How to prevent calibre reduction in citrus fruits

This year there has been a significant reduction in the calibre of citrus fruits in general, although it was more pronounced for early short cycle mandarins.

According to the producer and agricultural engineer Carlos Arenes, "the flowering was very good and we were expecting a very good harvest, but the high summer temperatures, above 30 degrees Celsius for many days, have prevented the production in many fields from reaching a satisfactory calibre. In some cases, the mandarins won't even be harvested," he affirms.

The specialist explains that with temperatures above 30 degrees, the plants close the stomata of their leaves to prevent dehydration, since they lose water at a faster pace than they can absorb it from the soil.

"It is a protection mechanism that plants naturally have. In those days when it was so hot, growers irrigated the soil for many hours, but the plants take very little of this water, as their stomata are closed. The fruits are unable to do this in their early stages; they dehydrate and die because they do not have any way to control dehydration," he explains.

A few years ago, Carlos Arenes developed a mixed control system against frost and heat where water can be applied above the plant.

"Each gram of water we evaporate above the plant 'steals' heat from it, since each gram of water we evaporate absorbs 590 calories per gram. If we apply 12 cubic metres per hectare per hour, we need 12,000 litres, which is 12,000,000 grams of water. If every gram absorbs 590 calories, we reach a total of 7.08 billion calories per hectare per hour."

Explained in a less technical manner - "We get the temperature of a plot on a day with 40 degrees Celsius down to approximately 24 to 25 degrees. If the plots are shaded, the temperature would be adjusted to just over 27 degrees, and so we have less sun and less evaporation. The sun is the main cause of evaporation, followed by the wind."

"With these figures, we can begin to understand things a little better," states Carlos. "It's not easy, because we need the maximum amount of water to evaporate without saturating the atmosphere with moisture. This system has water pulses with controlled droplet size to allow the water to reach the crops and evaporate above the ground without causing excess moisture."

With this system, fruits grow in size every day, whatever the temperature, with a considerable calibre increase and a precocity of even more than one week, both internally and externally. "In some varieties of clementine, we use water pulses at night to create a greater temperature differential and get an earlier colouring and a good balance between sweetness and sourness."