Have you ever wondered how a citrus fruit develops from a single flower into the fruit we recognise on supermarket shelves? And why do some fruits suddenly split open before harvest, causing serious losses for growers?
Understanding how citrus grows helps explain why fruit cracking occurs and why this issue has become increasingly difficult to manage in recent years.
Inside Citrus Growth: A Stage-by-Stage Overview
Citrus fruit development begins with a fragrant flower that may be self-pollinated or non-self-pollinated, with one ovary, divided into locules, which later become the fruit segments. Inside each locule are one or several ovules.
During pollination and fertilisation, pollen grains carrying sperm cells grow through the pollen tube until they reach the ovules. Once fertilised, the ovules develop into embryos.
Fruit development starts after fertilisation, but citrus can also produce fruit without fertilisation, resulting in seedless fruits, which are often preferred by consumers. Citrus also displays polyembryony, a process in which embryos can develop directly from the mother tissue without fertilisation.
How the Citrus Fruit Forms
In the early stages of development, the ovary transforms into a small green fruitlet through active cell division of the ovary wall. During this process, the different layers of the fruit are formed:
- Exocarp (Flavedo) – the outer coloured layer containing pigments and oil glands
- Mesocarp (Albedo) – the white, spongy middle layer
- Endocarp – the inner cell layers of the mesocarp, from which juice sacs are initiated at early stages of fruit development.
As development continues, the fruit enters the cell enlargement stage. Initially, growth is mainly driven by the expansion of the albedo. Later, the fruit increases in size as the juice sac cells enlarge through water accumulation, gradually filling the fruit segments. During the first half of development, acids accumulate inside these juice sacs.
Eventually, the fruit reaches the maturation stage. Growth slows down, acid levels decline, and sugars accumulate. The balance between sugars and acids determines the ideal harvest time. At the same time, the peel undergoes a process known as “colour break”, during which chlorophyll degrades and carotenoid pigments accumulate, changing the peel from green to orange or yellow. Similarly, pulp colour is also changed by carotenoids accumulation.
When Fruit Development Leads to Cracking
While citrus growth follows a highly coordinated process, it does not always progress smoothly. One of the most challenging disorders affecting citrus production is fruit cracking.
Cracking usually begins as a small split in the flavedo, the outer coloured skin of the fruit, often appearing at the end opposite the stem. Before the crack becomes visible, the affected area may start changing colour from green to yellow. As the problem develops, the crack spreads into the albedo, the inner white spongy layer of the peel, and may continue towards the middle of the fruit. This damages internal tissues, often causing the fruit to split open and fall from the tree. Cracking typically starts in late summer and can continue for three to four months.
Why Some Citrus Varieties Crack More Easily
The susceptibility to cracking is strongly linked to the fruit’s genetics and natural structure. Certain varieties, such as Nova mandarins, have long been known to be highly prone to cracking. Historical losses of around 10–15% increased dramatically over the past two decades, reaching 30–60% in many seasons and causing major economic damage as well as a decline in cultivation.
Similarly, Ori mandarins, widely grown in Israel, showed minimal cracking during the first years of commercial production. However, during the last decade, cracking has become a much more serious issue. In general, newer hybrid citrus varieties, especially those developed in Europe, tend to be more prone to cracking than older ones.
The main reason fruit cracking occurs is a mismatch in growth between the peel and the pulp inside the fruit. As the fruit develops, the pulp expands rapidly while the peel becomes relatively thinner. In many varieties, the peel can still stretch enough to accommodate this growth, but in more sensitive varieties it cannot, and the fruit splits. Larger fruits are more likely to crack, and fruits that are flatter rather than round are also more vulnerable.
The Role of Water, Nutrition and Climate
Water supply has a strong influence on cracking. Increased irrigation promotes faster pulp expansion, raising the likelihood of cracking, whereas reduced irrigation often lowers the risk.
Temperature and humidity are also believed to contribute, although their exact role is not yet fully understood. It is highly likely that climate change is intensifying cracking severity in many production regions.
Nutrient levels in the peel is another important factor. Low levels of potassium and calcium are frequently associated with higher cracking rates. Plant hormones also affect the process: auxin applications after fruit set can reduce cracking, while gibberellins may increase it because they encourage flatter fruit shapes.
Although some treatments can help reduce cracking, such as combining auxin with potassium, they are only partly effective. The severity of cracking varies greatly from year to year and between locations, making it a difficult problem to control completely.
Supporting Growers Through Better Prediction and Management
Since fruit cracking is influenced by multiple factors and can vary greatly between seasons and locations, predicting when and where it may occur remains a major challenge for growers. This is where the Integrated CrackSense Solution comes in.
The project is developing smart farming tools designed to help farmers better manage orchards, reduce yield losses and make more informed decisions throughout the growing season. As part of this work, a Spatial Decision Support System (SDSS) has been implemented through a web-based platform that enables farmers, advisers and policymakers to input field data and use predictive models to estimate the risk of fruit cracking under different conditions.
By combining data, monitoring and prediction tools, the system aims to support more efficient resource management and help growers respond earlier to conditions that may increase cracking risk.
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