Tolerant Rootstocks Research Articles

ASSESSMENT OF MORPHOLOGICAL AND PHYSIOLOGICAL RESPONSES OF PEACH ROOTSTOCKS UNDER DROUGHT AND ALUMINUM STRESS

The challenging constrain of peach production in highlands of northern Thailand is limited irrigation water and acidic soil. These problems could be alleviated by using a suitable rootstock. Field screening for drought and acidic soil tolerant rootstocks is time consuming and costly. Therefore, any parameter related to these traits detected in a greenhouse screening would be very useful as a selection index for a breeding program. In acidic soil, aluminum toxicity is one of the major confinements on plant growth. The aim of this study was to evaluate potential morphological (shoot fresh and dry weight, leaf size and leaf burn) and physiological (photosynthesis, chlorophyll a content, leaf conductance, leaf water potential and proline content) parameters related to drought and aluminum stress. A factorial experiment with two levels of the irrigation: 1) well watered (100% of evapotranspiration, ET) and 2) drought treatment (25-50% of ET), combined with three levels of aluminum: 1) no aluminum, 2) 50 mg kg-1 of aluminum and 3) 100 mg kg-1 of aluminum was done with four-week-old ‘Flordaguard’ rootstocks. The results showed that drought and aluminum stress caused leaf burn. Drought treatment significantly reduced shoot fresh weight (50%), shoot dry weight (35%), photosynthesis rate (80%), leaf conductance (90%) and leaf water potential (75%). Conversely, drought effect increased proline content by 10%. Aluminum stress caused linear decrease in shoot fresh weight and quadratic decrease in shoot dry weight and leaf size, but leaf burn was quadratic increased. For drought stress all parameters, except leaf size, leaf burn and chlorophyll content, were potentially selection index. Meanwhile, for aluminum stress, potential selection parameters were shoot fresh and dry weight, leaf size and leaf burn.

Sodium and Chloride Distribution in Grapevines as a Function of Rootstock and Irrigation Water Salinity

The salt tolerance of rootstocks is often assessed based on their ability to limit uptake of sodium (Na) and chloride (Cl) ions. Here, we evaluated the effects of three irrigation salinity levels (electrical conductivity of 1.2, 2.7, and 4.2 dS/m) on Cabernet Sauvignon grapes grafted on Ruggeri, Paulsen, 216/3, and 101/14 rootstocks. Growth parameters were affected by salinity level but not by rootstock, and yield was not affected by either variable. Ruggeri and 216/3 were most effective at limiting uptake and accumulation of Na and Cl in scion petioles, wood, and must. The rootstocks differentially excluded Na and Cl from vines; 216/3 and Ruggeri showed the best performance for Na and Cl, respectively. More Na than Cl accumulated in woody tissue. Mortality rates as high as 17.5% were found for poor salt-excluding rootstocks irrigated with the highest salinity water. The apparent breakdown of tolerance mechanisms, leading to salt damage and vine mortality, might be due to Na reaching critical levels in woody tissues. The ability to exclude Na and Cl from shoots and fruit was found to (a) increase wine quality by reducing concentrations of salt ions in must and wine, and (b) reduce mortality rates that result from long-term exposure to salt.

Some rootstocks improve pepper tolerance to mild salinity through ionic regulation

Grafting has been proposed as an interesting strategy that improves the responses of crops under salinity. In pepper, we reported increased fruit yield of the commercial ‘Adige’ cultivar under salinity when grafted onto accessions Capsicum chinense Jacq. ‘ECU-973′ (12) and Capsicum baccatum L. var. pendulum ‘BOL-58′ (14), whereas no effect was observed when grafted onto accession Capsicum annuum L var. ‘Serrano’ (5). We also analysed the physiological and biochemical mechanisms related to the tolerance conferred by these rootstocks. Responses to salinity (40mM NaCl) were studied in the different plant combinations for 30 days by determining water relations, mineral content, proline accumulation, photosynthetic parameters, nitrate reductase activity and antioxidant capacity. Higher salt tolerance was achieved when the ‘Adige’ cultivar was grafted onto the 12 genotype, which allowed not only lower Na+ and Cl− accumulation in the scion, but also ion selectivity maintenance, particularly Na+/K+ discrimination. These traits led to a minor negative impact on photosynthesis, nitrate reductase activity and lipid peroxidation in grafted scion leaves. This work suggests that using tolerant pepper rootstocks that maintain the scion's ion homeostasis is a promising strategy to provide salinity tolerance and can consequently improve crop yield.

Flooding and Phytophthora cinnamomi: Effects on photosynthesis and chlorophyll fluorescence in shoots of non-grafted Persea americana (Mill.) rootstocks differing in tolerance to Phytophthora root rot

Losses in the production of avocado (Persea americana (Mill.)) are incurred due to Phytophthora root rot (PRR), a disease of the feeder roots that results in tree-dieback and eventual tree death. Avocado is also a flood-sensitive species and flooding exacerbates the effects of PRR. The avocado industry relies on the use of rootstocks tolerant to PRR to minimise losses. The present study compared the gas exchange and chlorophyll fluorescence responses of avocado rootstock plants of ‘Dusa™’, the current South African industry standard, with ‘Duke 7’, and the selections R0.12 and R0.06 which show reduced and superior tolerance to PRR, respectively. A decline in stomatal conductance (gs) and net CO2 assimilation (PN) over the 30day evaluation period were early responses to flooding. ‘Dusa™’, the more tolerant rootstock plants, demonstrated a better recovery in PN and gs in response to inoculation; however, both rootstocks performed poorly under flooded conditions. A decline in PN in infected ‘Duke 7’ plants appeared to be associated with stomatal limitations due to reduced stomatal conductance. The decline in PN and gs was not apparent in infected ‘Dusa™’ plants. Non-stomatal limitations to PN in rootstock plants exposed to flooding were also evident as indicated by increases in the ratio of internal to atmospheric CO2 concentrations (Ci/Ca). Impaired photosynthetic capacity in flooded rootstock plants was reflected by reduced photosystem II efficiency and photochemical quenching. In comparison to ‘Dusa™’, R0.12 rootstock plants showed reduced PN and gs following inoculation with Phytophthora cinnamomi whereas the more tolerant R0.06 rootstock plants revealed sustained photosynthetic activity. Interestingly R0.06 was the only rootstock able to maintain PN and gs in non-inoculated, flooded plants.

Tomato susceptibility to Fusarium crown and root rot: Effect of grafting combination and proteomic analysis of tolerance expression in the rootstock

Grafting can enhance the tolerance of vegetable crops to soilborne diseases. The aim of this study was to investigate whether different tomato scion–rootstock combinations may affect the plant susceptibility to Fusarium oxysporum f. sp. radicis-lycopersici (FORL), the causal agent of crown and root rot. A proteomic approach was used to investigate whenever the protein repertoire of the rootstock can be modified by FORL infection, in relation to cultivar susceptibility/tolerance to the disease. To this purpose, plants of tomato hybrids with different vigor, “Costoluto Genovese” (less vigorous) and “Kadima” (more vigorous), were grafted onto “Cuore di Bue” and “Natalia” hybrids, sensitive and tolerant versus FORL infections, respectively. Disease symptoms, plant biomasses, and protein expression patterns were evaluated 45 days after FORL inoculation. The extent of vascular discoloration caused by FORL in tomato plants grafted on “Natalia” rootstock (0.12–0.37 cm) was significantly lower than that of plants grafted on sensitive “Cuore di Bue” (1.75–6.50 cm). FORL symptoms significantly differed between “Costoluto Genovese” and “Kadima” scions only when grafted on sensitive rootstock. Shoot FW of non-inoculated “Kadima”/“Cuore di Bue” combination was 35% lower than “Kadima”/“Natalia”, whereas no difference was manifested in inoculated plants. Shoot FW of inoculated “Costoluto Genovese”/“Cuore di Bue” combination was decreased of 39%, whereas that of “Costoluto Genovese”/“Natalia” of 11%, compared to control plants. Proteomic results showed a higher representation of proteins associated with pathogen infection in the tolerant rootstock, compared to the sensitive one, meaning a direct involvement of plant defence mechanisms in the tomato response to the pathogen challenge.

DEVELOPMENT OF AUSTRALIAN ROOTSTOCKS WITH ROOT-KNOT NEMATODE RESISTANCE AND LOW POTASSIUM TRANSPORT

Soils in some major Australian viticultural regions are generally high in potassium content, which is in contrast to viticultural regions in other parts of the world. There is a considerable body of evidence, both from commercial experiences and research results, that the widely adopted high vigour, root-knot nematode tolerant rootstock varieties contribute to negative impacts on wine quality associated with high potassium uptake, high pH and malate levels. Previous findings have shown that in a given year, grafted vine grape juice pH at harvest is highly correlated with the ungrafted rootstock petiole potassium level at anthesis. We demonstrated that this relationship is also present when multiple years of juice pH and ungrafted rootstocks petioles are compared. The mean harvest pH of ?Chardonnay? grafted on 6 commer¬cial rootstocks from 4 vineyards over two consecutive years had a strong correlation (adjusted R2= 0.755) with the 2 year mean potassium content of the ungrafted rootstocks in the field. Likewise, the 8 year mean pH of a rootstock trial with 5 commercial rootstock and 14 experimental rootstocks with ?Shiraz? scions was highly correlated (adjusted R2= 0.502) with the 2 year mean petiole potassium content of the ungrafted field grown rootstocks. To facilitate accelerated breeding of new rootstocks, we have developed a glasshouse based assay for the early selection of rootstocks with low potassium transport to aerial tissues. We demonstrated that the 8 year mean juice pH of grafted vines is also correlated with the ungrafted vine petiole potassium content for plants grown in the glasshouse with supplemented levels of potassium (adjusted R2= 0.501). In addition, we are also screening vines in the glasshouse for resistance to the root-knot nematode Meloidogyne javanica. We are currently screening potential and commercial germplasm for resistance to M. javanica, and have identified a Vitis cinerea accession that transmits resistance as a single dominant gene.

Rootstock alleviates PEG-induced water stress in grafted pepper seedlings: Physiological responses

Recent studies have shown that tolerance to abiotic stress, including water stress, is improved by grafting. In a previous work, we took advantage of the natural variability of Capsicum spp. and selected accessions tolerant and sensitive to water stress as rootstocks. The behavior of commercial cultivar 'Verset' seedlings grafted onto the selected rootstocks at two levels of water stress provoked by adding 3.5 and 7% PEG (polyethylene glycol) was examined over 14 days. The objective was to identify the physiological traits responsible for the tolerance provided by the rootstock in order to determine if the tolerance is based on the maintenance of the water relations under water stress or through the activation of protective mechanisms. To achieve this goal, various physiological parameters were measured, including: water relations; proline accumulation; gas exchange; chlorophyll fluorescence; nitrate reductase activity; and antioxidant capacity. Our results indicate that the effect of water stress on the measured parameters depends on the duration and intensity of the stress level, as well as the rootstock used. Under control conditions (0% PEG) all plant combinations showed similar values for all measured parameters. In general terms, PEG provoked a strong decrease in the gas exchange parameters in the cultivar grafted onto the sensitive accessions, as also observed in the ungrafted plants. This effect was related to lower relative water content in the plants, provoked by an inefficient osmotic adjustment that was dependent on reduced proline accumulation. At the end of the experiment, chronic photoinhibition was observed in these plants. However, the plants grafted onto the tolerant rootstocks, despite the reduction in photosynthetic rate, maintained the protective capacity of the photosynthetic machinery mediated by osmotic adjustment (based on higher proline content). In addition, water stress limited uptake and further NO3(-) transfer to the leaves. Increased nitrate reductase activity in the roots was observed, mainly in plants grafted onto the sensitive rootstocks, as well as the ungrafted plants, and this was associated with the lessened flux to the leaves. This study suggests that PEG-induced water stress can be partially alleviated by using tolerant accessions as rootstocks.

De Novo Sequencing, Assembly, and Analysis of the Root Transcriptome of Persea americana (Mill.) in Response to Phytophthora cinnamomi and Flooding

Avocado is a diploid angiosperm containing 24 chromosomes with a genome estimated to be around 920 Mb. It is an important fruit crop worldwide but is susceptible to a root rot caused by the ubiquitous oomycete Phytophthora cinnamomi. Phytophthora root rot (PRR) causes damage to the feeder roots of trees, causing necrosis. This leads to branch-dieback and eventual tree death, resulting in severe losses in production. Control strategies are limited and at present an integrated approach involving the use of phosphite, tolerant rootstocks, and proper nursery management has shown the best results. Disease progression of PRR is accelerated under high soil moisture or flooding conditions. In addition, avocado is highly susceptible to flooding, with even short periods of flooding causing significant losses. Despite the commercial importance of avocado, limited genomic resources are available. Next generation sequencing has provided the means to generate sequence data at a relatively low cost, making this an attractive option for non-model organisms such as avocado. The aims of this study were to generate sequence data for the avocado root transcriptome and identify stress-related genes. Tissue was isolated from avocado infected with P. cinnamomi, avocado exposed to flooding and avocado exposed to a combination of these two stresses. Three separate sequencing runs were performed on the Roche 454 platform and produced approximately 124 Mb of data. This was assembled into 7685 contigs, with 106 448 sequences remaining as singletons. Genes involved in defence pathways such as the salicylic acid and jasmonic acid pathways as well as genes associated with the response to low oxygen caused by flooding, were identified. This is the most comprehensive study of transcripts derived from root tissue of avocado to date and will provide a useful resource for future studies.

Growth rate of fine root systems influences rootstock tolerance to replant disease

Aims Plant tolerance to herbivory has often been linked to plant growth rate, with faster growing plants that present high tissue turnover rates expected to be more tolerant than slower-growing plants. We tested whether this relationship also holds for rootstock growth rate and tolerance to apple replant disease (ARD).

The physiological and nutritional responses of seven different citrus rootstock seedlings to boron deficiency

Carrizo citrange was the most tolerant citrus rootstock to B-deficiency and some physiological performance could be attributed to the decreased mineral nutrient concentrations caused by B-deficiency. Boron (B) is an essential microelement for normal growth and development in vascular plants, and adequate B nutrition is crucial for agricultural production. Although citrus plants are not classified as the most sensitive species to B-deficiency, the occurrence of B-deficiency has been reported in the major citrus producing countries of the world, including the east and south of China. In this study, in order to evaluate the effects of B-deficiency on plant growth, root-morphological traits, B and other nutritional responses of citrus rootstock and to investigate the relationship between this physiological performance and mineral nutrients seven common rootstock seedlings, including Trifoliate orange (TO), Carrizo citrange (CC), Chongyi tangerine (CT), Red tangerine (RT), Cleopatra mandarin (CM), Fragrant citrus (FC), and Sour orange (SO), were treated by B-deficiency (0 mg L−1) or moderate B (0.25 mg L−1). All the seedlings were grown in hydroponics situation with modified 1/2-strength Hoagland’s solution under greenhouse conditions for 10 weeks. The results showed that B-deficiency inhibited the growth and development of all tested citrus rootstocks, but substantial differences were observed among these rootstocks. Different visible symptoms were observed both in the leaf and root. Corking of the leaf veins and leaf yellowing symptoms were observed on all rootstock genotypes except on CC, which exhibited a little discoloration at the end of the experiment. In addition, root growth of the citrus seedlings were also decreased by B-deficiency, but the decreases were more obvious in TO and FC. It was worth noting that B-deficiency inhibited lateral root growth and development more significantly than tap root, but not in lateral root initiation. The different performance of these rootstock genotypes indicated that CC was the most tolerant while TO was the most sensitive to B-deficiency. In addition, under B-deficiency conditions, not only the B concentration, but also the other mineral nutrient concentrations were influenced, especially in Mg, Fe and Mn. This change in nutrient concentrations might partly contribute to the seedlings’ physiological performances under B-deficiency.

A Transcriptomic Analysis of Sensitive and Tolerant Citrus Rootstocks under Natural Iron Deficiency Conditions

Iron chlorosis is one of the most serious abiotic stresses affecting citrus (Citrus sp.) culture in the Mediterranean Basin. A trial was performed with potted tolerant and sensitive rootstocks that were grown in volcanic and calcareous soils. Microarray analysis allowed for the identification of differentially expressed genes putatively involved in iron (Fe) deficiency. Most of the differentially expressed genes isolated from the root tips were of unknown function; the remaining genes were related to the oxidative stress response (e.g., glutathione peroxidase), hormone metabolism and signaling (e.g., small auxin up RNA family protein genes), biological regulation, protein turnover, and the tricarboxylic acid cycle (e.g., aconitase). Additionally, the majority of the Fe stress-related genes expressed in the sensitive Swingle citrumelo (Citrus paradisi × Poncirus trifoliata) and tolerant Carrizo citrange (Citrus sinensis × P. trifoliata) rootstocks identified using real-time reverse transcription–polymerase chain reaction (RT-PCR) were related to regulation, the oxidative stress response, and hormone metabolism and signaling, thereby confirming the array data. Furthermore, validation of the differentially expressed genes in seven tolerant and sensitive rootstocks grown in a field trial under chlorotic conditions was performed. In general, the gene expression profiles reflect the different responses of rootstocks, possibly as a result of the various genetic mechanisms involved in the response to Fe deficiency. Moreover, the expression of aconitase was analyzed in the roots and juice to evaluate the implication of the different aconitase isoforms (Aco), which are derived from specific cellular compartments, in the different tissues. The involvement of the mitochondrial isoform (Aco2) was directly correlated with the acidity of the juice, whereas the cytosolic one (Aco3), which corresponds to the aconitase isolated during the microarray analysis, was found specifically in the roots.

Role of ascorbic acid in enhancing hypoxia tolerance in roots of sensitive and tolerant apple rootstocks

Ascorbic acid (AsA) has an important role in plant growth and development, protecting cells against oxidative challenges when exposed to environmental stresses. We investigated the mechanism by which exogenous AsA enhances hypoxia tolerance and alleviates oxidative damage in sensitive and tolerant apple rootstocks (Malus sieversii and M. hupehensis). Although such stress inhibited seedling growth in both species, the extent of this effect differed greatly between the two genotypes. Under stress, dry weights were significantly lower from the hypoxia-sensitive M. sieversii than from the controls, but they increased significantly with the addition of AsA. The contents of hydrogen peroxide (H2O2) and superoxide radicals (O2−) were significantly increased in roots of both species exposed to hypoxia. This led to oxidative damage, which was indicated by the accumulation of malondialdehyde (MDA). However, that reaction in M. sieversii was significantly suppressed when AsA was supplemented. Exogenously applied AsA enhanced the activities of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), glutathione reductase (GR), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in both species, albeit to a greater extent in M. sieversii than in M. hupehensis under hypoxia stress. Exogenously applied AsA raised its endogenous levels of both Malus species exposed to hypoxia, but to a greater extent in M. sieversii. AsA applications also slightly enhanced root glutathione (GSH) contents in stressed plants, but differences were not statistically significant between the treated and untreated plants. AsA possibly enhance hypoxia tolerance by elevating its endogenous level and improving the capacity of the ascorbate–glutathione cycle, thereby decreasing the occurrence of oxidative damage in the membranes.

A genome-wide expression profile of salt-responsive genes in the apple rootstock Malus zumi.

In some areas of cultivation, a lack of salt tolerance severely affects plant productivity. Apple, Malus x domestica Borkh., is sensitive to salt, and, as a perennial woody plant the mechanism of salt stress adaption will be different from that of annual herbal model plants, such as Arabidopsis. Malus zumi is a salt tolerant apple rootstock, which survives high salinity (up to 0.6% NaCl). To examine the mechanism underlying this tolerance, a genome-wide expression analysis was performed, using a cDNA library constructed from salt-treated seedlings of Malus zumi. A total of 15,000 cDNA clones were selected for microarray analysis. In total a group of 576 cDNAs, of which expression changed more than four-fold, were sequenced and 18 genes were selected to verify their expression pattern under salt stress by semi-quantitative RT-PCR. Our genome-wide expression analysis resulted in the isolation of 50 novel Malus genes and the elucidation of a new apple-specific mechanism of salt tolerance, including the stabilization of photosynthesis under stress, involvement of phenolic compounds, and sorbitol in ROS scavenging and osmoprotection. The promoter regions of 111 genes were analyzed by PlantCARE, suggesting an intensive cross-talking of abiotic stress in Malus zumi. An interaction network of salt responsive genes was constructed and molecular regulatory pathways of apple were deduced. Our research will contribute to gene function analysis and further the understanding of salt-tolerance mechanisms in fruit trees.

EFFECT OF PACLOBUTRAZOL AND PUTRESCINE ON ANTIOXIDANT ENZYMES ACTIVITY AND NUTRIENTS CONTENT IN SALT TOLERANT CITRUS ROOTSTOCK SOUR ORANGE UNDER SODIUM CHLORIDE STRESS

The effects of paclobutrazol (PBZ) and putrescine (Put) on antioxidant enzymes activity, proline contents and nutrients uptake were studied on salt tolerant citrus rootstock sour orange. Six-month-old nucellar seedlings grown in pots and subjected to three levels of PBZ and two levels each of salinity and Put for 90 days. Seedlings treated with PBZ or Put alone or in combination had higher anti-oxidant enzymes activities, accumulation of proline and nutrients contents like potassium (K+) and calcium (Ca2+) under both saline and non-saline conditions. Further, application of PBZ or Put alone or in combination also reduced the accumulation of both Na+ and Cl− ions in leaves and roots in NaCl stressed seedlings. A combined application of 250 mg L−1 PBZ and 50 mg L−1 Put proved to be more effective in improving proline and Ca2+ content and restricting accumulation of Na+ ions in leaf tissues.

Development of a Nested Quantitative Real-Time PCR for DetectingPhytophthora cinnamomiinPersea americanaRootstocks

Phytophthora cinnamomi causes Phytophthora root rot (PRR) in avocado (Persea americana), an important disease that causes severe economic losses to the avocado industry globally. To date, no PRR-resistant avocado rootstock variety has been discovered, although certain rootstock varieties have been shown to be more tolerant than others. In this study, we developed an accurate, low cost assay for in planta quantification of P. cinnamomi to evaluate disease tolerance. A nested real-time polymerase chain reaction assay was developed to sensitively detect pathogen DNA in plant tissues. Root samples from a highly tolerant (Dusa) and less tolerant (R0.12) rootstock were collected at 0, 3, 7, 14, and 21 days after inoculation with P. cinnamomi and used for pathogen quantification. Nested primers developed in this study were specific and sensitive and could detect P. cinnamomi in root tissues. The amount of P. cinnamomi quantified in roots was significantly higher in the less-tolerant R0.12 plants when compared with the highly tolerant Dusa plants at all time points. This study has confirmed the known status of disease tolerance of Dusa and R0.12 avocado rootstocks in a quantitative manner and provides a reliable molecular tool to assist with industry breeding programs for the selection of PRR-resistant avocado rootstock varieties.

Overcoming Salinity Barriers to Crop Production Using Traditional Methods

Salinity is a major problem in arid and semi-arid regions, where irrigation is essential for crop production. Major sources of salinity in these regions are salt-rich irrigation water and improper irrigation management. The effects of salinity on crops include inhibition of growth and production, and ultimately, death. There are two main approaches to alleviating the adverse effects of salinity on agricultural crops: (i) development of salt-tolerant cultivars by screening, conventional breeding or genetic engineering, and (ii) the traditional approach dealing with treatments and management of the soil, plants, irrigation water, and plant environment. The success of the first approach is limited under commercial growing conditions, because salt-tolerance traits in plants are complex. The present paper reviews, analyzes, and discusses the following traditional approaches: (i) improving the plant environment, (ii) exploiting interactions between plant roots and bacteria and fungi, and (iii) treating the plant directly. With respect to improving the plant environment, we review the possibilities of decreasing salt content and concentration and improving the nutrient composition and concentration in the root zone, and controlling the plant's aerial environment. The interactions between salt-tolerant bacteria or mycorrhizal fungi and root systems, and their effects on salt-tolerance, are demonstrated and discussed. Discussed treatments aimed at alleviating salinity hazard by treating the plant directly include priming of seeds and young seedlings, using proper seed size, grafting onto tolerant rootstocks, applying non-enzymatic antioxidants, plant growth regulators or compatible solutes, and foliar application of nutrients. It can be concluded from the present review that the traditional approaches provide promising means for alleviating the adverse effects of salinity on agricultural crops.

Class 1 non-symbiotic and class 3 truncated hemoglobin-like genes are differentially expressed in stone fruit rootstocks (Prunus L.) with different degrees of tolerance to root hypoxia

Root hypoxia produced by flooding or over-irrigation limits stone fruit tree development, particularly in orchards established on soils with restricted drainage. To overcome this problem, stone fruit trees are usually grafted on rootstocks (species or hybrid of the Prunus L. genus) with different degrees of tolerance to root hypoxia. However, the molecular base of such variability is largely unknown. In Arabidopsis thaliana (Heynh.), as well as in a number of crops and tree species, hemoglobin (Hb)-like genes stand out among hypoxia-related genes, but no such studies have been done with the Prunus species used as rootstocks. In this study, we analyzed the expression pattern of class 1 non-symbiotic Hb-like (nsHb) and class 3 truncated Hb-like (trHb) genes in Prunus rootstock roots with different responses to this stress. We observed that the putative Prunus nsHb and trHb genes were induced by root hypoxia in all analyzed Prunus genotypes, independently of their tolerance to hypoxia. However, Prunus nsHb and trHb genes had higher expression levels in roots of tolerant rootstocks. Prunus nsHb and trHb genes were also regulated by other abiotic stresses, such as salt stress and low temperatures. Our results suggest that changes in nsHb and trHb expressions could be part of the adaptive mechanisms that have evolved in the Prunus species to survive under hypoxia or other types of environmental stress that commonly challenge stone fruit tree orchards.

Evaluation of the protective capacity of new mild Citrus tristeza virus (CTV) isolates selected for a preimmunization program

The use of tolerant rootstocks and preimmunization has satisfactorily controlled losses associated with the Citrus tristeza virus (CTV). Several researchers have shown that CTV mild isolates that are selected in the same region where they are used are superior to isolates obtained from other areas. Thus, budwoods of 20 outstanding citrus trees were collected in north and northwestern Parana state (Brazil) citrus-producing areas and established to be used in a preimmunization program. These budwoods were tested to evaluate the potential protection of the inherently present viral complex. Based on biological indexing and molecular characterization of the capsid protein gene by RFLP (restriction fragment length polymorphism, which indicated that the plants were infected with mild isolates of CTV), some of the selected plants could be used in a preimmunization program. These potentially mild and protective isolates were challenged with severe 'Rolândia' isolate inoculations by grafting and by the brown citrus aphid (Toxoptera citricida Kirkaldy) vector, which was faster in transmitting the virus. Some isolates had a better protective value than others, particularly when challenged with the severe CTV isolate. The SSCP (single strand conformational polymorphism) molecular analysis was an excellent complementary tool for monitoring the performance of the experiments and the stability of the viral complex present in the plants. Isolate number 1, collected in the municipality of Cruzeiro do Sul (CS-1), was the most promising for protecting commercial Pera sweet orange (C. sinensis L.) orchards in northern and northwestern Parana. The Rolândia severe CTV isolate was stable and had a high genetic divergence among the severe isolates used as a control (Capao Bonito and Barao B) and all of the isolates tested.

Physiological and biochemical responses of the iron chlorosis tolerant grapevine rootstock 140 Ruggeri to iron deficiency and bicarbonate

Iron (Fe) deficiency chlorosis associated with high levels of soil bicarbonate is one of the main nutritional disorders observed in sensitive grapevine genotypes. The aim of the experiment was to assess both the independent and combined effects of Fe and bicarbonate nutrition in grapevine. Plants of the Fe chlorosis tolerant 140 Ruggeri rootstock were grown with and without Fe(III)-EDTA and bicarbonate in the nutrient solution. SPAD index, plant growth, root enzyme (PEPC, MDH, CS, NADP+ −IDH) activities, kinetic properties of root PEPC, organic acid concentrations in roots and xylem sap and xylem sap pH were determined. A factorial statistical design with two factors (Fe and BIC) and two levels of each factor was adopted: +Fe and −Fe, and +BIC and −BIC. This rootstock strongly reacted to Fe deficiency by activating several response mechanisms at different physiological levels. The presence of bicarbonate in the nutrient solution changed the activity of PEPC and TCA related enzymes (CS, NADP+-IDH) and the accumulation/translocation of organic acids in roots of Fe-deprived plants. Moreover, this genotype increased root biomass and root malic acid concentration in response to high bicarbonate levels in the substrate. Bicarbonate also enhanced leaf chlorophyll content. Along with a clear independent effect on Fe nutrition, our data support a modulating role of bicarbonate on Fe deficiency response mechanisms at root level.

Expression of defence-related genes against Phytophthora cinnamomi in five avocado rootstocks

Avocado (Persea americana) – a major fruit crop worldwide – is threatened by root rot caused by Phytophthora cinnamomi. This pathogen is known to infect the plant via the feeder roots leading to branch dieback, and eventually tree mortality. While it is known that different avocado rootstocks have varying degrees of susceptibility to Phytophthora root rot, little research has been done on the avocado–Phytophthora interaction. In this study, transcript abundance levels of defence-related genes coding for phenylalanine ammonia-lyase, lipoxygenase, pathogenesis-related protein 5, endochitinase, gluthathionine S-transferase and metallothionein were characterised and compared among five rootstocks with varying susceptibility to root rot, after exposure to P. cinnamomi. Root samples were collected at 0 h, 3 h, 6 h, 12 h, 24 h, 48 h and 72 h post-infection and transcript abundance of the defence-related genes was determined using quantitative real-time reverse transcription PCR. The results indicated the involvement of PR-5 and endochitinase in the defence response of all avocado rootstocks to P. cinnamomi but these genes could not be directly linked to the observed phenotypic resistance. PR-5 and endochitinase were highly upregulated at 72 h post-infection. Differences in transcript abundance of phenylalanine ammonia-lyase and lipoxygenase genes were seen when comparing tolerant and less tolerant rootstocks, which may suggest that transcripts of these genes contribute to resistance. These data provide important insights into plant defence and into how different avocado rootstocks may exhibit increased resistance to infection by P. cinnamomi.