Encyclopedia
Throughout history, new strategies and technologies have played a key role in promoting the development of agriculture. New strategies have led to substantial improvements in crop productivity and fruit quality. The cultivation of peach and apricot in controlled greenhouse conditions is one such strategy.
- Prunus
- stone fruit trees
- protected cultivation
- greenhouse
- dormancy
- temperature
- climate change
- fruit quality
1. Introduction
The cultivation of stone fruit trees (Prunus spp.) has nearly always been carried out in the field, outdoors, with some exceptions, such as trees grown in gardens or for ornamental purposes [1,2,3,4]. Stone fruit trees are typically grown in large production orchards in the open air, where there are many factors that can negatively impact the harvest, such as weather conditions, weeds, and pests, amongst others. In this type of orchard, farmers often opt for large-scale production, which often implies lower quality products, and the environment and fruit quality often get neglected. In conventional cultivation, fruit farms typically follow a common or habitual strategy rather than pursuing specific objectives to differentiate themselves from other farms. For more or less four decades, this has been the production strategy followed by the majority of fruit farms, despite appreciable differences in the degree of applied knowledge among the farms. In the analysis of a conventional fruit farm, the concept of productivity is of particular interest as an indicator of efficiency, both in terms of plantation and fruit production, since it implies a reference to yield [2,3].
In this conventional production strategy, not all interactions between production factors are considered in an integrated way, and, above all, achieving a balance in the planting environment is not considered a priority [3]. Likewise, little or no attention is paid to some important aspects of fruit production, since the goal is to obtain a large amount of fruit, without taking into account other factors, such as fruit health and size, organoleptic properties and pesticide residues. Among the most significant factors limiting fruit tree production, we can include a lack of sunlight, temperature inconsistency, a surplus or loss of moisture, weed growth, wind speed, reduced carbon dioxide levels in some areas, low caliber fruits in cold areas, pest and disease infection in open air conditions, the effect of strange odors and flavors, and late maturity. These restrictions, mainly in cold areas, are related to abiotic stress, due to changing climatic conditions, and biotic stress, and they can be minimized by protected fruit tree cultivation [5].
Protected fruit tree cultivation is a specialized form of horticulture. Using greenhouses makes it possible to extend cultivation cycles in order to produce more favorable conditions, increase productivity and quality, and extend cultivation to new areas. These factors are the keys to success in the cultivation of temperate Prunus species, mainly peach (P. persica (L.) Batsch) and to a lesser degree apricot (P. armeniaca L.), in forced greenhouse conditions. Specific characteristics of certain fruit trees, including their perennial nature, their size, or their need to accumulate cold hours to sprout, pose new challenges. In countries like Japan (Shizuoka and Nigata prefectures), Korea (Wanju County), China (in the Provinces of Henan and Shandong), Italy (Region of Naples and Sicilia), and Spain (in the Province of Huelva, Southwest of Spain), greenhouse cultivation is common, but mainly for tropical and subtropical trees, and to a lesser degree in the case of peach and apricot species (Figure 1).
Figure 1. Detail of peach cultivation in greenhouse conditions in Huelva, southwest of Spain (www.freshplaza.es ((accessed on 24 February 2021))).
China, the main producer, has approximately 20,000 hectares of peaches and several hundred hectares of apricot in protected cultivation in the provinces of Henan, Liaoning, Hebei, and Shandong [6,7,8]. In Japan, peach production is more limited with a total surface area of around 10,000 hectares spread around the country, and with high quality production in small paper bags in some cases, whereas in Korea commercial production is more limited [9,10]. In Spain, around 1000 hectares of peach production in greenhouses have been described in the Province of Huelva, with production of apricot starting in these conditions [11]. Finally, Italy is the other country where peach production has been described in the regions of Naples and Sicilia [12].
In these countries, peach and apricot production studies have taken a step beyond the evaluation of technological possibilities, and focused on the effect of certain parameters on the quality of stone fruits produced under greenhouse conditions [12,13,14,15]. However, there are few research references for Prunus cultivation under greenhouse conditions. During the 1970s, the first protected cultivation experiments were carried out in Italy using peach trees, and the crops were subsequently continued [12,14]. Later, further experiments were carried out, mainly in Japan and China, and mainly in apricot. At this moment, however, interest in this cultivation is increasing, in the context of climate change and new commercial opportunities derived from the securing supply in the face of demand.
Protected cultivation is a method of protecting fruit trees from adverse stresses and providing appropriate growth conditions. This cultivation technique provides numerous benefits, such as distributing labor more uniformly throughout the year owing to the diversified cropping season and climate conditions, minimizing the need for pathogen control, and avoiding adverse weather conditions. All of these benefits result in consistently good yields, excellent fruit quality, and early production, among other advantages [8]. The control of pests and diseases, and unique market opportunities are also important aspects of these cultivation systems. The high yields may make this production method a permanent alternative for fruit producers in cities, where production restrictions such as minimal land due to urbanization and pesticide use constraints make orchard production harder. The fact that greenhouse-grown fruit trees produce consistently better quality fruits and cost less than field-produced fruit trees thus makes greenhouse cultivation a promising alternative to field cultivation [15]. However, negative factors limiting apricot and peach production in greenhouse conditions have also been described, including light scarcity, a lack of cold for flower bud breaking, and excessive temperatures [9,15].
2. Main Factors Involved in the Cultivation of Fruit Trees in the Greenhouse
The establishment of new fruit plantations, driven, in most cases, by new technologies and irrigation systems, continues to promote the parallel development of other sectors and activities, with the subsequent development of the entire rural environment. New agricultural technologies developed in recent decades have contributed to an unprecedented growth in world food production. However, there is growing concern that the conventional production model and the trajectory of agricultural development may not be the best, or the only, alternative for the future. Fruit production systems have been evolving in parallel with agricultural development in general, but there are great differences between areas, as some have more innovations or very different cultivation possibilities. There is a highly diverse array of practices currently in use, ranging from those used by the ancient Chinese, Japanese, Greeks, and Romans to distinctly modern practices requiring satellites in orbit [16].
There are alternatives to extensive conventional cultivation, including the strategy we are discussing here: the organic cultivation of stone fruits in controlled greenhouse conditions. Fundamentally, this strategy entails cultivation without the application of synthetic chemicals such as fertilizers, pesticides, phytoregulators, etc. The main factors involved in the cultivation of fruit trees in greenhouse planting systems include environmental management (temperature, humidity, solar radiation, and CO2 concentrations); plant material selection (cultivar and rootstock); and tree management (planting system and substrates, pollination, pruning, and stem girding) (Figure 2).
Figure 2. Main factors involved in the cultivation of fruit trees in the greenhouse.
2.1. Planting Systems and Growth Substrates
In greenhouse fruit tree cultivation, owing to the high costs of the protective structures, it is very important to choose planting methods that take the tree size into account and make it possible to rapidly attain good fruit production and yields. One of the most efficient agricultural solutions thus far has been to decrease the amount of space between trees. Due to the correlation between shoot and root growth in fruit trees, root restriction in small spaces progressively reduces vegetative vigor in the canopy of tree. Furthermore, the relation between the decrease in vigor and the beginning of the reproductive period in young trees makes it possible to increase planting density, which has another significant effect: the early production of fruit trees [12].
For example, a planting method that has proven successful in greenhouse peach is that of Y-trained fruit trees that vary from 4.5 ± 5.0 m between rows and 1.0 ± 1.2 m between trees. This method enhanced the planting density from 1500 to 5000 trees per ha. Y-training has also proven to be an excellent training system for field-grown trees; the splitting of the tree canopy into two thin proliferous leaning walls magnifies light prevention, improves light distribution within the tree canopy, and enhances the delivery of photo-assimilates to the fruits. Thanks to these features, this training system produces great yields and good product quality. Moreover, the decrease in spacing between the plants along the rows did not lead to a decrease in red blush on the skin. The pruning carried out three weeks before harvest was surely a synergetic parameter of these results; this strategy in the orchard has proven to be very efficient for improving and modifying fruit color [12].
Soil moisture and relative humidity is primarily regulated by supplemental irrigation. Proper ventilation and irrigation management helps to limit foliar disease development. In addition, in planting systems for woody plants the substrate is the key to success, since it must provide sufficient aeration, texture, and moisture retention for adequate root development [17]. The substrates must have a field capacity that ensures the water needs of the crops are met. For peat substrate, for example, the porosity was found to be closely correlated with the plant growth of Prunus × cistena sp. [18]. Hydroponic cultivation is also the best alternative to peat. It is necessary to develop novel hydroponic or semi-hydroponic systems for woody trees that make it possible to control nutrition, growth, and fruit quality. The development of new substrate formulations that improve fruit tree nutrition in hydroponic and semi-hydroponic systems is therefore of great interest, together with the development of novel containers for hydroponic cultivation that enable us to control the nutrition and root growth of woody trees [19]. Abnormal growths of roots can occur due to the limited space in containers, as can unwanted constrictions due to the tree weight with a greater overloading during the phenological stage of fruit production. In addition, to achieve adequate crop development in different hydroponic and semi-hydroponic media, plant nutrition and irrigation must be fully controlled. It is necessary to develop automated irrigation technologies to efficiently control fertilizers in the irrigation network, injecting the adequate amounts to satisfy the nutritional needs of stone fruit trees in hydroponic and semi-hydroponic systems [20,21].
2.2. Rootstock and Cultivar Selection
The selection of adequate plant material (rootstocks and cultivars) is of utmost importance in greenhouse fruit tree cultivation, in terms of both production and yield [22]. A large tree size, for instance, has been one of the biggest limiting factors for growing fruit trees in greenhouses. As a result, greater control of tree size via dwarfing patterns and advances in training systems has opened new possibilities for this type of cultivation.
Regarding cultivar selection, since most fruit tree cultivars are self-incompatible [23], pollination problems will occur if these self-incompatible cultivars are planted in the greenhouse without bees or pollinizers within the rows [24,25]. In controlled conditions, the cultivation of self-incompatible Prunus cultivars must be compensated for with the introduction of natural pollinators. Alternatively, the cultivation of self-compatible Prunus genotypes is another solution to the lack of insect pollination in greenhouse conditions. A fruit tree variety that is self-compatible is therefore desirable for greenhouse cultivation [15,26]. Cultivars whose fruits get higher prices in the market are good choices for greenhouse cultivation, as are early maturing cultivars. Tree size is another aspect to consider. For fruit trees that accept shrub training or dwarf trees, a low roof is suitable. For example, a greenhouse less than three-meters high is suitable for fig trees [9].
Obtaining stone fruits through new intensive cultivation systems without soil also requires new cultivars with reduced chilling requirements for breaking bud dormancy. If these chilling requirements are not met, the buds will not blossom or the sprouts will not grow well. Forced chilling has been found to be effective when growers want to break dormancy in the short term. If forced chilling is performed, it is essential to closely monitor the temperatures inside the greenhouse and to open windows, doors, and vents at night, when external temperatures are favorable, and provide shade during the day, to ensure that the temperature does not rise above 7.2 °C for too long. A period of 6–8 weeks is enough to supplement chilling requirements and allow apricot trees to begin their fruiting phase sooner than they would outdoors [15].
Chilling requirements are another of the most important aspects in cultivar selection. Temperatures of between approximately 2 °C and 12 °C provide the chilling requirements for apricot trees, although the requirements vary between different cultivars, as well as in different years. Daily temperatures of 10 °C and higher for 4 or more hours can neutralize the chilling requirements a plant has received in the previous 24 to 36 h [15]. Cultivars with low chill requirements can be planted in colder areas under greenhouse conditions. In warmer areas very low chilling-requirement cultivars should be the solution, combining chill accumulation inside the greenhouse and chilling requirements of the Prunus cultivars [27]. In addition, the development and selection of new early market (low chill, short fruit development period) cultivars or very late cultivars (high chill, long fruit development period) with a high quality will expand the market window and help ensure ongoing profitability.
2.3. Pollination Management
Suitable pollination is necessary for optimal fruit set and production. The pollination of fruit tree flowers (Figure 3) is aided by insects and the wind. However, in a greenhouse, the activity of these natural factors is limited. If there is inadequate pollination at the time of flowering, productivity will be minimal. Honeybees play a vital role in the pollination for fruit trees like apricot trees, which are self-sterile and dependent on pollinators. If pollination were limited for some reason, decreased fruit set could occur. Adverse weather conditions may limit bee activity, which is highly climate-dependent. If bees fly infrequently between fruit trees, cross pollination by bees would logically diminish [15]. Additionally, high temperatures reduce the rate of floral differentiation, affecting the final yield in the following years in peach [28].
Figure 3. (A) Peach flowers recently opened in the controlled greenhouse cultivation of peach trees for research purposes in Murcia, southwest of Spain, in January. (B) Detail of peach fruits in the controlled greenhouse cultivation of trees for research purposes in Murcia, southwest of Spain, in March.
In greenhouse conditions, mason bees and bumblebees are used for pollination, as they tend to operate better in cooler temperatures and cloudy weather than honeybees. Mason bees and bumblebees are relatively calm and do not sting unless disturbed (mason bees never sting). As a result, honeybees have been replaced by mason bees or bumblebees in greenhouse fruit tree cultivation, and mason bee pollination has subsequently become a standard method. Proper pollinizers should be selected with care when beginning a greenhouse business. In the field, self-incompatibility in fruit trees is compensated for by planting pollinizers in close vicinity to the primary cultivars. This strategy should be followed in the greenhouse, and pollinizers should account for one-quarter of the number of primary cultivars within the plots [15].
Alternatively, as mentioned before, the use of self-compatible or self-fertile Prunus cultivars will produce a suitable pollination and production without a reduction of fruit attributes [29]. Although the use of natural pollinations should also be of interest from a production point of view [30].
2.4. Summer Pruning and Stem Girdling
Summer pruning regulates tree size and enhances flowering and fruiting. Researchers have demonstrated the effect of summer pruning on apricot tree flowering, and have found that responses to flowering and fruiting depend on the timing of top elimination. When the length of the new branches was 25–30 cm at the time of summer pruning, the formation of flower buds was greater than when the length of the new shoots was 40 cm or more. Summer pruning occurs in late June or early July in the Northern Hemisphere. Stem girdling in late June or early July has also shown a positive effect on controlling tree size and promoting flowering. Stem girdling restricted the growth of trees and enhanced the number of flower buds compared to non-girdled trees [31]. Besides pruning, tree size is limited by the artificial application of drought stress, root pruning, and the use of dwarfing and semi-dwarfing scion and rootstock cultivars. Other practices employed in this system include girdling in the fall [12]. Specific labor and equipment requirements are necessary to perform these tasks in controlled and reduced conditions. This mean a new culture of apricot and peach production in new conditions, with greater requirements linked with greater benefits [32].
From a commercial point of view, different practical measures need to be taken to maximize fruit yield and quality per unit of land area, while ensuring that trees do not get too big. One such practice is called postharvest canopy removal. This practice is a modified summer pruning technique. It begins immediately after harvest with the removal of all current shoots. New shoots that emerge thereafter are “tipped” several times to stimulate flower bud production [13].
2.5. Temperature and Humidity Control
After the chilling requirements of both the main cultivars and pollinizers have been fulfilled by natural chilling or forced chilling, the trees should be cared for following ordinary greenhouse management practices. In apricot, for example, the primary temperatures in the greenhouse should be maintained at 5–6 °C at night, and at about 26–28 °C in the daytime. Under such conditions, the trees will flower after approximately 5–6 weeks. The recommended temperature for full blooming is 22–25 °C in the daytime and not lower than 8 °C at night. The temperature of the greenhouse floor is lower at night than during the day, so covering the floor with white or black plastic mulch can increase the temperature in the greenhouse. Before flowering, relative humidity can be between 60–80%, but after flowering it should not be higher than 60% in apricot [15]. In addition, control of both temperature and humidity should be handled by retractable films or temporary structures, although this solution can increase production costs.
Greenhouse microclimate (i.e., illumination, temperature, water, humidity, etc.) should be monitored and controlled. The use of more transparent films on the outside of the structure, plus the use of reflective films on the greenhouse floor, enhances the light environment. Temperature regulation during bloom and during the fruit ripening period can result in ripening acceleration from 10 to 50 days [13].
2.6. Solar Radiation, Photosynthetic Capacity, and Carbon Dioxide Concentration
Due to the greenhouse structure, the intensity of the sunlight inside only reaches about 60–70% of that found outdoors. Moreover, the amount of sunlight varies in different parts of the greenhouse; the amount in the southern and middle parts (85%) is greater than that in the northern part (44%). The lower sunlight intensity will decrease fruit quality, especially in the northern section of the greenhouse. It is therefore necessary to improve the optical conditions by, for example, keeping the covering of the greenhouse clean so that the sunlight can easily penetrate [15].
Regarding photosynthetic capacity, nectarine (P. persica (L.) Batsch var. nucipersica or var. nectarine) has shown suitable acclimation to solar-heated greenhouse growth conditions. Compared to open-field-grown plants, a significant increase was found in the daily average net photosynthetic rate in the greenhouse-grown nectarines. In addition, the diurnal variation of maximum photochemical efficiency of Photosystem II (Fv/Fm) indicated that the plants grown in greenhouse conditions had less photoinhibition than in open-field conditions. A decrease in the chlorophyll (chl) a/b ratio and a significant increase in chlorophyll (chl) b content in the leaves of greenhouse-grown plants was also observed [33]. In apricot, greenhouse studies have also found an increase in photosynthetic capacity when the temperature was around 25 °C. Moreover, the functions of both photosystem I (PS I) and photosystem II (PS II) were improved by the high temperatures and intense light [34]. The protected environment was useful in maintaining a relatively high temperature, and avoiding leaf injury due to environmental factors, resulting in a longer period of photosynthetic activity, which increased the photosynthetic production [9].
Finally, the atmospheric carbon dioxide concentration in a greenhouse is about 300 mL.m−3, which is normal for plant growth. However, if the greenhouse lacks air exchange with the outdoors, and there is no extra supply of carbon dioxide when the sun rises, the concentration is often lower. Studies have shown that high carbon dioxide concentrations lead to increased growth and photosynthesis, especially when nutrients are sufficient for the plant. Plants can therefore improve their nutrient supply in the presence of high CO2 levels. The extra amount of carbon dioxide can increase yields by up to 15%, but it can be hard to keep the gas inside the greenhouse, as most are not well sealed, and there is significant potential for CO2 leakage. Using organic fertilizers in the greenhouse increases the concentration of CO2 to a certain degree, however, and is a widely used method [15].
This entry is adapted from the peer-reviewed paper 10.3390/agronomy11030474
