The academic from PUC, Marlene Ayala, recently awarded the Guacolda prize, has conducted evaluations on the effects of plastic coverings that go far beyond rain protection. She presented her findings at a seminar organized by UC DAVIS Chile, which are summarized in the interview below.
The use of plastic coverings on cherry trees began more than two decades ago. Initially intended solely to prevent fruit cracking caused by rain near harvest time, today their potential to modify harvest dates and mitigate other environmental stresses is being explored. Marlene Ayala, Ph.D., an academic from the Pontifical Catholic University of Chile (PUC), is one of the specialists who has studied this subject the most in Chile and was awarded the 2024 Guacolda de Frutas de Chile prize in the “Researcher” category.
In a presentation on this topic at the seminar “Abiotic Stress in Fruit Trees,” organized by the UC DAVIS Chile Life Sciences Innovation Center, she summarized some of her key findings related to abiotic factors. Without the “catastrophic” image of untimely rains blocking the fruits, these factors can have a serious impact on crop protection.
Image 1: Marlene Ayala.
How do abiotic factors produce these plant responses?
The higher the light intensity, the higher the photosynthesis. However, when a certain point of light saturation is reached, photosynthesis stops increasing, and under excessive light—above 1,000 micromoles (μm)/cm2/s in Central Chile—the cherry tree becomes saturated, begins to close its stomata, and thus reduces the rate of photosynthesis as a result of radiation stress.
Something similar happens with temperature, which has a beneficial effect on photosynthesis up to a certain point, as photosynthesis increases to a level where it stabilizes. Beyond approximately 30°C, photosynthesis begins to show increasingly negative effects, raising respiration. Similarly, photosynthesis is higher in a tree with sufficient water supply.
In contrast, under conditions of water stress or very low relative humidity, the stomata close (stomatal conductance decreases) and enzymatic variations occur. Some enzymatic substrates are depleted, or enzymes cease functioning, such as RuBP oxygenase-carboxylase (RUBisCO), an abundant enzyme in plants responsible for photosynthesis. The stressed tree depletes its energy, so to speak.
The results of her research on plastic coverings, she explains, have already demonstrated practical benefits in terms of fruit cracking protection compared to outdoor cherry trees. For instance, a study on the Royal Dawn cultivar measured 37% cracking outdoors versus 6% under low-density polyethylene macro-tunnels (2017, Maule region).
It is known that plastics affect temperature; what have you discovered?
In Molina, Maule region, temperatures recorded in a macro-tunnel (without proper ventilation) reached 43°C; under a tent, 35°C—both harmful to trees and their fruits. Outdoors, 32°C, which is above the threshold where heat begins to affect trees. In the early years of using these structures, environmental variables were not well monitored; fortunately, today growers have incorporated outdoor ventilation practices.
Is the temperature effect of plastic always harmful?
A trial on Santina compared the use of a macro-tunnel, a raffia tent, and the open air, aiming to study the accumulation of growing degree days to anticipate harvest. Harvest under the macro-tunnel occurred on November 21; under the tent on December 1.
The macro-tunnel accumulated approximately 30 more growing degree days between flowering and fruit growth compared to the open air, making this type of covering beneficial for offsetting low temperatures that hinder early-season growing degree day accumulation.
Flowering occurred earlier under the macro-tunnel, later under the tent, and even later in the open air. On the other hand, the maximum temperatures in December and January under the raffia tent were lower than outdoors. As a result, it can be used to mitigate high summer temperatures.
The differences in temperature under the macro-tunnel or the lower temperature under the tent compared to the open air on the same day are often greater than 2°C, causing significant effects on the trees. The key is knowing when to extend or retract the tent, monitoring the environment, and avoiding, for example, heat stress or excessive shading.
Image 2.
Is there management tied to these findings?
Of course, for example, the logical approach is to remove the macro-tunnel covering after the harvest. We kept it until December 30 for experimental purposes. The temperatures, as expected, proved much higher than outdoors, although some growers keep it longer to prevent radiation stress, always considering good ventilation.
On the other hand, keeping the tent between December and February had the favorable effect of lowering temperatures, but a prolonged period should be avoided as it reduces solar interception, impacting photosynthesis and flower differentiation.
In terms of frost mitigation, the macro-tunnel produced differences of about 1.5-2.0°C compared to the open air, which is useful when freezing temperatures are not overly severe. In the case of carps, no changes were observed compared to the open air.
In the soil, Dr. Ayala continues, surface temperatures outdoors between September and December can exceed 35°C; macro-tunnels and tents, however, record up to 10°C lower, meaning plastic coverings reduce surface soil temperature and in the top 40 cm.
At 40 cm depth, the open air shows the highest temperature, followed by the macro-tunnel and then the tent. Early in the season, roots begin to function when soil temperatures range between 12 and 14°C. The slightly higher soil temperature and greater accumulation of growing degree days during the sprouting and flowering period explain the earlier phenological stages achieved using the macro-tunnel.
Sometimes vapor pressure deficit is mentioned: what is it, and how is it affected by canopies?
The vapor pressure deficit (VPD) is an index representing the relationship between the moisture content of plant tissues, particularly the leaves, and the environment. This pressure difference drives transpiration in trees. When the VPD exceeds the threshold of 2.2 kilopascals (kPa), the cherry tree gradually starts to close its stomata completely or partially, reducing or halting photosynthesis and thus carbon fixation.
In the central zone, during September and October, while leaves develop and fruits begin to grow, the VPD is not limiting. However, from November onward, the VPD increases, with greater stomatal closure observed between 3:00 PM and 6:00 PM, coinciding with low relative humidity, high temperatures, and strong radiation. During critical summer months, the raffia canopy helps reduce heat and radiation stress to some extent.
Outdoor measurements indicate values of 2,500 PAR/cm2/s in mid-October and 2,900 μm/cm2/s in mid-December. As previously mentioned, the saturation limit is about 1,000 μm/cm2/s. The macro-tunneling keeps the values within a maximum approximate range of 1,000–1,400 μm/cm2/s.
With the raffia, it is difficult to reach 1,000 μm/cm2/s, which is favorable during the summer post-harvest period (from December to February) when solar radiation is highest. However, the reduced solar interception under the raffia canopy negatively affects morning and evening hours, which are particularly suitable for generating storage reserves and the floral differentiation processes that lead to the next season’s fruits.
What is your perspective on these technological tools?
The mitigation potential of plastic mulch against abiotic stresses is promising and requires further study. There are, of course, more options than those presented here, such as nets of various colors and different types of plastic with improved optical properties. It will be necessary
to determine the best alternatives based on the location (conditions differ between the north and south), seasonal characteristics, phenological stages, and the metabolic processes to be protected or promoted, such as root growth, leaf longevity, early flowering, or the prevention of double fruits.
How would you summarize your findings on the role of canopies in mitigating abiotic stress?
In summary, we already know the positive effects of macro-tunnels for cherry trees in terms of ambient and soil temperatures at the end of winter and the beginning of the growing season. Additionally, they help stabilize soil moisture and mitigate excessive solar radiation.
However, it is not advisable to keep the tunnels in place after the harvest due to high temperatures, unless ventilation is well managed. The raffia canopy significantly reduces solar radiation incidence and vapor pressure deficit compared to open air during summer.
Both the macro-tunnel and, to a greater extent, the raffia canopy reduce soil surface and root-level temperatures later in the season. Finally, it should be emphasized that abiotic stress negatively impacts photosynthesis, increases respiration, and reduces biomass accumulation.
This occurs not only during the season but also has a cumulative effect over the years. To maximize the benefits of plastic technology, it is important to revisit the physiological principles of cherry trees and apply them to understand their effects.
Source: Redagrícola
Images: SL Fruit Service; Redagrícola
Marlene Ayala
Ph.D., academic at Pontificia Universidad Católica de Chile (PUC)
07 Dec 2023
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