Alternate Bearing Research Articles

The Paradox of Self-Fertile Varieties in the Context of Self-Incompatible Genotypes in Olive.

Olive, representing one of the most important fruit crops of the Mediterranean area, is characterized by a general low fruit yield, due to numerous constraints, including alternate bearing, low flower viability, male-sterility, inter-incompatibility, and self-incompatibility (SI). Early efforts to clarify the genetic control of SI in olive gave conflicting results, and only recently, the genetic control of SI has been disclosed, revealing that olive possesses an unconventional homomorphic sporophytic diallelic system of SI, dissimilar from other described plants. This system, characterized by the presence of two SI groups, prevents self-fertilization and regulates inter-compatibility between cultivars, such that cultivars bearing the same incompatibility group are incompatible. Despite the presence of a functional SI, some varieties, in particular conditions, are able to set seeds following self-fertilization, a mechanism known as pseudo-self-compatibility (PSC), as widely reported in previous literature. Here, we summarize the results of previous works on SI in olive, particularly focusing on the occurrence of self-fertility, and offer a new perspective in view of the recent elucidation of the genetic architecture of the SI system in olive. Recent advances in research aimed at unraveling the molecular bases of SI and its breakdown in olive are also presented. The clarification of these mechanisms may have a huge impact on orchard management and will provide fundamental information for the future of olive breeding programs.

Nitrogen nutrition diagnosis for olive trees grown in super-intensive cropping systems

The results of a 4-year experiment combining irrigation, N, and K showed that to maintain productivity and tree growth it is necessary to attain a higher leaf N status than current diagnosis standards. To sustain high oil yield and quality and to avoid excessive vigor, a critical N concentration was established for olive trees of 1.94% for leaves sampled in July. A negative relationship was obtained between N leaf concentration and some quality parameters. Above 1.94% N, total phenolic content and oxidative stability fell below 220 mg kg−1 and 10 hr, respectively. In addition, above this N leaf concentration other factors including the alternate bearing index (ABI) and vegetative growth distorted productive and qualitative responses. Moreover, the differential productive response of trees related to alternate fruit bearing status implies that the interpretation of leaf nitrogen content can be more accurate when ABI suggests an ON tree status.

Growth and yield responses under different grove floor management strategies for water conservation in young grapefruit trees

Improving sustainable citrus production through soil and water conservation strategies such as plastic mesh groundcovers and raised planting beds has not been thoroughly evaluated. To determine the impacts of these practices on tree growth parameters, fruit yield, and soil environmental conditions we monitored a grapefruit grove (Citrus paradisi Macf. cv.‘Rio Red’) for five years after planting. Trees were planted under four grove floor management strategies that included flat beds with no plastic mesh groundcover (traditional practice), flat beds with groundcover, raised beds with no groundcover, and raised beds with groundcover. The use of ground cover increased soil temperature and moisture content and led to higher tree growth rates and yields relative to uncovered grove floors. Raising the planting bed to approximately 45 cm higher than the surrounding field reduced soil deposition from irrigation and improved soil properties, but caused fluctuations in soil moisture due to improved water infiltration of heavy clay soils compared to the flat beds. Despite the positive effects of the use of ground cover on yields, there was a trend of alternate bearing, smaller and sweeter fruit sizes for trees under this planting design. The traditional method of planting (flat bed with no groundcover) resulted in smaller trees as measured by tree height, canopy circumference, and trunk diameter. While yields were moderate, they were fairly consistent across years and increased slightly each year of the study. These results show that adoption of plastic mesh groundcovers in citrus or other tree fruit production could potentially improve water conservation while increasing production.

Role of endogenous polyamines in the alternate bearing phenomenon in pistachio

Alternate bearing is a significant economic problem adversely affecting the producers, consumers, and economy of a country. In the present study, we a...

Comparative RNA-Sequencing and DNA Methylation Analyses of Apple (Malus domestica Borkh.) Buds with Diverse Flowering Capabilities Reveal Novel Insights into the Regulatory Mechanisms of Flower Bud Formation.

In plants, DNA methylation (i.e. chromatin modification) is important for various biological processes, including growth, development and flowering. Because 'Fuji' apple trees are alternate bearing and have a long ripening period and poor-quality flower buds, we used bud types with diverse flowering capabilities to investigate the epigenetic regulatory mechanisms influencing flower bud formation. We examined the DNA methylation changes and the transcriptional responses in the selected apple bud types. We observed that in the apple genome, approximately 79.5%, 67.4% and 23.7% of the CG, CHG and CHH sequences are methylated, respectively. For each sequence context, differentially methylated regions exhibited distinct methylation patterns among the analyzed apple bud types. Global methylation and transcriptional analyses revealed that nonexpressed genes or genes expressed at low levels were highly methylated in the gene-body regions, suggesting that gene-body methylation is negatively correlated with gene expression. Moreover, genes with methylated promoters were more highly expressed than genes with unmethylated promoters, implying promoter methylation and gene expression are positively correlated. Additionally, flowering-related genes (e.g. SOC1, AP1 and SPLs) and some transcription factor genes (e.g. GATA, bHLH, bZIP and WOX) were highly expressed in spur buds (highest flowering rate), but were associated with low methylation levels in the gene-body regions. Our findings indicate a potential correlation between DNA methylation and gene expression in apple buds with diverse flowering capabilities, suggesting an epigenetic regulatory mechanism influences apple flower bud formation.

Dynamic Cytosine DNA Methylation Patterns Associated with mRNA and siRNA Expression Profiles in Alternate Bearing Apple Trees.

Cytosine DNA methylation plays an important role in plants: it can mediate gene expression to affect plant growth and development. However, little is known about the potential involvement of cytosine DNA methylation in apple trees as well as in response to alternate bearing. Here, we performed whole-genome bisulfate sequencing to investigate genomic CG, CHG, and CHH methylation patterns, together with their global mRNA accumulation and small RNA expression in "Fuji" apple trees. Results showed that "Fuji" apple trees have a higher CHH methylation than Arabidopsis. Moreover, genomic methylation analysis revealed that CG and CHG methylation were robustly maintained at the early stage of flower induction. Additionally, differentially methylated regions (DMRs), including hypermethylated and hypomethylated DMRs, were also characterized in alternate bearing (AB) apple trees. Intriguingly, the DMRs were enriched in hormones, redox state, and starch and sucrose metabolism, which affected flowering. Further global gene expression evaluation based on methylome analysis revealed a negative correlation between gene body methylation and gene expression. Subsequent small RNA analyses showed that 24-nucleotide small interfering RNAs were activated and maintained in non-CG methylated apple trees. Our whole-genome DNA methylation analysis and RNA and small RNA expression profile construction provide valuable information for future studies.

Effect of regulated deficit irrigation during the vegetative growth period on shoot elongation and oil yield components in olive hedgerows (cv. Arbosana) pruned annually on alternate sides in San Juan, Argentina

In super-intensive hedgerows, vegetative vigor must be controlled to allow access to harvesting machinery, particularly under Argentina conditions in which olive trees display excessive vigor. During shoot growth (spring–early summer), flowering and fruit set also take place and the potential yield (bud number) for the following season is defined. The effect of spring–early summer deficit irrigation was studied as a tool to reduce vegetative growth and its influence on inflorescence development, oil yield, and its components. During three seasons in an olive hedgerow (cv. Arbosana), we evaluated a control irrigated at 70% ETc over the season and two regulated deficit treatments irrigated at 50% (RDI-1) and 30% (RDI-2) ETc during the shoot growth period (from August to January) and then 70% ETc until harvest (May). Hedgerows were mechanically topped and pruned annually on alternate sides. We observed that RDI-1 and RDI-2 reduced hedgerow height and width increment after hedging by 15% and 20%, respectively, compared to control. Inflorescence structures were not affected by water deficit, but the control treatment showed on average 5.8 fruits per fruiting inflorescence, significantly higher than 2.4 fruits per fruiting inflorescence observed in RDI-2. After the third season, RDI-1 and RDI-2 were 174% and 146% more productive than control hedgerows, where the pruned sides showed excessive vigor with lower floral bud induction in the following seasons. Fruit size and oil accumulation were also higher in both RDI-1 and RDI-2 than in control, due to greater fruit exposure to irradiance in most deficit treatments. Compared with control, RDI-1 and RDI-2 allowed water savings of 17% and 35%, respectively, but RDI-1 was more productive and had lower alternate bearing than RDI-2.

Influence of antioxidative enzymes, phytohormones and pigments in alternate bearing of three olive cultivars

The olive tree (Olea europaea. L), is a characteristic species of the Mediterranean landscape. This species is widely known for its strong tendency for alternate bearing which severely affects the fruit yield from year to year. This phenomenon is widespread throughout many prennial trees and shrubs, but is not universal. Prennial fruit crops initiate flower buds for next season’s crop in the current season, and for most deciduous fruit species, the alternation of large and small crops is caused by competition between the current season’s crop and the coming season’s flower buds. Despite its agricultural importance, how the developing crop influences the following year’s return bloom and yield is not fully understood. This study was carried out in an olive orchard (Olea europaea L., cultivars Chemlali, Koroneiki and Arbosana) located in Sfax, Tunisia, during two consecutive years (2014–2015). Crop load alternation provoked good fruit and oil productions in the ON year (2014), followed by low crops in the OFF year (2015). For all the three cultivars, leaves and fruits of olive plants in the OFF year generally showed reduced pigments, decreased enzymatic and non-enzymatic antioxidant defences, and their abscisic acid (ABA) homeostasis was disrupted. All the studied factors can interact in determining the alternation of high and low crop load in olive tree and its effects on fruit/ oil yield and plant physiological status.

Irrigation of ‘Hass’ avocado: effects of constant vs. temporary water stress

The main objectives of the present study were to assess the water demand for heavy fruit load of ‘Hass’ avocado throughout the growth periods and to investigate the effects of deficit irrigation during sensitive phenological phases on yield. The experimental set-up allowed the comparison between trees responses to three irrigation strategies during the entire growth period (no water stress; excessive irrigation; constant water stress) as well as the comparison between regulated deficit irrigation (RDI) managements applied during the early or the late growth period. The yield of no water stress treatments during three experimental years was very high (25–31 t ha−1) while the yields of water-stressed trees were significantly lower (16–21 t ha−1). More importantly, the yield of no water stress trees was not susceptible to alternate bearing while the yield of water-stressed trees was considerably reduced during off-crop years. Irrigation rates and the actual evapotranspiration coefficient KL = ET/ET0 for the no water stress treatment may serve as a reasonable guide for irrigation management. Fruit load should be taken into account while planning irrigation and fertilization management and plant-based methods should be used for controlling the irrigation management (scheduling and quantities). Analyses of trunk diameter variation data that lead to evaluation of trunk growth rate and maximum daily shrinkage reflect phenological stages and periodicity of shoot, fruit and root growth, and also may provide an integrative, “holistic viewpoint” of overall tree status.

Relationship between alternate bearing and apical bud development in cranberry (Vaccinium macrocarpon)

Similar to other woody crops, cranberry (Vaccinium macrocarpon Ait.) exhibits alternate bearing or a tendency to produce heavier yields one year, followed by lighter yields the next year. Unfortunately, despite the occurrence in many fruit crops, this trait is not well understood. The variable differentiation of floral initials in cranberry uprights is a distinguishing characteristic associated with alternate bearing. This study evaluates bud morphology and the presence of floral initials through characterization of longitudinal sections of apical buds from vegetative and fruiting uprights of alternate and non-alternate bearing genotypes. Our results reveal that differentiation of floral initials in fruiting uprights only occurs in non-alternate bearing genotypes and after initiation in vegetative uprights. In addition, a strong positive correlation was found between the increase of bud width and the presence of floral initials. Lastly, uprights from the alternate bearing genotype exhibited significantly faster growth rates of the reproductive buds compared with the vegetative buds. In summary, our study shows marked differences in timing and growth rates of floral initials between uprights of the two genotypes, suggesting a possible correlation with resource allocation during the growth season, and thus could contribute to cultivar selection and management practices.

Association mapping of agro-morphological traits in European hazelnut (Corylus avellana)

More than half of European hazelnut (Corylus avellana) production occurs in Turkey. Despite this dominance, the yield of Turkish cultivars has remained stagnant over the past 10 years with Italian yield nearly double that in Turkey. This difference is due to Turkey’s unique cultivation system; hazelnuts are grown in bushy clusters (“ocak” system), not as single trees. Current hazelnut breeding efforts are shifting toward the development of materials for single plant orchards which are much higher yielding; thus, there is a need to explore germplasm for relevant agro-morphological traits and to determine their genetic control. The objectives of this study were to examine data for 44 such traits in 390 hazelnut accessions: 16 cultivars, 232 landraces and 142 wild individuals from nine provinces in Turkey and to map the loci associated with these characteristics using simple sequence repeat markers. Comparison of cultivars, landraces and wild hazelnut accessions revealed the effects of domestication and selection on the crop and indicated that useful alleles for traits such as cropping and reduced alternate bearing may exist in the wild germplasm. A total of 145 quantitative trait loci (QTL) were detected with the largest proportions identified for involucre (26%) and inflorescence (14%) morphology. Several markers co-localized with more than one trait including markers for male catkin abundance which were shared with plant vigor and height. Similarly, markers for female flower abundance co-localized with suckering and alternate bearing. Such markers and their linked QTL should be studied in more detail as they might help breeders select for plant vigor, decreased suckering and increased flower production: traits which will be extremely useful for Turkey’s transition to single plant orchards.

Temporal yield variability in subtropical table grape production

Inconsistent yield between seasons for table grape cultivars grown in subtropical climates in Australia presents a challenge to the industry in terms of marketing and economic sustainability. The pattern of this yield inconsistency has not been described. Marketable fruit yields of three cultivars (Red Globe (RG), Menindee Seedless (MS) and Flame Seedless (FS)) were acquired over 14 seasons, and individual vine yields of MS were collected for a four-season period from a vineyard in the Central Highlands of Queensland, Australia. Cultivars FS and RG did not show an alternate bearing pattern. Yields of MS varied up to four-fold in consecutive seasons. An alternate yielding pattern as measured by the indicators of an alternate bearing index (I) and associated probability calculated using a resampling process was evident in 7 (2009–2015) of the 14 monitored seasons. Individual vine yield over the four-season period 2014–2017 was irregular and 90% variation in vine yields was due to the variation in the number of bunches per vine. The seasonal yield variation of MS was partly explained (R2 = 0.31, P < 0.05) by the mean monthly temperature in October of the year preceding harvest. This result is ascribed to the effect of high temperature during inflorescence initiation and early development which occurs in October in the Central Highlands of Queensland.

An evolutionary platform for alternate bearing in fruit trees

Alternate bearing has been and still is a major problem in fruit trees. From the point of view of a horticulturist it is a tree productivity syndrome which must be addressed by agro-technical means in order to eliminate or, at least, mitigate the alternation cycle. ‘Masting’ is a widespread forest tree irregular sexual reproduction pattern that has attracted ecologists who wonder about the survival-advantage of this behavior. From an evolutionary perspective alternate bearing and masting seem to be related and may share a common bio-evolutionary mechanism, responsible for ‘yield alternation’. A recently developed resource budget model, based on nonlinear chaos dynamics, seems to account for a broad spectrum of yield alternation patterns. From an evolutionary standpoint, one may envision a domestication-evolutionary continuum from extreme masting, through intermediate alternate bearing, down to regular yield of fully-managed fruit trees. Domestication records of pecan (Carya illinoiensis) support this hypothesis. The alternate bearing of fruit trees appears to be mediated by a ‘fruit overload’ signal but resource depletion still plays a critical role in the alternate bearing of numerous tree crops.

Pistachio inflorescence bud abscission dynamics as a function of embryo weight, crop load and vegetative growth

Alternate bearing in pistachio (Pistacia vera L.) is particularly severe. The developing nuts are a major sink, draining branch carbohydrates, producing inflorescence bud abscission and decreasing the following year's production. We investigated inflorescence bud drop as a function of branch crop load, embryo weight, and vegetative growth. The main objective of this study was to investigate the mechanism of alternate bearing. We also tested the efficacy of eight different foliar nutrient combinations for reducing inflorescence bud abscission: Acadian A LSC®, Acadian B, low-biuret urea, Acadian A LSC® + low-biuret urea, Acadian B + low-biuret urea, MaxCell®, MaxCell® + low-biuret urea, and untreated control (sprayed with water). The trial was conducted during the 2016 growing season on eight-year-old Kerman pistachio trees planted at 5.2 m in-row and 6.1 m between rows, on PG1 rootstock, 8 replications for each treatment. Average tree yields as dry in-shell marketable split nuts were not significantly different among treatments ranging from 27 to 31 kg tree(-1)). The treatment did not affect inflorescence bud abscission. The bud abscission percentage was significantly higher in bearing branches (~70%) versus non-bearing branches (~15%). In fruiting shoots, the percentage of inflorescence bud abscission was significantly and positively correlated with embryo growth. Length, width and weight of new wood and one-year-old wood correlated significantly and positively with nut production, and negatively with inflorescence bud abscission. The data collected support the hypothesis suggested in the literature that alternate bearing is function of crop load and carbohydrate depletion. Results reported here are the first step toward characterization of optimal shoot parameters, length, width and leaf area, for optimal and regular crop production.

Behavior of leaf nutritional status in ‘on’ and ‘off’ year bearing trees of ‘Bengal’ litchi (Litchi chinensis)

The major problems reported in litchi in several countries include irregular flowering, poor fruit retention, small fruit size and alternate bearing. This study assessed the time course (all-year-round, on a monthly basis) of the leaf macro- and micronutrient concentrations, in ‘on’ and ‘off’ year bearing ‘Bengal’ litchi trees. In December 2012, fifty uniform trees, half of them with a heavy fruit load (‘on’ year) and the other half with a very light crop load (‘off’ year) were tagged to trace their nutritional status. The trees were twelve years old, with an approximately 5 m canopy diameter and 4 m in height, and have received the same cultural practices and fertilization since the orchard establishment. Leaves were sampled monthly from December 2012 to November 2013 and analyzed for nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn) and boron (B). Data were submitted to analysis of variance and means compared by multiple range test at 5% probability. Leaf analysis showed differences between ‘on’ and ‘off’ year bearing trees for N, K, Mn and Cu. Leaf macronutrients were higher in the 4th month and lower around the 10th month, whereas leaf micronutrients were higher around the10th month and lower in the 4th month. These results open the possibility to use leaf analysis as a tool to predict potential yield and manage fertilization accordingly to minimize alternate bearing in litchi.

Histological and proteomic approaches to study floral bud induction in relation to biennial bearing in apple

Biennial or alternate bearing in fruit trees leads to fluctuating yields, characterized by ‘On’ years with high crop loads followed by ‘Off’ years with little or no yield. To study the mechanism of biennial bearing in apple (Malus domestica Borkh.), the cultivars ‘Royal Gala’ as a regular bearer and ‘Fuji’ as a strongly alternating cultivar were investigated. ‘Off’ trees were completely thinned at full bloom, whereas ‘On’ trees were not thinned to maintain the natural high crop load. The first signs of floral bud initiation were detected for ‘Fuji’ at 70 days after full bloom (DAFB) and 120 DAFB for ‘Off’ and ‘On’ trees, respectively. In ‘Royal Gala’, initiated buds were observed not before 100 DAFB in both treatments. Hence, buds from ‘Fuji’ ‘Off’ trees committed to flowering about one month earlier than those from ‘Royal Gala’ ‘Off’ trees. In contrast, flower initiation of buds from ‘Royal Gala’ ‘On’ trees occurred 20 days earlier when compared to those from ‘Fuji’ ‘On’ trees. Electrospray ionization mass spectrometry (ESI-MS) coupled with ultra-high performance liquid chromatography (UHPLC) allowed the identification of 3011 proteins. A Student’s T-Test revealed 181 differentially expressed proteins between buds from ‘On’ and ‘Off’ trees. Specifically, 81 proteins were significantly upregulated in floral buds and 100 proteins in vegetative buds, respectively. The largest log2 fold change of 3.17 was identified for a zinc metalloprotease in buds from the ‘Fuji’ ‘On’ treatment. The protein with the highest up-regulation in ‘Fuji’ ‘Off’ buds was a nigrin b-like protein. Histological analysis of bud development has proven to be an essential and reliable tool to identify the time point of floral initiation in apple. Moreover, this study demonstrates that especially in biennial bearing cultivars, crop load heavily influences the onset of floral bud initiation and can differ for up to 50 days within the same cultivar depending on crop load. The identification of this critical period enables in-depth analysis of biochemical changes such as differences in protein profiles to further reveal the mechanism of biennial bearing in apple.

Influence of bud quality and wood age on fruit set and alternate bearing in ‘Conference’ pear (Pyrus communisL.)

Influence of bud quality and wood age on fruit set and alternate bearing in ‘Conference’ pear (<i>Pyrus communis</i>L.)

Seasonal changes in starch content in pistachio organs as related to crop load

The alternate bearing in pistachio (Pistacia vera L.) results from inflorescence buds abscission differentiated on current season growth during the year of heavy crop. Inflorescence bud drop has been directly correlated with the presence of fruits on 1-year shoot, and in particular with the kernel grow that acts as the major sink of nutrients. Experimental evidences suggest the involvement of competition between reproductive organs for the available resources. The annual carbohydrates storage was analysed in mature pistachio trees characterized by low, medium and high crop load (about 1, 7 and 11 kg tree(-1) of dry in-shell nuts). The experiment was carried out in 2013 in a commercial pistachio orchard, using ‘Bianca’ grafted onto P. terebinthus L. in Sicily (Italy, 37°30’ Lat N). Starch concentration was measured in sample tissues of trunk wood, current-year shoot, one-year-old and two-year-old shoots throughout the season from the dormancy phase (March) until the post-harvest period (September). During dormancy, the trunk represented the main storage organ having starch concentration ranging from 60 to 120 mg g(-1) of dry weight, whereas the tissues of the other organs examined showed values below 20 mg g(-1) of dry weight. The deepest decrement of starch concentration after bud-break until bloom was observed in the trunk, whereas, starch concentration started increasing after the initial growth flush. The patterns of starch in the tissues varied according with the crop load; trees bearing the highest crop load (11 kg tree(-1)) showed a constant starch concentration during the growing season, in contrast with the trees characterized by lower crop load values. Before the onset of dormancy, starch rapidly increased in all the shoots type and in the trunk wood and no effects of tree crop load were observed.

Deficit irrigation does not affect alternate bearing of ‘Valencia’ orange trees

Deficit irrigation does not affect alternate bearing of ‘Valencia’ orange trees

RNA-Seq analysis to investigate alternate bearing mechanism inPistacia veraL.

Pistachio (Pistacia vera L.) production suffers a high level of alternate bearing. The mechanism underlying this negative phenomenon is different from other species, such as apple and olive. Pistachio produces a high number of inflorescence buds every year that in heavy cropping trees (“ON”) mostly fall during the kernel development phase, which occurs in July-August. Primary metabolites (i.e., carbohydrates) play a key role in the signaling related to inflorescence bud abscission. In this work, RNA-Seq was used as a tool to investigate transcriptome of inflorescence buds and fruits, sampled from branches with low (“OFF”) and high (“ON”) crop load. Reference based RNA-Seq analysis using Arabidopsis thaliana genome was performed to discover differentially regulated genes in “ON” vs. “OFF” branches. In fruits developed on “ON” branches, photosynthesis, terpenoid and alpha-amylase-related genes were induced while tetrapyrrole was repressed; in inflorescence buds, photosynthesis was repressed while two genes involved in terpenoids pathways were still induced in comparison to the buds bore on “OFF” branches. A comparison with citrus transcriptomic data was performed. The data collected in the present study will be further utilized by the completion of pistachio genome and transcriptome de novo assembly.