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Size matters. Especially in citrus

4-5 hot weather events during the season can have a devastating effect on your bottom line. Here is how you can avoid it

Every year, about 850 million tons of fruits are produced around the world. Tomatoes are leading the pack regarding amounts, followed closely by Bananas and Citrus. Since Bananas are not trees but in fact a giant herbaceous plant (a herb, in other words), we can say that Citrus is the main fruit tree crop in the world. Not only that: the demand for Citrus and its by-products is increasing, mainly due to an increase in awareness of its health benefits and especially to its high Vitamin C content. 

The trend within Citrus production is a shift to producing more Lemons and softer kinds of Citrus, and fewer Oranges and Grapefruits. So Oranges, which used to be 60% of all Citrus production, are now less than 40%. 

One of the reasons for lower orange production is the spread of the “Citrus greening disease” in main Orange growing areas. Florida, for example, was hit hard with this disease and is expected to have the lowest orange production in the last 75 years. Other factors are related to climatic events such as droughts and Hurricanes that also affect main Orange growing areas. 

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A yield of 60 tons per hectare, will give us
about 240 thousand fruits.
Losing 1 millimeter of fruit size potential
is equivalent to 0.75 tons of fruit,
or losing roughly 250$ of income per hectare

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The changing weather is changing Citrus 

Climate and climate change have a major effect on the world’s Citrus production. The year-to-year change in production levels of different Citrus types, and the changes in the level of production in the main producing countries, are now attributed only to “favorable” vs “unfavorable” weather, and not to the Expansion of new plantations or market forces.  

Weather changes also have a major effect on the farm level, as instability in growing conditions puts pressure on farmers. That leads to them aiming to achieve higher prices for their produced fruits to compensate for “bad years” that are becoming more and more frequent and keep their business profitable.  

The price farmers receive for their Citrus is determined by quality factors that aim to answer consumer preferences. The main attributes that the consumers are interested in are flavor, appearance, and size. While flavor and appearance are mainly affected by the variety, fruit size is mainly dependent on the farmer’s field practice. 

Fruit size is affected by many parameters, so farmers use different methods to achieve the desired marketing size that yields the highest prices. 

One factor that affects fruit size is the load of fruits on the tree, as a high yield leads to competition for resources and may lead to smaller fruits. As a result, farmers use many techniques to limit fruit load and increase the size of the remaining fruits. These techniques include Flower suppression, excessive pruning, and chemical or manual thinning after fruit drop. 

those techniques have their downsides: chemical flower suppression is costly and can cause too many flower drops, later fruit thinning is also costly and wastes the resources the tree invested in those fruits, and chemical thinning can also cause too many fruit drops. 

Another technique to achieve higher fruit size is postponing the harvest until fruits reach the desired size, but late harvest timing can significantly impact the flowering and fruit set in the next season. 

 

The stages in fruit development 

Irrigation also has a major effect on Citrus fruit size. Depending on the growing region, farmers know they must irrigate regularly during the growing season to achieve better fruit size. But how exactly is irrigation affecting fruit size daily, what is the connection to the day-to-day weather conditions, and what are the short-term and long-term effects of short heatwaves? All those are far less understood. 

Citrus fruit development has three major stages, with different effects on final yield quality and fruit size, and different sensitivities to water stress and heatwaves. 

The first stage after the fruit set is a phase of rapid cell expansion. During this stage, the final number of cells in the fruit is determined. Here, the growth rate of the fruit is slow as most of the energy goes into the cell division process. During this stage The tree adjusts the number of fruits it can carry by aborting most of the fruits that were created after flowering, leaving only a small percentage.  

This stage is very sensitive to water stress as it increases fruit drops. Special care should be given to heat waves during this stage, as they pose a high risk of stress to the tree and lead to increased fruit drops. This often happens towards the end of this stage, when a large number of fruits are aborted as part of what is called the “June drop” in the northern hemisphere, or “November drop” in the southern. 

 

When fruits grow fast, then slow down 

The second stage in citrus fruit development is a stage of rapid cell expansion. During this stage, the water-soluble contents in the fruit cells increases, causing a constant flow of water into the fruit cells and a rapid increase in their size. According to our measurements in SupPlant, at this stage, some varieties can reach a growth rate of 0.5 mm a day, and a growth rate of 0.2-0.3 mm a day is very common. This stage has by far the largest effect on final fruit size. 

The third stage is the fruit maturation stage, in which the fruit starts to ripen, sugar content increases, acid content decreases, and color change takes place. During this stage growth rate is reduced, down to a few millimeters per month in late harvest varieties. 

So, we concluded that most of the citrus size accumulation is done during the second stage, and also most of the mistakes in irrigation limit the final fruit size, final yield – and final income. 

For one, we must understand that the fruit has a maximal growth rate potential, and that rate is only decreasing as the season progresses. This means that if we under irrigated our citrus for one week, and the fruit growth rate went down from 0.4 mm a day to 0.2, during that week we lost 1.4 mm of our fruit size and that cannot be undone: at best, the fruit will return to grow at a rate of 0.4 mm a day but it will never bring back what was lost. So, if our final fruit size could have been 68 mm, now it will be 66.6. there is no way to compensate and increase the growth rate to get that 1.4 back. 

this is the effect of small incorrect irrigation periods along the season, where it gets hot for a few days and the irrigation is not adjusted according to the change the plant needs, so fruit growth rate slows for a short while until conditions change. 

 

How heat waves affect fruit size 

But there are other events that have a more severe and long-lasting effect, and these are mainly short high-water stress events, usually caused by a “heat waves”- a combination of hot and dry days and nights, during these events the plant loses water much faster than usual, and without correct soil moisture management these events pose a real danger to our fruit size and income. 

During heatwave events the tree experiences high water stress, causing a surge of stress hormones from the roots to the tree’s tissues, that in turn puts into motion stress management mechanisms aimed at quickly adjusting to the new conditions. At the low-stress level, these mechanisms will only close the stomata to reduce water loss, but under extreme stress levels, like the levels reached during heat waves, these mechanisms can affect the cell’s ability to grow, putting a limitation on cells’ expansion ability from here until the end of the season. This is one of the tree’s natural “protection” mechanisms, but it has a big effect on production. 

This means that if we allow the Citrus tree to experience extreme stress, even for one weekend, we may be limiting the potential growth rate from this point until the end of the season. And if this happens during the rapid growth stage when fruits can grow 10-20 mm a month, we can end up losing 5-10 mm of our final fruit size, usually without even knowing it happened or why.  

 

Here we can see the effect of incorrect irrigation on fruit growth. Lack of irrigation combined with increasing temperatures from the 26th of December led to a reduction in fruit growth rate from a high of 0.5 millimeters a day, to 0.12. This created a potential fruit size drop that is equivalent to a yield loss of 900$ per hectare at harvest. We also see that after the event, and because of the stress,  the fruit will not return to the 0.5 millimeters per day growth rate, creating an even bigger loss of potential income
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How heat waves affect your income 

Losing fruit size affects income on two levels: 

The first is the price per kilogram. Citrus is marketed in different size categories that yield different prices – and generally, the high categories yield the best price per kilo. But therein is another effect that is less seen and understood that is very important. 

When we lose 1 mm of fruit size, the fruit is lighter: every millimeter is equivalent to a few grams. Take mandarin as an example: every 1 millimeter in size is equivalent to about 3 grams in weight. And if we are expecting a yield of 60 tons per hectare, we will have about 240 thousand fruits in one hectare. If we lost 1 millimeter of fruit size potential, that is equivalent to 0.75 tons of fruit, or roughly 250$ per hectare of income we lost from 1 millimeter of size lost along the way. 

Unfortunately, we have more than one heatwave event in a season: sometimes it’s four or five, and During other weeks, there are four or five weeks that are just hotter than usual, so farmers can easily lose 5-10 millimeters from their potential final fruit size, and This amounts to losing thousands of dollars per hectare. Farmers are unaware of this, and Many times, farmers will say “there was good flowering, but at the end the yield was low, we don’t know what exactly happened”. 

 

The difference between knowing and guessing is measuring 

It is possible to maintain constant fruit growth all along the season and maximize the income we get from every hectare every year. 

But Only when we measure the reaction of our trees to irrigation and weather conditions can we really know what our trees need instead of just guessing.  

In SupPlant we use an array of sensors, allowing us and the farmers to understand the full picture, and when you understand the full picture, you can give your trees what they need and in return get the most out of them. 

Our trunk growth sensors, called “dendrometers”, tell us if the tree is growing well or feeling stressed. Our dendrometer is measuring at a micron level every 30 minutes so we can see stressful events as they start to develop and react. For example, by using dendrometers we can prevent stress and reduce fruit drops during the first growth stage and increase the yield potential. 

Our fruit size sensors that measure the actual diameter of the fruit tell us what the growth rate of the fruit is every single day, and if this growth rate is following the expected curve. This enables us and our farmers to identify the conditions that limit their fruit growth on their farms and adjust accordingly. 

We use soil moisture sensors at various depths, to understand how much water is available to the plant at every stage according to the different plant requirements and weather conditions at every stage. 

On top of that, we add a layer of meteorological data, past data, current data, and future forecasts, allowing farmers to understand why things are happening. If we see that under a specific temperature and soil moisture our plant experienced stress last week, and the forecast tells us the same conditions will occur next week, we can change irrigation, avoid stress, and increase the final fruit size. 

But that doesn’t mean the farmer has to sit every day and analyze the data: we do it for him and give him the bottom line. We do that by using advanced algorithms that analyze the data every day and recommend what is the best irrigation practice for the next week We take into consideration plant data, fruit growth, soil moisture, and the coming weather. This way we foresee periods of high-water demand or extreme heat waves in advance so we can recommend how to schedule the irrigation in order to prepare for them and make it through them. With SupPlant, your trees can grow without stress and reach their maximal potential in yield fruit size and Quality. 

Nitzan Shatzkin, Chief Commercial Agronomist, SupPlant

 

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