Temperature is the basic factor that differentiates vegetation around the globe ^[1][2][1,2]. Trees and shrubs with edible fruits are no exception in this regard. Temperature, serving as a measure of the amount of heat, is one of the most important factors affecting the growth and development of plants. For most plants in temperate climates, the temperature optimum for growth is 20–30 °C [3]. Particular species of fruit plants exist in a range from minimum to maximum temperatures for their growth and development. Any excess results first in a decrease in immunity causing disease, and then death [4]. Depending on the cultivar, “safe” temperature for mango (Mangifera indica L.) at which no damage occurs to the fruits or leaves varies from 10 to 12 °C [5]. On the other hand, there are plants that survive severe frosts without or with minor damage [6]. Examples include plants growing in temperate climates. According to Proebsting and Mills [7], the flowers of the trees in full bloom survive temperatures below zero degrees Celsius. It is estimated that temperatures between −2.7 °C and −3.1 °C damage only 10% of flowers in full bloom in temperate climate species. Next to genetic predisposition [8], resistance to low, but also high temperatures depends on many factors, including chemical composition, structure, physiological adaptation, and geographical location ^[9][10][9,10].

Temperature affects the basic processes that occur in plants, such as photosynthesis, transpiration, and respiration. Moreover, it regulates the rate of transition of plants from the vegetative to generative phase ^[11][12][13][11,12,13]. The significance of the effect of temperature on yield quality is reported in most studies on fruit plants ^{[14][15][16][17][18][19][20][21][22]}[14,15,16,17,18,19,20,21,22].

At temperatures specific for them, plants show characteristic growth and development in particular physiological phases. Two temperature ranges are usually designated that affect plant development: low temperatures (chilling) and high temperatures (forcing) [23]. The induction of vegetative growth in a given season occurs after the chilling period, whereas growth and development depend on the range of higher temperatures. The range of low temperatures refers to a prolonged accumulation of chilling at which the plant is able to break winter dormancy. It ensures proper development of particular elements in the flower bud, and then development of fruits when higher temperatures occur. After the dormant period, the accumulation of heat forces their transition through subsequent phenological phases of the plant, and leads to reaching the phase of fruit maturity.

2. Grapevine (Vitis vinifera L.)

Grapevine is sensitive to the occurring air temperature at every stage of development (during flowering, growth, and fruit ripening). The harvest yield, uniform ripening, and consistent wine quality depend on temperature during flowering ^[24][137]. The proper course of flowering occurs at minimum mean daily temperatures of 15 °C. Lower temperatures prolong flowering, resulting in evident yielding reduction, and variable pollination terms increase the share of small and slowly ripening fruits ^[25][138]. Cultivars originating from Vitis vinifera have high thermal requirements, and need high temperatures already before flowering. Moreover, variable temperatures during ripening disturb the accumulation of reserve compounds: sugars and acids, strongly affecting the taste of the grapes through the disturbance of proportions between them ^[26][25]. The production of aromatic compounds is also affected, whereas the three aforementioned groups of compounds are key in wine production ^[27][139]. According to White et al. ^[28][140], calculation the growing season base at 10 °C growing degree day summation is of key importance in grape production. Its variability determines the variability of growth and yielding of plants in particular growing seasons. Grapevine cultivation has long employed the index of forcing accumulation during vegetation, i.e., SAT ^[29][74]. The criterion is particularly useful in the selection of the grapevine cultivar for cultivation in a given region. Particular cultivars considerably differ in requirements in terms of the SAT value until reaching full maturity. SAT is calculated when daily temperatures means are equal or higher than 10 °C from the period 1 April to 31 October. SAT and GDD values precisely determine the ripening potential of particular grapevine cultivars in a given region. GDD in viticulture is call Winkler index, and it is one among many indices that are used to describe temperature conditions adequate for grape-growing ^[30][141]. Several other indicators based on heat load (daily cumulative temperature above the 10 °C threshold for a certain period of time) and the temperature requirements of the vines were also developed ^[31][104]. One of them is the Heliothermal index of Huglin (HI). The HI provides information regarding heliothermal and sugar potential. According to Tonietto and Carbonneau ^[32][142], is more pertinent to the qualitative factors such as berry sugar potential. $$\mathrm{HI}={\displaystyle \sum }_{IV 1st\underset{}{\overset{}{IX 30th\underset{}{\overset{}{}}\frac{\left(\mathrm{Tmax}-10 °\mathrm{C}\left(\right)+\left(\mathrm{TD}-10 °\mathrm{C}\left(\right)\frac{2}{}\mathrm{d}\right)\right)}{}}}}^{}$$ TD—mean air temperature. d—length of day coefficient ranging from 1.02 to 1.06 between 40° and 50° of latitude. The increase in day length during the growing season increases potentially relative to an increase in latitude ^[33][143]. In order to improve the assessment of the quality potential of grapes, especially in relation to secondary metabolites (polyphenols, aromas) in grapes, the cool night index (CI) was introduced ^[32][142]. $$\mathrm{CI}={\displaystyle \sum }_{IX 1st\underset{}{\overset{}{IX 30th\underset{}{\overset{}{}}\frac{T min\frac{30\frac{}{}}{}}{}}}}^{}$$ Gladstones ^[34][144] used Biologically Effective Degree Days (BEDD or E °C) to classify grape varieties according to maturity. BEDD include heat accumulation that is defined by maximum and minimum temperature thresholds (between 10 and 19 °C), and the BEDD formula also modifies heat accumulation for diurnal ranges. The GDD formula has also been used for the determination of the values of base temperatures of important phenological phases for grapevines, namely bursting of buds and flowering. Oliviera ^[35][112] applied several statistical methods based on GDD, and found that the determination of the base temperature of the aforementioned phases is the most precise in the case of application of standard deviation, where GDD is calculated based on mean air temperature. According to the author, the base temperature of bud bursting is 8.7 °C, and flowering 10.7 °C. According to Jones et al. ^[27][139], due to global warming, regions of production of high quality grapes are on their climatic range boundaries. Koźmiński et al. ^[36][95], analyzing SAT, found that 60% of the territory of Poland has conditions favorable for intensive grape cultivation. Progress in global warming has resulted in a change in the current limit of intensive grapevine cultivation in the north of Poland (approximately 150 km). The greatest increase in the SAT value has been recorded in the south-west and west of Poland. A considerably lower increase has been observed in the south-east and east, probably due to the fact that it is an area of continental climate ^[37][145]. The study taken under the period of 1971-2020 showed an increase in the values of all agroclimatic indices and air temperature during the growing season, suggesting an increase in the thermal resources in the territory of Poland. ^[38][146]. As a result of changes in climate, most of Poland is currently suitable for cultivation of more demanding cultivars. Based on historical climatic data and model simulation of future climate conditions, Jones et al. ^[27][139] determined that the region of optimal cultivation in the south of Europe will continue to shrink. In some regions, warming may exceed the maximum temperature threshold specific for a given cultivar. The length of the growing season is very dependent on latitude. This was found to be a better indicator of climate suitability than the GDD system. Jackson ^[39][147] found that the LTI is better for comparing the suitability of a region for grape ripening than the use of degree days, especially for areas with cool climates.

3. Apple (Malus Domestica Borkh.)

Processes considered in the cultivation of apple trees are extended over time, due to the period from setting flower buds to yielding. The intensity of flowering is determined by the number of produced flower buds, and that process occurs in the preceding year, hence temperature conditions are of importance already at that time. Temperature affects the initiation of flower buds, and in the following year the date and intensity of flowering, pollination, and fruit setting. In addition, it determines the growth of fruits, their ripening, and the quality of the harvest. Many studies conducted under field conditions ^[40][41][42][148,149,150], as well as under controlled temperature [3], point to a strong positive correlation between temperature and the period from flowering to harvest. Stanley et al. ^[43][151], conducting research on ‘Royal Gala’ apple cultivar in New Zealand, evidenced that GDD at a base temperature of 10 °C (GDD 10), determined from the moment of pollination over the following 50 days, was strongly correlated with the weight of apples 50 days after pollination. This confirms the hypothesis that potential maximum size of apples is determined up to 50 days after flowering. It depends on the number of cells, the division of which is affected by temperature and tree nutrition. The authors also evidenced that in the period from 10 to 30 days after full bloom, GDD was strongly correlated with the duration of the period from pollination to harvest. According to Łysiak ^[44][85], the date of harvest of apples of the ‘Šampion’ and ‘Ligol’ cultivars can be determined by means of SAT. It only requires precise determination of the term of full bloom and continuous measurement of mean daily temperatures. He proposed 0 °C as the base temperature in the conditions of central-west Poland. For higher base temperatures, the standard deviation increased. SAT necessary for obtaining collective ripening of apples from the ‘Ligol’ cultivar was 2600 °C, and for ‘Šampion’ 2550 °C. This method, however, may not be as effective in another location, because the phenophases of particular cultivars may respond differently to environmental conditions ^[45][152]. Viškelis et al. ^[46][153], analyzing SAT, found the index to be strongly correlated with the ability to accumulate polyphenols in apples. The analysis of the content of polyphenolic compounds in fruit from the ‘Auksis’ and ‘Ligol’ cultivars showed that an increase in the content of these substances is inversely proportional to SAT. The index gradually decreased with a reduction in the growing period duration. According to research, greater accumulation of polyphenolic compounds occurs in stress conditions such as drought, diseases, and pest infestations, or lack of nutrients ^[37][47][48][145,154,155]. Estonia has less favorable conditions for apple tree cultivation than Poland, located approximately 400 km to the south. Due to the more difficult growth conditions, fruit of the ‘Auksis’ and ‘Ligol’ cultivars in Estonia accumulated 139% and 77% more phenolic compounds, respectively, than those grown in Poland ^[46][153]. This confirms the effect of temperature on changes in the patterns of accumulation of bioactive compounds. Another important feature related to temperature is the duration of the dormant period in apple trees. Dormancy of trees in temperate climate ends in spring, after the tree was subjected to appropriately long chilling during winter. It is determined by the minimum sum of chilling hours required to break the dormancy of vegetative buds of the apple tree ^[49][156]. The optimal chilling temperature depends on the cultivar, the location, and the intensity of dormancy ^[50][51][86,157]. Putti et al. ^[52][158] also report differences in base temperatures between apple tree cultivars. According to the authors, apple cultivars differ in the duration of the necessary low temperature period that initiates breaking dormancy. They identified cultivars with low requirements regarding chilling, where the range of low effective temperatures was from 3 to 12 °C, and cultivars with high chilling requirements, with a range of low effective temperatures from 3 to 6 °C.

4. Pear (Pyrus communis L., Pyrus pyrifolia Nakai)

Pear trees grow and yield well in temperate climate. Pear cultivation at a larger scale has developed in regions where temperature in winter does not fall below −27 °C, and in summer does not exceed 32 °C. Despite the thermophilic character of the species, proper development and fruiting of popular pear cultivars (originating from P. communis) requires approximately 400–800 chilling hours (temperature below 7 °C). Some cultivars, however, require more than 1050 chilling hours, e.g., ‘Bartlett’ ^[53][35], whereas others (originating from P. pyrifolia), e.g., ‘Patharnakh’ and ‘Punjab Beauty’ only need 150–200 h ^[54][159]. According to Łysiak ^[55][87], SAT can be useful for the determination of the term of pear harvest. The author evidenced that measurements are the most precise at a base temperature of 0 °C. The duration of the harvest window is minimum 5 days. Twelve years of research showed that pear of the ‘Conference’ cultivar in the conditions of west Poland was ready for harvest when the SAT value in the period from full bloom to harvest was 2469 °C, and the standard deviation was only 20°. Drapper et al. ^[56][160] defined the period of winter dormancy as chilling (in autumn), full dormancy (in winter) and forcing (in spring). In the study, they used the selected and dynamic + growing degree hour (GDH) phenological models and their sensitivity to parameter optimization. They noted that the acceleration of flowering under the influence of higher temperatures during winter dormancy was greater for pear cv. ‘Conference’ than for apple trees cv. ‘Jonagold’.

5. Peach (Prunus persica L.)

Despite its sensitivity to frost damage in winter, peach grows well in temperate climate, and the differences in thermal requirements between cultivars are substantial. Peach cultivars that require a short chilling period (50–300 h) at a temperature below 7.2 °C prefer low temperature in winter and high temperature (30 °C) in summer to obtain appropriately ripe fruit ^[57][38]. Cultivars with moderate chilling requirements need from 300 to 525 chilling hours, and those with high requirements from 525 to 1390 h ^[58][59][116,161]. In comparison to other stone fruits, peach is tolerant of higher temperatures in summer, and its fruit ripening requires temperatures of more than 24 °C. Climate changes involving an increase in temperatures in winter may result in shrinking of regions suitable for peach cultivation. In the south-eastern part of the USA, in the state of Georgia in 2017, drastic twice-lower yielding of peach was recorded in comparison to the preceding year due to the shortening of the chilling period necessary for appropriate development of the fruit by 200 h. Peaches respond with weaker resistance of buds to frost in winter, if in the period from 15 October to 31 December the maximum air temperature exceeds 18 °C. It also results in disturbances in entering the dormancy period. The resistance of cells to frost is directly determined by the accumulation of reserve substances in the vacuoles. Their high concentration decreases the freezing point of plant tissues [9]. A properly prepared peach tree for winter dormancy can survive temperature declines in winter of up to −25 °C. A warm winter caused early development of buds, making them more susceptible to spring damage. The assessment of the risk of excessively early development employed the GDD formula. It was determined that if from 15 February to the last day with a critical temperature of −1.7 °C the GDD value exceeds 335, the risk of damage of excessively developed buds or flowers is very high ^[60][82]. In the Czech Republic, Litschmann et al. ^[61][80] observed that the term of start of particular phenological phases in peaches is strongly dependent on temperature above 7 °C, measured from 1 January. This dependency was even observed in years with exceptionally high temperatures. The dependency between SAT 7 and bursting of flower buds (start of phenophase 01 according to BBCH) permits planning and scheduled spraying against peach leaf curl, even in seasons with different weather conditions. The authors emphasized a close correlation between the value of SAT 7 counted for 2 months from the onset of flowering, and the number of days from flowering to fruit harvest. This allows for precise determination of the term of peach harvest.

6. Sour (Prunus cerasus L.) and Sweet Cherry (Prunus avium L.)

Like other fruit trees in temperate climate, sour and sweet cherries require an appropriate amount of “chilling units” in autumn and winter to break winter dormancy. After meeting the chilling condition, the trees enter the period of relative dormancy during which dormancy is only determined by unfavorable environmental conditions (temperature, duration of the day, etc.). In that period, higher temperatures break winter dormancy and promote the development and growth of trees ^[62][115]. Guak and Neilsen ^[63][84] analyzed the effect of temperature in controlled and natural conditions on the end of the dormancy period in sweet cherry of the ‘Sweetheart’ cultivar. Shoots for the experiment were collected before the dormancy period, and subjected to a constant effect of 7 temperatures in a range from −2 to 16.8 °C. The study showed that the optimum chilling temperature for the analyzed sweet cherry cultivar is within the range from −2 to 7 °C, and temperature above 13 °C has no chilling effect. For all processes required for the dormancy period to occur in the plant, the value of chilling units needs to reach 740. Zavalloni et al. ^[64][97] developed a simulation model based only on temperature data that can be useful for producers in increasing the accuracy of decision making regarding pest control, fertilization, or irrigation. They developed formulas based on GDD values at a base temperature of 4 °C, permitting the determination of terms of particular stages of development of flower buds and fruits of sour cherry of the ‘Montmorency’ cultivar. The differences between the predicted and observed dates varied in a maximum of 4 days. Persely et al. ^[65][98] developed a similar simulation model for three Hungarian sour cherry cultivars, namely ‘Debreceni Bőtermő’, ‘Újfehértói Fürtös’, and ‘Kántorjánosi 3′. They employed the GDD formula at a base temperature of 2.5 or 5 °C. The analyzed cultivars in the study years did not differ in GDD values determined for bursting of flower buds, but significant differences occurred between years. In one of the seasons, bursting of flower buds occurred at GDD equal to 17.5 °C, and in another at 39.7 °C. Responses to a given temperature were determined to depend on the physiological stage of the tissues and previously occurring environmental conditions ^[66][67][162,163].

7. Strawberry (Fragaria x Ananassa Duchesne)

The yielding potential of particular strawberry cultivars strongly depends on the mutual effect of day duration and temperature. Strawberry cultivars show high genetic variability, and therefore different responses to the shortening of day duration and course of temperature. Temperature has a strong and variable effect of the generative processes in strawberries. At low temperatures (below 10 °C), the short day genotypes and those neutral towards day duration show no response to the photoperiod, and at higher temperatures, a reduction in day duration below 14 h induces the onset of flowering. Everbearing cultivars induce flowering in the conditions of a long day, i.e., throughout summer. In the conditions of a short day, at an increase in temperature to 24 °C, strawberries respond with intensified production of flower buds. The range of the inducing temperatures is from 12 to 22 °C. Below and above that range, the effectiveness of the inducive effect of a short day on the production of inflorescences in strawberries decreases. In the case of everbearing cultivars, inflorescences are initiated irrespective of day duration in a range of temperatures from 10 to 28 °C ^[68][164]. According to Tanino and Wang ^[69][165], flowering term is correlated with the accumulation of chilling hours, and fruit yield is correlated with cumulative chilling units. The authors concluded that strawberry may be affected by more complex environmental factors than its flowering. The effect of temperature during the chilling period on yielding is indirect, because it depends on the term of flowering, nutrition of the plants, and growth rate of the vegetative parts ^[70][166]. Moreover, different optimum chilling temperatures apply to the vegetative development, yielding, and fruit quality ^[71][47]. The subject of the study was also the determination of the dependencies of vegetative growth of strawberry on temperature. Description of the development of strawberry leaves applies the terms phyllochron and plastochron. According to Bonhomme ^[72][111], phyllochron is duration of time (usually in days) between the appearance of two subsequent leaves. Plastochron stands for the time duration between the initiation of two subsequent leaves. Mendonça at al. ^[68][164], using a model of linear regression between GDD at a base temperature of 7 °C and number of leaves in the crown of the strawberry, introduced the term phyllotherm expressed in °day⁻¹ (degree day). The studied cultivars varied in terms of the phyllotherm values from 60.38°day⁻¹ (cv. ‘Ventana’) to 199.96°day⁻¹ (cv. ‘Albion’). Based on GDD, Bethere et al. ^[73][93] developed a phenological model for strawberries. They then used it to predict the timing of phenological processes in strawberries in the period 1951–2099. The results of their research show that an acceleration of physiological processes can be expected in the future, contributing to a change in regionalization of strawberry cultivation.

8. Raspberry (Rubus idaeus L.) and Blackberry (Rubus fruticosus L.)

Raspberry and blackberry show an active response to temperature in adjusting the term of flowering and fruit ripening. American research has shown that the northern range of cultivation of primocane fruiting raspberries was closely correlated with the accumulation of forcing units at a base temperature of 5 °C ^[74][167]. Privé et al. ^[75][168] analyzed the effect of climatic conditions on the development and growth of the vegetative and generative parts of primocane-fruiting raspberries. The effect of climatic factors, such as insolation, duration of the day, availability of water, GDD, and soil and air temperature, were estimated based on the analysis of multiple regression coefficients. The effect of air temperature and insolation was the strongest during initiation of flower development, i.e., in June and July, and day duration was the most significant from June to October. Climatic conditions to the greatest degree determined parameters such as the number of fruits, their weight, and harvest yield, whereas the total number of nodes on a stem and term of harvest proved the least dependent. Particular raspberry cultivars showed different responses to climatic conditions. The ‘Autumn Bliss’ cultivar proved less sensitive than ‘Heritage’ or ‘Redwing’. The blackberry growing in the zone of temperate climate enters the dormant phase due to the shortening of the day and the low temperatures in autumn, and the phase ends after the completion of the necessary winter chilling ^[76][169]. Bursting of buds of floricane-fruiting blackberries occurs at the turn of March and April. Fruiting shoots grow out of the auxiliary buds on two-year shoots and lateral shoots. Over the following 4–5 weeks, the inflorescences develop on the fruiting lateral shoots, and flower buds on the inflorescence open from mid-May to mid-June. Jennings ^[77][75] compared the terms of flowering and fruit ripening of several genetically variable blackberries. The accumulated forcing units (temperature above 6 °C) proved strongly correlated with the term of fruit ripening. Based on phenological observations, Black et al. ^[78][90] tested linear and curve prediction models with the application of the range of cardinal temperatures. They determined that the flowering term under field conditions is best predicted by a linear model with a base and an optimal temperature ^[79][89] of 6 and 25 °C, and curve model with a base and an optimal temperature of 4 and 27 °C. Based on the linear increase in degree days (GDH), the authors determined that from the moment of flowering, the ‘Chicksaw’ cultivar needs 9200 GDH, and ‘Merton Thornless 18,900 GDH.

9. Other Species

Temperature indices can be useful in the introduction of new species and cultivars to cultivation. Ishchuk et al. ^[80][91] used GDD and SAT to describe the biorhythm of six species from genus Juglans, namely J. nigra, J. cinerea, J. rupestris, J. major, J. californica, and J. hindsii. Based on GDD, they compared the terms of flower bud swelling and bursting. SAT was applied in the comparison of term of onset and end of flowering, fruit setting, and fruit ripening. In combination with observations of resistance to frosts depending on the degree of winter dormancy, they determined the usefulness of the aforementioned species for plantings in the Ukrainian Right-bank Forest-Steppe. Based on SAT above 5 °C, Mirotadze et al. ^[79][89] divided cultivars of hazel (Corylus avellana) cultivated in Georgia into early ripening—SAT from 1800 to 2200 °C, medium early ripening—SAT from 2200 to 2600 °C, and late ripening—SAT from 2600 to 3000 °C.