The temperature on the ground has increased by approximately 0.7 °C in the 20th century, and the rate of increase has been higher during the last 50 years. These high temperatures, which are occurring more and more frequently in the central and central-southern areas of the country, would affect the accumulation of reserves for the following season in orchards and, in addition, would make it difficult for deciduous fruit trees to enter dormancy normally.
Inadequate cold accumulation during the winter dormancy stage causes a disorder in the differentiation of flower buds in summer, low budding level, lack of uniformity in budding, floral anomalies, flowering asynchrony of cross-pollinated cultivars and longer flowering periods, generating a decrease in production level (Figure 1).
In deciduous trees such as Prunus sp. (cherry, plum, nectarines, etc.), bud dormancy is a critical step in the phenological cycle, since a correct entry into dormancy and an optimal release of these will determine the yield of both flowering and fruiting.
Dormancy phases are divided into 3 phases, distinguishing between para-, endo- and eco-dormancy phases. Para-dormancy refers to the suppression of growth that other tree structures impose on particular organs (e.g. apical dominance) due to the production and/or action of inhibitory molecules. During endo-dormancy, growth is not possible even under suitable temperature conditions, as the buds have not received sufficient exposure to cold. Eco-dormancy refers to the subsequent period, in which tree shoots only need sufficient exposure to warm temperatures to resume growth.
Regarding how genetics relate to the dormancy process, studies have resulted in the discovery of a group of genes that regulate seasonal rest, namely the DORMANCY ASSOCIATED MADS-box (DAM) genes. Transcriptomic analyses have been able to link gene expression profiles with hormonal, metabolic and physiological changes during the progression of dormancy.
The hormone abscisic acid (ABA) plays a very important role in growth inhibition and dormancy entry, mainly because ABA has been proposed as an integral component in the DAM signaling pathway, and also ABA is antagonistic to the growth promoter gibberellic acid (GA), which shows a decrease during dormancy establishment. The ABA/GA ratio was found to be correlated with dormancy depth.
On the other hand, moderately low temperatures during autumn promote the establishment of dormancy and cold acclimation. Unlike photoperiod, temperature is sensed by the entire plant and affects multiple cellular components. Membrane fluidity and membrane-bound proteins act as temperature-sensing systems, as falling temperatures slow down the movements of proteins and lipids in membranes and make them stiffer. As a consequence, membrane stiffening leads to the activation of calcium channels in membranes, causing an influx of Ca2+ into cells.
The establishment of dormancy is accompanied by the cessation of photosynthesis and growth, leaf senescence, and tissue acclimation to cold. Sugars play a central role in each of these processes and are also directly involved in bud dormancy. Trees exhibit strong fluctuations between dormancy stages in the rates at which soluble sugars (i.e., glucose, fructose, among others) and starch are synthesized and degraded.
Significant increases in soluble sugars have been shown to be associated with the transition to and maintenance of dormant buds. Although changes in bud sugar concentrations during the fall have generally been attributed to cold acclimation (reducing free water content at the cellular level), some studies have suggested that changes in bud sugar metabolism may also be related to dormancy induction (see Figure 1).
Thus, during the growing season, the fruit tree produces sugars that are transported through the phloem from the source (mainly the leaves) to the sink structures (fruits, new growth units, storage organs). In late autumn and winter, carbohydrate synthesis progressively declines until leaf fall. During this period, nitrogen is remobilized from the leaves and transported to the storage organs. The main storage structures are the roots and wood, which reach a maximum level of reserves before leaf fall. In the buds, which show the maximum accumulation of starch after autumn, nonstructural carbohydrates act as reserve molecules and can support future growth.
Effect of carbohydrate deficiency on the performance of the following season:
Physiological floral abscission has been attributed to a shortage of carbohydrate status in deciduous fruit tree species. Flower buds erupt before vegetative shoots and therefore flower growth from budburst to early fruiting is dependent on the accumulation of reserves from the previous season.
This content has been linked to flower quality and initial ovarian growth and retention. Therefore, starch accumulated during ovary development is crucial for fruit set, maintaining an adequate glucose level that temporarily frees the ovary from competition with young leaves until they are photosynthetically active.
Strategy to support entry into dormancy and accumulation of reserves:
As previously mentioned, climate change is causing a physiological disorder in deciduous fruit trees, creating a problem for entering dormancy due to high temperatures in autumn, which hinder the accumulation of reserves, since the plants remain active using their nutrients for a longer period of time, and, in addition, the lignification of tissues is affected by the lack of the low temperature stimulus. In this sense, the use of foliar products helps to improve these processes that are being affected. For example, in the case of Molybdenum (Mo), which is used by selected enzymes to carry out important redox reactions such as the activity of nitrate reductase, xanthine dehydrogenase, aldehyde oxidase and sulfite oxidase. The loss of Mo-dependent enzymatic activity impacts on plant development, in particular, those processes that involve nitrogen metabolism and the synthesis of the phytohormones abscisic acid and indole-3-butyric acid.
The importance of ABA in stimulating dormancy was previously discussed, so molybdenum applications would benefit the production of this hormone, as well as Mo would allow for a better use and mobilization of nitrogen. On the other hand, Boron (B) applications would also bring benefits for an optimal accumulation of carbohydrates, since it has been shown that plants have the capacity to metabolize boric acid (BA)/borate (inorganic B speciation) and transform it into sugar borate esters (binding to carbohydrates to improve their transport). Sugar borates are crucial in plant development (for example, normal cell wall growth, membrane functions and lignin biosynthesis). Therefore, in addition to improving the transport of sugars for plant reserves, boron also allows lignification of tissues. A very important process in resistance to low temperatures, which has been affected by the lack of cold in the fall, which has caused frost damage to reproductive structures and also micro lesions that then allow the entry of pathogens into the fruit tree.
Products such as NITRATE BALANCE PLUS are an excellent tool to stimulate and improve the dormancy process in fruit trees, since in addition to containing Stoller technology, their formulation contains Boron and Molybdenum, essential for an efficient entry into dormancy. This product allows regulating vegetative growth and improving the accumulation of plant reserves prior to the winter break. It promotes the mobilization and accumulation of sugars and nutrients in reserve organs such as roots, buds and shoots, generating a physiological signal to the plant to begin its dormancy process. NITRATE BALANCER PLUS has been specially developed to naturally induce dormancy in deciduous fruit trees, without generating intoxication or stress that could decrease the quality of sprouting and the yield of the following season.
For more information about this and other Stoller products, visit their minisite at Smartcherry.cl or go to www.stoller.cl.
