1. Introduction
The recent rise in global warming across the world has caused severe challenges to crop production especially in arid and semi-arid areas, including Iran. Among others, the challenges include increase in air temperature, intensity of solar radiation [
1], considerable reduction in annual cold nights, increase in the number of annual warm nights [
2,
3], and scarce water resources [
4]. Thus, the extreme environment conditions result in a large increase in transpiration rate, often enhancing root water uptake rate even in well-irrigated soils following reduced photosynthetic activity and adverse effects on plant growth [
4]. Therefore, more sophisticated and modern practices are required to alleviate such challenges and dangerous climatic fluctuations in orchards with less energy cost in semi-arid and arid areas. The most effective method would probably be the use of shade nets, because they are able to alter environmental conditions. Photo-selective shade nets can change the light quality by extending the relative proportion of diffuse light (scattered) and also by absorbing different spectral bands [
5]. Additionally, the use of different colors of shade net can create desirable microclimates, protect against pests, fungi and physical damage [
5], and has the ability to extend the shelf life of fruit, thereby improving the quality of fruit and lowering postharvest losses [
6]. The net can also ensure that the air beneath the shade cloth stays humid and restricted, which is of further benefit to the plants by reducing wind damage to the crop and evaporation of soil moisture [
7].
“Colored-color nets” (red, yellow, green, and blue) and “neutral-color nets” (pearl, white, and gray) are the two categories of shade net color, absorbing spectral bands either shorter or longer than the visible range [
8]. The blue shade net is designed to absorb the Ultraviolet (UV), Red, and Far red (FR) spectral regions, while enhancing the blue spectral region. Meanwhile, the yellow shade net is designed to substantially decrease the UV and Blue, while elevating the Gray, Yellow, Red, and FR wavelengths [
9].
Several studies have reviewed the influence of photo-selective nets on plant vegetative growth, flowering, harvest time (early or late maturation), fruit quality, and yield [
8,
10]. Netting has developed noticeably in the last decade, and has been applied in different species including blueberry
(Vaccinium corymbosum L.) [
11], apple (
Malus domestica (Suckow) Borkh.) [
12], wine grape (
Vitis vinifera L.) [
13], orange (
Citrus sinensis. var Valencia) [
4], pomegranate (
Punica granatum) [
1], pepper (
Capsicum annum) [
14] and tomato (
Lycopersicon esculentum Mill.) [
8]. Grape vines under shaded conditions exhibit increased vigor, leaf surface and nitrogen content [
15]. Shaded apple trees show smaller trunk diameter but increased the number and length of their 1-year shoots under net colors [
16]. Additionally, the nets showed some negative effects with respect to the organoleptic characteristics of apples in Italy [
17]. The white net decreased soluble solids content (SSC) and flesh firmness and increased starch index in apple fruits at harvest, while reducing the SSC and flesh firmness after cold storage [
18]. Photoselective shade nets led to a notable reduced weight loss in tomatoes, red, yellow, and green sweet peppers after postharvest storage [
5]. Additionally, fruit firmness was higher in tomatoes and sweet peppers grown under photoselective shade nets after postharvest storage [
5,
19]. However, a white shade net did not influence weight loss of citrus fruit during storage at −0.6 and 4 °C for 34 days [
20].
The fig (
Ficus carica L.) belongs to the Moraceae family. It is one of the most important horticultural crops cultivated in arid and semi-arid regions of Iran, the largest fig production centers in the world with an annual production of over 87,000 tons [
21]. Estahban, located in the south-east of Fars province of Iran, is the biggest dry and fresh fig producer area in Iran. Cultivars “Sabz” and “Siah” are the well-known commercial cultivars of figs in Iran, and are consumed either dry or fresh, respectively. Studies describing the effects of a wide range of photoselective colored nets on the pre- and postharvest quality of cultivars “Sabz” and “Siah” of rain-fed figs are lacking. Therefore, the aim of this study was to investigate the effect of shade net color on nutritional elements and postharvest shelf life and quality attributes of the fresh figs.
4. Discussion
The production and delivery of high-quality fruit in the market remains the key factor in ensuring competitiveness in the fruit industry, thus driving producers to integrate new technology such as photo-selective nets to realize these targets. The influence of shade netting on fruit development and its impact on the postharvest potential of rain-fed fig fruits are chiefly unknown.
Blue shade nets had a positive effect on vegetative growth, manifesting in a significant increase in the current-year shoot growth and shoot diameter over the course of the study. Bastías and Corelli-Grappadelli [
29] reported that the shoot growth rate under blue nets was the highest among the different colored nets, which is in accordance with the current study. The longest current-year shoots and smallest shoot diameter under the blue net in cultivar “Sabz” could be explained by shade avoidance and the photoreceptor, phytochrome, located in meristematic tissue in the shoot tip. In the shade, plants redirect photo assimilates to shoot elongation and away from structures dedicated to resource acquisition and storage, at the expense of leaf development [
30]. In line with our study, an increase in mean number of year-old shoots per tree, mean diameter of one-year-old shoots, and total length of year-old shoots in ‘Pinova’ under 12% white, 14% red-white, 18% red-black and 23% green-black protective netting compared to an uncovered tree were reported by Solomakhin and Blanke [
16]. Changes in total solar radiation reaching the tree canopy and changes in leaf gas-exchange under color netting influenced vegetative growth such as shoot diameter. Thus, as blue nets exhibited a substantially smaller R/FR ratio compared to those of yellow color, it seems that this could enhance the mean diameter of the shoots in fig and other fruits such as apple and peach under blue nets [
31]. In a related study, it was reported that in warm regions, the decrease in midday leaf temperature and air vapor pressure shortage caused by shading nets increased leaf carbon assimilation, which resulted in an increase in vegetative growth [
32].
Though the nutritional characteristics of fig fruit have tremendous benefits in the human diet, the impact of shade netting on these compounds has received little attention.
Shade level affected rain-fed fig fruit mineral nutrient concentration, but the effect differed among nutrients. Fruit N, P, Ca, Mg concentrations increased with shade net, whereas that of K was unaffected (
Table 2). There are few studies on the effect of shading on fruit mineral nutrition compared to leaf mineral elements. For instance, a linear decline in water, N and K uptake, and an increase in foliar concentration of N, P, and K with increasing shade level was found in greenhouse tomato; however, fruit mineral nutrient content was not tested [
14,
33]. Shading increased augmented mineral nutrient concentrations in bell pepper [
14].
Leaf nutrient concentrations exhibited variations in species such as apple [
34], cherry [
35], and kiwifruit [
36]. These variations in concentrations could be associated with metabolic and structural functions [
37]. Since N, P, Ca and Mg are the mobile elements, they can translocate to fruit immediately, resulting in an increase in fruit size [
38]. Additionally, N, P, K, Ca, and Mg mineral concentrations increased by 20% on average in shaded plants; the increased mineral nutrients were closely associated with decreased carbohydrate accumulation [
15]. In line with the current results, the higher Ca
2+ concentration observed in shaded apple fruit tissue was thought to be attributed to lower dilution of the Ca
2+ accumulated in the fruit due to lower fruit growth rates [
39]. Therefore, other factors are also responsible for the higher Ca
2+ concentration observed in the cortical tissue of shaded fruit [
39]. Our results indicate that the highest phosphorus concentration belongs to cultivar “Sabz” covered with yellow net, so it seems that changes in phosphorus concentration can be cultivar dependent.
Starch accumulation in fruits under color shade nets resulted in less sweet fruit than those receiving full sun (
Table 3). Results from other studies also indicate that the lower sugar content of fruit produced in the colored shade nets may be associated with lower light intensity; nevertheless, the photosynthetic system should not be stopped. Moreover, greater light intensity is able to enhance the photosynthetic activity of the plants [
1,
40]. According to the findings of Solomakhin and Blanke (2010), the extreme tree vigor caused by the colored nets is related to reduction of red to far red (R: FR) ratio, which regulates the activity of phytochrome in leaf and fruit. Therefore, a decrease in invertase activity under low light intensity occurs [
40].
Photo-selective nets had significant effects on fruit firmness in rain-fed fig fruits (
Table 3), which was in agreement with previous research on tomato fruit [
41]. It has been reported that growing tomato plants under color nets and in optimal conditions can result in firmer fruits with a thicker pericarp and a better tolerance to transport than control [
41]. According to Campbell et al. [
42], a softer flesh was observed in apples under shading conditions. They suggested that a decrease in flesh firmness might be the result of poor activity in cell wall formation and a high water influx to fruit cortex cells.
The present study showed that photo-selective nets along with ripening stages decreased fruit weight loss in both fig cultivars resulting in extending shelf life. Weight loss due to water decrease results in a decrease of freshness and fruit firmness, and affects the shelf life and consumer acceptance [
5,
43]. It seems that when fruits are harvested at commercial maturity, the flesh firmness is the highest compared with the other ripening stages, resulting in an increase in the fruit shelf life (
Table 4).
In the current study, fig fruits harvested at the tree ripened and overripe stages showed the lowest weight loss. This lower percentage of weight loss could be ascribed to a lower degree of moisture loss due to the fruit ripening development known to be a characteristic of fruit ripening and customer acceptance. Additionally, during the postharvest period, weight loss mostly depends on the storage temperatures [
44], and our study confirmed that the weight loss was mainly affected by storage temperatures, whereby the fruits stored at 4 °C showed less weight loss than those stored at room temperature (
Table 4). Higher storage temperature may increase more moisture loss which is in line with the previous report on mandarin fruit [
43].
The relationship between Ca and fruit firmness in this study is thought to be related to an important function of Ca in plants in increasing the rigidity of the cell wall and promoting cohesion of neighboring cells. Magnesium can also play an important role in fruit firmness similar to calcium. However, this statement contradicts the findings of Hopkirk et al. [
45]. It seems that an increase in fruit firmness could be associated with high concentrations of starch, resulting in extended fig fruit shelf life. According to our results, the negative correlation between phosphorus and starch and sugar content may be due to its role in sugar metabolism. If inorganic phosphorus concentration is too high, it can show an antagonism effect, and CO
2 fixation can also be negatively affected [
37,
46].
Our results demonstrate that the effects of net colors are variable dependent, and are different among the studied parameters. Based on the factor analysis, calcium, firmness, total sugar, potassium and shelf life of fruit, as well as shoot length and diameter, showed the best response under net coverings and were considered the most effective variables.