They are characterized by a relatively stable multiannual yield, and usually possess efficient mechanism to control excess fruit production. A second strategy is also used by trees that bear a heavy fruit load in one year, which inhibits return bloom and vegetative growth the next year . Thus, the second year is characterized by low yield and high vegetative growth. Such trees, including olive, pistachio, mandarins, and many others, are defined as alternate or biannual bearers and they are usually characterized by low self-thinning ability . Therefore, chemical or manual fruit thinning are common practices in their cultivation . In citrus culture, low temperatures during the autumn and winter are a major factor in inducing flowering . Optimal flowering density is achieved only upon accretion of sufficient cool hours. It is assumed that a heavy fruit load prevents recognition of the low-temperature flowering inductive signal and/or blocks later stages of inflorescence, such as bud break . As expected, fruit load affects the expression of flowering control genes, FT, LFY, AP1, TFL, and miR156– regulated SQUAMOSA PROMOTER BINDING in leaves and buds of citrus as well as in mango and apple . The mechanism by which heavy crop load affects return bloom is not fully understood. The developing fruit provides a strong sink for photo assimilates.
It was therefore thought that depletion of photo assimilates, especially carbohydrates from the bud, drainage planter pot prevents flowering induction, a hypothesis known as the nutritional theory . Sucrose was shown to play a regulatory role in Arabidopsis flowering control , but whether sugars indeed play a regulatory role in flowering induction under various fruit loads in fruit trees has been a controversial issue for many years . Recent work has shown that trehalose metabolism and its product trehalose-6-phosphate were involved in flowering control in Arabidopsis . It was also shown that two genes encoding enzymes associated with trehalose metabolism were induced in OFF-Crop buds . In addition to the nutritional control of alternate bearing , it might well be that the fruit itself, or an organ which senses fruit presence, generates an inhibitory signal which moves into the bud and prevents flowering induction . Fruit thinning or complete removal from ON-Crop trees induces return bloom , thus providing support for this notion. Gibberellin is known to inhibit flowering in many perennials . However, while exogenous application of GA prevents flowering , the question of whether GA acts endogenously to inhibit flowering is still open. The involvement of abscisic acid in the regulation of return bloom is even less clear . Polar auxin transport from a dominant sink was also suggested as a possible mobile signal affecting flowering . Fruit load might act at various developmental stages such as flowering induction, transition of the shoot apical meristem, and subsequent stages of flower development and bud break . Regardless of the source of the AB signal and its nature, it must be recognized by its receptor in the bud which in turn must make the ‘decision’ of whether to proceed to inflorescence or not.
In order to investigate metabolic and regulatory processes taking place in the bud and affected by fruit load, the transcriptome of buds from ON- and OFF-Crop trees was recently compared during three developmental stages. Changes in metabolic and regulatory pathways, including photosynthesis, and in flavonoid and trehalose metabolism were identified . However, this work was biased due to the use of an Affymetrix Citrus Gene-Chip array that contained ~15 500 genes. In fact, with the exception of trehalose metabolism, no other regulatory pathways were identified. In the current work, a complementary approach was taken by comparing the transcriptome of buds of de-fruited trees with those of ON-Crop trees. The genomic analysis was non-biased, as it was based on RNA-deep sequencing. It was possible to identify an increase in ABA-metabolizing genes, accompanied by a decrease in ABA levels and those of its catabolites in buds of de-fruited trees. Moreover, a remarkable increase in the expression of genes encoding proteins associated with calcium-dependent auxin polar transport and a reduction in bud endogenous auxin levels following de-fruiting were identified. The results are discussed in light of the previously suggested auxin transport autoinhibition theory and its role in AB .Fruit removal has been reported to be effective in inducing return bloom . However, since annual variation and cultivar-dependent divergence may affect its effectiveness , fruit removal was carried out as early as the last 10 d of August. Indeed, the number of inflorescences and vegetative shoots, counted during the following spring , showed that those of DEF trees were similar to those of OFF-Crop trees, thus demonstrating the effectiveness of the treatment. Although it could be expected that the number of generative inflorescences would be higher in DEF than OFF-Crop trees , the actual number was identical, probably due to the low number of fruits on OFF-Crop trees .
De-fruiting resulted in relatively rapid changes in the expression of genes controlling and genes associated with trehalose and flavonoid biosynthesis, allowing determination of the time frame of the genomic analysis. As a rule, the expression of flowering control genes is induced in leaves, buds, and stems in association with the onset of the flowering induction period in regular bearer cultivars, and in AB cultivars during the OFF-Crop year, while GA treatment reduced their expression . CiSPL5 is an exception to this rule, probably due to its highly regulatory role, and it exhibited higher expression in buds from May until September . CiFT2 expression was usually earlier than the onset of the flowering induction period . Taking into account these expression patterns and the expected year to year alternation, the mRNA levels of CiFT2 and CiSPL5 were higher in OFF- than in ON-Crop buds. Therefore, the increase in the expression of CiFT2 and CiSPL5 in buds of DEF trees to levels similar to those in buds of OFF-Crop trees can be expected, and reinforces their role in return bloom. It is not surprising that during the time of the experiment, LFY did not show any difference in its expression between buds of ON- and OFF-Crop trees. Nevertheless, de-fruiting resulted in a 2-fold increase in its mRNA level, which returned to its basal level 4 weeks after treatment. Whether this temporary response has any relationship to return bloom requires further research. The differences between bud populations—those with a 55% chance to flower and those with a 96% chance —did not seem to be very high. However, genomic analysis resulted in numerous differentially expressed genes , allowing the partial identification of mechanisms that convert ON into OFF buds. Previously it was shown that the number of DEGs between ON- and OFF-Crop buds was considerably lower in September than in May . However, the present work showed that the number of these DEGs was quite high. This difference could be explained by year to year alternations, and differences in the methodologies used. Based on a cut-off of 50% coverage between sequences on the microarray and the currently identified sequences, and on at least 75% identity, it is estimated that only ~30–35% of the current sequences are present on the microarray, plant pot with drainage supporting this notion. Only 40% of the auxin transport-related genes were found on the microarray. Below, three identified mechanisms, common to OFF and DEF buds, which are altered during the conversion of DEF buds into OFF buds, and might play a role in the signalling mechanism of fruit load are discussed.In agreement with Shalom et al. , this study demonstrates that de-fruiting induces expression of photosynthetic genes in the bud. Although a recent proteomic analysis did not show an increase in photosynthetic proteins in OFFCrop trees , here the induced gene expression resulted in increased protein levels of four major genes. According to the C/N theory, the proteins of photosynthetic machinery represent the majority of leaf nitrogen which is directly related to photosynthetic capacity ; thus, the induced levels of photosynthesis proteins would suggest the induction of photosynthesis in OFF buds, although direct evidence is missing. Although bud photosynthesis was never measured in fruit trees, leaf photosynthesis in relation to fruit load has been measured in previous studies. While some workers found no change in photosynthesis between leaves of ON- and OFF-Crop trees , others reported increased photosynthetic and CO2 assimilation rates in fruit-bearing as compared with non-fruit-bearing trees . Vegetative growth is induced in buds of OFF-Crop and DEF trees , suggesting that increased photosynthesis may mark the initiation of vegetative growth. That is, due to fruit absence, the OFF-Crop and DEF trees are heavily loaded with photo assimilates, suggesting that by induction of its photosynthetic machinery, the bud signals to stop translocation of photo assimilates. The possibility that the flow of photo assimilates into the bud is reduced due to lower leaf photosynthesis in OFF-Crop trees, resulting in increased synthesis of photosynthesis proteins and higher CO2 assimilation, cannot be excluded.
Increased expression of three NCED-like genes, in buds of DEF and OFF-Crop trees compared with buds of ON-Crop trees, suggests the induction of ABA biosynthesis. However, direct measurements of ABA and its catabolites showed the opposite trend, namely reduced levels in buds of OFF-Crop and DEF trees. Direct biochemical evidence demonstrated that NCED3 cleaved 9-cis-violaxanthin to form xanthoxin, a precursor of ABA ; its expression paralleled ABA levels in the peel and during cycles of drought and re-watering of leaves and fruit . Therefore, one would expect higher ABA levels in OFF-Crop and DEF buds than in ON-Crop buds. A possible explanation of these apparently contradictory results is that the source of ABA in the ON-Crop bud is not within the bud itself, but external to it, and dependent on the presence of fruit. In OFF-Crop trees or following de-fruiting, the translocation of ABA from this source into the bud is blocked, at least partially, reducing the bud’sABA contents, and inducing NCDE3 expression in order to increase endogenous ABA production. Nevertheless, the possibility cannot be excluded that induced expression of NCED genes is futile, and has no physiological role. A closer look at ABA-responsive genes in the transcriptomic data did not solve this contradiction, as no common trend in their response was evident . Regardless, ABA levels in OFF-Crop buds and following de-fruiting were reduced, raising the question of its possible involvement in AB control. Consistent with the present results, buds of ON-Crop trees have been shown to contain higher levels of ABA or its isomer, t-ABA, than those of OFF-Crop trees . It has been suggested that elevated levels of ABA in ON organs may reflect a stress imposed by the fruit overload. Moreover, ABA might serve as an inhibitor of return bloom, since the local application of ABA to Citrus unshiu buds in late December inhibited bud sprouting and intensive flowering . Alternatively, the possibility that flowering promotes ABA activity has been suggested, since increased ABA levels were detected in leaves of OFF-Crop trees and following de-fruiting of ON-Crop trees in association with flowering induction . Whether ABA plays a role in AB control, or in other processes, such as maintaining the bud in an inactive state , requires further investigation.Refrigeration is the most effective tool to prevent post harvest losses of fruits and vegetables, however, its utilization is limited in cold-sensitive commodities, which typically originate from tropical and subtropical regions. Te term ‘post harvest chilling injury’ or ‘PCI’ is used to describe the group of symptoms and physiological alterations that compromise quality and promote spoilage when sensitive commodities are stored at temperatures between 0 and 15 °C . PCI contributes to post harvest horticultural crop loss, which is unsustainable given the need to produce more food for a burgeoning global population using fewer natural resources. Chilling injury has been studied in numerous species for more than 200 years, yet our understanding of the progress of its early stages and underlying causes at the molecular level is still incomplete, hindering the development of long-term solutions to this problem. Tomato is the second most important vegetable crop, ranking number one in terms of gross production value in the world. It is a key source of antioxidants for humans and is also a model organism for the study of freshy-fruited species.