By: José Miguel Nuntón León – Crop Manager, Blueberries, Hortus.

In blueberry cultivation, thermal stress cannot be assessed solely by the temperature recorded by a meteorological station. This data provides a general reference but does not describe what is truly occurring at the leaf and fruit level. The real physiological impact is defined in the microclimate of the canopy, where solar radiation, wind, and humidity interact, creating a thermal environment that is often more demanding than the measured ambient temperature.

When irradiance is high and air movement is limited, a determining phenomenon occurs: thermal decoupling. The fruit loses its ability to dissipate heat to the environment, and its surface temperature begins to rise above the surrounding air. Under these conditions, an environmental reading of 33–34 °C can translate into 40–44 °C on the fruit’s surface. This differential is sufficient to alter biochemical processes and compromise cellular stability.

The fruit tissue is not designed to sustain such temperatures for prolonged periods. Excessive heat affects the stability of structural and enzymatic proteins, alters cell membranes, and compromises the functionality of photosystem II. The consequence is not always immediately visible. Often, the fruit retains an acceptable commercial appearance in the days following the event, but internal deterioration continues to develop, manifested in reduced firmness, shorter postharvest life, and increased susceptibility to surface defects.

The phenomenon is cumulative. Repeated thermal events during the growing season result in progressive physiological wear. For this reason, thermal management must be preventive and sustained over time. Measuring and anticipating stress is more effective than intervening once the damage has already manifested.

From an operational standpoint, there are practical thresholds that guide decision-making. For green fruit, intervention should begin at 32 °C air temperature. For fruit at color change, around 35 °C. And if the leaf/air temperature differential (ΔT) exceeds 6 °C around midday, mitigation should occur without waiting for visible signs of damage. The ΔT is particularly useful because it reflects the real thermal decoupling and allows stress to be anticipated before structural damage becomes evident.

Incorporating systematic measurements allows trends to be identified and not just isolated events. A single hot day may not generate a significant impact, but the repeated occurrence of demanding conditions without proper management will

Vegetative Stage: Physiological Stability as a Foundation

The vegetative stage determines the plant’s ability to respond to subsequent thermal events. A plant with a balanced photosynthetic system and adequate hydration presents greater resilience.

Evaporative cooling is the main mechanism for thermal regulation. Transpiration allows heat dissipation and maintains internal stability. However, this mechanism depends on the water status and vapor pressure deficit (VPD). As the VPD increases, the synthesis of ABA rises, stomatal closure occurs, and leaf temperature increases. The assimilation rate decreases, and photosystem II begins to show signs of stress.

If this condition persists over several consecutive days, photosynthetic efficiency may progressively decline. This is not just a one-time response, but an accumulated loss of physiological performance.

Timely irrigation plays a strategic role beyond hydric balance. Ensuring water availability before the critical window of the day helps maintain active transpiration and keeps the natural cooling mechanism operational.

At the cellular level, the plant activates osmotic regulation mechanisms via proline, betaine, and soluble sugars. These compounds stabilize proteins and membranes, preserving cell volume against thermal dehydration. However, this response requires energy expenditure. Reducing radiant load decreases the pressure on these internal mechanisms.

In this context, the application of Protecsol (kaolin) in the vegetative-preflowering stage acts as a reflective film that reduces direct solar radiation absorption and lowers canopy temperature. By containing the ΔT, it aids in the recovery of Fv/Fm at dawn, an indicator that photosystem II returns to its baseline function after the thermal event. Anticipating the application is key to maximizing the preventive effect.

Preflowering and Flowering: Protecting the Reproductive Process

The reproductive phase is particularly sensitive to thermal stress. Processes such as pollen germination and pollen tube elongation require physiological stability. Excessive heat can interfere with these mechanisms, compromising fruit set and uniformity of production.

Priming applied 48–72 hours before the thermal event activates defensive mechanisms preventively. Bioactive seaweed extracts such as Martello (Ascophyllum nodosum) and Kelpura (Ecklonia maxima) induce heat shock proteins, modulate redox signaling, and increase osmoprotective sugars.

ROS, at controlled levels, play an adaptive signaling role that coordinates defensive responses. This process does not eliminate stress but enhances metabolic response capacity to the thermal event.

If the heatwave persists, reinforcing petal drop helps sustain the physiological activation initiated earlier. During the highest-risk day, micro-sprinkler pulses from 29–30 °C help maintain the canopy within operational ranges of 32–35 °C, reducing the air-fruit thermal decoupling.

Fruit Set and Green Fruit: Maintaining Structure and Metabolism

At this stage, the goal is to preserve the structural and metabolic integrity of the developing fruit. High temperatures affect structural and enzymatic proteins, disrupting internal processes.

Manvert Silikon provides silicon in the form of orthosilicic acid, the most assimilable form. Its stability depends on maintaining a pH that prevents polymerization. Silicon helps reinforce the cell wall, improve heat stress tolerance, and increase tissue stability.

Neogreen Up, a hydrolyzed product with a high content of glutamic acid, promotes protein resynthesis after the thermal peak, supporting metabolic recovery. Manvert Foliplus supports physiological processes related to growth and recovery after stress.

Consistency in the sequence of applications is critical. Unnecessary overlap can cause physiological interference and reduce efficiency.

From Color Change to Harvest: Direct Impact on Export Quality

Late thermal events directly affect commercial quality. Microcracks, superficial browning, and loss of firmness reduce export classification and postharvest life.

Comparing sublots and controls in terms of surface damage percentage, firmness, and °Brix allows validation of management effectiveness and quantification of its impact.

Calcium plays a key structural role. Due to its low mobility in the phloem, it relies on xylem flow. Movili-Ca, applied via irrigation, promotes fixation as calcium pectates, improving mechanical resistance, texture, and postharvest life. Reducing cracking, pitting, and accelerated loss of firmness contributes to greater uniformity and exportable proportion.

Continuous Measurement and Strategic Integration

Systematic measurement is an essential part of management. The leaf/air ΔT measured with an infrared thermometer in the mid-afternoon and Fv/Fm evaluated at dawn provide an accurate interpretation of the crop’s physiological state. Repeated values below 0.78 suggest cumulative physiological scars and require adjustments in radiant load reduction and water management.

In practice, the sequence matters more than the sum of isolated interventions. Balanced vegetative management, reproductive priming, structural support with silicon, and metabolic repair should be understood as an integrated strategy.

Release LPH (seaweed + fulvic acids via irrigation) helps sustain the rhizosphere and absorption efficiency. On-field synergy with Kelpura strengthens the comprehensive response to repeated thermal events.

Biostimulation against high temperatures is not a one-time action. It is a technical strategy based on anticipation, continuous measurement, and physiological coherence. The opportunity in each intervention and the orderly integration of tools determines whether the thermal event is managed or accumulated as structural quality loss.

Managing thermal stress in blueberries should not be understood as a reaction to a single hot day, but as an integrated strategy over time. The impact of heat is cumulative, and the real difference is built through anticipation, measurement, and intervention coherence.

The leaf/air ΔT allows for detecting thermal decoupling before the damage becomes visible, while Fv/Fm at dawn provides a clear signal of the photosystem II status. Repeated values below 0.78 indicate physiological scars that require reinforcing radiant load reduction and coordinating irrigation with VPD.

In the vegetative phase, the goal is to sustain physiological stability. Timely irrigation maintains active evaporative cooling, and the preventive application of kaolin reduces direct radiation absorption, alleviating pressure on internal osmotic regulation mechanisms.

In preflowering and flowering, priming 48–72 hours before the thermal event, using seaweed extracts like Martello and Kelpura, favors the induction of heat shock proteins, modulates redox signaling, and raises osmoprotective sugars, enhancing the metabolic response.

During fruit set and green fruit stages, structural support becomes crucial. Silicon reinforces the cell wall and tissue stability, while hydrolyzed products support protein resynthesis after stress. From color change to harvest, calcium via irrigation strengthens structure and improves firmness, reducing defects associated with heat.

Sequence matters more than isolated product application. Balanced vegetative management, preventive priming, structural support, and metabolic repair form a coherent strategy. Biostimulation, applied with measurement and opportunity, ensures that the thermal event is contained rather than accumulating as quality loss.