Salinity-calcium interactions on growth and ionic concentration of Citrus plants

Two-year-old Navel orange scions (Citrus sinensis (L.) Osbeck) budded to either Cleopatra mandarin (C. reticulata) and Troyer citrange (C. sinensis × P. trifoliata) rootstocks were used in this experiment. Cleopatra manda in rootstock was considered more tolerant to salinity than Troyer citrange, and this property was attributed to a greater capacity to exclude chloride ions.Plants were grown under glasshouse conditions and supplied with nutrient solution containing either no or 45 mM NaCl. Calcium concentration was increased from 3 to 30 mM. Sodium, potassium, calcium and chloride concentrations in plant organs were analyzed after 90 days of treatment.Supplemental Ca was found to mitigate the adverse effects of salinity on plant growth, defoliation or leaf injury.Chemical analysis indicated that in plants grafted on Troyer citrange Ca restricted uptake and subsequent translocation of Na to the leaves and increased K concentration in both roots and leaves. However, in Cleopatra mandarin-grafted plants increasing Ca levels seemed to reduce transport of Na from roots to leaves, and Na accumulation in roots was associated with reduced concentration of K in this rootstock.Organ chloride analysis showed that Cl accumulation in leaves of plants grafted on both rootstocks was reduced when external Ca concentration increased, whereas Cl concentration in roots remained constant or increased. The data of distribution of Cl in plants showed that a high external Ca level increased Cl accumulation in the basal stem and roots, and reduced the transport of Cl from roots to leaves.     

Effect of NaCl concentrations in irrigation water on growth and polyamine metabolism in two citrus rootstocks with different levels of salinity tolerance

Six-months-old, uniform sized seedlings of two citrus rootstocks; Cleopatra mandarin (Citrus reshni Hort. ex Tan) and Troyer citrange (Poncirus trifoliata × Citrus sinensis) were irrigated with half-strength Hoagland nutrient solution containing 0, 40 or 80 mM NaCl for 12 weeks. Shoot height, leaf number and fresh weights of the seedlings, and relative chlorophyll contents, chlorophyll fluorescence yields (Fv/Fm), net photosynthetic and respiration rates in the leaves decreased with the increase in salinity level in the irrigation water. The decrease was greater in Troyer citrange as compared to Cleopatra mandarin. The concentrations of sugars i.e. fructose, glucose and sucrose in the leaves of Cleopatra mandarin and both leaves and roots of Troyer citrange decreased with the increase in salinity level. However, the concentrations in the roots of Cleopatra mandarin increased with the increase in salinity level. Free proline content in the leaves of Troyer citrange and root tissue of Cleopatra mandarin also increased with the increased salinity level. Among the polyamines, spermine titer increased in the leaves of both rootstocks as a response to salinity treatments. Na⁺ concentrations were higher in leaf and root tissue of Cleopatra mandarin, while that of Cl⁻ were higher in Troyer citrange.     

Contrasts between Citrus species in response to salinisation: An analysis of photosynthesis and water relations for different rootstock-scion combinations

Leaf gas exchange, water relations and ion content were measured on two-year-old Valencia orange (Citrus sinensis [L.] Osbeck), Washington Navel orange (C. sinensis) and Marsh grapefruit (C. parodisi Macfad) scions budded to either Trifoliata (Poncirus infoliata [L] Raf) or Cleopatra mandarin (C. reticuLua Blanco) rootstoeks. Trees were watered with dülute nutrient solution containing either 0 or 50 _mM NaCl for 77 days. Leaf chloride concentrations (cell sap basis) were higher in all scions budded on “Trifoliata but sodium levels were lower than in equivalent foliage budded on Cleopatra mandarin rootstock. Foliar salt levels also varied according to scion. Leaves of Marsh grapefruit had higher levels of both sodium and chloride than leaves of either Valencia orange or Washington Navel orange on both rootstocks. Accumulation of sodium and chloride in salinised leaves caused a reduction in leaf osmotic potential of 0.2–1.4 MPa. and leaf water potential declined by as much as 0.5 MPa. Turgor pressure in salinised leaves was thus maintained at or above the control level. Osmotic potentials determined by psychrometry compared with pressure-volume curves were taken to imply that some accumulation of sodium or chloride in the apoplast of salinised leaves may have occurred. Despite turgor maintenance both _co2 assimilation and stomatal conductance were reduced by salinity. Following onset of leaf response to salinisation, gas exchange was impaired to a greater extent in scions budded to Cleopatra mandarin compared to those on Trifoliata. Amongst those scions. leaves of salt-treated Marsh grapefruit showed greater reductions in gas exchange than Valencia orange or Washington Navel orange budded on either rootstock. Increased sensitivity of 1Marsh grapefruit was correlated with a higher foliar sodium and chloride content in this scion. Scion differences in sensitivity of leaf gas exchange to solute concentration were independent of rootstock and appeared unrelated to leaf prolinebetaine concentrations. This implies an inherent difference between scion species with respect to salt tolerance, rather than variation in their capacity to acquire that type of compatible solute. In terms of rootstock effects, all scions proved more sensitive to salinity when budded to Cleopatra mandarin compared with Trifoliata. That response was attributed to a disproportionately higher concentration of leaf sodium in scions on Cleopatra mandarin.     

Effects of mixtures of citrus viroids as transmissible small nuclear RNA on tree dwarfing and commercial scion performance on Carrizo citrange rootstock

The term ‘transmissible small nuclear ribonucleic acids' (TsnRNAs) describes well characterised viroid RNA species that do not induce any disease syndromes in specific citrus hosts but rather act as regulatory genetic elements modifying tree performance. Twelve-year-old navel orange and 10-year-old Clementine mandarin trees on Carrizo citrange (Citrus sinensis×Poncirus trifoliata) rootstock treated with a mixture of three TsnRNAs (−Ia, syn. Citrus bent leaf viroid, +IIa, syn. Hop stunt viroid and +IIIb, syn. Citrus dwarfing viroid) were reduced in size by 33% and 43%, respectively. Clementine trees treated with a mixture of TsnRNA−Ia+IIa or −Ia+IIIb also had reduced canopy volume (CV) (∼38 and 31%, respectively), whereas trees treated with TsnRNA−IIa+IIIb showed little effect. The effects of the double TsnRNA treatments −Ia+IIa and −Ia+IIIb on Clementine canopy size and commercial performance were comparable and in some cases superior to that of the triple TsnRNA mixture. The TsnRNA−Ia+IIa treatment had the most attractive commercial traits with increased production of Clementine fruit per CV (23.6%), more fruit with high commercial value (31.7%), and more fruit optimally distributed in the canopy (68% of fruit between 0.5 and 2.5 m). None of the TsnRNA treatments affected the growth of Carrizo rootstock seedlings after 8 years in the field. Navel orange and Clementine scions treated with the same triple TsnRNA mixture expressed different trunk and fruit production patterns although effects on CV were similar.     

Comparative sensitivity of 'Prior Lisbon' lemon and 'Valencia' orange trees to foliar sodium and chloride concentrations

Abstract While citrus rootstocks differ in capacity for sodium and chloride ion exclusion, citrus scion species also vary in foliar sensitivity to NaCl salinisation. Of two common scions, ‘Lisbon’ lemon appears more sensitive, whereas ‘Valencia’ orange in less sensitive to leaf salt. In an attempt to explain this difference. ‘Valencia’ orange (Citrus sinensis [L.] Osbeck) and ‘Prior Lisbon’ lemon (Citrus limon [L.] Burm. F.) were budded to rootstocks known to differ in their ability to exclude sodium ions viz, the strong excluder Trifoliata (Poncirus trifoliata [L.] Raf.), and the weaker excluder Troyer citrange (C. sinensis×P. trifoliata); neither rootstock shows strong exclusion of chloride ions. Budded trees were held under a photosynthetic photon flux density of 450 μmol m ² S ¹ and watered with nutrient solution containing either 0 or 50 mol m ³ NaCl. Growth and photosynthetic responses were measured over 58 d following onset of salinization: salinity effects on leaf gas exchange were studied in relation to changes in leaf water status, compatible solutes and foliar content of sodium and chloride ions, over that same period. Once root-zone salinization began to influence leaf solutes (day 30 onwards), lemon showed a steeper increase in leaf chloride than occurred for orange. Although rootstock differences were without effect on this ingress of chloride ions for either scion, sodium ions were excluded from both scions to a larger extent by Trifoliata than by Troyer citrange. Carbon dioxide assimilation of scion foliage was reduced earlier and to a much larger extent by rootzone salinization in lemon than in orange. Furthermore, comparisons of CO₂ assimilation in relation to leaf tissue solutes between scions (on either rootstock) showed stronger responses for both sodium and chloride ions in lemon than in orange. Faster ingress of chloride into lemon leaves was identified as the crucial factor which predisposed towards expression of that contrast between scions. Although contrasts between scions in photosynthetic responses to salinization matched a faster ingress of chloride into lemon than into orange leaves, the sharper photosynthetic response of ‘Prior Lisbon’ lemon to salinity was not solely attributable to higher concentrations of chloride ions (cell sap basis). A difference between species in subcellular compartmentation of the chloride ion under saline conditions was invoked.     

Tetraploid citrus rootstocks are more tolerant to salt stress than diploid

Citrus trees are subject to several abiotic constraints such as salinity. Providing new rootstocks more tolerant is thus a requirement. In this article, we investigated salt stress tolerance of three tetraploid rootstock genotypes when compared to their respective diploid rootstocks (Poncirus trifoliata, Carrizo citrange, Cleopatra mandarin). Plant growth, leaf fall and ion contents were investigated. At the end of the experiment, leaf fall was observed only for diploid Poncirus trifoliata plants as well as chlorosis symptoms for Poncirus trifoliata and Carrizo citrange diploid plants. The diploid Cleopatra mandarin plants growth rate was not affected by salt stress and has even been increased for tetraploid Cleopatra mandarin. Ion contents investigation has shown lower accumulations of chloride ions in leaves of the tetraploid plants when compared to diploid plants. Our results suggest that citrus tetraploid rootstocks are more tolerant to salt stress than their corresponding diploid. To cite this article: B. Saleh et al., C. R. Biologies 331 (2008).     

Genotype Affects the Morphogenic Response in vitro of Epicotyl Segments of Citrus Rootstocks

The influence of light, hormones and explant orientation onin vitro regeneration in epicotyl cuttings was compared in fourCitrus species (C. aurantium, C. macrophylla, C. reshni andC.sinensis ) and the hybrid Troyer citrange (C. sinensis x Poncirustrifoliata). In all cases, explants planted vertically regeneratedshoots at the apical end by a process of direct organogenesiswithout callus formation. When the Troyer citrange explantswere incubated horizontally, regeneration at the apical endoccurred by an indirect organogenic pathway after callus formation.This change in the pathway of regeneration did not occur inany of the Citrus species, and incubation horizontally resultedin a reduction in the number of buds and shoots formed throughthe direct organogenic pathway. Shoot formation through thedirect organogenic pathway was inhibited by darkness, and thisinhibitory effect was counteracted by the cytokinin benzyladeninein Troyer citrange and, partly, in C. sinensis, but not in C.macrophylla. A non-organogenic callus formed at the basal endof most of the cuttings of C. reshni. InC. sinensis and C. aurantium,a non-organogenic callus formed only in a low proportion ofexplants. Troyer citrange formed an organogenic callus in whichbuds or roots differentiated depending on the auxin/cytokininbalance. C. macrophylla formed callus in the dark but not inthe light. Root formation occurred both in the presence of theauxin naphthaleneacetic acid or low concentrations (2.2 to 4.4µM) of the cytokinin benzyladenine, but no buds were formed.These qualitative and quantitative differences in the organogenicresponse indicate that the conditions for regeneration mustbe optimized for each genotype. Copyright 2000 Annals of BotanyCompany Benzyladenine, citrus, Citrus aurantium, Citrus macrophylla, Citrus sinensis, Citrus sinensis x Poncirus trifoliata, naphthaleneacetic acid, organogenesis, rooting, shoot regeneration, Troyer citrange     

Transmissible salt tolerance traits identified through reciprocal grafts between sensitive Carrizo and tolerant Cleopatra citrus genotypes

In this work, reciprocal grafts between the chloride-tolerant Cleopatra mandarin (Citrus reshni Hort. ex Tan.) and the chloride-sensitive Carrizo citrange (Citrus sinensis [L.] Osb. × Poncirus trifoliata [L.] Raf.) rootstocks were grown under saline conditions to identify major transmissible salt tolerance traits in citrus. The data indicate that lower chloride levels in leaves, attenuated shoot growth and smaller vessel size in xylem were the most important transmissible salt tolerance traits. Other tolerance attributes such as larger leaf area and lower transpiration rates were non-transmissible charac teristics. Leaf cation levels and gas interchange parameters were unrelated to salt tolerance. In com parison with sensitive Carrizo, tolerant Cleopatra plants showed reduced capabilities for water uptake as well as lower leaf Cl-concentrations. Carrizo on Cleopatra grafts also possessed these two attributes although they were slightly less tolerant than Cleopatra plants, which had higher shoot to root ratios than the grafted plants. Cleopatra on Carrizo plants showed high sensitivity to salt because they had higher ability for water uptake and accumulated higher Cl-concentrations in leaves.     

柑桔矮化砧木的生理生化预选指标研究

以７４－１积、紫花宜昌橙、Ｒｕｓｋ枳橙、Ｔｒｏｙｅｒ枳橙、江南柑、９２号红桔和蟹橙为标准系列砧木,研究了它们的生理生化特征,从中筛选出了茎还原性糖含量、叶片还原性糖含量、叶片氨基酸含量、茎蛋白质含量、叶片过氧化物酶活性５个柑桔矮化预选指标和叶片可溶性糖含量、叶片蛋白质含量２个辅助预选指标。     

Alleviation of salt stress in citrus seedlings inoculated with arbuscular mycorrhizal fungi depends on the rootstock salt tolerance

Seedlings of Cleopatra mandarin (Citrus reshni Hort. ex Tan.) and Alemow (Citrus macrophylla Wester) were inoculated with a mixture of AM fungi (Rhizophagus irregularis and Funneliformis mosseae) (+AM), or left non-inoculated (−AM). From forty-five days after fungal inoculation onwards, half of +AM or −AM plants were irrigated with nutrient solution containing 50 mM NaCl. Three months later, AM significantly increased plant growth in both Cleopatra mandarin and Alemow rootstocks. Plant growth was higher in salinized +AM plants than in non-salinized −AM plants, demonstrating that AM compensates the growth limitations imposed by salinity. Whereas AM-inoculated Cleopatra mandarin seedlings had a very good response under saline treatment, inoculation in Alemow did not alleviate the negative effect of salinity. The beneficial effect of mycorrhization is unrelated with protection against the uptake of Na or Cl and the effect of AM on these ions did not explain the different response of rootstocks. This response was related with the nutritional status since our findings confirm that AM fungi can alter host responses to salinity stress, improving more the P, K, Fe and Cu plant nutrition in Cleopatra mandarin than in Alemow plants. AM inoculation under saline treatments also increased root Mg concentration but it was higher in Cleopatra mandarin than in Alemow. This could explain why AM fungus did not completely recovered chlorophyll concentrations in Alemow and consequently it had lower photosynthesis rate than control plants. AM fungi play an essential role in citrus rootstock growth and biomass production although the intensity of this response depends on the rootstock salinity tolerance.     

Influence of Chloride and Transpiration on Net15NO3-Uptake Rate byCitrus Roots

Three-month-old Carrizo citrange (hybrid of Citrus sinensisL. OsbeckxPoncirus trifoliata Blanco) seedlings were grown incontrolled environment chambers in pots of fine sand. Plantswere irrigated with either non-saline or saline solutions overa 3-week period. After these treatments, plants were transferredto vessels containing a 5 m M¹⁵NO₃K (96% atom excess¹⁵N) solution,and transpiration as well as concentration of¹⁵N and Cl^-in roots,stem and leaves were measured after 24 h. Transpiration and¹⁵NO₃^-uptakerates were inhibited after exposure to NaCl and the concentrationof salt pre-treatment determined the intensity of this inhibitoryeffect. To determine the effect of transpiration on NO₃^-absorption,net¹⁵NO₃^-uptake rate was measured in salt stressed and non-stressedplants exposed to different light intensities or relative humiditiesand also in detached roots. Reduction in NO₃^-uptake was moreclosely related to Cl^-antagonism from salt stress than to reducedtranspiration rate. Copyright 1999 Annals of Botany Company Nitrate, absorption, inhibition transport system, salt, light and humidity.     

Relationship between salt tolerance and photosynthetic machinery performance in citrus

In citrus, salt stress has been related to the build up of chloride ions in plant tissues that affect photosynthesis, growth and yield. We investigated the effects of salt stress on the stability of the photosynthetic machinery with respect to the relative salt tolerance of different citrus genotypes including: Swingle Citrumelo, Carrizo citrange, C35 citrange, Cleopatra mandarin and Forner-Alcaide #5. Under identical salt-stress conditions, Forner-Alcaide #5 and Cleopatra mandarin accumulated less chloride ions in leaves than the other genotypes and showed a better plant performance. Chlorophyll fluorescence parameters indicated severe impairments of photosynthetic activity in salt-sensitive Citrumelo and citranges but Cleopatra and Forner-Alcaide #5 were less affected. In addition, differences in photosynthetic responses between these two moderately tolerant genotypes suggested different strategies to cope with salinity. The high tolerance to salinity shown by Forner-Alcaide #5 can be associated to the ability of keeping an active photosynthetic system at elevated saline conditions whereas the tolerance of Cleopatra was linked to rapid reductions of net photosynthetic rate, stomatal conductance, performance of PSII and photosynthetic efficiency.     

Leaf Water Potential Response to Transpiration by Citrus

This paper reports on further studies of a model for interpreting leaf water potential data for Citrus. Experimental data confirmed the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small. Data collected diurnally indicated that the relationship between leaf water potential and transpiration followed a sequence of steady states without hysteresis. No difference in water transport characteristics was found for Valencia orange on three rootstocks in well-watered soil, but the two rootstocks Cleopatra mandarin and Rangpur gave slightly greater leaf water stress in Valencia orange leaves than‘Troyer’ citrange rootstock at high transpiration rates under mild soil water deficits. In laboratory studies, previously unstressed seedlings had higher leaf water potentials than field trees at equivalent transpiration rates. After several drying cycles, however, leaf water potentials were similar to those observed in the field.     

Effects of cadmium on gas exchange and phytohormone contents in citrus

The effect of increased Cd²⁺ concentrations in the watering solution on citrus physiology was studied by using two citrus genotypes, Cleopatra mandarin and Carrizo citrange. Cadmium content in roots and leaves was tested together with measurements of leaf damage, gas exchange parameters, and hormonal contents. Citrus roots efficiently retained Cd²⁺ avoiding its translocation to the shoots and Cleopatra mandarin translocated less Cd²⁺ than Carrizo. With increasing Cd²⁺ concentration all gas exchange parameters were decreased more in Carrizo than in Cleopatra mandarin. Cd-induced increases in abscisic acid and salicylic acid contents were observed in leaves but not in roots of both genotypes.     

Susceptibility and Resistance of Citrus Genotypes to Alternaria alternata pv. citri

A survey of citrus cultivars in Israel in orchards where Alternaria brown spot was common on Minneola tangelos (mandarin × grapefruit), revealed the occurrence of the disease as typical foliar and fruit lesions on Dancy and Ellendale (mandarins), on Murcott tangor (mandarin × sweet orange), on Nova and Idith (mandarin hybrids), on Calamondin, and on Sunrise and Redblush (grapefruit). Isolates of Alternaria alternata from each of these hosts were proven to be pathogenic to Minneola tangelo.
The host range of A. alternata pv. citri from Israel was assayed by inoculating leaves of diverse citrus genotypes. Several mandarins and their hybrids (Dancy, Kara, King, Wilking, Satsuma, Minneola, Orlando, Mikhal, Idith, Nova, Page, Murcott), grapefruit (Marsh seedless), grapefruit × pummelo (Oroblanco), sweet orange (Shamouti, Valencia, Washington navel) Calamondin, and Volkamer citrus were susceptible. Several mandarins and their hybrids (Clementine, Avana, Yafit, Ortanique), Cleopatra, one sweet orange cultivar (Newhall), pummelo (Chandler), lemon (Eureka), Rough lemon, Rangpur lime, sweet lime, citron, limequat, sour orange, Troyer citrange and Alemow were resistant.     

Morphological factors determining salt tolerance in citrus seedlings: the shoot to root ratio modulates passive root uptake of chloride ions and their accumulation in leaves

The results presented in this work were obtained with two citrus genotypes, the chloride-tolerant Cleopatra mandarin (Citrus reshni Hort. ex Tan.) and the chloride-sensitive Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.]. The data show that chloride uptake under salinization is driven by passive forces. In both species, net rates of chloride root uptake increased linearly, without saturation, with the increase of external NaCl concentrations (30–240 mol m^–3). Uptake rates, on a μ g g root dry weight^–1 h^–1 basis, in Cleopatra and Carrizo decreased (from 38 to 21) and increased (from 21 to 35), respectively, with the increase (about three-fold) of the shoot to root ratio. With the appropriate shoot to root ratio in each genotype, it was demonstrated that at identical external doses of NaCl, Cl^– uptake rates and Cl^– xylem concentrations in the two species were very similar. Root pruning and defoliation showed that the amount of chloride taken by the plant was a function of the size of the root system, whereas leaf chloride concentration, the parameter responsible for salt damage, was dependent upon leaf biomass. Measurements of water transpiration suggested that chloride root uptake and leaf accumulation might be linked to water absorption and transpiration rates, respectively. The data indicate that plant morphology is a crucial factor determining salt-tolerance in citrus.     

Fertile fruit trees obtained by somatic hybridization: navel orange (Citrus sinensis) and Troyer citrange (C. sinensis x Poncirus trifoliata)

Summary Nucellar cell suspension protoplasts of navel orange (Citrus sinsensis Osb.) were chemically fused with mesophyll protoplasts of Troyer citrange (C. sinensis x Poncirus trifoliata) and cultured in hormone-free Murashige and Tucker medium containing 0.6 M sucrose. Two types of plant were regenerated through embryogenesis. One type showed intermediate mono-and difoliate leaves and the other types was identical to Troyer citrange. The regenerated plants with intermediate morphology were demonstrated by chromosome counts and rDNA analysis to be amphidiploid somatic hybrids. Five clones of these somatic hybrids were grafted in the field. After 4 years, they set flowers having a morphology intermediate between those of the two parents. The pollen grains showed high stainability and sufficient germinability, and were larger than those of Troyer citrange. The fruits of the somatic hybrids were large and spherical with thick rinds. Most of them contained seeds with normal germinability. These results indicate that somatic hybridization is a useful tool for Citrus breeding.     

The Response of Phaseolus vulgaris L. to Root-zone Anaerobiosis, Waterlogging and High Sodium Chloride

Phaseolus vulgaris L. grown at a range of external concentrationsof NaCl (0 to 80 mM) responded differently to gaseous anaerobiosis(N₂ gas) in nutrient solution or stagnant waterlogging of theroot-zone. With similar patterns of distribution of Na⁺ andCl^- occurring in the plants with comparable NaCl treatments,and similar final concentrations of Na⁺ and Cl^- in plants grownunder both root-zone conditions, rates of uptake of Na⁺ andCl^- were much higher in plants with the stagnant waterloggedrootzones. After 72 h stagnant waterlogging, plant tops fromplants grown at 40 mM NaCl contained 1.42 per cent Na⁺ and 3.44per cent Cl^- (d. wt basis) while after 9 days exposure to NaClwith gaseous anaerobiosis, leaf tissue contained 1.49 per centNa⁺ and 4.28 per cen Cl^- (d. wt basis). Plants exposed to 40mM external NaCl were severely damaged within 72 h when grownwith stagnant waterlogged root-zones; those grown with N₂ anaerobiosiscontinued growth and development over the 9 d period. Plantsgrown in nutrient solution showed changes in distribution andconcentration of Na⁺ and Cl^- when oxygen concentration was reducedbelow 21 per cent O₂ (full aeration). Phaseolus vulgaris. L., bean, mineral salt distribution, anaerobiosis, salinity, waterlogging     

Factors affecting Citrus tree isozyme-gene expression

Ten enzymatic systems of Citrus species and cultivars have been evaluated for identification purposes and for genetic variability studies. The following factors that could affect their expression were studied: season of sampling, location, rootstock, position of the branch, infection, and age of the tree. Differences involving the presence-absence of the Cu/Zn SOD within the same tree were found. This difference is mainly related to the position of the leaf relative to the sunlight. No change was observed at any of the ten enzymatic systems assayed regarding the location, the rootstock, the growing condition, the season, or the infection with most virus and virus-like pathogens. Viroids induced noticeable changes on 6PG and PRXa zymograms in C. medica. A new peroxidase (not present in healthy plants) was detected that could be related to appearance of symptoms. This may induce errors when trees without sanitary control are characterized by this enzymatic system. On the other hand, it provides a new possibility for studying the plant response to the presence of viroids. An effect of age, from 3 months up to 12 years, was observed on citrange Troyer and mandarin Cleopatra PRX, MDH and 6PG patterns. An important change occurs around the first year, most likely related to the end of the seedling stage. This is followed by a long transition phase, the end of which (around 9 years later) coincides with a change in the PRX pattern. These age-related changes seem to involve post-translational modifications of pre-existing isozymes.