Richard Bastías ([email protected]), Nicol Romero and Gustavo Soto O. Fruit Growing Laboratory, Faculty of Agronomy, Universidad de Concepción.

Karen Sagredo ([email protected]). Laboratory of Deciduous Fruit Trees, Faculty of Agronomic Sciences, Universidad de Chile.

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LEVELS OF CHERRY SPLITTING

Fruit splitting damage due to rain in periods close to harvest continues to be the main cause of economic losses for the cherry industry in Chile and worldwide, both due to decreased yields in orchards and a reduction in fruit packing percentage in processing plants. This type of damage produces fruit splitting at three levels, including fracture formation in the cuticle, epidermis and the mesocarp, in the case of more severe damage (Figure 1).

DEVELOPMENT OF FRUIT AND SPLITTING

Although split damage is directly related to rainfall incidence, there are numerous research papers to date, dating from the 1970s, which would demonstrate how complex is the physiological mechanism involved behind this damage. Phenomenon in which diverse environmental, varietal and management factors interact.

The most widespread argument is that damage is caused by the entry of rainwater through the fruit’s epidermis increasing its volume to such level that the epidermis is unable to withstand the internal pressure of the fruit. Water enters the fruit through the cuticle due to the difference in osmotic potential between rainwater covering the fruit surface (near zero osmotic potential) and the water content (juice) of the fruit (negative osmotic potential). This difference of potentials destroys the vacuoles, with the consequent collapse of the epidermal cells and the components of the cell wall, causing the split, which would partly explain why the fruit becomes more sensitive to damage as it ripens on the tree. On the basis of this argument, it has been demonstrated in Chile

that the induction of split damage in mid-season and highly susceptible cherry cultivars such as ‘Bing’ begins to be evident at approximately 50 days after full bloom (DAFB), while in less susceptible, late-harvest cultivars such as ‘Kordia’ this damage begins to be evident after 65 DAFB (Figure 2).

Split damage in cherries is also closely related to the growth stage of the fruit. Cherry fruit has a double sigmoid pattern. This type of growth is common to all stone fruit trees and is characterized by three well-defined phases called stage I, II and III. In stage I, there is an active division and growth of the pericarp and mesocarp cells. In stage II, there is a slowdown in the fruit growth because in this stage the endocarp (stone) is lignified and the embryo develops. Finally, in stage III, the fruit resumes an accelerated growth due to the active elongation of the mesocarp cells.

Studies carried out in Chile for the ‘Bing’ cherry variety have shown that these fruit stages can be defined based on the variation that exists in the absolute growth rate (AGR) of the fruit on the tree. Thus, for example, it has been found that the end of stage I and the beginning of stage II coincides with the first peak of AGR and occurs at approximately 20 DAFB (Figure 3). After that, the AGR decreases to a second peak at 50 DAFB, which coincides with the beginning of stage III (Figure 2). In this sense, stage III has been suggested to be the most sensitive to splitting, which is in line with the results in terms of split induction. In addition, after 50 DAFB, the first fruits with symptoms of damage appeared for the same cultivar studied, ‘Bing’ in this case (Figure 1).

CELL DEVELOPMENT AND “MICRO CRACKING”

It has been found that during stage III of fruit growth, the cells of cherry mesocarp reach a size 300% larger than the cells that are under active cell division during stage I of fruit growth (Figure 4). This increase in mesocarp cell expansion is contrasted with the decrease in deposition of structural components of the cuticle, which makes it less elastic. As a result, several studies show that during stage III of fruit growth, a series of micro-fractures develop at the cuticular level and can only be seen at microscopic level. This type of fracture is commonly known as “micro-cracking” and would seemingly be related to the incidence of splitting at harvest.

The cuticle is the outermost layer of the fruit, being the first barrier to the entry of water into it. Research supported by electron microscopy shows that these micro fractures or “micro-cracking” in their initial stage of development cross the cuticle layer, but unlike larger fractures they do not continue through the underlying cell layers of the epidermis or hypodermis (Figure 5). In technical terms, “micro-cracking” can be defined as microscopic imperfections that occur at cuticle level and that only under a rain event increase in size, thus having an important impact in the development of cherry splitting.

SPLITTING OF VASCULAR ORIGIN

While there is broad consensus that splitting damage in cherries is primarily caused by water entering through the cuticle, there is also a significant component of this damage that is caused by water entering through the vascular route and through the roots.

Numerous works have shown that there is a significant number of split fruits on cherry trees protected from the rain by plastic covers. In addition, it has been established that one of the most important factors in cherry splitting is the sudden increase in soil moisture during stage III of fruit development, especially if water deficiency has existed previously. It is therefore highly advisable to always maintain uniformity of irrigation in the orchards, avoiding abrupt changes in soil moisture content.

Recent work has shown that location and severity of damage varies depending on splitting origin. Thus, those splits that are located at the pedicellar and distal zones of the fruit are normally associated with the transport of water through the cuticle after rainwater deposition on fruit surface.

It was also determined that splits located in the lateral zone of the fruits and whose cracking severely compromises their mesocarp, are normally related to the transport of water by the vascular route and through the roots (Figure 6).

Varieties such as “Sweetheart”, handled in a similar way from the agronomic point of view, have shown a very different distribution of damage by type of split when compared to two localities with different soil and climate conditions. While in one condition lateral damage prevails, in the other one pedicellar type damage predominates, which would indicate the effect of environmental conditions and probably of soil moisture on the incidence of damage of vascular origin (Figure 7)

CUTICLE PROTECTORS

The use of lipidic protectors of natural origin, such as carnauba wax and vegetable oils, have been used in Chile and worldwide with different degrees of effectiveness for the control of cherry splits. Due to their lipidic nature these protectors help in some way to waterproof the cuticle, thus acting as a physical barrier to the transport of water from the fruit surface to its interior. The latest formulations act as true cuticle supplements (CS), thus also allowing to improve the stability of this membrane.

Recent tests carried out in Chile have shown that the application of CS-type protectors is effective in controlling splitting and that such effectiveness is enhanced when applied in combination with mineral salts such as calcium chloride (Ca Cl2). These CS-type protectors also work in programs that consider an early application in the freshly set fruit stage, followed by a complementary application in the state of straw-yellow fruits (Figure 8).

A relevant aspect to be considered before establishing an application program of any cuticle protector type has to do with the capacity of coverage and adherence of the product on the fruit surface. In this sense, a study carried out in Chile showed the need to use products with the right amount of adherent agent in their formulation to ensure a better distribution of the droplets at the time of application. This prevents loss of product due to excessive surface run-off and promotes uniform protection from the pedicellar cavity to the distal area of the fruit (Figure 9).

MODEL FOR SPLITTING PREDICTION

In the need to seek new tools to handle and manage splitting by cherry producers is that researchers are currently working on the fundamentals for developing a model to predict the incidence of splits in the orchards based on physiological and environmental information. In a first stage, the behavior of varieties in different coverage environments is being tested as study variables. The results to date are promising because some adjustments in damage kinetics have been identified based on these factors (Figure 10).

ACKNOWLEDGEMENTS

To “Center for Research and Innovation in Fruit Growing for the Southern Zone” Technological Program (16PTECFS-66647) and its project “Technological package for the sustainable production of export cherries in the central-southern zone”, both supported by CORFO.

The cuticle supplementation studies were made possible by funding from Cultiva LLC. For more information visit: www.centrofruticulturasur.cl