Effects of Covering Mature Avocado ‘Pinkerton’ Trees with High-density Shading Nets during Cold Winters on Microclimate, Chlorophyll Fluorescence, Flowering, and Yield

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Lior Rubinovich Northern Agriculture Research and Development, MIGAL–Galilee Research Institute, P.O. Box 831, Kiryat Shmona, Israel

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Carmit Sofer-Arad Northern Agriculture Research and Development, MIGAL–Galilee Research Institute, P.O. Box 831, Kiryat Shmona, Israel

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Simon Chernoivanov Northern Agriculture Research and Development, MIGAL–Galilee Research Institute, P.O. Box 831, Kiryat Shmona, Israel

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Nitzan Szenes Agricultural Extension Service, Israel Ministry of Agriculture and Rural Development, Beit Dagan 50250, Israel

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Abstract

Avocado (Persea americana Mill.) is a subtropical fruit tree with high commercial value and increasing global demand. Most avocado cultivars are vulnerable to cold climates, which may reduce yields and restrict their geographical expansion. This includes the green-skinned avocado cultivar Pinkerton, which accounts for 45% of the avocado cultivated in northeastern Israel. Shading nets can protect agricultural crops from cold environments. Therefore, we evaluated the effect of covering mature ‘Pinkerton’ trees with high-density shading nets during the winter. Trees were covered with silver-colored 50% or 70% shading nets during three consecutive winters, and uncovered trees served as controls. Photosynthetically active radiation in plots covered with the silver 50% or 70% nets was significantly lower than that for the control by 52% and 90%, respectively. The minimum air temperature was similar between treatments. The maximum air temperature was generally lower under the shading nets compared with that of the control. The ratios of variable fluorescence to maximum fluorescence (Fv/Fm) measured in February 2019 and February 2020 were 0.72 and 0.8 for the control trees, 0.79 and 0.83 for the silver 50% trees, and 0.81 and 0.84 for the silver 70% trees, respectively. Flowering intensity of the net-covered trees was lower than that of the control by up to 42%. Interestingly, the 3-year average yield of trees covered with the silver 50% or 70% nets was insignificantly higher by 27% and 38%, respectively, compared with the control trees. These results suggest that the reduction of daytime solar irradiance in the winter by the shading nets may mitigate adverse effects of cold and increase yield. Additional long-term studies should examine the effects of shading nets and other shading strategies on different avocado cultivars.

Avocado (Persea americana Mill.) is a subtropical fruit tree with high commercial value and increasing global demand. In 2021, avocado world production was ∼8.7 million tons, with a gross production value of approximately $US 8.56 billion (www.fao.org; accessed Jul 2023). Most avocado cultivars are vulnerable to cold stress, which may reduce yields and restrict their geographical expansion (Schaffer et al. 2013). Extreme cold events are becoming more frequent as climate change intensifies, even in regions where they were previously rare. As a result, it is imperative to develop solutions to reduce cold damage of mature avocado plantations. The green-skinned avocado cultivar Pinkerton is considered as cold-sensitive as Hass and accounts for 45% of the avocado cultivated in northeastern Israel. The majority (∼70%) of the Israeli ‘Pinkerton’ are exported to Europe (mainly Eastern Europe), whereas ∼30% are marketed in the domestic market (data from the Israeli Fruit Board). In this region, minimum winter temperatures may drop below 0 °C (frosts) or approach, but remain higher than, 0 °C (chilling events). Although frost can cause mild to severe visible damage to leaves, buds, flowers, and fruit, the consequences of chilling have been much less described because the plant tissues show almost no visible damage (Chernoivanov et al. 2022). Chilling temperatures can cause photoinhibitory damage to photosystem 2 that can be quantified by measuring a decrease in the ratio of variable to maximum chlorophyll fluorescence (Fv/Fm). Whiley et al. (1999) showed that for avocado and mango, low winter temperatures below 10 °C combined with high light intensity resulted in a decrease in Fv/Fm values because of chill-induced photoinhibition of leaves (Whiley et al. 1999). Additional studies showed that the Fv/Fm rate is a reliable quantitative indicator of cold stress damage (Rizza et al. 2001). Shading nets can protect agricultural crops from harsh environments and cold damage (Chernoivanov et al. 2022; Zait et al. 2020). We hypothesized that covering avocado trees during the winter with high-density shading nets would reduce adverse effects of cold. Therefore, the main objective was to evaluate the effect of covering mature ‘Pinkerton’ trees with different shading nets during the winter. Specifically, we evaluated the capacity of the shading nets to reduce the risks of cold damage and increase tree production.

Materials and Methods

Experimental site.

The experiments were conducted during 2017–18, 2018–19, and 2019–20 at a commercial ‘Pinkerton’ orchard in Kibbutz Dan in northwest Israel (lat. 33°23′N, long. 35°66′E, 190 m above sea level). ‘Ettinger’ trees served as pollinizers. The trees were grafted on ‘Shiller’ seedling rootstocks and planted in 1996, with 6-m spacing between rows and 4 m between trees. The rows were oriented northeast/southwest. Chilling temperatures occurred in the experimental orchard mainly from December to mid-February.

Shading nets and experimental design.

From early December through mid-March (before the flowering period) of 2018, 2019, and 2020, silver-colored nets with 50% or 70% shading levels (silver 50% and silver 70%; Ginegar Plastic Products Ltd., Ginegar, Israel) were placed over the trees and supported by their canopies. Trees from the control plot were not covered with shading nets (control trees). The experimental plots were completely randomized, with four repeats for each treatment (control and shading nets, n = 4). The area of each repeat was 0.1 ha, and only the eight middle trees of each repeat (experimental trees) were measured.

Temperature and light measurements.

Miniature waterproof, single-channel Hobo temperature data loggers (cat. no. UA-001–64; Onset Corp., Bourne, MA, USA) were used to measure air temperatures in the field. They were positioned 1.5 m above the ground and shielded from direct sunlight by the canopy. The air temperature was measured continuously at 10-min intervals from December until mid-March of each year. Three temperature data loggers were placed for each treatment. Photosynthetically active radiation (PAR) was measured 1.5 m above the ground (∼3 m beneath the nets) using a Field Scout quantum meter (#3415F; Spectrum Technologies Inc., Aurora, IL, USA). At least 10 measurements were performed for each treatment with different nets and controls.

Analysis of chlorophyll a fluorescence.

Chlorophyll a fluorescence was measured soon before sunrise in complete darkness with a FluorPen FP100 portable fluorometer (Photon Systems Instruments, Drasov, Czech Republic) and Fv/Fm was calculated (Chernoivanov et al. 2022). Measurements of at least 3 leaves of each of the experimental trees were performed during Feb 2019 and Feb 2020, after the swelling of inflorescence buds and before flowering.

Estimation of flowering intensity and yield measurements.

Flowering intensity was appreciated as described previously during peak bloom in Apr 2018, 2019, and 2020 (Ziv et al. 2014); during a blind test, two surveyors independently assessed each experimental tree from 0 (no flowering) to 5 (high flowering intensity). To determine fruit yield, during the commercial harvest period of each year (winter), all the fruits on the experimental trees were manually picked and weighed separately for each tree. The 3-year mean yield of each treatment was calculated by averaging the yields of each of the 3 years of the experiment (n = 3).

Statistical analysis.

All results were subjected to a one-way analysis of variance, followed by the Tukey honestly significant difference test using JMP software (version 11.0.0; SAS Institute, Cary, NC, USA).

Results

Effect of shading nets on PAR and air temperature.

PAR of plots covered with the silver 50% and silver 70% nets were significantly lower (P < 0.001) by 52% and 90% compared with those of the control, respectively (Supplemental Fig. 1). Thus, the actual shading rate of the silver 70% shading nets was slightly higher than expected. PAR in plots covered with the silver 50% net was significantly higher (P < 0.001) than that of those covered with the silver 70% net. During Winter 2017–18 and Winter 2018–19, the minimum air temperature in the orchard reached 2.52 °C (Fig. 1A) and 0.42 °C (Fig. 1B), respectively; it did not fall below 0 °C. In Winter 2019–20, the minimum air temperature reached below 0 °C (−1.38 °C), but only during 1 night (Fig. 1C). The minimum air temperature was similar between treatments (Fig. 1A–C). However, the maximum air temperatures under the shading nets were generally lower than those of the control (Fig. 1D–F).

Fig. 1.
Fig. 1.

Minimum and maximum daily air temperatures in the control plots and under the different shading nets during the Winters of 2017–18 (A, D), 2018–19 (B, E), and 2019–20 (C, F). The average of three data loggers represents the mean temperature for each treatment.

Citation: HortScience 58, 10; 10.21273/HORTSCI17337-23

Effect of shading nets on Fv/Fm.

In Feb 2019 and Feb 2020, the Fv/Fm rates of the leaves of trees from the control treatment reached 0.72 and 0.8, respectively (Supplemental Fig. 2A). During both years, the Fv/Fm rates were significantly higher (P < 0.05) under the silver 50% and silver 70% shading nets, with no significant differences between them (P > 0.05).

Effect of shading nets on flowering intensity and yield.

The baseline (background) yield before the deployment of the nets was meager and similar between treatments (Supplemental Fig. 2B). In Apr 2018, flowering intensity was very high, with no significant differences (P > 0.05) between the treatments (Fig. 2A). In Apr 2019, flowering intensity in the control trees was high and significantly higher (P < 0.05) than that of the trees covered with the silver 70% net. In Apr 2020, flowering intensity was highest in the control trees, but with no significant differences (P > 0.05) between treatments. At the end of 2018, fruit yield was significantly higher (P < 0.05), by 71%, in the trees covered with the silver 70% net than in the control trees (Fig. 2B). There were no significant differences (P > 0.05) between the yields of trees covered with the silver 50% net compared with both the control trees and trees covered with silver 70%. At the end of 2019, fruit yield was very low, with no significant differences (P > 0.05) between treatments. However, the yields of trees covered with the silver 50% and silver 70% nets were higher by 109% and 62%, respectively, compared with the control trees. At the end of 2020, the average fruit yield was similar, with no significant differences between treatments (P > 0.05). The 3-year average yields of trees covered with the silver 50% and silver 70% nets were insignificantly (P > 0.05) higher by 27% and 38%, respectively, compared with the control trees.

Fig. 2.
Fig. 2.

Flowering intensity was assessed and scored in Apr 2018, Apr 2019, and Apr 2020, using a scale of 0 to 5, with 0 representing no apparent flowering and 5 representing maximum bloom (A). During the commercial harvest period of each year, fruits were picked and weighed separately for each tree (B). Values are means ± SE of four repeats (n = 4), with each comprising eight trees. Values of the 3-year average yield (2018–20) are means ± SE of the yields of each year of the experiment (n = 3). Different letters above a column indicate significant differences within each year (Tukey’s honestly significant difference, P < 0.05).

Citation: HortScience 58, 10; 10.21273/HORTSCI17337-23

Discussion

A decrease in Fv/Fm may indicate chill-induced photoinhibition and cold stress damage (Rizza et al. 2001; Whiley et al. 1999). Thus, the higher Fv/Fm values observed in trees covered with either of the shading nets compared with the control suggested a reduction in cold stress. A previous study showed similar results for young avocado ‘Reed’ trees covered with the same shading nets during cold winters, when temperatures were similar but above 0 °C (Chernoivanov et al. 2022).

Flowering intensity was generally lower in the net-covered trees. Floral induction of avocado grown in the northern hemisphere occurs during late fall, before net deployment (Ziv et al. 2014). Thus, it is possible that the reduction in flowering intensity in the net-covered trees was attributable to the reduction in winter daytime temperatures and/or solar irradiation, affecting floral bud development, which occurs during winter (Acosta-Rangel et al. 2021). This assumption requires substantiation. It is also possible that alternate bearing, which is manifested by a decrease in flowering intensity after high yield (Lovatt 2010), was the reason for the reduction in flowering intensity in the net-covered trees in Apr 2019. During the years of the experiment, despite the lower flowering intensity, there was no reduction in fruit yield with both net treatments. In particular, at the end of 2018, the yield of the net-covered trees was higher than that of the control and was inversely related to the flowering intensity of Apr 2018. Thus, it is possible that the use of these shading nets during winter enhances avocado flowering quality (i.e., the functionality of floral organs) rather than flowering intensity, leading, in some cases, to higher yields. It is also possible that the use of shading nets resulted in a delay in flowering, which may be advantageous to fruit yield in cases of low spring temperatures, which impair bee pollination (Stern et al. 2021). Covering the avocado trees during winter may also enhance the chlorophyll content and photosynthesis rate, thus increasing carbohydrate production required for fruit set and development (Alon et al. 2022; Chernoivanov et al. 2022; Mditshwa et al. 2019). These possible effects were not examined during this study; therefore, they warrant further investigation.

The relatively low yields in 2019, after the high yields of the previous year, can be attributed to the tendency of avocado to alternate bearing (Cohen et al. 2023). Because the yield of the net-covered trees in 2018 was higher than that of the control, it could be expected that the yield during the following year of these trees would be lower than that of the control. The fact that the yield of the net-covered trees was higher further supports the effect of the shading nets on fruit yield.

It is important to note that, similar to previously reported results, the shading nets did not increase minimum winter temperatures; however, they reduced the daytime maximum temperatures and solar irradiation (Chernoivanov et al. 2022). High light intensity has an important role in overall stress during chilling events (Wise 1995). Thus, the effect of the nets on Fv/Fm values and fruit yield might be attributed to the reduction of daytime solar irradiance during the winter rather than changes in the minimum air temperature. This assumption must be further investigated by determining the mechanism by which the shading nets exert their effects, such as their impact on light-adapted chlorophyll fluorescence, leaf temperature, and gas exchange parameters.

More broadly, previous studies have shown that high-density shading nets can protect banana plants from frost damage (Zait et al. 2020). Interestingly, high-density shading nets also improve photosynthetic performance of mature ‘Pinkerton’ trees during extreme heat events (Alon et al. 2022). Thus, using shading nets in avocado orchards should be considered as a potential measure to reduce a wide range of climate-related abiotic stresses. However, excess shading may have adverse effects. For instance, photosynthetic performance may decrease on days with regular fall temperatures (Alon et al. 2022). Furthermore, bee activity and pollination may be impaired in trees grown under shading nets (Mditshwa et al. 2019). Hence, long-term follow-up studies should be conducted to establish proper shading-management protocols using shading nets and other shading strategies, such as over-canopy solar panels, with different avocado cultivars and in countries with different light regimes.

Conclusion

In conclusion, covering ‘Pinkerton’ trees with high-density shading nets during cold winters could have a negative effect on flowering but a positive effect on chlorophyll fluorescence and fruit yield. Although the mechanism for these effects remains unclear, it is possible that the reduction of daytime solar irradiance is involved. Additional studies should be conducted to further examine their effect mechanism and evaluate their ability to reduce tree damage during extreme cold events such as frosts.

References Cited

  • Acosta-Rangel A, Li R, Mauk P, Santiago L, Lovatt CJ. 2021. Effects of temperature, soil moisture and light intensity on the temporal pattern of floral gene expression and flowering of avocado buds (Persea americana cv. Hass). Scientia Hortic. 280:109940. https://doi.org/10.1016/j.scienta.2021.109940.

    • Search Google Scholar
    • Export Citation
  • Alon E, Shapira O, Azoulay-Shemer T, Rubinovich L. 2022. Shading nets reduce canopy temperature and improve photosynthetic performance in ‘Pinkerton’ avocado trees during extreme heat events. Agronomy (Basel). 12(6):1360. https://doi.org/10.3390/agronomy12061360.

    • Search Google Scholar
    • Export Citation
  • Chernoivanov S, Neuberger I, Levy S, Szenes N, Rubinovich L. 2022. Covering young ‘Reed’ avocado trees with shading nets during winter alleviates cold stress and promotes vegetative growth. Eur J Hortic Sci. 87(1):110. https://doi.org/10.17660/eJHS.2022/007.

    • Search Google Scholar
    • Export Citation
  • Cohen H, Bar-Noy Y, Irihimovitch V, Rubinovich L. 2023. Effects of seedling and clonal West Indian rootstocks irrigated with recycled water on ‘Hass’ avocado yield, fruit weight and alternate bearing. N Z J Crop Hortic Sci. 51:3951. https://doi.org/10.1080/01140671.2022.2098779.

    • Search Google Scholar
    • Export Citation
  • Lovatt CJ. 2010. Alternate bearing of ‘Hass’ avocado. California Avocado Society Yearbook. 93:125140.

  • Mditshwa A, Magwaza LS, Tesfay SZ. 2019. Shade netting on subtropical fruit: Effect on environmental conditions, tree physiology and fruit quality. Scientia Hortic. 256:108556. https://doi.org/10.1016/j.scienta.2019.108556.

    • Search Google Scholar
    • Export Citation
  • Rizza F, Pagani D, Stanca AM, Cattivelli L. 2001. Use of chlorophyll fluorescence to evaluate the cold acclimation and freezing tolerance of winter and spring oats. Plant Breed. 120(5):389396. https://doi.org/10.1046/j.1439-0523.2001.00635.x.

    • Search Google Scholar
    • Export Citation
  • Schaffer B, Gil P, Mickelbart M, Whiley A. 2013. The avocado: Botany, production and uses. CABI, Wallingford UK. https://doi.org/10.1079/9781845937010.0000.

  • Stern RA, Rozen A, Eshed R, Zviran T, Sisai I, Sherman A, Irihimovitch V, Sapir G. 2021. Bumblebees (Bombus terrestris) improve ‘Hass’ avocado (Persea americana) pollination. Plants. 10(7):1372. https://doi.org/10.3390/plants10071372.

    • Search Google Scholar
    • Export Citation
  • Whiley AW, Searle C, Schaffer B, Wolstenholme BN. 1999. Cool orchard temperatures or growing trees in containers can inhibit leaf gas exchange of avocado and mango. J Am Soc Hortic Sci. 124(1):4651. https://doi.org/10.21273/jashs.124.1.46.

    • Search Google Scholar
    • Export Citation
  • Wise RR. 1995. Chilling-enhanced photooxidation: The production, action and study of reactive oxygen species produced during chilling in the light. Photosynth Res. 45(2):7997. https://doi.org/10.1007/BF00032579.

    • Search Google Scholar
    • Export Citation
  • Zait Y, Elingold I, Londener A, Gal E, Or G, Galpaz N. 2020. Banana frost protection by thermal nets, p 21–26. Acta Hortic. International Society for Horticultural Science. https://doi.org/10.17660/ActaHortic.2020.1272.3.

  • Ziv D, Zviran T, Zezak O, Samach A, Irihimovitch V. 2014. Expression profiling of FLOWERING LOCUS T-like gene in alternate bearing ‘Hass’ avocado trees suggests a role for PaFT in avocado flower induction. PLoS One. 9(10):114. https://doi.org/10.1371/journal.pone.0110613.

    • Search Google Scholar
    • Export Citation

Supplemental Materials

Supplemental Fig. 1.
Supplemental Fig. 1.

Effect of winter shading nets on photosynthetically active radiation (PAR). PAR measurements were obtained in Feb 2018, during the late morning, in the control and net-covered plots. Values are means ± SE of 10 replicates (n = 10). Different letters above columns indicate significant differences (Tukey’s honestly significant difference, P < 0.05).

Citation: HortScience 58, 10; 10.21273/HORTSCI17337-23

Supplemental Fig. 2.
Supplemental Fig. 2.

Chlorophyll a fluorescence parameters were recorded in Feb 2019 and Feb 2020, in the dark, soon before sunrise to calculate the maximum quantum yield (Fv/Fm) (A). Values are means ± SE of four repeats (n = 4), with each comprising eight trees; three leaves of each tree were measured. In Jan 2018 (the commercial harvest of season 2017–18; baseline), fruits were picked and weighed (B). Values are the means ± SE of four repeats (n = 4), with each comprising eight trees. Different letters above columns indicate significant differences (Tukey’s honestly significant difference, P < 0.05).

Citation: HortScience 58, 10; 10.21273/HORTSCI17337-23

  • Fig. 1.

    Minimum and maximum daily air temperatures in the control plots and under the different shading nets during the Winters of 2017–18 (A, D), 2018–19 (B, E), and 2019–20 (C, F). The average of three data loggers represents the mean temperature for each treatment.

  • Fig. 2.

    Flowering intensity was assessed and scored in Apr 2018, Apr 2019, and Apr 2020, using a scale of 0 to 5, with 0 representing no apparent flowering and 5 representing maximum bloom (A). During the commercial harvest period of each year, fruits were picked and weighed separately for each tree (B). Values are means ± SE of four repeats (n = 4), with each comprising eight trees. Values of the 3-year average yield (2018–20) are means ± SE of the yields of each year of the experiment (n = 3). Different letters above a column indicate significant differences within each year (Tukey’s honestly significant difference, P < 0.05).

  • Supplemental Fig. 1.

    Effect of winter shading nets on photosynthetically active radiation (PAR). PAR measurements were obtained in Feb 2018, during the late morning, in the control and net-covered plots. Values are means ± SE of 10 replicates (n = 10). Different letters above columns indicate significant differences (Tukey’s honestly significant difference, P < 0.05).

  • Supplemental Fig. 2.

    Chlorophyll a fluorescence parameters were recorded in Feb 2019 and Feb 2020, in the dark, soon before sunrise to calculate the maximum quantum yield (Fv/Fm) (A). Values are means ± SE of four repeats (n = 4), with each comprising eight trees; three leaves of each tree were measured. In Jan 2018 (the commercial harvest of season 2017–18; baseline), fruits were picked and weighed (B). Values are the means ± SE of four repeats (n = 4), with each comprising eight trees. Different letters above columns indicate significant differences (Tukey’s honestly significant difference, P < 0.05).

  • Acosta-Rangel A, Li R, Mauk P, Santiago L, Lovatt CJ. 2021. Effects of temperature, soil moisture and light intensity on the temporal pattern of floral gene expression and flowering of avocado buds (Persea americana cv. Hass). Scientia Hortic. 280:109940. https://doi.org/10.1016/j.scienta.2021.109940.

    • Search Google Scholar
    • Export Citation
  • Alon E, Shapira O, Azoulay-Shemer T, Rubinovich L. 2022. Shading nets reduce canopy temperature and improve photosynthetic performance in ‘Pinkerton’ avocado trees during extreme heat events. Agronomy (Basel). 12(6):1360. https://doi.org/10.3390/agronomy12061360.

    • Search Google Scholar
    • Export Citation
  • Chernoivanov S, Neuberger I, Levy S, Szenes N, Rubinovich L. 2022. Covering young ‘Reed’ avocado trees with shading nets during winter alleviates cold stress and promotes vegetative growth. Eur J Hortic Sci. 87(1):110. https://doi.org/10.17660/eJHS.2022/007.

    • Search Google Scholar
    • Export Citation
  • Cohen H, Bar-Noy Y, Irihimovitch V, Rubinovich L. 2023. Effects of seedling and clonal West Indian rootstocks irrigated with recycled water on ‘Hass’ avocado yield, fruit weight and alternate bearing. N Z J Crop Hortic Sci. 51:3951. https://doi.org/10.1080/01140671.2022.2098779.

    • Search Google Scholar
    • Export Citation
  • Lovatt CJ. 2010. Alternate bearing of ‘Hass’ avocado. California Avocado Society Yearbook. 93:125140.

  • Mditshwa A, Magwaza LS, Tesfay SZ. 2019. Shade netting on subtropical fruit: Effect on environmental conditions, tree physiology and fruit quality. Scientia Hortic. 256:108556. https://doi.org/10.1016/j.scienta.2019.108556.

    • Search Google Scholar
    • Export Citation
  • Rizza F, Pagani D, Stanca AM, Cattivelli L. 2001. Use of chlorophyll fluorescence to evaluate the cold acclimation and freezing tolerance of winter and spring oats. Plant Breed. 120(5):389396. https://doi.org/10.1046/j.1439-0523.2001.00635.x.

    • Search Google Scholar
    • Export Citation
  • Schaffer B, Gil P, Mickelbart M, Whiley A. 2013. The avocado: Botany, production and uses. CABI, Wallingford UK. https://doi.org/10.1079/9781845937010.0000.

  • Stern RA, Rozen A, Eshed R, Zviran T, Sisai I, Sherman A, Irihimovitch V, Sapir G. 2021. Bumblebees (Bombus terrestris) improve ‘Hass’ avocado (Persea americana) pollination. Plants. 10(7):1372. https://doi.org/10.3390/plants10071372.

    • Search Google Scholar
    • Export Citation
  • Whiley AW, Searle C, Schaffer B, Wolstenholme BN. 1999. Cool orchard temperatures or growing trees in containers can inhibit leaf gas exchange of avocado and mango. J Am Soc Hortic Sci. 124(1):4651. https://doi.org/10.21273/jashs.124.1.46.

    • Search Google Scholar
    • Export Citation
  • Wise RR. 1995. Chilling-enhanced photooxidation: The production, action and study of reactive oxygen species produced during chilling in the light. Photosynth Res. 45(2):7997. https://doi.org/10.1007/BF00032579.

    • Search Google Scholar
    • Export Citation
  • Zait Y, Elingold I, Londener A, Gal E, Or G, Galpaz N. 2020. Banana frost protection by thermal nets, p 21–26. Acta Hortic. International Society for Horticultural Science. https://doi.org/10.17660/ActaHortic.2020.1272.3.

  • Ziv D, Zviran T, Zezak O, Samach A, Irihimovitch V. 2014. Expression profiling of FLOWERING LOCUS T-like gene in alternate bearing ‘Hass’ avocado trees suggests a role for PaFT in avocado flower induction. PLoS One. 9(10):114. https://doi.org/10.1371/journal.pone.0110613.

    • Search Google Scholar
    • Export Citation
Lior Rubinovich Northern Agriculture Research and Development, MIGAL–Galilee Research Institute, P.O. Box 831, Kiryat Shmona, Israel

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Carmit Sofer-Arad Northern Agriculture Research and Development, MIGAL–Galilee Research Institute, P.O. Box 831, Kiryat Shmona, Israel

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Simon Chernoivanov Northern Agriculture Research and Development, MIGAL–Galilee Research Institute, P.O. Box 831, Kiryat Shmona, Israel

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Nitzan Szenes Agricultural Extension Service, Israel Ministry of Agriculture and Rural Development, Beit Dagan 50250, Israel

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Contributor Notes

We thank the Avocado-GAL Corporation and the Israeli Fruit Board for financial support. We thank the Kibbutz Dan avocado team and Michael Noy for their invested effort in this study.

L.R. is the corresponding author. E-mail: liorr@migal.org.il.

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  • Fig. 1.

    Minimum and maximum daily air temperatures in the control plots and under the different shading nets during the Winters of 2017–18 (A, D), 2018–19 (B, E), and 2019–20 (C, F). The average of three data loggers represents the mean temperature for each treatment.

  • Fig. 2.

    Flowering intensity was assessed and scored in Apr 2018, Apr 2019, and Apr 2020, using a scale of 0 to 5, with 0 representing no apparent flowering and 5 representing maximum bloom (A). During the commercial harvest period of each year, fruits were picked and weighed separately for each tree (B). Values are means ± SE of four repeats (n = 4), with each comprising eight trees. Values of the 3-year average yield (2018–20) are means ± SE of the yields of each year of the experiment (n = 3). Different letters above a column indicate significant differences within each year (Tukey’s honestly significant difference, P < 0.05).

  • Supplemental Fig. 1.

    Effect of winter shading nets on photosynthetically active radiation (PAR). PAR measurements were obtained in Feb 2018, during the late morning, in the control and net-covered plots. Values are means ± SE of 10 replicates (n = 10). Different letters above columns indicate significant differences (Tukey’s honestly significant difference, P < 0.05).

  • Supplemental Fig. 2.

    Chlorophyll a fluorescence parameters were recorded in Feb 2019 and Feb 2020, in the dark, soon before sunrise to calculate the maximum quantum yield (Fv/Fm) (A). Values are means ± SE of four repeats (n = 4), with each comprising eight trees; three leaves of each tree were measured. In Jan 2018 (the commercial harvest of season 2017–18; baseline), fruits were picked and weighed (B). Values are the means ± SE of four repeats (n = 4), with each comprising eight trees. Different letters above columns indicate significant differences (Tukey’s honestly significant difference, P < 0.05).

 

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