0 m x 0.8 m (6,250 plants ha -1 ), in 12 L pots containing a commercial substrate of pine bark and fertigation.Plants were pruned to a height of 20 cm, at five months before starting the evaluations.The new shoots that grew after pruning were evaluated from May 2014, regarding the following variables: Growth and sprouting: one shoot per plant was selected from each cultivar and tagged after counting 12 nodes from the meristematic apex downwards, being its length measured every four weeks from the previously marked point to the terminal pair of leaves, using a tape measure.The average monthly shoot growth rate, expressed in cm month -1 , was calculated from the difference between consecutive shoot length evaluations in 30 shoots per cultivar.The average monthly shoot sprouting rate, expressed as the mean number of sprouts per month, was calculated from the counts of lateral sprouts emitted on the tagged shoots, as well as those emitted from any lateral sprout grown in the tagged shoot portion; Characterization of the flowering and fruiting phases: in each cultivar, two branches were selected per plant and the number of flower buds and flowers were evaluated fortnightly until the petals fell off; Fruit yield: the number of fruits per plant was estimated from the average number of productive branches per plant, counted in seven plants of each cultivar, and the average number of fruits per branch, recorded in 60 branches sampled per cultivar.Fruit yield (kg plant -1 ) was estimated by multiplying the estimated number of fruits per plant by the average individual fruit mass (1.5 g); Productivity: calculated by multiplying the estimated fruit yield (kg plant -1 ) by the density of 6,250 plants ha -1 , being expressed in kg ha -1 .

RESULTS AND DISCUSSION
'Emerald' blueberry plants presented two main periods of shoot growth, with the highest peak observed in June and a smaller one in October, both characterized by the emergence of new shoots (Figure 1A).'Jewel' blueberry plants also had two periods of shoot growth, but in both the shoots were shorter than those of the 'Emerald' plants, with the first peak occurring also in June, and a second one recorded between November and December.For the 'Jewel' cultivar, three peaks of lateral sprouting were identified on the tagged shoots, the first in October 2014 and the other one in January and February 2015 (Figure 1B).
In 'Emerald' blueberry plants, lateral sprouting peaks were recorded from August onwards (Figure 1A), after the main shoots had ceased to grow.This process was likely enhanced by the occurrence of high temperatures from August onwards, in the experimental site (Table 1), that increased metabolic rates and promoted shoot sprouting.Zheng et al. (2017) found that increases in temperature enhanced dramatically the growth of four Southern Highbush cultivars through the warming effect, demonstrating an optimal temperature ranging from 29.0 ºC to 32.6 ºC.They also found that physiological parameters, such as leaf photosynthesis, stomatal conductance and transpiration, had similar trends as plant growth in response to temperatures, suggesting  that the biochemical and photochemical processes of blueberry plants are boosted by the optimal growth temperature.The emergence of new lateral sprouts allows an increase in the plant productive potential, since it guarantees the renewal of the productive branches and the continuity of fruit production in time.For the 'Jewel' cultivar, two peaks of lateral sprouting were recorded in October 2014 and February 2015 (Figure 1B).In this study, the 'Emerald' and 'Jewel' cultivars had different periods of flowering and fruiting (Figure 2).For the 'Emerald' plants, about 50 % of the flower buds emerged between late May and late June 2014, resulting in a first flowering peak between mid-June and late July (Figure 1A).For this cultivar, the duration of the flowering stage was 56 days, although 42 % of the flowers emerged at the end of this phase, between the second and fourth week of July.Harvesting began in August, with a peak at the beginning of September.A total of 75 % of the fruits were harvested between the second week of August and the third week of October.A second peak of flower buds occurred in the third week of September until the first week of October, which produced fruits from the second half of November until the beginning of January, 2015.The main harvest period observed in the 'Emerald' cultivar is particularly interesting for fresh fruit export, as it may be able to supply the North hemisphere's markets during the off-season period extending from August to March.
'Jewel' plants produced flower buds later than 'Emerald' plants, with a main peak of flower buds registered in the third week of September, that came into flowering at the first week of October.For this cultivar, a second peak of flower buds was observed in early February 2015; however, these buds did not complete their development and aborted, most likely due to the high temperatures recorded during this period (Table 1).
Flowering of the 'Jewel' blueberry cultivar (Figure 2B) extended over 30 days and was more concentrated and shorter than that of the 'Emerald' cultivar (Figure 2A).For the 'Jewel' cultivar, fruit harvest happened later, but in a more concentrated way (Figure 2B) than in the 'Emerald' cultivar (Figure 2A), starting at the end of October 2014 and ending at the second week of January 2015, with 85 % of the fruits being harvested in this period (Figure 2B).Distinctly, a second fruit crop of smaller intensity was harvested from the 'Emerald' plants in January 2015 (Figure 2).
Other studies involving 'Jewel' and 'Emerald' blueberry cultivars grown in warm climates in Hawaii (Hummer et al. 2007) and Florida (Lyrene 2008), USA, also described a later flowering period of 'Jewel' plants, as compared to the 'Emerald' cultivar.In Florida, 'Emerald' blueberry plants flowered from January to February (corresponding to the months of July to August in the southern hemisphere), and produced large, firm and sweet fruits from April to May (October to November in the southern hemisphere) (Lyrene 2008), with more concentrated and different fruiting periods than those observed in this study.
In Florida, both cultivars required at least 100 h of air temperature below 7.2 ºC, in order to flourish (Lyrene 2008).Such temperature threshold did not occur at the experimental site in Piracicaba (Table 1), and it is not necessary to stimulate flowering in both studied blueberry cultivars.
In Hawaii, Hummer et al. (2007) stated that the lack of low temperatures necessary for breaking bud dormancy did not affect the productivity of Southern Highbush genotypes, including 'Jewel' and 'Emerald'.In the United States, Norvell & Moore (1982) concluded that temperatures between 1 ºC and 12 ºC were sufficient to meet the cold needs of Southern Highbush blueberry plant genotypes.

Figure 1 .
Figure 1.Average shoot growth rate (black) and number of sprouts per shoot (blue) in 'Emerald' (A) and 'Jewel' (B) blueberry plants.