At the 2017 Olive Exposium, a presentation made by Paul Vossen, Emeritus Olive Consultant, brought up a pertinent point about hormone deficiency and the role they play in unproductive olive trees. This is the first time some of the delegates heard this concept. Here is an abstract and details about a study from Turkey and Spain:
Nutrition Metabolism Plays an Important Role in the Alternate Bearing of the Olive Tree (Olea europaea L.)
The olive tree (Olea europaea L.) is widely known for its strong tendency for alternate bearing, which severely affects the fruit yield from year to year. Microarray-based gene expression analysis using RNA from olive samples (on-off years leaves and ripe-unripe fruits) are particularly useful to understand the molecular mechanisms influencing the periodicity in the olive tree. Thus, we carried out genome wide transcriptome analyses involving different organs and temporal stages of the olive tree using the NimbleGen Array containing 136,628 oligonucleotide probe sets. Cluster analyses of the genes showed that cDNAs originated from different organs could be sorted into separate groups. The nutritional control had a particularly remarkable impact on the alternate bearing of olive, as shown by the differential expression of transcripts under different temporal phases and organs. Additionally, hormonal control and flowering processes also played important roles in this phenomenon. Our analyses provide further insights into the transcript changes between ”on year” and “off year” leaves along with the changes from unripe to ripe fruits, which shed light on the molecular mechanisms underlying the olive tree alternate bearing. These findings have important implications for the breeding and agriculture of the olive tree and other crops showing periodicity. To our knowledge, this is the first study reporting the development and use of an olive array to document the gene expression profiling associated with the alternate bearing in olive tree.
Although an olive tree typically produces a lot of flowers when it blossoms, only a small percentage of them become fruits in the “on” year. In fact, it has been found that when an olive tree enters an ‘on’ year, the flower and inflorescence density is not correlated to the fruit density .
Interestingly, the fructification process alters both the floral bud differentiation and the flowering induction, being recognised as an inhibitor of the flowering in the fruit trees as previously indicated. Thus, the “off” year leaves showed significantly increased expression levels of the genes cift (citrus flowering locus T) and soc1 (suppressor of overexpression of constants 1), which are responsible for flowering, as compared with those from the “on” year leaves in mandarin . However, the microarray data of the present work indicates that the expression level of the only one transcript related to flower development (GO243632_1) was altered among the “on” and “off” year leaves. The expression of GO243632_1 was two-fold higher in the “off” year leaves as compared to the “on” year ones in the olive tree. Nevertheless, the microarray data suggests that the flowering is not the main factor in the alternate bearing in the olive tree.
Endogenous Plant Hormones
Previous studies have indicated that the endogenous plant growth hormones influence the alternate bearing. In fact, significant differences were revealed in the presence of some endogenous plant growth hormones including the abscisic acid (ABA), gibberellins like the gibberellic acids (GA3 and GA4) and auxins like the indole-3-acetic acid (IAA), between the “on” and “off” years for the olive tree samples , . Thus, the floral formation was inhibited in the presence of high GA3 levels, whereas the application of high concentrations of GA4, ABA and cytokinins resulted in elevated levels of flower formation in the olive tree . It has been stated that high levels of GA3 caused vegetative growth, negatively affecting the generative bud development in the following year. The “on” and “off” year buds had equal amounts of ABA in orange, being suggested that the ABA was not related to the alternate bearing. On the other hand, the 2-trans-abscisic acid (t-ABA) was almost twice the concentration of the ABA in the “on” year buds, with the difference decreasing at later sampling dates. Consequently, the impact of the t-ABA on the bud dormancy was proposed in orange . On the contrary, it has been found that the “off” year olive trees produced more ABA than the “on” year ones .
The olive tree microarray analyses of this work showed that among 246 differentially expressed genes between the “on” and “off” year leaves, irrespective of their developmental stage, only 14 genes were found to be associated with hormone regulation (Table S4). Of these, five genes (probes fgenesh4_pg.C_scaffold_19987000001, eugene3.00160596, eugene3.00020895, eugene3.00170500 and FL683585_1) showed significantly elevated expressions in the “on” year leaves, as compared to the “off” year ones. The other nine transcripts (probes grail3.0111003801, GO245518_1, GO245517_1, GO243107_1, GO245994_1, FN998690_1, GO244677_1, GO243685_1 and gw1.VII.2355.1) revealed opposite expression patterns. Lavee  reported that the phytohormones were present at lower levels in the olive tree than in other fruit trees. Consequently, our results indicate that although the endogenous hormones had an influence on the alternate bearing at a certain level, they were not the key determinants of this phenomenon in the olive tree.
In summary, a total of 136,628 oligonucleotide probe sets were arrayed in this first microarray gene expression profiling of six O. Europaea samples from fruits and leaves. The gene expression profiles with regard to the different tissues and developmental stages were examined. The expression of the transcripts greatly varied among the six studied libraries, indicating the involvement of diverse processes in response to bearing. The expressions of the transcripts for different organs under different developmental phases indicated that the nutrition metabolism had a remarkable impact on the olive tree alternate bearing. Additionally, the hormonal control also played relevant roles in this complex phenomenon.