Top Netting Reduces Citrus Fruit Cracking

Abstract

Fruit cracking is a fruit peel physiological disorder. In citrus, it typically results in fruit splitting and abscission. Cracking is related to plant water factors and the irrigation regime; heavy or non-regular irrigation is correlated with elevated cracking incidence. In addition, cracking is more common in larger fruit than in smaller ones. Some cultivars are more prone to cracking, suggesting that the phenomenon has a genetic background. However, it has become evident that climatic conditions also affect fruit cracking. As a result, some years are characterized by heavy cracking incidence, and this may be associated with yearly climate variations. Top netting is a useful tool to mitigate microclimates, and it can potentially reduce cracking intensity. We conducted a five-year experiment in a commercial ‘Valencia’ orange orchard, using three different photoselective nets and three distinct irrigation regimes: the recommended level (100%), and two reduced regimes (80% and 60%). Out of the five experimental years, cracking occurred in three years with various intensities. As expected, reduced irrigation in control trees resulted in decreased cracking intensity, and there was a positive relationship between the fraction of cracking and larger fruit size. In netted trees, the cracking intensity was reduced regardless of net color, irrigation regime or fruit size. During the summer, canopy temperature was reduced under the nets, and relative humidity increased. The possible involvement of climatic parameters, modified under the nets, in altering cracking intensity is discussed. Results of this experiment demonstrate the potential of top-netting technology to mitigate cracking intensity in citrus.

Introduction

Fruit cracking is a peel physiological disorder that occurs in many species, such as tomato, pomegranate, persimmon and citrus (Khadivi-Khub, 2015; La Spada et al., 2024). It usually initiates as a microcrack on the fruit surface, which extends into the internal layer of the peel. In most cases, the crack remains in the peel, but in citrus and pomegranate, it may lead to fruit splitting. The citrus peel is composed of two layers: an internal white layer, termed albedo (which degrades in mandarins), and an external colored layer, termed flavedo (Sadka et al., 2019). Cracking starts as a small crack in the flavedo, usually at the stylar end of the fruit (Cronje et al., 2013; Juan and Jiezhong, 2017; Kalatippi et al., 2024; Krajewski et al., 2022). The crack extends into the albedo and then toward the fruit equator, tearing the vascular bundles and usually resulting in pulp cracking and fruit drop; it starts in late summer and continues for ~3–4 months. Citrus cultivars that are more prone to cracking include ‘Valencia’ and ‘Navel’ oranges, and ‘Nova’, ‘Murcott’ and ‘Mor’ mandarins (Cronje et al., 2013).

Splitting-prone cultivars, such as the ‘Nova’ mandarin, have been suggested to possess three characteristics associated with cracking. One, which is most likely a major factor contributing to cracking, is the non-uniform growth of the peel and pulp, especially during the second phase of fruit development, when pulp volume increases and peel thickness is somewhat reduced (Cronje et al., 2013; Spiegel-Roy and Goldschmidt, 1996). Whereas the reduction in peel thickness in cracking-resistant cultivars is followed by secondary peel thickening, no such growth is usually evident in sensitive cultivars and therefore their peel cannot resist the pulp pressure. Second, ‘Nova’ mandarin has a cavity at the base of the style, just below the point where it breaks following fruit set (Garcia-Luis et al., 1994, 2001). Third, splitting-prone cultivars are usually oblate, whereas resistant ones are typically spherical (Garcia-Luis et al., 2001). Whether these traits are indeed associated with cracking in all or most cultivars is still an open question. For instance, ‘Valencia’ oranges have relatively thick peels but still suffer from cracking in some years. ‘Ori’ mandarins, grown commercially for over 25 years, have been exhibiting this phenomenon in Israel for only a decade for unknown reasons. Climate change may well affect the phenomenon’s intensity (Fischer et al., 2021; Manzoor et al., 2024).

Tree water status plays a major role in fruit growth and pulp expansion, and thus cracking is strongly affected by irrigation (Cronje et al., 2013; La Spada et al., 2024; Opara et al., 1997). Relatively heavy irrigation and non-regular fluctuations in irrigation regimes, including rain events, induce the phenomenon, whereas reduced irrigation usually results in a lower number of affected fruit (Goldschmidt et al., 1992; La Spada et al., 2024; Mesejo et al., 2016; Zur et al., 2017). On a particular tree, large fruit are more sensitive to cracking than small ones (Cronje et al., 2013; Garcia-Luis et al., 2001), and this may be associated with their water status, as the larger fruit absorb more water.

Cracking frequencies vary greatly among years and geographical locations, with high cracking incidence being most common in hot and humid regions (Cronje et al., 2013). In most years, 10–25% of the fruit are lost, but in some years (“cracking years”), 50–60% yield loss can occur in cracking-prone cultivars. Such years are sometimes also characterized by the cracking of cultivars that usually do not suffer from this phenomenon, such as lemons and some mandarins. This indicates that cracking is strongly affected by as yet undetermined environmental factors, which affect some cultivars regardless of their peel thickness or anatomy. Cracking incidence in Israel has been on the rise in recent years, and could be related to the extreme heat waves that occur in early summer, when fruit growth is most rapid. We recently presented a machine learning model aimed at predicting the effects of climate parameters (temperature, radiation and humidity), along with management and environmental characteristics (Abekasis et al., 2024). It showed that among other parameters, such as cultivar and tree age, temperature, in the 40% quantile, plays a role in cracking induction.

Current practical means for reducing fruit cracking, based on auxin and potassium treatments, have had only limited success (Cronje et al., 2013; Greenberg et al., 2006; Habibi et al., 2021; Stander et al., 2014). Therefore, climate mitigation may offer a more effective alternative for minimizing cracking incidence. Various netting technologies have been shown to improve the microclimate in the open field, including citrus orchards (Manja and Aoun, 2019; Mditshwa et al., 2019; Tanny, 2013; Vuković et al., 2022; Wachsmann et al., 2012). Depending on the type of construction and net/screen used, improvements include wind-breaking effects, reduced radiation, protection against hail, mitigation of extreme air temperatures and increased humidity (Mupambi et al., 2018). Photoselective (colored) nets reduce sun irradiation, induce light scattering and change the spectrum of the light reaching the plant. Therefore, in addition to the general microclimate-mitigation effect, colored shade nets also affect different physiological pathways that respond to altered light properties (Rajapakse and Shahak, 2006; Shahak, 2008; Stamps, 2009). Photoselective netting has been suggested as a tool to mitigate cracking in vegetables (Champaneri and Patel, 2022).

In this study, the effect of top netting with photoselective nets on cracking incidence in ‘Valencia’ oranges was investigated in a-five year experiment; the yield data, net spectral properties and water levels are described elsewhere (Dovjik et al., 2021; Nemera et al., 2023). Over the five years, three years were characterized as “cracking years”. Although use of the nets resulted in improved tree water status and increased fruit size, two factors generally associated with higher cracking, the fraction of split fruit significantly decreased in trees grown under the nets. The possibility of cracking reduction through microclimate mitigation is discussed.

Materials and Methods

Description of the experiment

The study was conducted in a 25-year old commercial orchard containing of ‘Valencia’ orange (Citrus sinensis L. Osbeck) grafted on sour orange (Citrus aurantium L.) located in southern Israel (31.546 N, 34.616 E). A schematic presentation and a photograph of the experiment are provided elsewhere (Dovjik et al., 2021). Tree spacing was 4 m and 6 m in and between rows, respectively, with 19 trees per row. The study was designed as a split-plot experiment with different types of nets replicated across years, based on a 4 × 3 factorial-type design with three types of nets plus one control (no net) and three irrigation regimes. The experimental units were arranged into three or four netting blocks with five or six sampled trees per block, as follows: control trees (4 blocks), trees covered with red (3 blocks), transparent (4 blocks) or pearl (3 blocks) nets subdivided into three irrigation subplots. The irrigation levels were: 1) 100%—defined as the recommended irrigation in the experimental region, or ‘control irrigation’ according to a table of values for 10 day periods during the irrigation season. The table was based on 10-day crop factors recommended by the Israeli Ministry of Agriculture’s extension service applied to 5 years of Penman–Monteith reference evapotranspiration (2005–2009) from the Dorot meteorological station, located 5 km southeast of the orchard. The crop factors appear in pamphlets published by the extension service and are available upon request; 2) 80%—irrigation reduced by 20% from the control, and 3) 60%—irrigation reduced by 40% from the control. Irrigation in each treatment was measured and controlled by a Gal Pro DC4 irrigation controller (Galcon, Kfar Blum, Israel). The irrigation volume of each treatment was also monitored and recorded using ECM water meters with electrical output (Arad, Kibbutz Dalia, Israel). Irrigation, applied twice a week, was initiated about one month after the last major rain event of the season and was continued until the first major rain event of the season. Changes in the applied water amount were made every 10 days, according to the standards. Trees were sampled only from the middle row of three rows with the same irrigation level. Fertigation, applied through the irrigation system, was proportional, and therefore the three regimes are hereafter referred to as fertigation regimes, as previously described (Dovjik et al., 2021). The nets used in the experiment (Ginegar Plastic Products Ltd., Ginegar, Israel) were: 1) transparent—a commercial Crystal Leno net with 12–15% primary shading, 2) pearl—a commercial Pearl Leno net, with 18–22% primary shading, and 3) red—a custom-made net utilizing both red and transparent filaments with 15–17% primary shading. Other spectral properties of the nets are shown elsewhere (Dovjik et al., 2021). The nets were placed horizontally at a height of 6 m above the ground in December 2013.

Fruit cracking

Cracked fruit assessment was conducted in the three seasons in which fruit cracking was observed: 2014–2015, 2015–2016, and 2017–2018 (in 2016–2017, no cracking occurred). Hand monitoring of all cracked fruits from each sampled tree was performed from the beginning of September (in the 2014–2015 season) or October (for the 2015–2016 and 2017–2018 seasons). Fruit cracking was defined as the fraction of split fruit out of the total fruit count, as determined from the commercial harvest by hand monitoring.

Statistical analysis

Statistical analyses were performed by one-way analysis of variance (ANOVA) followed by Tukey–Kramer’s multiple comparison test, with P ≤ 0.05, using JMP software, version 16 (SAS Institute).

Canopy microclimate

Air temperature and relative humidity (RH) in the canopy of netted and control trees were monitored with EC650 MicroLogs (Fourtec - Fourier Technologies, Tel Aviv, Israel). Three MicroLog units were installed in each plot (in three separate nets and three separate control plots) at a height of 1.5 m above the ground inside the plant canopy in the middle row of the plot to prevent heating by direct radiation.

Results

Cracking incidence was mitigated under reduced fertigation in control trees

The experiment, consisting of four top-netting treatments—control, red, pearl, and transparent nets, each with three fertigation regimes—standard (100%) and two reduced rates (80% and 60%), was carried out from 2014 to 2018, over four full seasons from flowering induction until harvest.

Cracking occurred in three of the four seasons: 2014–2015, 2015–2016, and 2017–2018. The final fraction of split fruit out of the final fruit harvest is presented for all treatments in Figure 1 (upper panels). Control trees fertigated with the 100% regime showed about 25%, 16%, and 12% cracking incidence in 2014–2015, 2015–2016, and 2017–2018, respectively. In these trees, in the 2015–2016 and 2017–2018 seasons, 60% fertigation resulted in reduced cracking intensity as compared to 100% fertigation, with 80% fertigation producing intermediate values. A similar trend was observed in the 2014–2015 season, but the results were not significant. The pattern of cracked-fruit accumulation seemed to be quite linear in all seasons, with most of the cracked fruit detected from the beginning of October until harvest time (Fig. 2). One exception was in 2014–2015 with 60% fertigation, where there was a lag in cracked-fruit accumulation, with most of the cracking occurring from the end of October to the beginning of January.

Fig. 1

Deficient fertigation resulted in mitigated cracking intensity in control trees, and use of photoselective nets reduced cracking regardless of fertigation. Percent final cracking for all netting × fertigation combinations in the indicated seasons (A–C). Percent cracking for netting treatments (combined data; net) in the indicated seasons (D–F). Data are mean values of 20 trees (60 for net treatment) ± SE. Different letters indicate significant difference according to the Tukey–Kramer test (P ≤ 0.05).

Fig. 2

Fruit cracking increased linearly with time. The cumulative cracking percentage on the indicated dates (day/month/year) for all netting treatments combined (net) and control trees under the different fertigation regimes in the indicated seasons. Data are mean values of 20 trees ± SE (control) and 60 trees ± SE (net).

Cracking was induced in larger fruit

Reduced fertigation in control trees resulted in reduced fruit size. In 2015, this was only evident for the 60% fertigation regime compared to the 100% and 80% regimes. In 2016, fruit under 80% fertigation were smaller than those under 100% fertigation, and in 2018, there was a gradual decrease in fruit size with decreasing fertigation (Fig. 3A). Fruit of control trees displayed a positive, albeit low, correlation between their diameter and cracking percentage (Fig. 3B).

Fig. 3

Reduced fertigation reduced fruit size and netting increased it. (A) Average fruit diameter of control, non-netted trees and trees grown under the nets (combined data; net), fertigated at 100%, 80%, and 60% levels. Average of 20 control trees and 60 netted trees per treatment ± SE. Different letters indicate significant difference according to the Tukey–Kramer test (P ≤ 0.05). (B) Relationships between cracking intensity and fruit diameter in control trees under all fertigation regimes.

Netting reduced cracking incidence regardless of fertigation regime or fruit size

All netting treatments generally resulted in a reduced rate of cracking under all fertigation treatments: in 2014–2015 and 2017–2018, all net–fertigation combinations, with the exception of red net–60% fertigation, had significantly lower cracking rates than the control–100% fertigation treatment, but they were not significantly lower than for the control–80% or 60% treatments. In 2015–2016, most of the netting–fertigation combinations were significantly lower than the control–100% and control–80% fertigation treatments. Because the various nets did not show any significant differences among them, data of all nets were combined in further analyses (defined as “net” treatment). In 2014–2015 for all fertigation regimes, the net treatment showed significantly lower cracking intensity than the control–100% and control–80% fertigation combinations. In 2015–2016, the net treatment resulted in a lower cracking percentage than controls under all fertigation regimes, whereas in 2017–2018, only the control–100% combination was significantly different from the net treatment with 100%, 80% and 60% fertigation (Fig. 1, lower panels).

As already noted, cracking incidence tends to increase with increasing fruit size. As compared to control trees, net treatment usually increased fruit size; in 2015, this was the case for 100% and 60% fertigation; in 2016, only net–80% fertigation showed significantly larger fruit than its respective control, and in 2018, fruit size increased under the net treatment compared to the control for 80% and 60% fertigation (Fig. 3A).

Net effect on microclimate

Canopy air temperature and RH were measured for the control treatment and under the nets, both with 100% fertigation, during the summers of the three experimental years, 2015–2017. The various nets did no show any significant differences in temperature or RH; therefore, only data for red net and control trees are shown. In 2015, maximal and minimal temperatures during the first half of July were considerably lower than in 2016 and 2017. The maximum summer canopy temperature under the net was consistently lower than for control trees by 1–1.5°C (Fig. 4, upper panels). Differences in minimal temperature were similar in 2016 and 2017, but only very slight in 2015. Maximal and minimal RH under nets increased by 2–5% in all tested years (Fig. 5). Even though the effect of the net on the absolute temperature and RH of the canopy was relatively small, it became substantial when considering the cumulative number of heat hours (taken here as > 32°C) or RH hours (taken here as < 55%). Thus, as shown for the hottest week of July 2015 and August 2016, control trees were subjected to 0.5–2 more heat hours per day than netted trees (Fig. 6A). Similarly, control trees were under the threshold RH for 1–3 more hours than the netted trees (Fig. 6B).

Fig. 4

Netting reduced canopy temperature. Maximum (top panels) and minimum (bottom panels) canopy temperature of control trees and trees under a red net under 100% fertigation on the indicated dates (given as day/month/year) in the indicated years. With the exception of minimal temperature in 2015, differences between the treatments were significant on all measuring dates according to t-test (P ≤ 0.05).

Fig. 5

Netting increased canopy relative humidity (RH). Maximum (top panels) and minimum (bottom panels) canopy RH of control trees and trees under a red net under 100% fertigation on the indicated dates (given as day/month/year) in the indicated years. On all measurement dates, differences between the treatments were significant according to t-test (P ≤ 0.05).

Fig. 6

Cumulative hours of netting altered the microclimate. Cumulative heat hours > 32°C and relative humidity (RH) hours < 55% over one week in July 2015 (A) and August 2016 (B) in control trees and in trees under a red net. Mean values of 12 trees ± SE. Dates are given as day/month/year.

Discussion

Cracking timing differs for different cultivars, and does not seem to depend on maturation time or peel width. In ‘Nova’ mandarin, harvested in November, cracking starts from the end of August up to October, whereas in ‘Ori’ mandarin, harvested in January, cracking also starts in October and ends in December–January (Zur et al., 2017). ‘Valencia’ orange, with its thicker peel, matures in April–May, but its cracking timing is similar to that of ‘Ori’ mandarin.

Cracking intensity was reduced under deficient fertigation in control trees

Data presented here are in good agreement with previous reports showing that reduced or deficient irrigation mitigates cracking intensity (Cronje et al., 2013). Deficient irrigation results in a lower pulp percentage by volume in the fruit and increased peel width, as also demonstrated by the dry matter of the peel (Conesa et al., 2014; Huang et al., 2000; Zur et al., 2017). The lower pressure exerted by the pulp on the peel may account for the reduced cracking observed at lower irrigation levels. Also worth mentioning is that irrigation frequency affects cracking; under daily irrigation, cracking frequency was reduced as compared to alternate irrigation (Mesejo et al., 2016). Here, fertigation was applied twice a week, which may have increased the number of affected fruit.

Netting reduced cracking intensity regardless of increased fruit size and improved water characteristics

Fruit size is associated with fruit cracking: the larger the fruit, the higher its probability of cracking compared to smaller fruit (Cronje et al., 2013). Irrigation is a major factor affecting fruit size, although the total number of fruit per tree also affects their final size (Carr, 2012). The effect of netting on fruit size has also been well documented, and can be due to the modified microclimate under the nets, which contributes to improved tree water status and physiological performance (Cohen et al., 2005). Our data showed that although the net treatment increased fruit size, thus increasing its probability of cracking, the opposite effect was observed. Moreover, unlike in control trees, there was no relationship between cracking intensity and fruit size under the nets (not shown). Our previous reports clearly demonstrated that water characteristics under nets, especially red ones, are considerably improved, based on all measured parameters (Nemera et al., 2023). Sap flow (tree transpiration/ET₀ (reference evapotranspiration)) under the nets was significantly lower than in control trees under both 100% and 60% fertigation. Stem water potential was higher under the nets and with all netting treatments, 100% fertigation showed higher values than for controls with 100% fertigation. Trunk maximal daily shrinkage was lower under the nets than in controls under both 100% and 60% fertigation. In addition, leaf conductance under the red net was significantly higher than in control trees. As a result of the improved water characteristics, water-use efficiency based on yield was higher under the nets than in control trees (Dovjik et al., 2021). It can be assumed that these improved water characteristics could further increase the probability of fruit cracking, as results from increased irrigation; however, this was not the case and no increase in cracking was detected under increased fertigation, although fruit size was usually larger under 100% fertigation as compared to 80% (2015) or 60% (2016 and 2018).

Does the modified climate under nets reduce cracking intensity?

While results shown here demonstrated the effect of netting on temperature and humidity, our previous publication demonstrated its effect on other parameters. Wind speed under the top-netting structure is reduced by at least 70% (Wachsmann et al., 2012). Total light under the net is reduced, whereas scattered light increases, resulting in a higher scattering percentage (Dovjik et al., 2021). Considering the above factors, we suggest that the mitigated microclimate under the nets played a major role in reducing cracking intensity. Five variables were altered under the nets: wind speed, canopy air humidity, canopy air temperature, total radiation and spectral irradiance, and they were also affected by the various net colors used in the experiment. Obviously, some of these variables are interconnected, so their possible roles in mitigating cracking intensity cannot always be separated. For instance, protective nets alter the dynamics of canopy air temperature and the soil underneath as a result of reduced wind speed and solar radiation reaching the plant canopy and the soil (Mditshwa et al., 2019; Mupambi et al., 2018). Nevertheless, these factors are discussed below in relation to the data in the literature. To the best of our knowledge, the relationship between cracking and climatic factors in fruit trees in general, and in citrus in particular, have not been based on experimental data, but on the incidence of phenomena in growing regions characterized by various climatic conditions, and climatic differences between growing seasons.

Light spectrum: Net color has been shown to affect fruit size in pear, citrus, apple and table grapes (Bastias et al., 2012; Dovjik et al., 2021; Shahak, 2008). However, to the best of our knowledge, this is the first report of an altered spectrum generated from various net colors affecting fruit cracking.

Wind speed: High wind speed may cause external injuries to the fruit surface (Gravina et al., 2011), but to the best of our knowledge, no relationship between wind velocity and fruit cracking has been demonstrated to date.

Temperature: As already noted, citrus cracking is induced by warmer and more humid conditions, as commonly detected with other fruit types, such as pomegranate (Krajewski et al., 2022; Singh et al., 2020). ‘Valencia’ orange fruit grown in warmer temperatures tend to have thinner peels than those grown in cooler areas and as such, are more prone to cracking (Cronje et al., 2013). Furthermore, it is well established that the fruit growth rate accelerates under warmer temperatures, especially during stage II when the pulp expands, thus causing more pressure on the peel and increasing its probability of cracking (Cronje et al., 2013). Increased cracking intensity in pomegranate was associated with the side of the tree facing higher temperatures (Drogoudi et al., 2021), and in sweet cherry, there was a linear increase in cracking intensity with increasing temperature (La Spada et al., 2024; Simon, 2006). In litchi, cracking intensity was induced by temperatures in the range of 35–40°C, and was more intense in fruit located in canopy positions exposed to higher temperatures (Mandal and Mitra, 2018). Accordingly, the use of top sprinklers, which reduces canopy temperature, resulted in reduced cracking (Nirala and Suresh, 2022). The effect of temperature on cracking has also been demonstrated in vegetables. Exposure to extreme temperatures resulted in severe cracking symptoms in bell pepper (Moreshet et al., 1999). In tomato, exposure to direct sunlight, which increases pericarp temperature, resulted in cracking induction (Liu et al., 2023 and references therein). Use of near-infrared reflective film reduced temperature and tomato cracking intensity (Yamaura et al., 2022). Similarly, a thermal barrier film reduced the temperature and cracking intensity of tomato in a greenhouse, as compared to polyolefin film (Nakayama et al., 2021). Although fruit temperature was not recorded, it is reasonable to assume that it was somewhat lower under the nets. If so, higher fruit temperature in control trees may have raised the gas and hydrostatic pressures of the pulp on the tomato skin, resulting in cracking, as shown by Peet (1992). On the other hand, there have been cases in apple, bell pepper and passion fruit in which higher cracking intensity was associated with low temperatures, which probably limited transpiration and induced fruit turgor (Aloni et al., 1998; Fischer et al., 2021). While considering the temperature effect, a cumulative effect over time should also be considered, as presented in Figure 6A. While to the best of our knowledge no direct data associating between heat hours accumulation and fruit cracking are available, cumulative heat or cold hours are crucial for many traits, such as flowering induction, a decrease in the fruit acid level, and color break of the peel (Agustí and Primo-Millo, 2000; Tadeo et al., 2000). Therefore, it is likely that cracking intensity is also dependent on hours of heat accumulation.

Humidity: A few studies in sweet cherry have highlighted the importance of increased humidity on higher cracking incidence. High humidity induces microcracks in the cuticle, followed by development of macrocracks (Knoche and Winkler, 2019; Winkler et al., 2016, 2020). Rain induced fruit cracking when it occurred 10–25 days prior to harvest (Salvadores and Bastias, 2023). Moreover, when humidity was over 75%, the frequency of cracks increased exponentially (Salvadores and Bastias, 2023). Similarly, fruit grown in more humid areas have a thinner peel and are thus more prone to cracking (Cronje et al., 2013). In line with these results, a relatively rapid shift from dry soil to heavy rainfall induced cracking in pear and citrus, and heavy rainfall was associated with increased cracking intensity in grape (La Spada et al., 2024).

Radiation: As already noted, high irradiation also increases temperature. Exposure to extreme radiation resulted in severe cracking symptoms in bell pepper (Moreshet et al., 1999). In Camellia, high irriadiation due to pruning increased cracking intensity (Si et al., 2024). Lower cracking intensity was detected in shaded compared to unshaded tomatoes (Fischer et al., 2021; Ulinnuha et al., 2020). Shading and particle film, alone or in combination, reduced cracking intensity in pomegranate (Sharma et al., 2018; Tarabih, 2020). Use of various photoselective nets reduced cracking intensity in tomatoes (Ilic et al., 2012; Kittas et al., 2012).

Conclusions

Netting resulted in reduced transpiration and improved water status of the trees. This most likely resulted in the increased fruit growth under the nets. Improved water characteristics and increased fruit growth are both expected to promote fruit cracking. Even so, the fraction of cracked fruit was lower under the nets regardless of the fertigation regime. Moreover, netting enhanced canopy humidity, but reduced the temperature, two changes with contrasting effects on cracking intensity. It is therefore proposed that temperature mitigation plays a more significant role in reducing cracking than the inductive effect of humidity. However, factors other than climatic variables may have also played a role in cracking induction in control trees. For instance, higher transpiration of uncovered trees may have led to more solute accumulation in the fruit, which would attract more water from other tree organs to the fruit, increasing their turgor pressure and resulting in cracking induction (Peet, 1992). In citrus, data regarding the shading effect on total soluble solids (TSS) is not in agreement, with some studies showing induced TSS in non-shaded fruits, as compared to shaded ones, and some showing no effect of shading (Harrison et al., 2013, and references cited therein). Over five experimental years, the data gathered in this study were not conclusive (not shown). Therefore, the above hypothesis warrants further investigation. Regardless, the results presented here further demonstrate the potential of netting in citriculture, especially in the context of climate change and higher temperatures.

Acknowledgements

We thank Earthbound, LLC for their support of this project.

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