Alternate bearing is also perhaps one of the oldest known plant behaviors, as written descriptions of it can be found going back over several millennia. Most often these references are simple observations of trees that flower in one year but not the next , or as one early account of cider production in England elegantly described it, the trees would “. . . provide a full and complete blessing every second year” . Other written accounts offer advice on how to get sterile trees back into production, though it is not always clear if these early authors were aware of the full two-year cycle. Nevertheless, the prescribed regimes of girdling, branch twisting, and pruning are well-matched to its existence nonetheless: by removing a lot of branches or inflicting specific types of damage, this advice would stimulate the trees to produce a dense crop of new branches, which go on to bear fruit in the following year. Other accounts instead try to explain the biennial pattern by speculating that the trees require a period of “rest” before or after a large crop. Alternatively, vertical tower for strawberries the cycle has been suggested to be the result of rainfall, variation in cross-pollination efficiency, and/or total the flower numbers.
Not everyone was inclined to such speculation though, as Japanese farmers reportedly accepted the cycle as nature’s way, without trying to intervene at all.Despite the long history of this tree behavior, there have been relatively few attempts to learn how or why the plants produced the cycle. Even when descriptions of such experiments have survived, they tend to be hearsay accounts published by 3rd parties, making it difficult to determine what exactly transpired. For example, one horticultural manual from the 1800’s recognized that an unequal growth trade-off between the fruits and branches was a significant symptom of alternate bearing trees, and recommended that farmers artificially restore that balance by cutting off excess vegetative growth. In another example twenty five years later, a reporter for a popular horticultural magazine visited a nearby orchard and held a walking interview with the owner. In the process, we learn that the proprietor had performed an experiment some years earlier, in which he had removed all the flowers from a single tree one season, and found an abundance of fruit in the next. In contrast, academic interest in alternate bearing was essentially non-existent until the late 1800’s, when the subject apparently received a major boost from the establishment of state sponsored agricultural experiment stations. This was apparently a global trend, as England adopted a private research station that began as early as 1843, Japan obtained one by official decree in 1871, and United States began building its research stations following the Hatch Act in 1887.
By the turn of the century, these and other experiment stations had begun to produce a large number of agricultural publications, quickly establishing the foundation of crop science as we know it today. Research in the alternate bearing field lagged behind somewhat, perhaps by the need to establish experimental orchards, but began to produce a burst of new research starting around 1900. Thus began a roughly 30 year period of clever experimental designs and careful scholarship, ultimately producing the tools and basic concepts of alternate bearing research. Beginning with the development of chemical sprays in the 1940’s, the focus of this research began to shift away from basic inquiries and instead frequently focused on practical farming issues. Over the next few decades, this work made slow but steady progress towards understanding the physiological mechanism that propagates the cycle, while also identifying a broad array of species with similar physiological symptoms. The past few decades were also an extremely productive time for alternate bearing research, as this student estimates that the field has accumulated more than 1200 papers that mention the cycle by name, as well as another 3- 4000 papers in closely related subjects, with less obvious key words. In contrast, work to incorporate modern genetic tools is a much more recent phenomenon, beginning with just a handful of papers in the early 2000’s , then escalating rapidly by the end of the decade.
Generally such genetic work has attempted to observe when and where key developmental genes respond to the cycle, because it is not yet possible to make an entirely predictive model of alternate bearing, based on known developmental pathways from other species. Such knowledge is however, is expected to help farmers precisely control the behavior in popular tree crops, either through selective breeding , transgenic modification, or predictive mathematical models. A genetic understanding may even provide a way to clarify current ideas about the ecology and evolution of the cycle, while also identifying key developmental differences that separate annual and perennial species. In many cases alternate bearing is described largely in terms of fruit production, but close observations have shown that the actual phenotype is much more complicated. Similar two-year cycles have also been found in leaf area, flower numbers, branch lengths, and trunk diameter, just to name a few . Several physiological parameters have also been found to change in phase with the cycle, including mineral nutrients and carbohydrates, which tend to accumulate in the fastest growing organ, and are depleted in slower growing organs. Plant hormones have also been correlated with specific parts of the cycle, and are important regulators of floral induction, dormancy, fruit ripening, and abscission. Collectively, these findings outline an inverse relationship between fruit and vegetative growth that is broadly applicable to all species: When fruits are abundant, the vegetative growth is reduced. Conversely, when fruit are rare, the vegetative growth is abundant. This pattern is commonly known as a growth trade-off. Such trade-offs have been known to plant specialists for almost two centuries, and there are numerous documented examples where the fruits are negatively correlated with growth elsewhere in the plant, container vertical farming and occasionally with tissue death, especially of the meristem region. The existence of a growth trade-off however, does not immediately explain how this contributes to a biennial cycle. Most temperate trees shed their fruits in the fall, and there is usually nothing left to affect growth at the start of the second season. With the exception of more tropical trees like Avocados, this suggests the plants actually retain a “memory” of the fruit load from the previous season, a hypothesis that immediately raises questions about how that information is stored and transmitted over time. While the idea that plants may retain that memory in terms of their gene expression patterns has never been tested, the available evidence instead suggests that this information is recorded in the plant’s physical anatomy. For example, most trees produce flowers from their axillary buds a few months to a year after the buds are first produced. The number of such buds is proportional to the vegetative growth in the previous season, and since vegetative growth is often coeval with immature fruit, it is easy to see how a growth trade off might be recorded directly by the number of axillary buds. In addition, immature flowers and inflorescences typically begin growing while still inside the buds many months before they burst open, so the growth trade-off has the potential to affect the size of the inflorescence at a very early stage in their development.
Any initial differences in size incurred in this way are then likely magnified in the second season, when both inflorescence and vegetative growth resumes at an accelerated rate. Together the combination of inflorescence size and number can substantially change the number of flowers in subsequent season, and thus have an indirect control over number of subsequent fruits. In fact, when branch lengths and flower numbers are mapped to a generic branch structure, it is possible to derive a reasonably descriptive model of alternate bearing anatomy, summarized in Figure 1.0. In the “ON” year, the presence of a large number of fruit is thought to cause the reduction in vegetative branch length and leaf numbers, providing fewer axillary buds for the return bloom in the next season. In the following “OFF” year, the axillary buds produce fewer flowers and fruit, allowing the vegetative longer branches to grow longer and bear more leaves. The cycle then repeats itself when the axillary buds produce a large number of flowers in the 3rd season, creating yet another “ON” year. Once started, this mechanism is thought to be sufficient to propagate the biennial cycle indefinitely, barring intervention by the environment or anthropological factors. The variation in branch length is also permanently recorded in the plant anatomy, and with a few caveats, several years worth of cycles can be seen simply by noting the distance between successive sets of bud scars, or the remnants of old cones or inflorescences. The simplicity of this model is perhaps its greatest selling point, as it can readily incorporate information from several other areas of research. Following the discovery of plant hormones for example, it was soon found that gibberellic acid was an important regulator of flower numbers in many species, either as an activator or a repressor, and has its strongest effect during the period of floral induction. The response to auxin apparently changes over the course of the season, as mid-season auxin treatments can increase fruit abscission rates, while applications to immature inflorescences and mature fruit tend to prevent abscission. Ethylene seems to have a dual function, enhancing the number of flowers in some cases, while stimulating abscission of young fruits in others. The role of abscisic acid is not quite as clear, but the concentration of this hormone is known to fluctuate with the cycle in citrus trees, yet it was not correlated with abscission of immature pistachios nuts. In addition to the hormones, the distribution of starches, soluble carbohydrates and even inorganic mineral content can be mapped to anatomically different phases of the cycle. The result is a comprehensive physiological model of alternate bearing, which can be used to make reliably accurate predictions for both practical and theoretical applications. The basic physiological model is also surprisingly compatible with more recent concepts of plant development. For example, most vegetative branches do not grow continuously, but are instead produce in regular bursts commonly known as a “flush”. Also variously known as a “growth unit” “iterative growth” or even polycyclic growth, a flush typically occurs once per year in temperate climates, but tropical trees can produce as many as 3-4 flushes per year. Each flush is composed of at least two discrete phases of growth, known as the juvenile and adult phases. Confusingly, these two terms are also used to describe growth patterns related to the age of the plant in years, so for the sake of clarity it is necessary to borrow a slightly different terminology and refer to the variation along a single branch as “seasonal heterophylly”. The first leaves of the flush are distinguished from the later leaves by subtle-to-significant differences in trichome density, color, size, and leaf morphology, and typically, the first leaves are also abscised while still immature. Typically a flush is thought to begin and end with SAM dormancy, and as a result, alternate bearing anatomy can easily be outlined as the sum of two consecutive flushes, attached end to end.The flowers in contrast, are born on a completely different type of flush, usually called an inflorescence. These are typically produced from dormant axillary buds, though some plants also use the SAM as well. All such buds have the option of producing vegetative or reproductive structures, and the deciding factor is determined by a combination of information derived from the external and internal environments. Once the correct combination for each species is recognized though, the buds are said to be “induced”, irrevocably committing them to a reproductive fate. The resulting Inflorescence Meristem then produces the branches, bracts and flowers of the mature inflorescence. While the flowers are an obvious indication of reproductive growth, the inflorescence branches may also be distinguished from vegetative branches by their unique patterns in color, diameter, and trichomes. In addition, the inflorescence is also a determinate structure that senesces at maturity. The resulting abscission zone or boundary with dead tissue then provides a clear indication of which tissues were reproductive and which were vegetative .