By: Carlos Tapia, Agricultural Engineer, Ms.C. – Avium Technical Director -Bruno Tapia, Agricultural Engineer – Avium Technical Coordinator
When the harvest is over, it may seem at first glance that there is a lack of activity in the orchards, but that is when the cherry season really begins. During the post-harvest, important processes occur such as floral induction and differentiation, accumulation of carbon and nitrogen reserves, recovery of the root system, among others.
The management carried out during this long summer period is of utmost importance, as it will be crucial for a good start to the following season. In this context, we will analyze the relevance of detecting abiotic stress, that is, the negative impacts of non-living factors (drought, humidity, temperature) on cherry plants, their possible consequences on them and how to deal with it.
High temperatures, above 30 degrees Celsius, are one of the main causes of the so-called thermal water stress in post-harvest, which is related to premature stomatal closure of leaves in fruit trees; a poor supply of irrigation and an inadequate vegetative state of the plants also contribute.
Thermal water stress can be determined by measuring different parameters, including: taking the temperature of the underside of the leaf, measuring carbon and nitrogen reserves in roots and fruit centers (at the end of the season) and measurements of stomatal conductance of the leaves.
If, for example, the temperature measurement results in 30ºC or more, we are talking about water stress in the plant. In turn, it is possible to detect this condition through the porometer that measures the transpiration of the leaves. The lower the transpiration level, the greater the water stress.
An orchard subjected to thermal water stress can have serious consequences for the following season; one of them is the malformation of the fruits, because as we have already mentioned, once the harvest is over, very important physiological processes occur, such as the periods of induction (IF) and floral differentiation (DF) (Fig. 1), in addition to all the sugar reserve mechanisms in the different organs of the plant.
Floral induction (FI) at the stage (Fig. 2, S1), begins at the end of spring in mid-December in the southern hemisphere and continues to advance until the beginning of floral differentiation (FD) at the stage (Fig. 2, S2, S3, S4) during summer, up to even the first days of March, passing through an evolution of morphological and reproductive structures, mainly recognized by the human eye in darters. (Fig. 2).
If we think about the construction of the productive potential of an orchard, we must consider that this is the product of the interaction of hormonal, nutritional and environmental factors. The most important hormone in this process is Absic Acid (ABA), due to the role it plays in regulating processes related to abiotic stress, protecting the plant, controlling the opening or closing of the stomata in the event of high UV radiation, high temperatures and water deficit.
This allows the plant's gas and water exchange process to remain active in direct relation to the atmosphere, understood as water flow from the soil via the xylem to the atmosphere, through the leaves, without impediment or resistance from the stomata to expel water (transpiration). In this way, the plant manages to capture CO2 as the main basis for carbon fertilization and generation of reserve sugars through the photosynthetic process (Fig. 3).
If the plant does not have the capacity to make continuous gas exchange to the atmosphere due to early stomatal closure in its leaves, either due to high temperatures, less vegetative development and/or with a deficient irrigation, it cannot capture CO2 for the generation of sugars, which has a direct impact on the malformation of flower primordia in the fruit centres, generating loss of fertility and a consequent malformation in fruits; this considering that the floral induction period (IF) begins approximately 70 days after full flower (DDPF) and the floral differentiation period (FD) begins approximately 100 DDPF, both processes being highly dependent on water, light and thermal stability in the plant, at times of greater atmospheric demand.
The recovery of the root system after harvest plays a fundamental role in maintaining a system in constant balance. This process extends between approximately 90 and 120 DDPF (Fig. 1), starting at the end of December in central Chile.
All of these processes, including root system recovery, water flow stability and stress prevention, are directly linked to proper irrigation management and scheduling at this stage of crop development. It is essential to monitor by inspecting test pits, weather stations, probes, etc., without neglecting the visual assessment of the appearance of the plants and, in turn, monitoring the correct functioning of the irrigation system, as well as the times and frequency, in order to achieve the objective of maintaining the soil-water-plant system in balance.
As regards nutrition, and when dealing with orchards in full production and especially with a high productive potential, it is necessary to incorporate agents that enhance root recovery; these will be responsible for accumulating a large part of the carbon (starch) and nitrogen reserves that will support the beginning of the following season. Remember that in spring the initial development of roots is recognized, not before 25-30 DDPF (Fig. 1), generally coinciding with soil temperatures above 15ºC (approximately mid-October in the central zone of Chile).
These agents that contribute to this process are known as rooting agents; they are phytohormones that contain growth regulators, mainly auxins (AIB, AIA, ANA, etc.), and that provide a direct signal to the plant to further enhance the creation of new roots and their sustained development; their application should be carried out in the second "flush" of growth (80-90 DDPF).
The use of this type of enhancing agents, whenever the garden requires it, provides significant differences in terms of the increase in root mass expressed in grams of roots per m³ of soil; the use of this type of compounds enhances the development of the roots and generates new growth points. The development of the root system, measured in weight, represents the presence of a greater amount of accumulation of carbon (starch) and nitrogen reserves in the plants.
Sunblocks
There are different strategies to improve or reduce thermal water stress, several of which have already been mentioned above; however, the use of sunscreen is also an optimal tool to contribute to the control of this complex scenario, as it generates light reflectance, which helps to reduce the temperature of the trees and the impact of this unfavorable condition on them.
Its application should be carried out immediately after harvest, and repeated 25 to 30 days later to prevent the high temperatures of January and February from affecting the correct development of the orchards. The use of this type of product prevents the premature closure of the stomata, allowing a correct gas exchange between the fruit plants and the atmosphere; if this process is continuous, everything that the plant takes as water from the soil is transported via xylem ducts or vascular bundles to the atmosphere by releasing water through said stomata.
When plants are exposed to optimal temperature and irrigation conditions, the perfect balance of water release into the atmosphere, CO capture, and oxygen absorption occurs.2 and the transformation into sugars, through physiological cycles. On the other hand, if thermal water stress is triggered, there is no continuous flow of water from the soil to the atmosphere, therefore, the uptake of water by the plant is limited and stomatal closure is imminent. This last condition generates the impossibility of releasing water into the atmosphere, which means that there is no correct uptake of CO2.
Recent research carried out by the Avium R&D team on the use of sunscreens in cherry trees has shown that there are marked and important differences in the accumulation of starch as the main carbon reserve in saplings. Although the measurements have been carried out in fruit centres and roots, the great difference in the accumulation or greater accumulation of starch with sunscreens vs. the control is seen in the saplings. This greater accumulation of starch in the saplings brings with it several positive factors that are related to a possible greater resistance to low temperatures in spring, because the saplings are much better formed in terms of sugar accumulation. In turn, the flowering processes, such as initial setting and fruit development, are benefited from the point of view of the quality and condition of the fruit.
In the following figure (Fig. 4) it is possible to see how the temperature curve of the leaves without sunscreen follows the same shape as the ambient temperature curve, however, the curve of a leaf treated with sunscreen shows markedly lower temperatures, especially after noon.
In parallel, other research carried out in Chile on the correct use of sunscreens has been able to prove their usefulness in dealing with the risks of water stress in the face of high summer temperatures. Its use alone is a good tool, however it is recommended to reinforce it with agents that help the plant to deal with thermal stress, such as seaweed extracts, for example. Their combined use has been investigated for years, leading to the conclusion that the application of kaolinites at concentrations of 3% reduces the temperature of the leaves compared to an untreated control, improving resistance to stress, as well as indirectly reducing malformations of floral primordia when the ambient temperature increases above 30°C.
Controlling abiotic stress in cherry orchards, using the different existing mechanisms, is a task that cannot wait; any anomaly that occurs from the point of view of stress during this period will directly impact the accumulation of reserves and the formation and development of floral primordia in the fruit centres that will participate in the production of the following season. There is a lot at stake!
References.
Bonomelli, C., Bonilla, C., Acuña, E., and Artacho, P. 2012.Seasonal pattern of root growth in relation to shoot phenology and soil temperature in sweet cherry trees (Prunus avium): A preliminary study in central Chile. Hundred. Inv. Agr. 39(1).
Quero-García, J., Iezzony, A., Pulawska, J., Lang, G. 2017.Cherries: botany, production and uses. Boston, MA: CABI, 2017.
Salazar, C., Hernández, C., Pino, M. 2015.Plant water stress: Association between ethylene and abscisic acid response. Chilean J. Agric. Res. Vol.75. suppl1. August 2015. Chillán, Chile.
Taiz, L. & Zeiger, E. 3rd. Edition, 2006.Plant Physiology, Water Balance of Plants. Sinauer Associates. Sunderland, Ma., USA.
Tapia, C. 2017.Utilization, mode of action and experiences of different growth regulators that influence cherry production. Red Agrícola Magazine, August 2017 edition. pp. 30-31. Santiago, Chile.Villar, L., Lienqueo, I., Llanes., A., Rojas, P., Péez, J., Correa, F., Sagredo, B., Masciarelli, O., Luna, V., Almada, R. 2020.Comparative transcriptomic analysis reveals novel roles of transcription factors and hormones during the flowering induction and floral bud differentiation in sweet cherry tres (Prunus avium L. cv. Bing). PLoS One. 2020 Mar 12;15(3).
