Reference experiences in the use of measuring equipment.
In the cherry production industry, it has become increasingly understood that the most important period is the post-harvest, since this is decisive for the production of the following season. It is during this period of time that nutritional corrections must be made according to the extraction of the previous season's production and the background information provided by the different laboratory analyses, such as fruit, foliar, and soil analyses, among others. At the same time, in post-harvest management, it is essential to condition the plants so that they are able to better tolerate the different types of stress, both abiotic and biotic, that could substantially reduce their physiological condition and, consequently, prevent the expected productive potential from being achieved.
Heat stress is defined as an increase in temperature for a sufficiently long period that can cause irreversible damage to the metabolism and development of plants (Porch and Hall, 2013). Most plants are sensitive to the stress caused by high temperatures and suffer when these are too low or too high with respect to the thresholds defined for each one. In the case of the cherry tree, the ideal development range is between 18 and 24 °C; at temperatures above 36 °C, its metabolism ceases and oxidative damage begins to occur in the different organs of the plant (Lemus, 2005). Several authors agree that stomatal closure is imminent above 30ºC of ambient temperature.
Under ideal conditions, the structures called stomata are usually open and tend to maintain a perfect balance between the release of water vapor into the atmosphere, and in turn, in that same process, they capture CO2 and incorporate it into the plant to be transformed into sugars, through the physiological cycles inherent to photosynthesis (Tapia, 2019). Stomatal regulation plays a fundamental role in the balance of water and CO2 of plants, through different physiological processes such as transpiration and photosynthesis. Stomatal regulation is a reaction of plants that helps mitigate the harmful effects caused by water deficit and/or thermal stress. This regulatory process results in a reduction in the rate of transpiration and gas exchange due to the closure of stomata (Blaya et. al. 2021).
As a way of quantifying this process in a dynamic way, the concept of stomatal conductance or conductivity is used. Stomatal conductance (g)s) measures the vapor flow that is expelled through the transpiration process by means of the stomata, it is usually measured in mmol m⁻² s⁻¹. When there is a decrease in stomatal conductance it means that the stomata are closing, and therefore the loss of water through transpiration is also reduced. This results in a decrease in the photosynthetic rate, therefore, all processes dependent on photosynthesis, including sugar reserves will have a negative impact as the main carbon source.
The normalized condition of stomatal conductance in fruit plants is dependent on some factors such as ambient temperatures, soil water status, among others. This is graphically explained in figure 1 according to Salisbury & Ross in 1994.
Figure 1. Diagram summarizing stomatal behavior under various environmental conditions (Modified from Salisbury & Ross 1994).
The research carried out aims to understand the physiological responses of cherry trees under post-harvest thermal stress conditions. Different strategies have also been observed that may be interesting to carry out in the pre-harvest period.
As a pre-harvest background, in the 2021-22 season, a research was carried out that consisted of the application of a potassium (K)-based product with a biostimulant action in cv. Regina on Gisela®12 rootstock in the commune of Molina, VII region, where records of stomatal conductance (g) are shown.s), leaf and ambient temperature (°C) every hour for 8 hours a day to understand the dynamics that exist between the different indices (Fig. 2).
Measurements corresponding to stomatal conductance (mmol m⁻² s⁻¹) and leaf temperature (°C) were taken with a Meter® brand device, model SC-1 Leaf Porometer, and ambient temperature records were extracted from an Instacrops meteorological station located in the field approximately 200 m from the research site.
Measurements were made on fully extended adult leaves from two-year-old wood, between 1.4 and 2.2 m high, on two dates 7-10 days after each application of the foliar biostimulant. It should also be noted that each measurement includes an average of 5 leaves per hour and each of the treatments.
Figure 2. Stomatal conductance curves (mmol m⁻² s⁻¹), leaf temperature and ambient temperature (°C) in cherry cv. Regina with foliar application of a biostimulant in spring.
Figure 2 shows that conductances decrease as a result of stomatal closure while temperatures rise during the day. In addition, it can be seen that regardless of the treatment, leaf temperatures remain similar to each other. In treatment 1 (T1) conductances are higher than those of treatment 0 (T0) for most of the day, where better indices were reported in fruit hardness harvest measured as durofel (UD), dry matter (%) and a better caliber distribution curve of T1 (Unpublished data).
In the records in Figure 2 it can also be inferred that the dynamics of leaf temperatures is similar between T0 (Orchard Treatment) and T1 (biostimulant product) in both graphs, and that the highest points of gs They 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 of 26.10.2021.
Meanwhile, in the afternoon on 11/04/2021, the T1 curves oscillate above the T0 curve, marking maximum measurements of 415 mmol m⁻² s⁻¹ when leaf temperatures are at approximately 25°C, which contrasts with T0 where values of 315 mmol m⁻² s⁻¹ are obtained at a temperature of 26 °C, which is probably a response of K that strongly participates in the regulation of stomatal opening and closing.
On the other hand, the use of products for the mitigation of thermal stress in post-harvest periods is a strategy that is becoming increasingly popular among many producers, mainly using sunblocks such as kaolinites (95% kaolin) and transparent sunscreens of different formulations, whose effect can be reinforced with biostimulant agents that help the plant to cope with thermal stress, where the dose strategies and application frequencies depend on the type of biostimulant to be used (Tapia, 2019). This has been reported in research carried out in past seasons, and its main result was to achieve a decrease in leaf temperatures that generally ranged between 0.5 to 1.5 °C compared to untreated plants.
During the current season, g measurements have continued to be made.s in cherry trees cv. Regina on Gisela® 6, where the measurement curves during 06.01.2023 can be seen that the g recordss were higher in plants treated with 2.5% kaolin (T1) in contrast to untreated plants (T0) (Fig. 3).
Figure 3. Stomatal conductance curves (mmol m⁻² s⁻¹), leaf temperature (°C) in cherry cv. Regina with application of a foliar product based on kaolinites (95% formulation of kaolin) at 2.5% during the summer of 2023.
In post-harvest measurements (Fig. 3), from 10:00 hrs. leaf temperatures began at an average of 28 °C, obtaining the highest g recordss at 11:00 am, and that indeed the g recordss In this case they are in a negative correlation with those of gs versus leaf temperatures as they rose to values above 32 °C leaf temperature.
A study published in 2019 on the response of young cherry trees to drought events shows that under induced irrigation situations for severe water stress (subjected to two water retention cycles), the gs show values between 100 and 190 mmol m⁻² s⁻¹, while the treatment with full irrigation replacement shows values of gs about 315 mmol m⁻² s⁻¹, with values of 55 and 73% higher than severe stress (Blaya-Ros, PJ, et al 2021). Trees subjected to this water deficit were unable to promote a foliar osmotic adjustment that had the capacity to obtain a high turgor pressure and hydrate the plant to levels similar to trees with full irrigation.
Now the question is: At what point can we define thermal stress in cherry cultivation? This could probably be seen in situ in the field when a “ is seen“curl” of the leaves as a fairly obvious condition of thermal/water stress, a typical physical effect, since the stomata are located on the underside of the leaves. Understanding this condition based on different measurement tools and being able to conclude is undoubtedly one of the inherent challenges as an industry that must be taken with attention.
After several seasons of study with a large number of measurements, the reference values of g are shown.s with respect to leaf temperatures and the thresholds defined for crop stress conditions (Table 1).
Table 1. Reference table of leaf underside temperature (°C) and stomatal conductance in cherry trees in the Curicó area. Avium 2023.
Porometer measurements as stationary equipment in the gs They are sensitive to the environmental conditions of each particular location as they could vary due to temperature, humidity, wind and light with respect to the water potential measurements with the pressure pump, so it is important to carry out a large number of measurements to reduce their uncertainty.
Attention must be paid to the signals that must be given in the event of stressful situations during the summer, since the practices of using strategies to control or rather mitigate thermal stress should be aimed at carrying them out in a preventive manner and that these can even be evaluated pre-harvest in the event of very sensitive events for the crop.
Thus, an adequate irrigation replacement 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/biostimulant route, can be an interesting proposal to mitigate thermal stress events in cherry cultivation as a comprehensive proposal from spring to post-harvest.
Fountain:
-Blaya-Ros, PJ, 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, NF, & Gutiérrez-Soto, MV (2017). Heat stress responses in crops. I. Molecular, biochemical and physiological aspects. Mesoamerican Agronomy, 28(1), 237-253.
-Kole, C. (Ed.). (2013). Genomics and breeding for climate-resilient crops. New York: Springer.
-Lemus, G. 2005. Cherry cultivation. Institute of Agricultural Research, Chile, INIA Bulletin No. 133, 256 p.
-Porch, TG, & Hall, AE (2013). Heat tolerance. in Genomics and breeding for climate-resilient crops (pp. 167-202). Springer, Berlin, Heidelberg.
-Tapia, C., (2019). Correct use of sunscreens in cherry trees would lead to a greater accumulation of reserves with a positive impact on their productive potential. 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/.
