The declining trend began before significant numbers of senescent leaves were observed, and continued unperturbed even after most leaves had been lost In comparison, rosette leaf initiation rates revealed a nearly constant production rate at about 1.1 leaves per day, which is about the same as the initial rate of flower bud production. Meristem growth stopped between 26-28 days after induction, indicated by the simultaneous plateau in both immature green bud numbers and total flower numbers . In many cases the buds around the arrested meristem remained green for several days, during which time they can be induced to resume growth by removing the fruits or following the senescence of the old fruits. Flowers produced during the transition to this quiescent phase were often small, infertile, and had petals that did not exceed the sepals. The visible symptoms of senescence eventually appeared as an abrupt color change, proceeding from pale green, yellow, dark red in less than 48 hours, and eventually became brown as the tissue dried out. Senescence affected multiple tissues within the apical region simultaneously, including the SAM, 1-2 mm of the subtending stem, and all attached flower buds within that region, while tissues immediately below remained green . No abscission layer was detected. One possible trigger for such apical senescence may come from the leaves, which showed an increasing trend of senescence prior to meristem arrest . To study the possibility that apical senescence might be triggered by mobile signal produced by senescent leaves, growing strawberries vertically a further experiment was performed that removed leaf tips from adult-phase leaves before they began to display senescent-related color changes.

Removal was timed to coincide with the maximum leaf length growth rate, well before senescence could occur. The results however, show that this actually had the opposite effect, as meristem arrest occurred on average 4.6 buds, or about 1.8 days sooner in leaf-tipped plants than it did in the controls . Alternatively, the fruits might be the source of a growth inhibitor, as predicted by the alternate bearing inhibitor hypothesis. To eliminate this possibility, the flowers were removed on a daily basis in order to observe changes in meristem activity and the time of growth arrest. Because previous results had shown that the rate of immature bud production and total stem height were closely correlated during the flowering period , internode lengths were measured as a proxy for meristem activity in A. thaliana. The resulting data revealed that both the controls and de-fruited plants displayed a similar declining trend of meristem activity that eventually reached zero growth . However, de-fruited plants clearly offset the time of meristem arrest through an extended period of almost linear bud production rates. This linear trend began a day before the control meristems slowed to zero growth, and lasted foral most a week before resuming the declining trend that the control plants had already completed. To verify the effects of fruit load in Avocados, SAM tissue was collected from branches with and without subtending fruit, four months after anthesis. To test the dominance and inhibitor hypothesis, the expression profile of four genes were measured with qPCR, yet this analysis found no significant patterns related to the presence of the fruit .In the carbohydrate competition model, the fruits are thought to consume the majority of available nutrients, leaving little if any for the rest of the plant. The remaining parts must either reduce their growth in direct proportion to the limiting nutrient, or trigger starvation responses in order to survive short-term depletions.

In most plants, the two most commonly encountered nutrient depletions involve carbohydrates and nitrogen, both of which have well-characterized response patterns that also show a significant degree of overlap. Under carbohydrate starvation conditions, for example, the plant tissues typically suppress respiration and growth while consuming their starch reserves. Eventually both proteins and lipids are degraded, releasing free amino acids and nitrogen in the form of ammonia. Some nitrogen is recovered in glutamine or asparagine biosynthesis, while the rest either diffuses into the media or is consumed by the nearest available sink tissue. Under prolonged conditions, large portions of the cytoplasm are consumed by autophagy, in which even organelles are degraded in lytic vacuoles containing cysteine/serine proteases. These vacuoles eventually coalesce until the cell consists of little but the nucleus and a large vacuole, which is followed by cell death under extreme cases. This pattern of responses is also strongly reflected in the present study. Starch breakdown is predicted both by trehalose signaling and direct digestion by AMY1. Although an increase in asparagines biosynthesis was not detected, the methionine gamma-lyase enzyme is known to release ammonia, and two nitrate transporters were upregulated, perhaps reflecting a decrease in free nitrates. Autophagy is consistent with the upregulation of an array of cystein/serine proteases, the near-lack of DNA laddering, and the presence of MC3. Carbohydrate starvation has also been reported to reduce osmolarity and membrane permeability, which parallels the observation of a weak plasmembrane, and the up-regulation of desiccation responses found by the present study.

Nitrogen starvation in contrast, is associated with the suppression of most chloroplast and photosynthesis related enzymes, loss of starch reserves, and the increase in uptake transporters and various storage compounds, including asparagine, glutamine, and various organic acids, while reducing losses that occur through degradation. Prolonged shortages typically result in anthocyanin production. Autophagy also seems to be an important part of nitrogen recycling in senescent leaves, though this may not have been detected in published studies that rely on short term starvation experiments.In the present data, the symptoms of nitrogen starvation are equivocal. Two photosystem subunits were reduced , which is broadly consisted with the degradation of the chloroplasts predicted by FZL and ELIP2. Autophagy signals do occur in the data, but the collected tissues were not obviously senescent, suggesting that autophagy may be more closely related to carbohydrate starvation. The increase of PAP1 is consistent with the biosynthesis of flavonols and other red pigments under nitrogen stress, though the much stronger increases in GL3, PAP2, and MYB12 reported by were not detected in the present study. The nitrate importer NRT1.2 was increased, but this conflicts with the earlier interpretation of amino acid catabolism in the present data. Thus the IM does not appear to be synthesizing storage proteins as predicted by nitrogen starvation studies. In consideration that the growth conditions used in this study included supplemental nitrogen fertilizer, the existence of a nitrogen shortage is unlikely. Instead, the relatively weak nitrogen starvation signal found here is potentially a consequence of the much more significant carbohydrate starvation response.Among the candidate induction genes that might be inhibited by high fruit loads, the results found that none of the floral induction pathways were reduced. Instead the expression of FT was strongly increased , as were several downstream targets including SOC1, AGL71, and perhaps also AGL44. One possible explanation for this pattern might be found in the expression pattern of FT, which based on microarray data is also produced by the ovaries and immature fruits, in addition to the leaf and stem vasculature. The failure to remove all immature flower buds during tissue collection might then bias the results in favor of these induction genes. In avocado trees, the TFL1 and DAM1 paralogs showed no significant response to fruit load . The bio-synthetic enzymes that produce phloridzen are currently unknown, so the presence of this compound cannot be evaluated with the present data.The fruit dominance hypothesis also does not seem to be strongly supported by the present study, drainage planter pot as all detected parts of the auxin response pathway were suppressed by high fruit load conditions. However, Arabidopsis is among the minority of species that do not have strong apical dominance responses, which could imply that this finding is an artifact. The failure to abscise immature fruits like many alternate bearing trees, for example, may indicate the lack of a functional fruit dominance system. However, the observation that isolated nodes of Arabidopsis can suppress axillary bud growth following auxin treatment , and that the lower portion of Arabidopsis branches initiate growth in a basipetal pattern, suggest that the dominance mechanism has not been lost, but is instead suppressed. A likely candidate for this suppression is the plant hormone strigolactone, which is able to suppress branching in the shoot when expressed in the roots.

In A. thaliana, the widespread expression of the strigolactone biosynthesis gene MAX1 in the vasculature is consistent with the broad suppression of dominance pathways in the species. Given that strigolactones appear to inhibit bud growth by blocking their ability to export auxin, this system has interesting parallels with the 2nd fruit drop, king fruit dominance, and even the number of flowers produced by axillary inflorescences. Immature fruits are already known to export auxin, suggesting that the plant may control their numbers by secreting root or shoot derived strigolactone into the inflorescence. The up regulation of strigolactone production under starvation conditions is certainly consistent with the increase in MAX1 expression levels found in the present microarray data. Such a role may explain the different alternate bearing amplitudes that were found when avocado trees were grafted to different rootstocks. Expression of ARF2 and RMS2 paralogs in Avocado trees however, did not reveal a significant response to fruit load .Another potential candidate for fruit inhibition might also include the trehalose signaling pathway. Although the precise role of this signaling sugar is not well understood, biosynthesis mutant are lethal, and high concentrations of trehalose-6-phosphate do seem to be correlated with starch biosynthesis. Accumulating evidence however, suggests that trehalose may have a central role in controlling the broad details of cellular metabolism. Several TPP genes are upregulated in response to nitrate treatments , and also by carbohydrate starvation. This mirrors the findings of the two trehalose phosphate phosphatases that were identified in this study, and in a recent profiling experiments with alternate bearing trees. Trehalose has also been shown to inhibit the kinase activity AKIN10/11, which has broad effects throughout the cell, and may explain how trehalose can regulate pathways such as cell size and stress tolerance. Intriguingly, the vegetative-adult phase change has also been implicated to be a product of carbohydrate supply, which also involves trehalose signaling.Although the data supports the existence of a massive senescence response, much of this signal disappears when the symptoms of carbohydrate starvation are considered. This is consistent with observations of the tissue during collection, which was often still green and displayed no external symptoms of senescence. The only remaining sign of biologic stress is the slight increase of AtRLP54 expression levels.The inflorescence meristem of Arabidopsis is a determinate growth, known to stop functioning after producing a predictable number of flowers in the Ler ecotypes. Under the growth conditions used by this study for the Col-0 ecotypes, the time of meristem arrest was slightlymore variable, but otherwise displayed a nearly identical pattern. Growth arrested meristems however, do not immediately terminate their activities, but instead exists in a quiescent state for several days, during which they can resume growth when the subtending fruits are removed. The present data suggests that this behavior is driven to a large extent by the re-allocation of carbohydrate resources. Interpreted in this way, the quiescent state is comparable to the survival phase exhibited by excised maize root tips, where growth could be resumed by adding supplemental sugar to the media. However, starvation isn’t sufficient to explain all of the behaviors of the inflorescence meristem. In contrast to previous report of a linear rate of anthesis, a closer examination of meristem activities revealed a number of subtle trends. When measured in terms of anlagen/day, the vegetative meristem is found to have a nearly constant rate of production that does not change with the juvenile/adult phase transition, which occurred between leaves 6-8 . After induction, the inflorescence meristem revealed a rapid rise in flower buds/day, a period of time that corresponds to an enlarged SAM diameter, and an increase in gibberellic acid biosynthesis. As anlagen production requires a finite surface area in order to develop, the increase rate of bud production may be related to the larger diameter of the meristem during this time.