One of our Trutina users contacted us in the beginning of March, 2024 with an aim to reduce the quantity of cracked cherries. The client is a hydroponic farmer, producing cherries based on the guidance of Trutina Berry (our professional greenhouse monitoring system, adapted to the production of various berries).
The existing system measures the weight of containers, quantity of irrigation, EC, ΔEC, temperature, and the level of drain as well. It is capable of calculating the water uptake speed, and of measuring the irradiation, water capacity, humidity and temperature, too – aggregating and presenting all these values on Trutina’s user-friendly online platform.
Though these are indeed a lot of essential values for professional growing, we knew that dendrometers can offer additional help to achieve less cracks - and of course, Trutina is able to accept such data. We decided to connect 2 dendrometers with 0,002mm resolution – and installed one on the fruit, the other one on its branch. The sensors started working with a data gathering frequency of 5 seconds.
Why dendrometer?
Dendrometers are used to measure the diameter / circumference of the branches of trees or other plants. Using such data, farmers are able to monitor the growth and physiology of plants in the long term, and also, to measure osmotic pressure – which was our first guess as to what may cause cracked fruits.
Our hypothesis was that a better control of osmotic pressure can avoid cracks during the growth of fruits. Read on and we’ll elaborate all the related terms, required to understand the connection between osmotic pressure and its potential negative effects.
What makes cherries crack?
According to cherry growers and experts, cracked cherries usually have the followingreasons:
• Sudden raining: cherries tend to crack after heavy rains, especially when the fruits are close to ripening. Rain water quickly accesses the fruits, increasing water content and internal pressure, leading to cracks. As our client has a closed greenhouse, this cannot be the reason.
• Quick growth: when the ripening process is quick, the skin of the fruits cannot keep up with it and expands slower, so the internal pressure can create cracks on the skin. According to our measurements, the speed of fruit growth always decreased close to ripening – so most likely it is not related to the current case.
• Sensitivity of species: some cherry species have a higher tendency to crack than other species. Those with thicker skin are less prone to cracks, while species with thinner skin crack easier. We are familiar with these differences, and consider that the species in question may also have a tendency for cracks, but as it is a common and popular species, distributed in large quantities, this tendency should not be too significant. Otherwise it would not be suitable for mass production.
• Environmental stress: extreme ups and downs in temperature, strong sunlight or wind can also lead to cracks. In Hungary, these environmental circumstances are always present from March to May, however, this could also mean that all cherries, growing outside, would have cracks. Of course, this is not the case.
• Extreme changes of humidity: high humidity can precipitate on the surface of fruits, and it can be easily absorbed – having a similar effect as the rain, increasing internal pressure, leading to cracks. These plants grow in a greenhouse with regulated ventilation, which are controlled in order to avoid any quick changes in humidity and temperature, during the growth of fruits. Unfortunately, external factors could still contribute to humidity and temperature changes, which had to be compensated: the client needed to turn on heating for 8-10 days during this period. However, we were not able to point out a definite correlation between high humidity and cracks.
• Irrigation practices: when irrigation is not uniformly distributed – for example, a high level of irrigation follows a dry period, fruits tend to crack. Roots soak up a large amount of water, and fruits cannot keep up with such sudden pressure changes. Most growers and experts agree on this: the irrigation strategy definitely has a significant effect on fruit cracks.
As we learned the possible reasons for fruit cracks, let’s proceed and explain two important terms, for a better understanding.
Osmotic pressure
Osmotic pressure occurs inside the plant and controls the flow of water through the cell membrane. The level of osmotic pressure depends on concentration of solutes (such as salts, sugars) in the cells. If the concentration is higher inside the cell than externally, water flows into the cell – called osmosis – to achieve a balance. Osmotic pressure helps maintain turgor pressure, which is elementary for plants to keep structural integrity and growth.
There are several factors that take effect on osmotic pressure – the most important factors are:
• The concentration of solutes: the level of osmotic pressure is proportional to the concentration of solutes in the plant cells and outside of the cells. Higher concentration results in higher osmotic pressure.
• Temperature: increasing temperature causes increasing osmotic pressure, as molecules start moving quicker and having more energy, increasing the intensity of osmosis.
• Permeability of cell membrane: when the membrane lets water molecules transfer easier than solutes, osmotic pressure gets more significant.
• Difference in water potential: water always flows from parts of higher water potential to parts of lower water potential.
• Tenseness of cell membrane: the flexibility or tenseness takes effect on turgor pressure that, in cooperation with the osmotic pressure, helps maintain the structure of the cell.
All of these factors contribute to the osmotic pressure and play a vital role in the nutrient and water supply of plants.
Root pressure
The hydrostatic pressure in the roots is called root pressure, which helps the plant absorb and transfer water and various solutes in its fibers. This pressure is a result of the active ion transfer that works through the cell membranes of the roots, created by the osmotic gradient. Osmosis transfers the water into the cells and creates a positive pressure in the xylem of the roots.
The importance of root pressure are as follows:
• Water uptake: root pressure contributes to transferring the water upwards, from the roots to the branches and leaves, especially during the nighttime or when transpiracy is low. It helps maintain the constant hydration of plants.
• Guttation: as a result of root pressure, small water drops can appear on the leaves and other parts of the plants, which is often referred to as guttation. It usually occurs in the early morning hours.
• Ion transport: root pressure helps the absorption of nutrients (such as ions and minerals) from the soil, and contributes to their transfer to various parts of the plants.
• Support during low transpiracy conditions: when evaporation is slow or minimal, for example at night or when humidity is high, root pressure plays a crucial role to maintain water transfer inside the plant.
To sum up, root pressure is a critical mechanism that lets plants gather water and nutrients and to maintain their internal water balance, especially when transpiracy is not enough for ensuring the necessary water transfer.
What is the relation between root pressure and osmotic pressure?
There is a strong connection between root pressure and osmotic pressure, as they cooperate to maintain the water and nutrient uptake and transfer of plants. Root pressure takes effect on osmotic pressure in the following actions:
• Cumulation of ions: plant cells have active ion pumps that transfer ions (like potassium, natrium) to the xylem, from the space between cells. The cumulation of ions increases the level of solutes in the xylem, which increases osmotic pressure.
• Water intake: increased osmotic pressure helps water flow into root cells thanks to osmosis, as water always flows from lower to higher osmotic pressure. This movement creates hydrostatic pressure, or, in other words, root pressure.
• Nutrient and water transfer: positive hydrostatic pressure (thanks to root pressure) helps water and nutrients transfer upwards in the xylem, from the roots to the upper parts. This process is particularly important during low transpiracy conditions (for example at night or during low humidity), when water flow would not be enough without the suction force by transpiracy.
• Keeping osmotic balance: the cooperation of root pressure and osmotic pressure makes sure that plant cells have enough water and nutrients are transferred efficiently. Root pressure supports water transfer (generated by osmotic pressure), especially when environmental conditions are not ideal for transpiracy.
When root pressure is too high, it can take effect on osmotic pressure and the general balance of water and nutrients in the plant. Here’s how high root pressure affects osmotic pressure and the activity of the plant:
• Differences in water potential: high root pressure induces hydrostatic pressure in the cells of the root and the xylem. This increased pressure helps water transfer from the roots to the stalk and leaves - resulting in a quicker balancing of the concentration of solutes in the cells.
• Turgor pressure in cells: when the root pressure is too high, turgor pressure (pressure occurring on the membrane) can also increase. This pressure helps maintain the tension of the cells and the structural integrity of the plant.
• Guttation: in case of high root pressure, water appears on the surface of leaves and other parts of the plant - called guttation. It occurs when the speed of transpiracy is low and water is forced through the pores of the plant.
• Distribution of nutrients: high root pressure makes water and nutrient transfer faster in the xylem. It is a positive effect, as the necessary nutrients arrive quicker to various parts of the plants. However, in case the pressure is too high, osmotic balance can break and the cells cannot control the water and nutrient intake, resulting in stress.
• Cell damage: when the root pressure is too high, cells can fill up with too much water, leading to cell damage or even cell tear. This is a result of extreme water intake, as cells cannot withstand the increased hydrostatic pressure.
Our strategy to avoid cell damage
It was our main intention to avoid any cell damage, explained above. We managed to observe that the branches (blue line) and the fruits (red line) both show a significant pulsating movement during 24 hours. See the diagram below, showing a one-week period. Our conclusion was that the pulsating is generated by changes of osmotic pressure:
• in order to protect the fruits, we need to avoid irrigation when the osmotic pressure is high - and any other activities that would further increase osmotic pressure
• we need to make sure that there are no sudden changes in osmotic pressure
• we set the climate computer to produce slow effect, with the right amount of vegetative impulses
This chart below shows how the berry grows on the tree - marked by red (one-week period)
In the next chart, you can see that we started the irrigation when the diameter of the branches started to reduce, and did our best to stop the irrigation before any increase in diameter.
Below you can see a whole day, showing how osmotic pressure decreases in the branch and the fruit. The blue columns on the bottom represent the irrigations: 26 times in 6 hours! We decided to have lower amounts but with higher frequency to avoid the increase of root pressure and osmotic pressure.
This irrigation strategy is clearly based on osmotic pressure, from flowering to harvesting - constantly making sure that there is enough drain and the ΔEC and PH have appropriate values.
IN SUMMARY,
our irrigation strategy has proven successful, and the amount of cracked fruits has been minimized.
Sensitivity to cracking between the different varieties has also been observed. The Trutina system, along with the dendrometer sensor, provided excellent data to achieve our goal, and it will become one of our main sensors in the future for growing fruit trees and shrubs.
FUTURE RESEARCH PLANS
An interesting observation to explore in the next season: by removing the flesh of the cherry fruit, it became possible to measure the pulsing of the seed, and I would like to report on this in the next section.
Let’s talk about your greenhouse
Is there anything in common with you and the greenhouse described above? Do you have problems with creating the best irrigation strategy, increasing your yield and having better control over your plants? Trutina can help: don’t hesitate to book a free consultation with one of our experts and talk about the possibilities:
gremonsystems.com/contact-us/