Recognition of this pattern of senescence thus suggests that many trees do not produce one large inflorescence, but instead produce many smaller ones.Another phenomenon that has frequently been correlated with alternate bearing is the abscission of immature fruit. Descriptively called a “fruit drop”, the abscission of immature fruit often appears to occur in three distinct cycles over a single season, typically numbered 1-3. Where this has been studied in tomatoes, the first drop appears to be the result of abscisic acid buildup in unpollinated ovaries, which is normally reversed by fertilization. The second drop occurs a few weeks later when apparently healthy immature fruits are shed, much to the consternation of the farmer who was expecting a bountiful crop up until this point. Internallythough, the abscised second drop fruits often have dead or dying ovules, which is potentially the result of inter-fruit competition. The exact cause for this behavior is not completely understood, but it appears to reflect the competition for nutrients between the individual fruits, hydroponic vertical garden carried out though a complex array of hormone pathways and involves different responses by different tissues in the fruit anatomy.
Finally, the 3rd drop occurs at the end of the season when the fruits become mature, and is often associated with ethylene induced ripening. Of the three drops, the 2nd seems to have the most significant influence on the alternate bearing cycle. Current evidence suggests that this process is regulated by the seeds, as seeded fruits tend to be retained and grow larger, while the seedless fruits are smaller and are frequently abscised. The abscission caused by Pistachio fruits appears to be even more severe, as this species is known to cause entire immature inflorescences to abscise, even before the fruits are present. Such intra-fruit competition is known as correlative inhibition, which at least in some species, has been linked to auxin-based apical dominance mechanism. Although there has been relatively little research to investigate why the 2nd drop occurs at all, it has been suggested to be a self-pruning mechanism, allowing the plant to control the final number of progeny.In most plants, the distribution of carbohydrate and other nutrients is commonly thought to rely on the bulk flow of phloem sap between source and sink tissues. Within this framework, the competition hypothesis predicts that the ratio of sink strengths directly determines the ratio of nutrients that are provided to each organ, regardless of the overall supply. Thus when the fruits are present, the vegetative portion of the plant may receive so few nutrients that it suffers from nutrient starvation, forcing the branch to slow down or stop growing all together.
Interestingly such effects have actually been observed in living plants, as the excessive consumption of nutrients by the fruit; or even large branches, can lead to visible signs of nutrient deficiency, stress, and even the death of large portions of the plant. This hypothesis might also explain why fertilizer treatments have been repeatedly found to partially alleviate alternate bearing symptoms, and why the removal of excess fruit tends to increase the size of the remaining fruits. The result of girdling a branch can also be explained as a temporarily increase the relative amount of carbohydrates in isolated branches , as this practice blocks sugar export to other parts of the tree. Alternatively, it is also possible to exacerbate the competition by removing leaves, which are usually the primary source of carbohydrates. In fact, organ removal is commonly used experimental tool, both for leaves, and fruits. Identification of limiting nutrients can be difficult though, because the limiting one appears to change with the seasons, and there are variations caused by differences between species, soils, cultivation practices, and even the vascular anatomy of the plant in question. In most cases, carbohydrates and nitrogen are by far the most important nutrients relevant to alternate bearing, which together are often referred to as the C/N ratio. In rare cases though, the cycle can be affected by micro-nutrient deficiencies.This hypothesis is based on the idea that the plant produces a signaling compound that is able to block growth elsewhere in the plant, even in the face of adequate nutrition.
Exactly how this works on a biochemical level is not well understood, in part because this idea actually seems to encompass more than one phenomenon. Many early accounts use the term “inhibited” to describe situations where the plant grew less than expected, suggesting that the definition has changed over time. With the benefit of hindsight, it is possible to recognize that some reports actually describe plant behaviors that would later be known as dormancy and apical dominance. More recently, there have also been many reports showing that plants sprayed with the gibberellic acid can prevent flower formation, implying the endogenous levels of this hormone could play a similar role in the alternate bearing cycle. Whatever the identity of the inhibitor might be, current evidence suggests that it has a local effect, with a range of a few centimeters. The source of the inhibitor has been speculated to reside in the leaves, though it has been suggested to be produced by the fruits themselves. Remarkably, attempts to extract such a substance from apple leaves identified a common glycoside known as phloridzin, which displays growth inhibitory effects in Avena coleoptile tests. The mechanism of inhibition was later revealed by the breakdown products of phloridzin, one of which is phloretic acid, a known auxin response inhibitor. Thus at least within the spur-shoot of apple trees, the inhibitory hypothesis has a plausible mechanism that resembles apical dominance, though phloridzin has received scant attention in other species. The role of plant hormones however, has received much more attention, and many of them have roles both promoting and inhibiting growth. Exogenous sprays of gibberellic acid or its inhibitor, paclobutrazol, are known to regulate flower numbers in many species . The seeds are also known to be significant sources of auxin, and gibberellic acid. Interestingly, radio-labeled tracer experiments have repeatedly shown that hormones and sugars can move out of the immature fruit and into the subtending branch. However the reported values suggest that less than 1% of the radioactivity leaves the fruit and enters the SAM or nearby leaves, making it unclear if this is movement is enough to have a physiological effect. So faras this student is aware, only a single report has attempted to identify radio-labeled substances after they were secreted, which found both sucrose and malic acid. The export of common metabolites from a major sink tissue is an unexpected finding, but it might be related to the effects of transpiration, as it has been shown that Pisum sativum fruits can temporarily supply water to the rest of the plant late at night. It would thus be of some interest to repeat this experiment in fruit-bearing trees, where the large size of apples, oranges, or avocados might exacerbate such an effect. Rather than being a pure chemical though, there is evidence to suggest the inhibitor might be a protein that affects the floral induction pathway. Many plants for example, have been found to produce a short-range signal in their leaves, which in Perilla is capable of inducing flower production in nearby axillary buds. Known as the “florigen”, this substance was eventually traced many years later to FLOWERING LOCUS T , a protein that is produced by the vasculature in the leaves and stems of A. thaliana. Once produced, FT has been shown to travel through the phloem to the SAM, where it helps trigger the process of floral induction. Induction in turn, is known to involve an array of other proteins, such as FLOWERING LOCUS D , SUPPRESSOR OF THE OVEREXPRESSION OF CONSTANS1 , SHORT VEGETATIVE PHASE , and AGAMOUS-LIKE24 , which help establish inflorescence meristem identity, vertical vegetable tower and later the expression of LEAFY, which initiates individual flowers. Thus rather than directly inhibiting flower production, it is possible that the inhibitory model might function by blocking the activity of one or more of these genes. Interestingly a known inhibitor of FT function, TERMINAL FLOWER 1 , is expressed in the seeds of orange trees.
If CuTFL1proteins are sufficiently non-cell autonomous, this also holds the potential to propagate the alternate bearing cycle by inhibiting floral induction in nearby buds. A similar paralog in apples,MdTFL1, is expressed in apical buds before the reproductive transition, and is known to extend the period of vegetative growth when expressed in A. thaliana. Another way in which the fruits are thought to suppress growth is by taking advantage of a mechanism that already exists in most plants: apical dominance. As commonly understood, growth of axillary buds along the sides of the stem is suppressed due to the rootward transport of auxins produced by the apical meristem. When the SAM is removed, the lateral buds sprout in basipetal sequence reflecting the depletion of the auxin flow. A similar suppression of growth also occurs in the inflorescence, as the first formed fruit suppress the growth later formed fruits in tomato trusses, which is also known as king fruit dominance. The term correlative inhibition has also been used to describe dominance between fruits in the same inflorescence. As a result, it is not hard to imagine how this auxin-based mechanism might be used to suppress the growth of the SAM and other vegetative structures, where it is known as fruit dominance. This idea has some support, as plants with parthenocarpic fruits have higher fruit set rates than seeded ones, and removal of the SAM improves seed set in peas. One prediction of the fruit dominance model suggests that the flow of auxin between the fruit and apical bud would be reversed, and this reversal has actually been observed following the injection of radio-labeled auxin into various plant organs. The mechanics of dominance however, are still poorly understood. Studies in both A. thaliana, and in the P. sativum models have identified the importance of the auxin transporter PIN1, and a potential root-derived chemical inhibitor of dominance. This root-derived substance was later identified as strigolactone , a molecule whose biosynthesis and perception is at least partially regulated by MORE AXILLARY MERISTEM genes , and also by several CARATENOID CLEAVAGE DIOXYGENASES genes.Interestingly, strigolactone molecules also bear a passing structural resemblance to phloridzen, suggesting that the latter may be a functional analog of this plant hormone. Much more recently, sugar distribution was found to be a significant part of establishing dominance, suggesting that all three hypothesis may be needed for a full explanation of growth trade-off.In alternate bearing theory, there are three competing hypothesis that attempt to explain how the fruit negatively influence vegetative growth. The competition hypothesis suggests that the demand between two sink tissues determines the flow of nutrients to each organ, the inhibitor hypothesis suggests that either the leaves or the fruits suppress flower development even when nutrient supplies are adequate, while the dominance hypothesis suggests that the fruits reverse the apical dominance mechanism, suppressing the apical meristem and subsequent vegetative growth. Although the competition hypothesis is favored by the growth trade-off observed in many alternate bearing species, there been few attempts to determine which of the mechanisms predominates in actual growing tissues. One way of doing this would be to observe the expression profile of actively growing meristems subjected to a heavy fruit load, as each of the hypothesis could be expected to produce a unique signature of up and down regulated genes. For example, a recent genetic study of alternate bearing apple trees was able to demonstrate several genes related to auxin and gibberellin hormone pathways were located in alternate bearing quantitative trait loci. The presence of auxin genes could be used to support the fruit dominance hypothesis, while gibberellin and floral induction genes might indicate the presence of an inhibitor pathway. An impressive set of microarray data from alternate bearing mandarin scions found that several glucan and trehalose sugar-related genes were activated during the ON year. The authors further argued that the FT paralogs CiFT1 and CiFT3 were involved in suppressing vegetative growth, but recommended more work to validate this idea. One aspect not captured by either study is the degree of fruit load, which varies continuously over the annual production cycle and may even produce a concentration gradient in the case of the inhibitor theory.