Experimental study on cold tolerance thresholds in field grown subtropical fruit trees
Article ID: D-24-00013
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Article ID: D-24-00013
To evaluate suitability for growth of tropical fruits under global warming in Japan, we revealed cold tolerance threshold of subtropical fruit trees by performing field and chamber experiment and developed an extrapolatory method that can predict the cold tolerance of subtropical fruit trees without conducting extensive field experiments. Results of the field experiments estimated cold tolerances of -1.5 and -2°C for ‘Summer Queen’ passion fruit and ‘Sata’ lychee, respectively, and -5, -4.5, and -4°C for ‘Mexicola,’ ‘Bacon,’ and ‘Fuerte,’ avocados, respectively, indicating the dependence of cold tolerance on the variety of the tree species. The cold tolerance obtained from the field experiment was higher than the chamber experiment by 1°C for passion fruit and avocado and 2°C for lychee, which is explained by the difference between air and leaf temperature of field trees caused by radiative cooling on a clear and calm night. Therefore, cold tolerances of passion fruit and avocado can be estimated by adding 1°C to the cold tolerances obtained from the chamber experiment and that of lychee can be estimated by adding 2°C to the cold tolerance obtained from the chamber experiment. Moreover, even for other tree species, which were not consider in this study, the cold tolerance of the trees grown in fields can be estimated based on the results of the chamber experiment and using estimations of the difference between air and leaf temperatures on a clear and calm night.
The average surface air temperature increases by 1.35°C per 100 years in Japan due to global warming (JMA, 2022), which has an adverse impact on domestic agricultural productivity. As perennial crops are less adaptive to climate compared to annual crops, fruits trees are considered particularly vulnerable to climate change (MAFF, 2015). In Japan, the effects of global warming such as poor coloring, coloring delay, sun burn, and poor germination significantly affect many fruit trees (Sugiura et al., 2007; Sugiura et al., 2009). On the other hand, global warming also has positive effects on the productivity of fruit trees. As the southern part of Japan is in the tropical to subtropical climate zone, the area of growth of subtropical fruit trees is considered to expand northward with global warming, which provides farmers an opportunity to initiate the production of new species. The government of Japan has incorporated the policy of “switch to tropical and subtropical crops” in the National Plan for Adaptation to the Impacts of Climate Change that was effected on December 27, 2018. This implies that the shift toward fruit trees for adapting to climate change is now becoming a public mission. Fruit trees require several years from the planting stage to achieve stable production. Therefore, to minimize yield loss, the shift to growing tropical trees should be based on careful consideration of their suitability over future decades.
In countries like Japan that experience a cold season, the cold tolerance of trees restricts suitable areas for their field cultivation (Ledin, 1957). Uchino et al. (2019) reported that avocado, litchi, and papaya trees grown in Kagoshima Prefecture suffered damage caused by low temperature on December 18, 2015 and January 25, 2016, which recorded minimum temperature of -1.2°C and -6.3°C, respectively. This report provided valuable information on the cold tolerance of subtropical fruit trees in Japan. However, there is no evidence that the minimum temperatures obtained in these studies indicate the lethal low temperature, that is, the trees might have suffered cold damage even at higher nighttime temperatures compared to the recorded minimum temperatures. In general, evaluation of cold tolerance of fruit trees in the field is difficult as we cannot always measure the field air temperature. In addition, in tropical or subtropical areas, as these tropical trees rarely experience such fatal low temperatures, there are no records rely on. Therefore, most studies place materials into a growth chamber and simulate desirable temperature conditions to investigate cold tolerance (hereafter, referred to as the chamber experiment). Shimada et al. (2018) investigated cold tolerance of different varieties of avocado through low-temperature treatment of potted plants in a growth chamber. However, as the treatment was done using detached leaves and branches, the cold tolerance of the entire tree could not be evaluated. In low-temperature treatment using detached leaves or branches, since water is not sufficiently absorbed from the soil by roots and stems, physiological response such as transpiration and photosynthesis reacts improperly, which affects sugar accumulation and subsequent cold tolerance enhancement of the tree. Furthermore, the most crucial problem in the chamber experiment is the radiative environment. In the chamber experiment, a tree receives infrared radiation from the structures or walls around the tree. Therefore, a tree in a chamber receives more infrared radiation than that in the field, which may suppress radiative cooling on the tree surface.
Thus, the cold tolerance of subtropical fruit trees has not been ascertained clearly so far. The most simple and reliable method for the estimation of cold tolerance of trees is to simultaneously expose them to low temperature conditions in fields during the winter season in multiple regions with various climatic condition (hereafter, referred to as the field experiment). Although this is an ideal method, it requires considerable effort and much cooperation. An essential prerequisite for the field experiment is the selection of appropriate regions that are subjected to various temperatures.
Japan extends from the south (20°N) to north (45°N) and has adequate climatic diversity. This geographical feature is ideal for performing field experiments to understand the cold tolerance of subtropical fruit trees. We conducted field experiments of cold tolerance of tropical fruit trees during two cold seasons at ten regions covering many areas in Japan. The ten fields of study provide us with conclusive results.
In addition, we conducted chamber experiments to investigate the difference in the cold tolerance between the field and chamber experiments. Based on the results of these two different approaches, we developed a simple method that can predict the cold tolerance of the subtropical fruit trees without conducting extensive field experiments.
The introduction of subtropical fruit trees by taking advantage of changes caused by global warming has significant benefits for fruit production in Japan and other countries that undergo similar climatic changes. The aim of our study is to 1) estimate the cold tolerance of important subtropical fruit trees and 2) develop a simple method that can predict the cold tolerance of subtropical fruit trees without conducting extensive field experiments.
We evaluated the cold tolerance and tree temperature of passion fruit, lychee and avocado in field and chamber by conducting three experiments. In experiment 1, the cold tolerance of tropical fruit trees in field was assessed. In experiment 2, tree temperature of the trees in field were measured. In experiment 3, the cold tolerance and tree temperature of the trees in chamber were investigated. The experiment details were described as follows.
2.1 Experiment 1: Field experiment for the evaluation of cold tolerance of subtropical fruit treesCold tolerance of subtropical fruit trees was investigated through filed experiments in the winters from 2016 to 2017 and 2017 to 2018. Figure 1 shows the locations of the field experiments: Tsukuba, Gifu, and Kyoto are inland; Chiba, Mie, Kagoshima, Amami, and Ishigaki are in coastal regions. In each field, one-year-old potted plants of ‘Bacon,’ ‘Mexicola,’ and ‘Fuerte’ avocados, ‘Summer Queen’ passion fruit and ‘Sata’ lychee provided from the same nursery were used for the experiments. The plants used in the experiments were grown in fields from October 2016 to May 2017 and September 2017 to May 2018, respectively. For each variety, at least three plants were tested, and the pots were surrounded with soil to prevent radiative cooling from the pot surface. Air temperature (Ta) at the experimental field was recorded every 5 min using an automated data logger (Ondotori TR-52i, T&D Corp., Tokyo, Japan). Thermometers were kept in radiative shelters with well natural ventilation to prevent over- and under-estimate of temperature due to the effect of direct solar insolation and radiative cooling, which were mounted at a height of 1 m from the ground.
Fig. 1. Study area of experiment 1.
The cold tolerance of each tree was evaluated using visual observation of cold damage and measurements of the minimum temperature in winter. Since budbreak is a vital phenomenon which leads to the subsequent growth of plants, the cold damage was assessed based on bud emergence of plants in the following spring. Note that, in the case of our test plants, as buds are hidden in leaf axils during winter and the buds burst in coming spring, we evaluated the effect of cold damage by judging the existence of budbreak in spring based on the assumption that normal budbreak can lead the following growth regardless of any cold damage in part of leaf or branch during winter. Thus, in this study, plants that showed normal budbreak at the beginning of May were judged as ‘overwintered’ and that without budbreak were judged as ‘killed.’ In this criterion, plants of which the bud break delayed compared to normal phenology were also treated as “killed”, since these kinds of plants can delay the following vegetative or reproductive growth. The killed rate (Pd) was defined as an index representing the degree of cold damage and it was calculated as follows:
Pd = (Nd / Na) × 100 (1)
where, Nd is the number of ‘killed’ plants for each variety in each experimental field, Na is number of total plants of each variety in each experimental field. Pd > 0% and Pd = 0% were considered ‘killed’ and ‘overwintered,’ respectively. The minimum temperature during the experimental period (Twin) was calculated for each field. The highest Twin in the fields at which results of ‘killed’ were obtained for a variety in the two years was defined as the temperature of cold tolerance (Tct).
2.2 Experiment 2: Field experiment for the evaluation of tree temperature.As described above, bud emergence of plants at spring is an important event that indicates their wintering ability. Bud growth is mainly affected by environmental factors, especially, the temperature of the bud. Therefore, we estimated characteristic bud temperatures of ‘Mexicola’ avocados, ‘Sata’ lychee, and ‘Summer Queen’ passion fruit during winter nights. This study focused on leaf temperature (Tl) as a replacement for bud temperature, as the buds are hidden in leaf axils until late winter. In addition to Tl, we measured branch temperature (Tb) near the bud. Ta, Tl, and Tb were measured from sunset to the following morning under a clear and calm weather condition at the Institute of Fruit Tree and Tea Science, NARO, Tsukuba, Japan. Experiments on passion fruit were held during the nights of March 16 and 24, 2020, on lychee on February 6 and 27, 2020, and on avocado on February 17 and 18, 2021.
Ta was measured every 10 min in the field using a thermometer (Ondotori TR-52i, T&D Corp., Tokyo, Japan) fixed in an instrumental shelter to prevent the effect of direct solar insolation and radiative cooling. In addition, temperatures of mature leaves (Tl) oriented toward the sky and branches (Tb) were measured every 10 min using 0.2-mm T-type thermocouples (Hypodermic Needle Probes HYP0-33-1-T-G-60-SMPW-M, OMEGA Corp., Norwalk, Connecticut, U.S.A.). The detection parts of the sensors were attached on the adaxial side of a mature leaf around the leaf vein and surface of branches using medical tape as described by Wakiyama (2002).
2.3 Experiment 3: Chamber experiment for evaluation of cold tolerance and tree temperatureIn experiment 3, cold tolerance of ‘Summer Queen’ passion fruit, ‘Sata’ lychee, and ‘Mexicola’ avocado trees were investigated using chambers at the Institute of Fruit Tree and Tea Science, NARO, Tsukuba, Japan in the winters from 2019 to 2020 (experiment 2020) and 2020 to 2021 (experiment 2021). The chambers are about 5 to 50 m3 in size, which are put in the experimental facility. During experiments, dark conditions were maintained, and temperatures in the chamber were controlled so that its variations were within ± 0.6°C. Trees grown in the same conditions as in experiment 1 were used. The cold damage was assessed based on bud emergence within a month after the chamber treatment. “Presence” and “absence” of budbreak of the test plant was judged as ‘overwintered’ and ‘killed,’ respectively. Using these results, Pd was calculated for each treatment section.
During the experiment, air temperature in a chamber was measured every 10 min using a thermometer (Ondotori TR-52i, T&D Corp., Tokyo, Japan) with air well circulating in the chamber. In addition, leaf and brunch temperatures of experimental plants were measured at a height of approximately 70 cm (middle height of trees) at 10-min intervals using 0.2-mm T-type thermocouples (Hypodermic Needle Probes HYP0-33-1-T-G-60-SMPW-M, OMEGA Corp., Norwalk, Connecticut, U.S.A.). The detection parts of the sensors were attached on the adaxial side of a mature leaf around the leaf vein and surface of branches using medical tape. The experimental methods of each tree species are as follows.
2.3.1 Passion fruitsIn the experiments of 2020, two-year-old ‘Summer Queen’ passion fruit plants were planted into 20-L pots filled with nursery soil (Ikubyo baido, TAKII Co., LTD, Kyoto, Japan) in the beginning of May 2019. After planting, the plants were managed in a glasshouse until November 2019. Temperature in the glasshouse was controlled to be the same as that of the outside air temperature. Then, the plants were managed in a heated glasshouse in which the temperature was maintained at 15 to 25°C (average approximately 20°C). Chilling treatment was performed on January 31, 2020. In this experiment, 15 treatment sections were made using a combination of five different temperature levels of 7, 4, 1, -2, and -5°C and three different treatment durations of 3, 6, and 9 h in five chambers. Three plants were used for each treatment. The plants were moved immediately to a heated glasshouse (average 25°C) after the treatments.
In the experiments of 2021, additional cold treatments were made. Three- to five-year-old ‘Summer Queen’ passion fruit plants were used in experiment 3. The plants were managed in a glasshouse until November 2020. The temperature in the glasshouse was controlled to be the same as that of the outside air temperature. After November 2020, the trees were managed in a heated glasshouse with a temperature of 15 to 25°C (average approximately 20°C). Chilling treatments were performed on January 22, February 2, and March 12, 2021. In this experiment, one to two plants were used for three treatments and were exposed for 3 h to temperatures of -3, -3.5, and -4°C, respectively, on different days. After the chilling treatments, the plants were moved immediately to a heated glasshouse (average 25°C).
2.3.2 LycheeIn the experiments of 2020, two-year-old ‘Sata’ lychee were planted into 20-L pots filled with nursery soil (Ikubyo baido) at the end of June 2019. After planting, the plants were managed in a glasshouse until November 2019. The temperature in the glasshouse was controlled to be the same as that of the outside air temperature. Following this, the plants were managed in a heated glasshouse with a temperature of 15 to 25°C (average approximately 20°C). Chilling treatments were performed on February 28 and March 3, 2020. In the experiments, 12 treatment sections were made by a combination of four different temperature levels of 1, -2, -5 and -8°C and three different treatment durations of 3, 6 and 9 h in two chambers. Three trees each were used for the -2 and -5°C treatments and two trees each were used for the 1 and -8°C treatments. After the treatments, all the trees were moved immediately to a heated glasshouse (average 25°C).
In the experiments of 2021, additional cold treatments were made based on the results of the 2020 experiments. Two-year-old ‘Sata’ lychee trees were planted into 20-L pots filled with nursery soil (Ikubyo baido) at the end of June 2019. After planting, the trees were managed in a glasshouse until November 2020. The temperature in the glasshouse was controlled to be the same as that of the outside air temperature. After November 2020, the plants were managed in a heated glasshouse with a controlled temperature of 15 to 25°C (average approximately 20°C). Chilling treatments were performed on January 22, February 9, and March 12 and 26, 2021. In this experiment, two plants were divided for five treatments and were exposed for 3 h to temperatures of -3.5, -4, -4.5, and -5°C, respectively, on different days. After the chilling treatments, the plants were moved immediately to a heated glasshouse (average 25°C).
2.3.3 AvocadoThree-year-old ‘Mexicola’ avocado trees were planted into 20-L pots filled with nursery soil (Ikubyo baido) at the beginning of February 2020. After planting, the plants were managed in a glasshouse until November 2020. Temperature in the glasshouse was controlled to be the same as that of the outside air temperature. Then, the plants were managed in a heated glasshouse with a controlled temperature of 15 to 25°C (average approximately 20°C). Chilling treatments were performed on January 22, February 2 and 9, and March 12, 2021. In this experiment, two plants each were used for four treatments and were exposed for 3 h to temperatures of -4, -5, -6 and -8°C, respectively, on different days. After the chilling treatments, the plants were moved immediately to a heated glasshouse (average 25°C).
Result of experiment 1 was shown in Table 1. In passion fruit, the ‘Summer Queen’ variety grown in the field with Twin ≥ 1.2°C overwintered and that with ≤ -1.8°C died. Although, there is possibilities that the cold tolerance of passion fruit ranges from -1.8 to 1.2°C with lack of data between them, Tct≒-0.5°C can be obtained by averaging 1.2 and -1.8°C.
Table 1. Relationship between Twin and Pd obtained from experiment 1. Pd of each species and variety (%) is shown at each experimental site in the order of increasing Twin (°C). Solid double lines indicate Tct obtained from experiment 1.
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In lychee, the ‘Sata’ variety grown in the field with Twin ≥ -1.8°C overwintered and that of ≤ -2.2°C died. Therefore, the Tct of ‘Sata’ is approximately -2°C.
In avocado, the ‘Mexicola’ variety grown in the field with Twin ≥ -4.6°C overwintered and that ≤ -5.5°C died. Therefore, Tct of ‘Mexicola’ is approximately -5°C. ‘Bacon’ grown in the field with Twin ≥ -4.4°C overwintered and that ≤ -4.5°C mostly died. Therefore, Tct of ‘Bacon’ is approximately -4.5°C. ‘Fuerte’ grown in the field with Twin ≥ -3.5°C overwintered and that ≤ -4.4°C mostly died. Therefore, Tct of ‘Fuerte’ is approximately -4°C.
The tree temperatures obtained in experiment 2 are shown in Table 2. In Table 2, net radiation on a leaf (Rn) during the experimental nights was calculated for grasping radiative cooling intensity. According to Kondo (2000), Rn is calculated as follows:
Rn = (1-ref) S↓ - ε (σTs4-L↓) (2)
where, S↓ is the downward shortwave radiation, L↓ is the downward longwave radiation and Ts is the leaf temperature. The ref is albedo on leaf, which is designated as ref = 0.12 by referring to the value of the evergreen broadleaf tree Hollinger et al. (2010). ε is the emissivity, which is defined as ε = 1 against L↓ and ε = 0.97 against σTs4 according to Takiuchi and Hashimoto (1977) and Wang et al. (1994). Rn is a positive value when there is more incoming radiation than outgoing radiation; then, the air layer on the leaf is heated. In this study, S↓ and L↓ were measured at Tateno by the Japan Meteorological Agency (JMA). Ts was measured in our experimental field using thermocouples. Rn of experimental nights estimated using equation (2) are shown in Table 2. During our observation days, fine weather and calm conditions were observed each night and Rn was -80 to -100 W m-2, indicating strong radiative cooling. Tl of passion fruit was 1.1 to 1.5°C (average 1.3°C) lower than Ta. Tl of lychee was 1.9 to 2.4°C (average 2.2°C) lower than Ta. Tl of avocado were 0.9 to 1.2°C (average 1.1°C) lower than Ta. Tb of passion fruit were 0.4 to 0.7°C (average 0.6°C) lower than Ta. Tb of lychee was the same or slightly lower than Ta. Tb of avocado was 0.5 to 0.8°C (average 0.7°C) lower than Ta. These results suggest that Tl drops remarkably compared to Tb under clear and calm weather conditions.
Table 2. Tree temperature and calculated Rn obtained from experiment 2. Ta is the daily minimum air temperature (°C), Tl is the daily minimum leaf temperature (°C), Tb is the daily minimum branch temperature (°C). Rn is calculated net radiation at 21:00 on a leaf (W m-2). W_ss and W_nm are weather conditions at the sunset and the next morning, respectively. ‘○’, ‘◎’ and ‘●’ indicate fine, cloudy and rainy weather, respectively. Wind is mean wind speed from 18:00 to 6:00 of next morning (m s-1).
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3.2 Cold tolerance and tree temperature obtained in the chamber experiment
Result of experiment 3 was shown in Table 3. Pd of ‘Summer Queen’ passion fruit was 0% at actual mean temperature during the experiment in chamber (Tact) of 7.3, 4.9, 1.6, and -2.3°C, 50% at Tact of -3.3°C, and 100% at Tact of -3.5°C, -3.8°C, and -4.8°C (Table 3), suggesting that Tct is -2.3 to -3.3°C. By average -2.3 and -3.3°C, we can obtain -2.8°C as Tct. As the trees in the -3°C treatment (Tact = -3.3°C) were four to five years old, which are the older trees of experiment 1, there is a possibility that Tct of trees in experiment 3 is more than that in experiment 1. Therefore, Tct of ‘Summer Queen’ passion fruit obtained by experiment 3 is around -2.5°C. Pd of ‘Sata’ lychee was 0% at Tact of 0.9°C, -2°C, -3.3°C, and -3.8°C, 50% at Tact of -4.4°C, and 100% at Tact of -4.8°C, -5.6°C, and -8.1°C. Therefore, Tct of ‘Sata’ lychee obtained by experiment 3 is around -4°C. Pd of ‘Mexicola’ avocado was 0% at Tact of -3.3°C and -5.6°C, and 100% at Tact of -6.4°C, and Tact of -8.2°C. Average of lower limit Twin of overwintered plants (-5.6°C) and higher limit Twin of killed plants (-6.4°C) is -6°C. From these results, Tct of the ‘Mexicola’ avocado is around -6°C. Note that, cold damage surveys in experiment 3 were performed one month after the low- temperature treatment; however, most of the trees suffered heavy cold damage in the day after the treatment.
Tl-Tact and Tb-Tact in experiment 3 are also shown in Table 3. Although chamber specifications such as structure or cooling methods are slightly different, the mean temperature difference between Tl and Tact was -0.1 to 1°C in passion fruit, lychee, and avocado.
Table 3. Results of Pd and tree temperature in each treatment temperature and hour of experiment 3. Pd of each species and variety (%) is shown at each treatment temperature and hour. Ttre is target value of treatment temperature. Tact is actual mean temperature during the experiment. DT is duration of the treatment. A solid double line indicates Tct obtained by experiment 3. Tl is leaf temperature (°C), and Tb is surface temperature of branch (°C). Values shown in Table 3 is mean temperature during the treatment (°C).
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In experiment 1, the cold tolerance of fruit trees in the field was investigated for each tree species and variety. Tct of ‘Summer Queen’ passion fruit was estimated at around -0.5°C by averaging lower limit of Twin in overwintered plants and higher limit of Twin in killed plants. In this regard, however, since an experimental result is absent in Kagoshima_B 2017 (Twin = -1.5°C), there is possibility that Tct of passion fruit in field can range from around -1.5 to 1°C. Until now, little research on the cold tolerance of field grown passion fruit has been conducted. Kawasaki (1999) mentioned based on their experience that places with a temperature of > -2°C are preferable for the cultivation of passion fruit. Yonemoto (2009) reported that purple passion fruit can withstand -2°C for several hours and overwinter in fields such as in the southern part of Kyushu and Shionomisaki Peninsula in the Wakayama Prefecture. Extreme minimum temperature during 2011-2020 recorded by the automated meteorological acquisition system (AMeDAS) operated by JMA at Shionomisaki was -1.8°C in 2016. Similarly, -1.5°C was recorded in 2011 and low temperature events with approximately -1°C occurred in 2012 and 2018. In our chamber experiment (experiment 3), Tct of passion fruit was around -2.5°C. Moreover, difference of Ta and Tl in passion fruit was around -1°C in experiment 2. Considering that Tct of our plants obtained from field experiment tends to be higher than that from chamber experiment by the difference between Ta and Tl as described in the following part of 4.3, reasonable value of Tct of passion fruit in field seems to be around -1.5°C, which is consistent with Tct in previous reports. Tct of ‘Sata’ lychee was estimated at -2°C in experiment 1. Previous studies have reported that Tct of lychee is -1 to -2°C in mature trees (Uto, 1980), -2 to -3°C (Paull and Duarte, 2011) and 0 to -2°C (Crane et al., 2019) in young trees. Tct of lychee, thus, had a range of 0 to -3°C in previous studies, which could be specified by our study. Tct of avocado in experiment 1 was approximately -5 in ‘Mexicola,’ -4.5°C in ‘Bacon,’ and -4°C in ‘Fuerte,’ which indicated the dependence of cold tolerance on variety. Yonemoto (2016) reported that Tct of ‘Mexicola’ and ‘Bacon’ avocado is -5 to -6°C and -4 to -5°C, respectively. Avocado is classified into three races: Guatemalan, Mexican and West Indian. In these species, Mexican is the most cold-hardy race (Yonemoto, 2016). As the ‘Mexicola’ variety is one of the Mexican races, its cold tolerance is considered stronger than the ‘Bacon’ and ‘Fuerte’ varieties. Through experiment 1, specific difference of cold tolerance of avocado on the variety of the species was clarified, which can contribute to our better understanding on cold tolerance of avocado.
Most of the perennial plants have the ability to acquire cold tolerance by experiencing low temperatures from fall to the beginning of winter, this process is called ‘hardening’ (Young 1961; Utsunomiya, 1990). In tropical fruits trees, Yamada (1989) investigated the effect of low temperature during the pre-freezing treatment period on the hardening of one-year-old potted ‘Yellow Passion Flower’ and ‘Oomino Passion Flower’ passion fruit and two-year-old potted ‘Walter Hole’ avocado by managing them in facilities with having different temperatures. They reported that the avocado in the lower temperature facility before freeze-treatment showed stronger cold hardiness compared to that in a higher temperature facility. However, they reported that this effect is unclear in passion fruit. McKellar et al. (1983) periodically sampled leaves of avocados and mangos, each of which were grown at two different temperature regions in the Florida Peninsula, USA during October to February. Following the low-temperature treatment of the sample leaves, they measured electrolyte leakage of the leaves and investigated the relationship between temperature during the period before sampling and cold hardiness of each species. Results indicated that low temperature before sampling strengthened cold hardiness in avocado but not clear in mango. These studies suggest that the effect of low temperature during the hardening period on the acquisition of cold tolerance vary among tropical tree species. Figure 2 shows scatter plots comparing Twin and mean minimum temperature from September 1 to the end of February (Taut) at each site of experiment 1. In Figure 2, plots of cross and circle shape indicate ‘killed’ and ‘overwintered’ plants in experiment 1, respectively. In ‘Mexicola’ avocado, ‘Sata’ lychee, and ‘Summer Queen’ passion fruit, ‘killed’ and ‘overwintered’ plots were distributed lower and higher than the Tct (a vertical dashed line), respectively. In ‘Bacon’ and ‘Fuerte’ avocado, some ‘overwintered’ plots were distributed lower than the Tct. As Taut of the plots tend to be lower than the ‘killed’ plots in the same Twin, low temperature during the fall season seems to be one of factor that strengthen cold tolerance at least in ‘Bacon’ and ‘Fuerte’ avocado. At this moment, since we have insufficient data to confirm our hypothesis, further research is required for clarifying the impact of low temperature including its duration or timing to the cold tolerance acquisition.
Fig. 2. Relationship among Twin, Taut and wintering of test plant
Twin indicates the minimum temperature during the experimental period. Taut indicates mean minimum temperature from September 1 to the end of February. Plots of ‘×’ and ‘○’ indicates ‘killed’ plants (Pd > 0%) and ‘overwintered’ plants (Pd = 0%), respectively. Vertical broken line indicates Tct obtained from experiment 1.
Field and chamber experiments enabled us to investigate the effects of the differences in the experimental methods to cold tolerance and tree temperature.
Tct obtained in experiment 1 was around -1.5°C for ‘Summer Queen’ passion fruit and -2°C for ‘Sata’ lychee. In avocado, Tct was -5, -4.5, and -4°C in ‘Mexicola,’ ‘Bacon,’ and ‘Fuerte,’ respectively. On the other hand, Tct estimated in experiment 3 was around -2.5°C for ‘Summer Queen’ passion fruit, -4°C for ‘Sata’ lychee and -6°C for ‘Mexicola’ avocado. Comparing these results, Tct obtained from experiment 3 was approximately 1°C lower than that from experiment 1 for passion fruit and avocado, and approximately 2°C lower for lychee. In experiment 2, Tl was approximately 1°C lower than Ta for ‘Summer Queen’ passion fruit and ‘Mexicola’ avocado and 2°C for lychee. On the other hand, in experiment 3, Tl was almost the same as Tact. Therefore, the difference in Tct obtained in the field and chamber experiments should be due to the difference in Tl between field and chamber experiments. In the chamber experiment, as a test tree was surrounded by walls on all sides, the leaf oriented toward the sky receives longwave radiation emitted from the walls. In addition to this, in the chamber, air is continuously blown to keep the inside temperature constant. These factors suppress radiative cooling on the tree surface, which reduce the temperature difference between Tl and Tact. On the other hand, in the field experiment, Rn was -80 to -90 W m-2 on each experiment day, indicating the occurrence of radiative cooling on the leaf. A strong radiative cooling (Rn = -97.3 W m-2) occurred on the night of May 5; however, the difference between Ta and Tl was small, probably because of the forced convection heat transfer on the leaf induced by the nighttime strong wind (Takechi, 1968). From these results, we concluded that Tl drops significantly compared to Ta on clear and calm nights due to radiative cooling.
4.3 A simple method for estimating cold tolerance of trees growing in the fieldOur experimental results clarified that Tct obtained from the field experiment is higher than the chamber experiment by 1°C for passion fruit and avocado and 2°C for lychee, which is explained by the difference between Ta and Tl of field trees caused by radiative cooling. Therefore, Tct of passion fruit and avocado can be estimated by adding 1°C to Tct obtained from the chamber experiment and that of lychee can be estimated by adding 2°C to the Tct obtained from the chamber experiment. Since leaf of lychee is thin compared to passion fruit and avocado, which might affect to radiative cooling intensity on the leaf and cause difference of Tl - Ta between lychee and the other species. As energy exchanges such as radiative, sensible heat, and latent heat transfers on a leaf are to be not substantially different among the varieties, the cold tolerance prediction of the other varieties of passion fruit, avocado, and lychee might be possible by adding the temperature differences to Tct obtained in the chamber experiments. Moreover, even for other tree species, once the estimation of Tct is made through the chamber experiment and the difference between Ta and Tl is measured on a clear and calm night, Tct of trees growing in fields can be estimated without conducting extensive field experiments.
With the intensification of global warming, more varieties of tropical fruit trees are expected to be grown in Japan, leading to the formation of a new industry. The method of estimation of cold tolerance developed in this study can be utilized for precisely and efficiently identifying suitable areas for the growth of various tropical species in the future.
The cold tolerance of subtropical fruit trees was investigated by exposing plants to winter daily low temperatures in fields of ten regions in Japan (field experiment). In the field experiment, the cold tolerances of ‘Summer Queen’ passion fruit and ‘Sata’ lychee were estimated at -1.5 and -2°C, respectively. The cold tolerances of ‘Mexicola,’ ‘Bacon,’ and ‘Fuerte,’ avocados were estimated at -5, -4.5, and -4°C, respectively, indicating the dependence of cold tolerance on the variety of the tree species.
Additionally, cold tolerance in tropical trees were evaluated through chilling treatments in chambers (chamber experiment), which enabled us to investigate the effects of the differences in the estimated cold tolerance based on the experimental method. The cold tolerance in the chamber experiment was approximately 1°C lower than that of the field experiment for passion fruit and avocado and approximately about 2°C lower for lychee, which could be explained by the effect of radiative cooling on the surface of the leaf under conditions of a clear and clam night in the field.
Using the difference in the cold tolerances estimated in the field and chamber experiments, we developed a simple method for the estimation of cold tolerance of trees that are grown in the field without conducting field experiments. Accordingly, the cold tolerance of passion fruit and avocado can be estimated by adding 1°C to the cold tolerance obtained from the chamber experiment and that of lychee can be estimated by adding 2°C to the cold tolerance obtained from the chamber experiment. As energy exchanges such as radiative, sensible heat, and latent heat transfers on a leaf are not substantially different among varieties, the prediction of cold tolerance of the other varieties of passion fruit, avocado, and lychee might be possible by adding the temperature difference to the cold tolerance obtained in the chamber experiment. Moreover, even for other tree species, which were not consider in this study, the cold tolerance of the trees grown in fields can be estimated based on the results of the chamber experiment and using estimations of the difference between air and leaf temperatures on a clear and calm night.
This research was performed by the Environment Research and Technology Development Fund (JPMEERF20S11806) of the Environmental Restoration and Conservation Agency of Japan. We express our gratitude to all researchers who supported our experiments.
