Peach-almond Hybrid Research Articles

Seasonal Dynamics and Distribution of Xylella fastidiosa in Infected Almond Trees.

This research focused on studying the dynamics of the bacterial pathogen Xylella fastidiosa in almond trees across different developmental stages. The objective was to understand the seasonal distribution and concentration of X. fastidiosa within almond trees. Different tree organs, including leaves, shoots, branches, fruits, flowers, and roots, from 10 X. fastidiosa-infected almond trees were sampled over 2 years. The incidence and concentration of X. fastidiosa were determined using qPCR and isolation. Throughout the study, X. fastidiosa was consistently absent from fruits, flowers, and roots, whereas it was detected in leaves as well as in shoots and branches. We demonstrate that the absence of X. fastidiosa in the roots is likely linked to the inability of this isolate to infect the peach-almond hybrid rootstock GF677. X. fastidiosa incidence in shoots and branches remained consistent throughout the year, whereas in leaf petioles, it varied across developmental stages, with lower detection during the early and late stages of the season. Similarly, viable X. fastidiosa cells were isolated from shoots and branches at all developmental stages, but no successful isolations were achieved from leaf petioles during the vegetative and nut growth stage. Studying the progression of almond leaf scorch symptoms in trees with initial infections showed that once symptoms emerged on one branch, symptomless branches were likely already infected by the bacterium. Therefore, selectively pruning symptomatic branches is unlikely to cure the tree. This study enhances our understanding of X. fastidiosa dynamics in almond trees and may have practical applications for its detection and control.

Linking diverse salinity responses of 14 almond rootstocks with physiological, biochemical, and genetic determinants

Fourteen commercial almond rootstocks were tested under five types of irrigation waters to understand the genetic, physiological, and biochemical bases of salt-tolerance mechanisms. Treatments included control (T1) and four saline water treatments dominant in sodium-sulfate (T2), sodium-chloride (T3), sodium-chloride/sulfate (T4), and calcium/magnesium-chloride/sulfate (T5). T3 caused the highest reduction in survival rate and trunk diameter, followed by T4 and T2, indicating that Na and, to a lesser extent, Cl were the most toxic ions to almond rootstocks. Peach hybrid (Empyrean 1) and peach-almond hybrids (Cornerstone, Bright’s Hybrid 5, and BB 106) were the most tolerant to salinity. Rootstock’s performance under salinity correlated highly with its leaf Na and Cl concentrations, indicating that Na+ and Cl- exclusion is crucial for salinity tolerance in Prunus. Photosynthetic rate correlated with trunk diameter and proline leaf ratio (T3/T1) significantly correlated with the exclusion of Na+ and Cl-, which directly affected the survival rate. Expression analyses of 23 genes involved in salinity stress revealed that the expression differences among genotypes were closely associated with their performance under salinity. Our genetic, molecular, and biochemical analyses allowed us to characterize rootstocks based on component traits of the salt-tolerance mechanisms, which may facilitate the development of highly salt-tolerant rootstocks.

Study of the influence of peach-almond hybrid rootstocks on the growth and productivity of three peach cultivars grown in southern Bulgaria

Study of the influence of peach-almond hybrid rootstocks on the growth and productivity of three peach cultivars grown in southern Bulgaria

Scion × Rootstock Response on Production, Mineral Composition and Fruit Quality under Heavy-Calcareous Soil and Hot Climate

Since there are many different factors which affect peach orchard profitability, it is necessary to conduct long-term studies to find the best scion/rootstock combination within the constraints imposed by local climate and economic conditions. The influence of six peach-almond hybrids (“Adafuel”, “Adarcias”, Garnem”, “GF 677”, “PADAC 9902-01” and “Rootpac 70”); one P. persica × P. davidiana hybrid: “Cadaman”; five hexaploid plums (“Adesoto 101”, “PM 44 AD”, “PM 105 AD”, “PM 150 AD”, and “Penta”); three plum × peach-almond hybrids (“PADAC 04-01”, “PADAC 04-03” and “PADAC 99-05”), and one plum x almond hybrid (“Rootpac R”) on agronomic, basic fruit quality parameters, and flower and leaf mineral nutrients of the “Big Top” nectarine was tested. The highest vigor was induced by “Adafuel”, while the lowest vigor was exhibited by “PM 105 AD”, although the latter did not differ from the rest of the rootstocks. Yield efficiency was higher for “Adesoto 101” and lower for “Adafuel”. Regarding the soluble solids content (SSC), “PADAC 04-01” rootstock induced the highest average content, although it did not differ from “Adarcias”, “Adesoto 101”, “PADAC 99-05”, “PM 105 AD”, and “Rootpac R”. In addition, significant effect of rootstock was found on the flower and leaf mineral analysis traits evaluated. The medium (“Adarcias”, “Adesoto 101”, “PM 105 AD”) to high vigorous plum based hybrid rootstocks (“PADAC 04-01”, “PADAC 99-05” and “Rootpac R”) showed the tendency to induce a better fruit quality, based on higher concentration of SSC, and other yield attributes, demonstrating their commercial interest as new rootstocks for peaches. Considering its overall performance, “PADAC 04-01” appears as a new promising rootstock alternative to peach-almond hybrids on heavy and calcareous textured soils.

Sugars and organic acids profile and antioxidant compounds of nectarine fruits influenced by different rootstocks

Thirteen Prunus rootstocks with different genetic background [five peach-almond hybrids: Adafuel, Adarcias, GF 677, PADAC 9902-01 and Rootpac 70; one P. persica x P. davidiana hybrid: Cadaman; three hexaploid P. insititia plums: Adesoto 101, PM 105 AD, and PM 150 AD; three plum x peach-almond hybrids (PADAC 04-01, PADAC 04-03 and PADAC 99-05), and a plum-almond hybrid (Rootpac R)] were budded with the ‘Big Top’ nectarine and evaluated in typical Mediterranean growing conditions. Biochemical fruit compounds, such as sugars and organic acids profile, and antioxidant compounds were assessed to enhance fruit quality and beneficial effects of ‘Big Top’. For three consecutive years (2014–2016), sugars and organic acids were analysed by high-performance liquid chromatography and phenolic compounds by spectrophotometry. Statistical analyses showed that rootstocks affected strongly fruit phytochemical composition, although it was also influenced by year. Compared with peach-based rootstocks, the hexaploid ‘Pollizo’ plums Adesoto 101 and PM 150 AD can increase the sweetness of the low-acid ‘Big Top’ nectarine, inducing higher contents of sugars and organic acids. Similarly, beneficial antioxidant effects can be enhanced on diploid plum based hybrids PADAC 04-01, PADAC 99-05 and Rootpac R, inducing higher contents on sugars and bioactive compounds (total phenols, flavonoids and vitamin C) and antioxidant capacities. High significant correlations were found between all studied compounds. New plum based rootstocks seem to induce higher fruit quality based on sugars and organic acids profile, and antioxidants compounds.

First Report of the Peach Root-Knot Nematode, Meloidogyne floridensis Infecting Almond on Root-Knot Nematode Resistant 'Hansen 536' and 'Bright's Hybrid 5' Rootstocks in California, USA.

In April-August 2018, samples of galled roots with rhizosphere soil were collected from almond orchards in Atwater, Merced County and Bakersfield, Kern County, California. Almond trees (Prunus dulcis) grafted on ‘Hansen 536’ and ‘Brights Hybrid®5’ (peach-almond hybrid) rootstocks showed strong symptoms of growth decline. Extracted root-knot nematodes were identified by both morphological and molecular methods as M. floridensis. Meloidogyne floridensis was initially found in Florida, USA, and has not been reported from any other states and countries. This is a first report of M. floridensis in California and outside of Florida.

Graft compatibility for new releasedPrunusrootstocks

Graft compatibility for new Prunus rootstocks recently introduced into the Spanish market or under selection in public and private programmes is assessed in nursery trials. Part of the evaluated material belongs to the Aula Dei Experimental Station (EEAD-CSIC) selection program in Zaragoza, Spain. Another group of rootstocks under test is from the private nursery Agromillora Iberia, S.L. Some of those rootstocks showed good adaptation to Mediterranean growing conditions and resistance to root-knot nematodes. As control, peach-almond (‘GF 677’), peach × P. davidiana (‘Cadaman’), other interspecific plum-based rootstocks (‘Damas GF 1869’, ‘Ishtara’) and different plum species (P. cerasifera: ‘Adara’; P. domestica: ‘Constanti 1’; P. insititia: ‘Adesoto’ and other ‘Pollizo de Murcia’ clones, ‘Saint Julien A’) are used. New evaluated rootstocks include: ‘Densipac R 20’ (P. besseyi × P. cerasifera), ‘Nanopac R 40’ (P. dulcis × P. persica), ‘Replantpac RR’ (P. dulcis × P. cerasifera), ‘AP 45’ [(P. besseyi × P. salicina) × P. armeniaca], ‘Aprimed AP 65’ [(P. armeniaca L. × P. cerasifera Ehrh) × P. pumila L. var. besseyi], and ‘LC 52’ (P. cerasifera × P. maackii), among others. To determine graft-compatibility, commercial cultivars of different Prunus species were budded on some of the evaluated rootstocks. They include: ‘Moniqui’ and ‘Portici’ (apricot); ‘Lauranne’, ‘Nonpareil’, ‘Penta’ and ‘Soleta’ (almond); ‘Angeleno’ and ‘Golden Japan’ (Japanese plum); ‘Big Top’ and ‘Sumergrand’ (nectarine) and ‘Aida’, ‘Carmen’, ‘Cordia’, ‘Ferrovia’, ‘Kovics’, ‘Krupnoplodnaja’, and ‘Sandor’ (cherries). Several degrees of graft incompatibility have been observed for the ‘translocated’ type of incompatibility. In general, peach-almond hybrids and slow-growing plums (P. domestica and P. insititia plums as ‘Adesoto’ and other ‘Pollizo de Murcia’ clones) were graft-compatible with peach, almond and Japanese plum tested cultivars. However, in the case of P. cerasifera, P. besseyi and interspecific hybrids with these species, performance differed substantially depending on the evaluated genotype.

Long-term graft compatibility study of peach-almond hybrid and plum based rootstocks budded with European and Japanese plums

The introduction of some new rootstocks requires knowledge of the potential ‘translocated’ and ‘localized’ graft incompatibilities before releasing these rootstocks into the market. Trials were established at the Aula Dei Experimental Station (Zaragoza, Spain) to assess graft compatibility between five European plum (Prunus domestica L.) and six Japanese plum (Prunus salicina L.) cultivars with thirty-eight rootstocks belonging to peach × almond hybrids, as well as slow-growing, fast-growing and interspecific hybrid plum groups. Some of these rootstocks are under process of selection and others are commercial rootstocks already introduced into the European market. External visual diagnosis of the scion-rootstock graft combinations was performed by observing symptoms of the ‘translocated’ incompatibility in the nursery. Visual symptoms of ‘translocated’ incompatibility were only found on the Japanese plum cv. ‘Golden Japan’ budded on the plum-apricot hybrid AP-45. The ‘localized’ graft incompatibility was assessed after internal macroscopically examination of graft unions. In the case of the European plum cvs., ‘President’ and ‘Reine Claude Tardive of Chambourcy’ exhibited good graft compatibility with all the tested rootstocks, with the exception of ‘President’ budded on the pentaploid plum hybrid rootstock Damas GF 1869. ‘Reine Claude Verte’ cv. showed ‘localized’ incompatibility with Myrobalan B and Myrobalan GF 3-1 since the second year after budding. ‘Stanley’ cv. showed ‘localized’ incompatibility with the six evaluated peach × almond hybrid rootstocks, although it was compatible with all plum based rootstocks. In the case of Japanese plums, four cultivars (‘Angeleno’, ‘Black Amber’,’ Delbarazur’ and ‘Songold’) did not show any graft incompatibility, but ‘Friar’ showed ‘localized’ incompatibility on Myrobalan B. In the case of peach-almond and P. cerasifera based rootstocks (Adarcias, GF 677, Damas GF 1869, Myrobalan B, Myrobalan 29 C, Myrobalan GF 3-1 and Marianna GF 8-1), ‘Golden Japan’ trees showed similar symptoms to those caused by TomRV in Myrobalan and peach rootstocks and ‘localized’ incompatibility. Finally, this study confirmed the interest of P. insititia specie as well as several interspecific Prunus hybrids as a good source of rootstocks for the plum industry.

Effects of leaf wetness duration and temperature on infection of Prunus by Xanthomonas arboricola pv. pruni.

Xanthomonas arboricola pv. pruni is the causal agent of bacterial spot disease of stone fruits and almond. The bacterium is distributed throughout the major stone-fruit-producing areas of the World and is considered a quarantine organism in the European Union according to the Council Directive 2000/29/EC, and by the European and Mediterranean Plant Protection Organization. The effect of leaf wetness duration and temperature on infection of Prunus by X. arboricola pv. pruni was determined in controlled environment experiments. Potted plants of the peach-almond hybrid GF-677 were inoculated with bacterial suspensions and exposed to combinations of six leaf wetness durations (from 0 to 24 h) and seven fixed temperatures (from 5 to 35°C) during the infection period. Then, plants were transferred to a biosafety greenhouse, removed from bags, and incubated at optimal conditions for disease development. Although leaf wetness was required for infection of Prunus by X. arboricola pv. pruni, temperature had a greater effect than leaf wetness duration on disease severity. The combined effect of wetness duration and temperature on disease severity was quantified using a modification of the Weibull equation proposed by Duthie. The reduced-form of Duthie’s model obtained by nonlinear regression analysis fitted well to data (R = 0.87 and R2adj = 0.85), and all parameters were significantly different from 0. The estimated optimal temperature for infection by X. arboricola pv. pruni was 28.9°C. Wetness periods longer than 10 h at temperatures close to 20°C, or 5 h at temperatures between 25 and 35°C were necessary to cause high disease severity. The predictive capacity of the model was evaluated using an additional set of data obtained from new wetness duration-temperature combinations. In 92% of the events the observed severity agreed with the predicted level of infection risk. The risk chart derived from the reduced form of Duthie’s model can be used to estimate the potential risk for infection of Prunus by X. arboricola pv. pruni based on observed or forecasted temperature and wetness duration.

Agronomic and physicochemical fruit properties of ‘Big Top’ nectarine budded on peach and plum based rootstocks in Mediterranean conditions

Agronomic and physicochemical fruit properties of ‘Big Top’ nectarine were evaluated in the Ebro Valley (Zaragoza, Spain), under replant and heavy-calcareous soil conditions typical of the Mediterranean area. Fruits were harvested at commercial maturity from trees budded on four peach-almond hybrids (Prunus amygdalus×Prunus persica): Adarcias, Felinem, GF 677 and Mayor; two P. persica×Prunus davidiana hybrids: Barrier and Cadaman; two hexaploid plums: Adesoto (Prunus insititia) and Tetra (Prunus domestica); and one almond-plum hybrid (Prunus cerasifera×P. amygdalus): Replantpac. Significant differences among the different rootstocks were found for most of the fruit quality traits analyzed at harvest maturity. The average of the four years of study showed that the brightest (high L* value) and most red colored skin fruits (high a* and b* values) were found on GF 677, while the lowest b* and h were recorded on Tetra and Felinem, respectively. Adesoto, GF 677 and Replantpac induced good fruit quality traits based on anthocyanins, Vitamin C, relative antioxidant capacity and/or total phenolic content (TPC). GF 677 also showed higher values on cumulative yield (CY) and yield efficiency (YE), with no significant differences with Replantpac for CY and Adesoto for YE. Barrier and Cadaman seem to delay maturity of ‘Big Top’ fruits based on flesh firmness (FF), titratable acidity (TA) and CIELAB parameters, and showed lower TPC and anthocyanins, which could be partly explained due to their worst adaptation to calcareous soil conditions. Results from this study underlined the important relationships between plant adaptability and fruit quality traits.

In silico search and biological validation of microRNAs related to drought response in peach and almond.

Plant responses to drought stress are regulated at the transcriptional and post-transcriptional levels through noncoding endogenous microRNAs. These microRNAs play key roles in gene expression, mainly by down-regulating target mRNAs. In this work, an in silico search and validation for microRNAs related to drought response in peach ('G.H. Hill'), almond ('Sefied') and an interspecific peach-almond hybrid ('GN 15') has been performed. We used qPCR to analyse the gene expression of several miRNAs described as being related to drought response in peach, including miR156, miR159, miR160, miR167, miR171, miR172, miR398, miR403, miR408, miR842 and miR2275 under mild and severe water deficit. These miRNAs were in silico selected on the basis of previous works, their conservation in plants and their drought response. qPCR analysis confirmed the implication of these miRNAs in the dehydration stress response in the three assayed genotypes. Comparison of miRNA expression patterns in the three evaluated genotypes indicated that the hybrid GN 15 showed higher expression levels of specific miRNAs which should be related to the observed drought tolerance. mRNA target transcripts of the miRNAs studied were predicted using the Rose database, which includes transcription factors that regulate plant growth and development. In addition, results showed that the promoter region contains responsive elements to hormone-mediated regulatory elements. Network analysis not only unravelled the interaction between miRNAs and their predicted gene targets but also highlighted the roles of miRNAs in response to drought stress.

Influence of peach–almond hybrids and plum-based rootstocks on mineral nutrition and yield characteristics of ‘Big Top’ nectarine in replant and heavy-calcareous soil conditions

The agronomic performance and leaf mineral nutrition for ‘Big Top’ nectarine budded onto twelve Prunus rootstocks were evaluated. Seven Prunus amygdalus×Prunus persica hybrids (Adafuel, Adarcias, Felinem, Garnem, Monegro, GF 677, and Mayor), two Prunus davidiana×P. persica hybrids (Barrier, Cadaman), a Prunus insititia plum (Adesoto), a Prunus domestica plum (Tetra), and another selection considered to be an hybrid of Prunus cerasifera×P. amygdalus parentage (Replantpac). Rootstocks were budded during the summer of 1999, and trees were established in a replant site in March 2001. The trial was located in the Ebro Valley (Northeastern, Spain) on a heavy-textured and calcareous soil typical of the Mediterranean area which supported a previous peach orchard until 2000. At the thirteenth year after budding, growing conditions generated varying levels of tree mortality, the highest with peach-almond hybrids: Adafuel, Garnem and Monegro. In contrast, all Replantpac trees survived well and the mortality rate was low on the other rootstocks. Adesoto, Tetra, and Adarcias proved to be the most dwarfing rootstocks, while Cadaman and Replantpac were the most invigorating and generated greater cumulative yields. However, the highest yield efficiency was recorded on GF 677, although it did not differ significantly from other peach-almond (Adarcias, Felinem) and plum (Adesoto, Tetra) rootstocks. The highest fruit weight was observed on Barrier and the lowest on Felinem and Mayor, but they did not differ significantly from the rest of rootstocks. Leaf mineral analysis of trees showed all rootstocks induced N and Fe deficiency and P optimum value according to reference values. Nevertheless, the tendency of plum Adesoto to induce higher Fe leaf concentration could indicate higher tolerance to iron-chlorosis in calcareous soils. The most invigorating rootstock Replantpac seems to induce higher SPAD values and adequate K, Mg and Mn values according to reference values. Tetra induced the best balanced nutritional values (ΣDOP), especially when compared with Barrier and Cadaman, although it did not differ significantly from GF 677 and Mayor.

Asymmetrasca decedens (Cicadellidae, Typhlocybinae), a natural vector of ‘Candidatus Phytoplasma phoenicium’

Abstract‘Candidatus Phytoplasma phoenicium’ is associated with a lethal disease of almond, peach and nectarine named almond witches'‐broom disease (AlmWB). The disease spread rapidly in Lebanon from coastal areas to elevations exceeding 1200 m, killing over 150,000 trees in a span of two decades. The mode of spread suggested the involvement of efficient vector(s) and Asymmetrasca decedens (Hemiptera, Cicadellidae) was suspected as it is the most abundant leafhopper species present in Lebanese stone fruit orchards. Living A. decedens specimens were collected from fields heavily infested by AlmWB and used in transmission trials on healthy peach almond hybrid GF‐677 and peach GF‐305 seedlings with an inoculation‐access period of 30 days. PCR analysis supported by sequencing showed that A. decedens is a carrier of the phytoplasma, and that the phytoplasma was detected in insect salivary glands and in some inoculated GF‐677 and GF‐305 seedlings. One year post‐inoculation, ‘Ca. P. phoenicium’ was detected in newly emergent leaves of inoculated seedlings. However, the characteristic symptoms of witches'‐broom were not observed. PCR amplified fragments from phytoplasma‐positive seedlings and from A. decedens samples showed 99.9% nucleotide identity in their 16S RNA region and phylogenetic analysis using a neighbour jointing tree confirmed that the phytoplasmas detected in both insects and inoculated seedlings belonged to 16SrIX‐B (D). The present manuscript is the first known report for a leafhopper vector of ‘Ca. P. phoenicium’ and shows that the incubation period of the disease in plants may be longer than 1 year. The importance of phytosanitary control measures, the adoption of a national strategy and regional cooperation in order to contain the further spread of the disease are discussed.

Changes Induced by Fe Deficiency and Fe Resupply in the Root Protein Profile of a Peach-Almond Hybrid Rootstock

The changes in the root extract protein profile of the Prunus hybrid GF 677 rootstock (P. dulcis × P. persica) grown in hydroponics as affected by Fe deficiency and short-term (24 h) Fe resupply have been studied by 2-dimensional gel electrophoresis-based techniques. A total of 335 spots were consistently found in the gels. Iron deficiency caused above 2-fold increases or >50% decreases in the relative abundance in 10 and 6 spots, respectively, whereas one spot was only detected in Fe-deficient plants. Iron resupply to Fe-deficient plants caused increases and decreases in relative abundance in 15 and 16 spots, respectively, and one more spot was only detected in Fe-resupplied Fe-deficient plants. Ninety-five percent of the proteins changing in relative abundance were identified using nanoliquid chromatography-tandem mass spectrometry. Defense responses against oxidative and general stress accounted for 50% of the changes in Fe-deficient roots. Also, a slight induction of the glycolysis-fermentation pathways was observed in GF 677 roots with Fe deficiency. The root protein profile of 24 h Fe-resupplied plants was similar to that of Fe-deficient plants, indicating that the deactivation of Fe-deficiency metabolic responses is slow. Taken together, our results suggest that the high tolerance of GF 677 rootstock to Fe deficiency may be related to its ability to elicit a sound defense response against both general and oxidative stress.

Agronomical and fruit quality traits of two peach cultivars on peach-almond hybrid rootstocks growing on Mediterranean conditions

The agronomical and fruit quality trait influence was evaluated for five almond×peach hybrid and one Prunus davidiana×peach hybrid rootstocks. The six rootstocks, Adafuel, Adarcias, Cadaman, Felinem, Garnem and GF 677, were budded with ‘Tebana’ peach and ‘Queen Giant’ nectarine cultivars during the summer of 1997, and trees were established in two adjacent plots during the winter of 1998–1999. The trial site was located in the Ebro Valley (Zaragoza, Spain), on a heavy and calcareous soil typical of the Mediterranean area.At the twelfth year after budding, growing conditions generated varying levels of tree mortality, the highest with Felinem and Garnem rootstocks. In contrast, all Adarcias and GF 677 trees survived and the mortality rate was low in Adafuel and Cadaman. The lowest vigour was induced by Adarcias for both cultivars, a 37% and 48% reduction in trunk cross-sectional area (TCSA) for ‘Tebana’ and ‘Queen Giant’ respectively compared to vigour on GF 677. For ‘Queen Giant’, cumulative yield was greater on Felinem, although no significant differences were found with Garnem. Other rootstocks that showed high cumulative yields were Adafuel and GF 677. The highest yield efficiency was recorded on Cadaman rootstock with both varieties, although differences were not significant with Felinem for ‘Queen Giant’.On average, the highest fruit weight was recorded on Adafuel and Cadaman for both cultivars. For ‘Queen Giant’, the greatest soluble solids content (SSC) was recorded on Adarcias and Cadaman, and the lowest on Garnem and GF 677. The highest titratable acidity was also induced by Cadaman rootstock but it did not differ significantly from Adarcias. Correlations between some agronomical and fruit quality traits were found. The less vigorous rootstocks seem to induce a better fruit quality to the studied cultivars based on fruit sugar content. Our results show the relationship between the characteristics on plant adaptability and development, such as yield, vigour or fruit weight, and the factors of fruit quality value.

Physiological responses and differential gene expression in Prunus rootstocks under iron deficiency conditions

Two Prunus rootstocks, the Myrobalan plum P 2175 and the interspecific peach–almond hybrid, Felinem, were studied to characterize their biochemical and molecular responses induced under iron-Deficient conditions. Plants of both genotypes were submitted to different treatments using a hydroponic system that permitted removal of Fe from the nutrient solution. Control plants were grown in 90 μM Fe (III)-EDTA, Deficient plants were grown in an iron free solution, and plants submitted to an Inductor treatment were resupplied with 180 μM Fe (III)-EDTA over 1 and 2 days after a period of 4 or 15 days of growth on an iron-free solution. Felinem increased the activity of the iron chelate reductase (FC-R) in the Inductor treatment after 4 days of iron deprivation. In contrast, P 2175 did not show any response after at least 15 days without iron. The induction of the FC-R activity in this genotype was coincident in time with the medium acidification. These results suggest two different mechanisms of iron chlorosis tolerance in both Strategy I genotypes. Felinem would use the iron reduction as the main mechanism to capture the iron from the soil, and in P 2175, the mechanism of response would be slower and start with the acidification of the medium synchronized with the gradual loss of chlorophyll in leaves. To better understand the control of these responses at the molecular level, the differential expression of PFRO2, PIRT1 and PAHA2 genes involved in the reductase activity, the iron transport in roots, and the proton release, respectively, were analyzed. The expression of these genes, estimated by quantitative real-time PCR, was different between genotypes and among treatments. The results were in agreement with the physiological responses observed.

Differential Susceptibility of Prunus Germplasm (Subgenus Amygdalus) to a California Isolate of Xylella fastidiosa

Seedling peach (Prunus persica Batsch) and clonal peach–almond hybrids are popular rootstock choices for commercial almond growers in California. In this study, clonal replicates of peach and almond [P. dulcis (Mill.) D.A. Webb] rootstock germplasm and a first-generation peach–almond hybrid created from them were challenged with Xylella fastidiosa isolate M23. Clonal replicates were needle-inoculated with M23 and maintained in a greenhouse environment for a growing season. Typical almond leaf scorch disease symptoms began to develop on M23-inoculated almonds 11 weeks after inoculation. No leaf scorch symptoms were observed on M23-inoculated peach or peach–almond hybrids. Quantitative real-time polymerase chain reaction revealed consistent levels of X. fastidiosa DNA among inoculated almond replicates, whereas X. fastidiosa DNA was undetectable in replicates of peach–almond hybrids. A trace level of X. fastidiosa DNA was detected in a single peach replicate, and statistical analysis demonstrated that this level differed significantly (P < 0.001) from that detected in almond replicates. Selected almonds were further sampled sequentially along their meristematic axes to examine bacterial titer throughout the trees. Selected almond trees differed significantly (P = 0.036) in bacterial titer, but no significant differences were noted in levels of X. fastidiosa from different vertical sections of the main growth axes. The results suggest that peach and peach–almond hybrid rootstock germplasm used by commercial almond tree nurseries in California are not primary inoculum sources for X. fastidiosa-induced diseases.

Elemental 2-D mapping and changes in leaf iron and chlorophyll in response to iron re-supply in iron-deficient GF 677 peach-almond hybrid

Iron is an essential micronutrient for plant growth and development, involved in key cellular processes. However, the distribution of Fe in plant tissues is still not well known. In the so-called Fe chlorosis paradox, leaves of fruit trees grown in the field usually have high concentrations of Fe but still are Fe-deficient. Leaves of the Prunus rootstock GF 677 (P. dulcis × P. persica) grown in hydroponics have been used to carry out two-dimensional (2-D) nutrient mapping by synchrotron radiation-induced X-ray fluorescence. Iron-deficient leaves accumulated more Fe in the midrib and veins, with Fe concentration being markedly lower in mesophyll leaf areas. The effects of Fe deficiency and Fe re-supply on leaf chlorophyll concentration and on the distribution of Fe and other nutrients within different plant tissues have been investigated in the same plants. After Fe re-supply, leaf Fe concentrations increased largely in all leaf types. However, whereas re-greening was almost completely achieved in apical leaves, in some expanded leaves the increase in chlorophyll concentration was only moderate. Therefore, after Fe re-supply Fe-deficient expanded leaves of the Prunus rootstock GF 677 had significant increases in Fe concentration but were still chlorotic. This is similar to what occurs in leaves of peach trees in field conditions, opening the possibility that this system could be used as a model to study the Fe chlorosis paradox.

Advanced generation peach-almond hybrids as seedling rootstocks for almond: first year growth and potential pollenizers for hybrid seed production

For decades, seedling peaches have been used as the standard rootstock in California almond orchards. Vigorous, deep rooted trees are needed in almond orchards for maximum yields and to withstand the annual tree-shaking at harvest. Currently, researchers are actively evaluating rootstocks for almonds in field trials and in various screening protocols. In this study, seedling rootstocks, obtained from male-sterile advanced generation peach-almond (PEAL) hybrid mother trees, were compared with ‘Nemaguard’ peach seedlings for emergence in the nursery row, trunk caliper at propagation time, and end of season dormant above ground tree weight. Seedling emergence was affected significantly (P ≤ 0.05) by seed source, as was trunk caliper and end of season dormant above ground tree weight. Trunk caliper and dormant above ground tree weight were also affected significantly (P ≤ 0.05) by planting year. Results obtained in this study demonstrate the enhanced first year growth from seedlings of advanced generation PEAL hybrids, as compared to ‘Nemaguard’ seedlings. Seedling emergence in the rootstock bed was not affected significantly by planting year. Advanced generation PEAL hybrid seedlings were ready for June-budding at an earlier date compared to ‘Nemaguard’ seedlings, providing the potential for larger-sized finished nursery stock by the end of the growing season. Due to the male-sterile status of the advanced generation PEAL mother trees, bloom periods of several root-knot nematode resistant rootstock cultivars were examined for their degree of synchronicity with the mother trees. The examined rootstocks and mother trees varied in both chill hour and post-chill heat requirements necessary to effect bloom. ‘Flordaguard’ peach rootstock began bloom in advance of the male-sterile mother trees, whereas the bloom period of ‘Tsukuba No. 4’ occurred well after, suggesting they would not be effective synchronous pollenizers for consistent hybrid seed production. Based on more limited flowering period data, better bloom synchronicity was achieved with a Tsukuba No. 4 X Flordaguard hybrid.

Efficacy of the Non-Pathogenic Agrobacterium Strains K84 and K1026 against Crown Gall in Tunisia

The non-pathogenic Agrobacterium radiobacter strain K84 and its genetically modified (GEM) strain K1026 were tested for their effectiveness against local Tunisian strains and two reference strains (C58 and B6) of the crown gall bacterium Agrobacterium tumefaciens. Tests in planta were carried out on herbaceous plants (tomato and tagetes) and on some sensitive rootstocks (bitter almond, peach almond hybrid GF677 and quince BA29). In vitro tests showed that both K84 and K1026 were effective and that the difference between these strains was not statistically significant. On tomato and tagetes, strain K84 was effective against all crown gall isolates with the exception of the A. tumefaciens reference strain B6. GEM strain K1026 was very effective against all isolates from Tunisia and against the reference strains. Both antagonistic strains significantly reduced the percentage of galled plants as well as the number of galls per plant. Under field conditions, both antagonists controlled crown gall effectively. Best results were obtained on the bitter almond-tree rootstock. Antagonist effectiveness was less evident on quince BA29 and peach almond GF677 rootstocks. The genetically modified strain K1026 is of interest in controlling crown gall disease in Tunisia.