Sunscreens: how, when and where to use them

Sunscreens: how, when and where to use them

This tool, which prevents summer stress in key moments for the formation of flower buds and other important processes in the plant is being increasingly used in cherry orchards in the post-harvest stage: how does sunscreen work and what factors should be considered when utilizing it? We talked about it with the Specialist Advisor in cherry production and Founder of Avium, Carlos Tapia.

When several producers have already finished harvesting and the bulk of production is at its peak, it is extremely important to think about post-harvest and how to recover the orchards to ideal condition after an arduous season of work.

In this context, the use of sunscreens in cherries is quite relevant and has been a well-studied matter in Chile and other places around the world. These analyses have usually shown pretty consistent signs concerning their operation. 

To begin, it is necessary to know what sunscreens do: they prevent – speaking from a physiological scope – premature closure of stomata, which are pores that are invisible to the naked eye and are on the underside of leaves. They allow, among other functions, gaseous exchange between the plant and the atmosphere, allowing the plant to cool and also to generate sugars. In other words, these stomata are in charge of releasing water through the plant and, in so doing, capturing CO2; the same CO2 that is a raw material for sugars, which are mostly stored as reserves during this period, through the photosynthesis process. Therefore, it is a very important process for the generation of said reserves.

But aside from knowing how they work, it is always important to consider which factors should be taken into account when making the decision to apply sunscreen to a cherry orchard. In general, this type of tool for the prevention of thermal-hydrological stress is used on weak rootstocks, less vigorous rootstocks or some less vigorous orchards in general (not framing “vigor” as a value that only applies to weak rootstocks, as we can recognize in some areas orchards that, even when using vigorous rootstocks, present weak responses.) It is important to remember that vigor is measured through the recovery of annual growth, meaning if we recognize an annual growth above 50-70 centimeters, we can say that we are referring to an orchard with good vigor, or a balanced orchard. It is also important not to confuse large orchards with vigorous orchards – there can exist a giant, bulky orchard that also has no vigor.

“We shouldn’t just frame this as a strategy we use to prevent thermal and hydrological stress only in low-vigor rootstocks. In this framework, we need to look at which rootstocks could respond to this need: obviously the Gisela series, among them Gisela 5, Gisela 6, Gisela 12; even when Gisela 12 is the most vigorous in the series, in many areas its use could be justified. Let’s not forget Gisela’s close relatives, which are the sour cherry versions. So, even though we have and can recognize different vigor statuses for sour cherry trees, we must recognize that there is a large number of sour cherry orchards that fall into the less-vigorous category, rather than the more-vigorous one,” Carlos Tapia, Founder of Avium, explained.

The Advisor specializing in cherry production also added that: “This goes for the series MaxMa, among them MaxMa 14, MaxMa 60 and their direct “relatives”; some Mahaleb, Santa Lucia and Pontaleb. On one hand, vigor is determinant in making this decision, and on the other hand we also believe that the decision should be made considering seasons like the current one with high-heat zones, or zones where there is an important hydrological deficit; where the product could be helpful in offsetting this deficit.”

It is decidedly important to stress, when considering these products, that the most important factor in preventing problems associated with abiotic stress is the efficient use and programming of irrigation. THERE IS NO OTHER more foolproof method to avoid problems associated with hydrological and/or thermal stress in plants. All of the sunscreen or sun filter treatments out there are conceptualized as a companion to these basic forms of management in crop development.

What do records for the last few years tell us about sunscreen applications in Chile? The most commonly seen experience is the use of kaolinites at 95 percent – there are four or five commercial names for it in our country. 

“The use of these products is somewhere between 2.5 and 3 percent. Even though some indicate lower concentration on their labels, we believe that their use could also be effective and we have proven in recent years that orchards applied with this type of product keep their leaves at a lower temperature throughout the day, in the different moments in which we’ve performed measurements. Additionally, stomatal flow measurements show positive differences after midday, translated into better conductance in stomata where sunscreen treatment has been done.

We have also gotten to know colorless sunscreens, considering that they can be interesting tools for applying in the pre-harvest stage. In our preliminary research we can effectively recognize that some of them have shown interesting results in preventing direct harm to the fruit, such as pedicle dehydration, sun shock in fruits and even improving quality indicators in post-harvest. Even though formulation is very different from conventional sunscreens, these are also a valid alternative for the post-harvest period“.

The Department of Research and Development at Avium has performed a series of trials regarding the use of sunscreens in cherry trees; about five or six seasons ago, they incorporated into their kaolinite applications certain additives or ingredients that allowed for the improvement or support of kaolinite use, through the biochemical method known as thermal-hydrological stress control. They performed tests with algae extracts (from the type Ascophyllum nodosum) and even with some amino acids. 

“Results have been super consistent. Orchards with kaolinite alone show a decrease in leaf temperature in regard to the witness. And when these kaolinites are coupled with some additive that allows for a biochemical improvement of this stress prevention, that drops temperatures even further and leaf temperature is maintained at a fresher point, because the concept doesn’t revolve around decreasing temperature, but rather keeping it from increasing, thus preventing premature stomatal closure. We have understood, in time, that not only Ascophyllum nodosum could get that job done; it could also be performed by products that had folcisteine in their formulas, and also some amino acids could be used, specifically those that had in their formula proline, which is also directly linked to thermal-hydrological stress prevention,” said Carlos Tapia, Founder and Technical Director at Avium.

On the subject of when to start applying sunscreens in orchards, the ideal moment is immediately after the harvest and up to, maybe, a week after that.

“The ideal scenario is for us to repeat this strategy or these applications every 25 to 30 days. In other words, we can perform two applications in the springtime or at the end of spring, and in that case we have detected and studied that there is a more optimal leaf temperature regarding the witness. And consistently there is a better operation of carbonated reserves in the shoots when measured in the winter. This is for orchards and the treatments that we have performed on them by applying some type of sunscreen, when measuring reserves in shoots. Which reserves? Total nitrogen, total protein, amino acids; some amino acids and starch. When we have measured these reserves in the wintertime, we have consistently found there to be an increase in starches in the shoots, unlike other reserve ingredients”, Tapia warned.

These temperature control treatments in bodies, performed with sunscreens, allow for the stomata to be open for longer periods of time, thus having the possibility of performing greater gaseous exchange, catching more CO2 and transforming it into sugar. Thusly, when we measure the sugar that best represents reserves within the plant, which is starch, there is a greater presence of it. 

“We do believe it to be an important, interesting strategy which we will continue to study. We have more research in progress, because there is also a current trend of observing what the possibilities are of incorporating these same pre-harvest strategies, but with some type of sunscreens that are colorless, to be applied pre-harvest in orchards of less vigor that might come out later in the season. That way we can prevent the effect of thermal-hydrological stress before the fruit is harvested,” Carlos Tapia pointed out

We must consider that the month of January lines up with the flower differentiation period in Chile’s Central Zone, therefore it is quite relevant that during that period the plants are in the best possible conditions to ensure that this physiological process, which consists of the formation of flower buds, takes place when the plant is hydrologically, thermally and metabolically balanced.

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