3.1. Climatic Conditions
A selection of the climatic conditions (average temperatures and precipitation) at the experimental location in the study years and in the multi-year period of 1981–2010 are presented in
Table 1 and
Figure 2 In
Table 2, we present data for spring ground frosts in 2019–2022.
In Poland,
Elaeagnus multiflora blooms from April to the end of May and ripens at the end of June or at the beginning of July [
16]; therefore, the distribution of meteorological conditions in this period is significant.
The analyses of the climatic conditions in the particular years the research was conducted on the yields and morphological characteristics of selected biotypes and cultivars found that the highest mean temperature of the vegetative season from flowering (April) to fruit harvest (July) (
Table 1) occurred in 2019 and reached 14.8 °C. The lowest mean temperature during the vegetative season (13.3 °C) was recorded in 2020. In the year 2021, especially low temperatures were reported in winter months: −3.2 °C in February and −1.6 °C in December. In turn, the temperatures of the summer months June and July were the highest in 2021, and the summer temperature during the research period (20.3 °C, mean of two months) was 3.6 °C higher compared with the multiannual period.
During the four-year experiment, the mean temperatures recorded in April ranged from 5.9 °C in 2021 to 9.3 °C in 2019. The closest value to the multiannual mean, which was 7.3 °C, was recorded in 2020 (7.5 °C). In all experimental years, the mean temperatures recorded in May were lower than the multiannual mean of 1981–2010, which was 12.8 °C. The highest mean temperature in May (12.1 °C) was recorded in 2022, and the lowest in 2020 (10.2 °C). In June, on the other hand, in all the analysed monthly temperatures were higher than the multiannual mean (15.5 °C) and ranged from 17.9 °C in 2020 and 2022 to 20.9 °C in 2019.
The
Table 2 shows spring ground frost occurring in April and May. The beginning of
Elaeagnus multiflora flowering was noted in late April or early May and lasts from 15 to 20 days. In each of these months in 2019, there were 18 (April) and 4 (May) days with a temperature below 0 °C. Similarly, in subsequent years, i.e., 2020, 2021 and 2022, the number of days with ground frosts in April and May were 20 and 6, 16 and 4, and 14 and 7, respectively.
Minimum temperatures below zero on three consecutive days in May were recorded in 2019 as follows: −5.1, −2.8 and −3.4 °C. According to Faust [
63], when occurring during the bloom or post-bloom stages, temperatures from −2 to −5 °C injure the flowers or young fruits. In 2019, the latest frost also occurred, which was on 30th of May (−1 °C). The ground frosts (−2.2 °C and −4.5 °C) on 14th and 15th of May in 2020 threatened to freeze flowers. In 2021, the fewest days with frost were recorded. In 2022, slight frosts (from −0.5 to −1.3 °C) were recorded from 3 May to 5 May, and on 10 May, a frost of −2.5 °C posed a high risk. In this year, the frosts occurred in the second half of May, and on 19th of May, the minimum temperature at the ground was recorded at −1.1 °C.
Large differences in precipitation during the vegetative season (from April to July) were observed in different years (
Table 1). In April 2019, no precipitation was recorded, whereas in May of the same year, the monthly precipitation was the highest and amounted to 134.8 mm. Low precipitation (4.5 mm) was also recorded in April 2020, whereas in May, the total precipitation level significantly exceeded the multiannual mean and reached 104.8 mm. In 2019 and 2020, the precipitation, both during the growing season from April to July and throughout the year, was higher than the mean precipitation for these periods between 1981 and 2010.
In March 2022, no precipitation was recorded, and the total precipitation in the season from April to July was the lowest in the experimental years and was also lower than data for the period 1981–2010 (
Table 1,
Figure 2).
The course of the climatic conditions from April to July had a strong influence on the yield and some morphological features of the biotypes and cultivars of the
Elaeagnus multiflora fruits tested (
Table 3 and
Table 4).
Precipitation in June had a negative influence (
p < 0.01) on the weight, length, and width of fruits for the tested biotypes and cultivars of
Elaeagnus multiflora (
Table 4). On the other hand, temperatures in May had a significant (
p < 0.05) positive effect on the length of the fruit and the length-to-width ratio of the fruit. In the discussed vegetative season, both temperature and precipitation had a significant positive influence on the width of fruit (
Table 3). The strongest positive impact of these weather factors on the width of fruit was demonstrated for July, that is, just before fruit harvest. A statistically significant (
p < 0.01) positive influence of precipitation during June and July on the width of the seeds was found (
Table 3 and
Table 4). Precipitation in April had a significant positive influence on the ratio of seed length to width (
Table 4). The dependencies testify to the relatively high water needs of the studied species, and high rainfall affects the quality parameters of the fruit.
Table 3 and
Table 4 demonstrate significant negative correlations between temperatures and precipitation of this season and yield. There was a highly significant negative influence on yield from temperatures in June (
p < 0.001) and July (
p < 0.01) (
Table 3) and precipitation in May (
p < 0.001), and in June and July (
p < 0.01) (
Table 4). Environmental conditions influence flower bud development. The summer temperature may also determine the rate of flower bud initiation. In apples in shaded areas where less than 30% of the sunlight penetrates, practically no flower bud development occurs [
63]. Heide et al. [
64] showed that at 12 °C, flowering in apple seems to be limited by low temperature depression of growth and leaf production, whereas at 27 °C, flowering is blocked by inhibition of the floral initiation itself. Intermediate temperatures of 18–21 °C, on the other hand, seem to satisfy the requirements for both processes. For good yield and development,
Elaeagnus multiflora requires large amounts of sunlight [
16,
36].
Elaeagnus multiflora has a long vegetative season, and leaf loss begins immediately after the first autumn frosts.
Table 5 shows the effects of temperatures and precipitation in the months after fruit harvest from August to March on the yield in the following year. The correlation analysis shows that the temperatures from August to October had a significant negative impact on yield in the next year. However, temperatures in November and March, did not have a significant effect on yield. The correlation analysis also shows that the temperatures in the winter months from December to February had a significantly negative impact on the yielding of the tested biotypes and cultivars of
E. multiflora grown in the conditions of north-eastern Poland. Analysis of the interaction of precipitation with
E. multiflora yields in the following year shows that precipitation in November had a particularly significant positive impact, whereas precipitation in October had a significantly negative impact on yield (
Table 5). Analysing the interaction of temperature and precipitation throughout the year (January to December) prior to fruiting on yield, a significantly negative effect of temperature and statistically significant (
p < 0.05) effect of precipitation (r = 0.36) were found (
Table 5).
A study conducted in Ukraine [
65] showed that in November, the fall of leaves can be caused by a significant night frost. All leaves shed infrequently. Usually, one or more leaves can remain on the tops of annual shoots for a very long time. The late fall can cause shoots to freeze during severe winters.
E. multiflora is a plant with low resistance to frost and high regenerative properties [
36]. According to Grygorieva et al. [
65], shrub shoots, even completely frozen, regenerate well and grow from the root neck in significant amounts. After cold winters, there are many young shoots with vegetative buds on the bush, which thicken the bushes, thereby protecting the buds that form on older shoots from the external environment. In
E. multiflora, flower buds occasionally develop one or two at a time on the axils of the lower leaves of replacement shoots (
Figure 3).
On one shoot of E. multiflora, flower buds are formed gradually, synchronously with its growth and development. On the same shoot, as well as on the same tree, the fruits ripen at different times, which corresponds to the gradual development of generative buds.
According to the data presented in
Table 3,
Table 4 and
Table 5, the climatic conditions of Poland during the growing season from April to July, as well as in the year before the fruiting from August to October, had a significant impact on the yield of the biotypes and cultivars of
Elaeagnus multiflora. Environmental factors affect flower development. The period of flower bud formation in
E. multiflora. begins after fruit harvest in Poland’s climatic conditions, usually in July and during flowering in April and May, and any severe heat or moisture stress hinders normal flower growth. Inadequate winter chilling limits cell division and spring development, or in severe cases, the flower buds simply drop from the tree [
66]. Rodrigo and Herro [
67] showed that flower drop can vary not only between cultivars, but also for the same cultivar, depending on the year or site. After dormancy and during the pre-blossom period, flower buds are exposed to variable climatic conditions from the end of winter to the beginning of spring. In this period, frost temperatures can easily occur in most temperate zones, and the stage of development is the most important factor in the resistance to frost injury [
68]. Therefore, most of the work conducted on frost damage in the reproductive organs of fruit trees concentrates on either the endodormancy [
69] or post flowering periods [
70].
The data in
Table 1 show that in the period preceding fruiting in 2019 and 2021, that is, from August to September 2018 and 2020, the average monthly temperatures were higher than the multiannual mean of 1981–2010. In 2019 and 2021, the yields were lower than in 2020 and 2022, which may indicate that the differentiation of flower buds
E. multiflora requires lower temperatures and more precipitation (
Table 1 and
Table 5). According to Westwood [
66], biennial bearing of most tree fruits results from poor flower initiation during a heavy crop year, which can also be seen in our research (
Figure 4). Research conducted by scientists in Ukraine [
65] shows that
E. multiflora has abundant and regular fruiting. In our experiment we observed relatively low yields of
Elaeagnus multiflora cultivars and biotypes (
Table 6,
Figure 4). The lowest yields were obtained in 2021 and the highest (6.5 kg per shrub) in 2022. The mean yields of biotypes and cultivars from the period 2019–2022 ranged from 1.13 kg for the cultivar ‘Sweet Scarlet’ to 4.02 kg for biotype B11. Biotype B11 was significantly different in terms of yield from the other biotypes and cultivars tested (
Table 6). Statistical analysis of the yield results showed that six biotypes (B0, B3, B4, B8, B7, B8) and two cultivars (‘Jahidka’ and ‘Sweet Scarlet’) form the largest homogeneous group, with the lowest mean values of 1.13 to 3.26 kg (
Table 6). Slightly more than 3 kg of fruit were harvested from the shrubs of the B1, B2 and B0 biotypes, but the least-yielding biotype was B5 with 1.99 kg per the shrub, and the ‘Jahidka’ cultivar yielded 2.59 kg per the shrub (
Figure 4).
E. multiflora begins to bear fruit in the fourth to fifth year. The most productive fruiting occurs at the age of 8 and lasts at least 12–15 years [
61]. In our investigation, the biotypes and cultivars were at production age. The shrubs started to fructify in the third year after planting, so it was their ninth year of fructification in 2019. In this experiment we obtained relatively low yields.
As shown in
Figure 5 and
Figure 6, the influence of temperatures and precipitation in the experimental years were negatively correlated with yield. The graphs of the linear relationship show a stronger negative impact of precipitation than temperatures in the years of the research, indicating that higher rainfall reduces the yield of
Elaeagnus multiflora. According to Chawla et al. [
71], rain during flowering washes out the pollen from the stigmas of flowers, resulting in poor or no fruit setting. Heavy rainfall in areas of poor drainage reduces oxygen availability in the soil, leading to reduced growth of beneficial microorganisms. Additionally, due to water-logged conditions, many insect-pests and diseases occur which affect crop yield.
The harvest yields of fruit plants are dependent on both genetic characteristics and the climatic conditions in a given vegetative season [
4,
22,
36,
72,
73,
74,
75,
76,
77]. The climate of north-eastern Poland, which is determined mainly by air masses flowing in from the eastern border of the country, has been observed to change in recent years. One of the reasons for this change is global warming, which has led to an increase in the mean annual temperature and, consequently, to decreases in the total annual precipitation and soil humidity [
57]. According to the available literature [
78,
79], climate change and the potential for more extreme temperature events will affect the phenology and productivity of plants. Consideration of the effects of climate conditions on yielding and quality of fruit of
Elaeagnus multiflora are particularly important to enable proper regionalization of cultivation, and choice of cultivars adapted to the diversified environmental conditions of the north-eastern European production area.