The stress index is defined as an objective atmospheric indicator that allows for the determination and quantification of stress accumulation through a correlative number, directly relating the atmosphere to the plant. The accumulation of stress units is conditioned by the relationship between two climatic variables: ambient temperature (T/°C) and relative humidity (RH/%). To accumulate stress units, the following conditions must be met for each variable: temperatures must be equal to or greater than 10°C, and relative humidity must be equal to or less than 75%. If either variable does not meet this condition, no stress units accumulate.

It has been observed that, in most cases, the main trigger in the accumulation of stress units is the percentage of relative humidity in the environment. When comparing areas with similar ambient temperatures, the area with a lower percentage of relative humidity experienced a greater accumulation of stress units.

This method of accumulating stress units derives from the simplification of the vapor pressure deficit (VPD) formula, which calculates the difference between saturation pressure and actual pressure. The greater the difference between the two, the higher the plant’s water demand. The concept and calculation of this method can be complex; however, the stress index formula helps estimate and interpret stress conditions more easily.

The accumulation of stress units directly impacts the physiological behavior of cherry plants because there is a direct relationship with the generation of carbon structures through photosynthesis. In other words, the greater the accumulation of stress units, the lower the photosynthetic rate and carbohydrate reserves for the next season. When plants experience atmospheric stress, they tend to use part of their reserves to recover their damaged photosystems.

As mentioned earlier, for cherry cultivation, it is crucial to prepare the plants for the next season, which involves maintaining low-stress conditions. This is because, at least until the phenological stage of petal fall, the plants rely solely on the carbon reserves accumulated during the previous season, stored in their roots and vegetative structures during the post-harvest period.

Since the accumulation of stress units is directly related to the climatic conditions during the season, our only way to cope with this condition is to mitigate this environmental stress through cultural practices aimed at reducing this effect and associated problems. This will result in better reserve accumulation for the next season.

Cultural Practices to Mitigate Problems Attributed to the Stress Index (Sepúlveda, 2020)

1. Avoid excessive fruit load and weak foliar development of the tree. This ensures more carbohydrates are available for the plant’s defense systems.
2. Maintain adequate water status for the tree. Greater damage has been observed in trees with irrigation deficits.
3. Ensure orchard health. Any biotic stress (pests, diseases, weeds) will compete for assimilates from photosynthesis.
4. Apply algae and/or sun blockers during periods of high stress accumulation.

References:

• Sepúlveda, A., Arenas, L., Yuri, J. (2019). Climate and Apple Quality. Center for Pomaceous Fruits – University of Talca.
• Sepúlveda, A. (2020). Environmental Stress and Fruit Production. Climate Information No. 54, Center for Pomaceous Fruits – University of Talca.