Referential Experiences in the Use of Measurement Equipment
In the cherry production industry, there is a growing understanding that the most critical period is the post-harvest phase, as it plays a decisive role in the next season’s production. During this time, nutritional corrections should be made based on the yield of the previous season and the information provided by various laboratory analyses, such as fruit, leaf, and soil analyses, among others. In addition, it is essential to condition the plants during post-harvest management to enable them to better tolerate different types of abiotic and biotic stresses, which could significantly affect their physiological condition and, consequently, their expected productivity potential.
Heat stress, or thermal stress, is defined as a prolonged period of elevated temperature that can cause irreversible damage to the metabolism and development of plants (Porch & Hall, 2013). Most plants are sensitive to stress caused by high temperatures and suffer when temperatures are too low or too high compared to the defined thresholds for each plant species. In the case of cherries, the ideal range for development is between 18 and 24°C, temperatures above 36°C cause a cessation of metabolism and oxidative damage in different organs of the plant (Lemus, 2005). Several authors agree that stomatal closure is imminent at temperatures above 30°C.
Under ideal conditions, the structures called stomata are usually open and tend to maintain a perfect balance between releasing water vapor into the atmosphere and simultaneously capturing CO2 for conversion into sugars through the physiological cycles inherent in photosynthesis (Tapia, 2019). Stomatal regulation plays a fundamental role in water and CO2 balance in plants through various physiological processes such as transpiration and photosynthesis. Stomatal regulation is a plant’s reaction that helps mitigate the harmful effects caused by water deficit and/or thermal stress. This regulatory process results in a reduction in transpiration rate and gas exchange due to stomatal closure (Blaya et al., 2021).
To quantify this process dynamically, the concept of stomatal conductance or stomatal conductivity is used. Stomatal conductance (gl) measures the flow of vapor expelled through transpiration by stomata and is generally measured in mmol m⁻² s⁻¹. When stomatal conductance decreases, it indicates that the stomata are closing, resulting in reduced water loss through transpiration. This leads to a decrease in photosynthetic rate, and consequently, all processes dependent on photosynthesis, including sugar reserves, are negatively affected as the main source of carbon.
The normalized condition of stomatal conductance in fruit trees depends on factors such as ambient temperatures, soil water status, among others. This is graphically explained in Figure 1, according to Salisbury & Ross in 1994.
Research conducted aims to understand the physiological responses of cherries to thermal stress during post-harvest and has also observed different strategies that may be interesting to implement during the pre-harvest period.
As a pre-harvest background, an investigation was conducted in the 2021-22 season, consisting of the application of a potassium-based product with bio-stimulant action on cv. Regina on Gisela®12 rootstock in the Molina commune, VII region, Chile, where stomatal conductance (gl) records, leaf temperature, and ambient temperature (°C) are displayed every hour for 8 hours of the day to understand the dynamics that exist among the different indices.
The measurements corresponding to stomatal conductance (mmol m⁻² s⁻¹) and leaf temperature (°C) were taken using a Meter® brand equipment, model SC-1 Leaf Porometer, and the ambient temperature records were extracted from an Instacrops weather station located in the field about 200 m from the research site.
The measurements were carried out on fully extended mature leaves from two-year-old wood, with tree circumference ranging from 1.4 to 2.2 m in height, during two dates 7-10 days after each foliar bio-stimulant application. It is also worth noting that each measurement represents an average of 5 leaves per hour and per treatment.
It was observed conductance decreases as a result of stomatal closure while temperatures rise during the day. Additionally, it can be seen that regardless of the treatment, leaf temperatures remain similar to each other. In Treatment 1 (T1), conductance values are higher than those in Treatment 0 (T0) for a large part of the day, where better harvest indexes were reported, such as fruit hardness measured as durofel (UD), dry matter (%), and a better size distribution curve for T1 (Unpublished data).
It can also be inferred that the dynamics of leaf temperatures are similar between T0 (Orchard Treatment) and T1 (bio-stimulant product) in both graphs, and the highest gl points are associated with leaf temperatures ranging from 25 to 29 °C, with records of up to 620 mmol m⁻² s⁻¹ in the measurement taken on October 26, 2021.
In the afternoon on November 4, 2021, the T1 curves oscillate above the T0 curve, reaching maximum measurements of 415 mmol m⁻² s⁻¹ when leaf temperatures are around 25°C. This contrasts with T0, where values of 315 mmol m⁻² s⁻¹ are obtained at a temperature of 26 °C. This is probably a response to potassium (K), which plays a strong role in the regulation of stomatal opening and closure.
On the other hand, the use of products to mitigate thermal stress during the post-harvest period is a strategy that is gaining more popularity among many producers, mainly through the use of sunscreens such as kaolins (95% kaolin) and transparent sunscreen filters of different formulations. This effect can be reinforced with bio-stimulant agents that help the plant cope with thermal stress, where the dosage and frequency strategies of the applications depend on the type of bio-stimulant used (Tapia, 2019). This has been reported in previous seasons, and the main result achieved was a decrease in leaf temperatures, which generally ranged from 0.5 to 1.5 °C lower compared to untreated plants.
During the current season (2022-23), measurements of gs have continued to be taken in Regina cherry trees on Gisela® 6 rootstock. It can be evidenced from the measurement curves on January 6, 2023, that gl records were higher in plants treated with 2.5% kaolin (T1) compared to untreated plants (T0).
In the post-harvest measurements, starting from 10:00 am, leaf temperatures began at an average of 28 °C, reaching the highest gl values at 11:00 am. It is observed that gl values in this case have a negative correlation with leaf temperatures, increasing as the temperatures rise above 32 °C.
A study published in 2019 on the response of young cherry trees to drought events showed that under induced severe water stress conditions (subjected to two water retention cycles), gl values ranged from 100 to 190 mmol m⁻² s⁻¹, while the treatment with full irrigation replenishment showed gl values above 315 mmol m⁻² s⁻¹, which were 55% to 73% higher than under severe stress conditions (Blaya-Ros, P. J., et al., 2021). The trees subjected to this water deficit were unable to promote a foliar osmotic adjustment that would allow for high turgor pressure and hydration levels similar to those in fully irrigated trees.
Now, the question is: When can we define thermal stress in cherry cultivation? This can probably be observed in situ in the field when leaf curling occurs, which is a clear manifestation of thermal/hydraulic stress, as the stomata are located on the underside of the leaves. Understanding this condition based on different measurement tools and drawing conclusions is undoubtedly one of the inherent challenges that the industry must pay attention to.
After several seasons of study with a large number of measurements, reference values of gl regarding leaf temperatures and defined thresholds for the crop stress condition are shown in Table 1.
The porometer measurements as stationary equipment for gl are sensitive to the specific environmental conditions of each location, as they can vary due to temperature, humidity, wind, and light compared to measurements of water potential with a pressure chamber. Therefore, it is important to take a large number of measurements to reduce uncertainty.
Attention should be paid to the signals that indicate stress situations during the summer seasons since the practices of controlling or rather mitigating thermal stress should be oriented towards preventive measures. These measures could even be evaluated pre-harvest during highly sensitive events for the crop.
Thus, an adequate irrigation replenishment strategy, supported by the use of different formulations based on free amino acids, algae extracts, phospholipids, and/or products whose formulations are based on the nutritional/bio-stimulant pathway, can be an interesting proposal to mitigate thermal stress events in cherry cultivation as a comprehensive approach from spring to post-harvest.
-Blaya-Ros, P. J., Blanco, V., Torres-Sánchez, R., & Domingo, R. (2021). Drought-Adaptive Mechanisms of Young Sweet Cherry Trees in Response to Withholding and Resuming Irrigation Cycles. Agronomy, 11(9), 1812.
-Chaves-Barrantes, N. F., & Gutiérrez-Soto, M. V. (2017). Respuestas al estrés por calor en los cultivos. I. Aspectos moleculares, bioquímicos y fisiológicos. Agronomía Mesoamericana, 28(1), 237-253.
-Kole, C. (Ed.). (2013). Genomics and breeding for climate-resilient crops. New York: Springer.
-Lemus, G. 2005. Cultivo del cerezo. Instituto de investigaciones Agropecuarias, Chile, Boletín INIA n° 133, 256 p.
-Porch, T. G., & Hall, A. E. (2013). Heat tolerance. In Genomics and breeding for climate-resilient crops (pp. 167-202). Springer, Berlin, Heidelberg.
-Tapia, C., (2019). Una correcta utilización de bloqueadores solares en cerezos, impulsaría a una mayor acumulación de reservas con impacto positivo en su potencial productivo. https://smartcherry.cl/manejos-agronomicos/nutricion/una-correcta-utilizacion-de-bloqueadores-solares-en-cerezos-impulsaria-a-una-mayor-acumulacion-de-reservas-con-impacto-positivo-en-su-potencial-productivo/.