Root and stem hydraulic conductivity as determinants of growth potential in grafted trees of apple (Malus pumila Mill.)

The anatomy of the graft tissue between a rootstock and its shoot (scion) can provide a mechanistic explanation of the way dwarfing Malus rootstocks reduce shoot growth. Considerable xylem tissue disorganization may result in graft tissue having a low hydraulic conductivity (k(h)), relative to the scion stem. The graft may influence the movement of substances in the xylem such as ions, water and plant-growth-regulating hormones. Measurements were made on 3-year-old apple trees with a low-pressure flow system to determine k(h) of root and scion stem sections incorporating the graft tissue. A range of rootstocks was examined, with different abilities of dwarfing; both ungrafted and grafted with the same scion shoot cultivar. The results showed that the hydraulic conductivity (k(hroot)) of roots from dwarfing rootstocks was lower compared with semi-vigorous rootstocks, at least for the size class of root measured (1.5 mm diameter). Scion hydraulic conductivity (k(hs)) was linked to leaf area and also to the rootstock on to which it was grafted, i.e. hydraulic conductivity was greater for the scion stem on the semi-vigorous rootstock. Expressing conductivities relative to xylem cross-sectional areas (k(s)) did not remove these differences suggesting that there were anatomical changes induced by the rootstock. The calculated hydraulic conductivity of the graft tissue was found to be lower for grafted trees on dwarfing rootstocks compared to invigorating rootstocks. These observations are discussed in relation to the mechanism(s) by which rootstock influences shoot growth in grafted trees.     

盐胁迫下番茄砧木对嫁接苗生物量、氨基酸含量和活性氧代谢的影响

为探讨砧木对提高番茄嫁接苗耐盐性的作用机理,以耐盐性较敏感的‘中杂9号’(S)为接穗,耐盐性较强的‘OZ-006’(R)为砧木,采用劈接法形成嫁接苗(RS)以及接穗自嫁接苗(SS)、砧木自嫁接苗(RR)3个试验材料,在175 mmol·L^-1 NaCl处理下测定植株生长、Na⁺积累、氨基酸含量和活性氧代谢的变化。结果表明: NaCl胁迫导致番茄幼苗的盐害指数和Na⁺含量均显著提高,幼苗生长速率和叶绿素含量显著降低,但不同嫁接苗的类型差异显著,在盐害表型上表现为SS>RS>RR的规律。NaCl胁迫诱导嫁接苗的叶片和根系总氨基酸含量显著提高,其中RR、RS叶片有9种、根系有8种氨基酸含量显著高于对照,以脯氨酸含量变化最为显著,而SS叶片中仅有2种、根系中仅有4种氨基酸含量显著高于对照;幼苗间的氨基酸含量呈现RR>RS>SS的规律,RR、RS叶片的氨基酸含量分别比SS叶片上升了32.8%、16.6%,根系分别比SS上升了53.1%和32.5%。NaCl胁迫造成活性氧代谢的变化,幼苗叶片和根系的抗氧化酶活性、超氧阴离子产生速率、丙二醛含量均显著提高,以RR叶片和根系中抗氧化酶活性的增幅最大,其次为RS;SS和抗氧化酶活性的增幅最小,品种间活性氧水平表现为SS>RS>RR。综上,砧木通过抑制Na⁺向上运输、提高氨基酸水平和抗氧化酶活性缓解了盐胁迫对嫁接苗的伤害,但不同砧穗组合的耐盐性差异较大,以RR的耐盐性最强,其次为RS,SS最弱。因此,番茄嫁接苗的耐盐性主要受砧木耐盐性的影响,其次为接穗,同时,其与番茄体内的氨基酸和活性氧代谢调控密切相关。     

柑桔接穗对砧木生长及若干生理生化特性的影响

以体细胞杂种红桔+枳、红桔+粗柠檬和有性杂种Troyer枳橙、Swingle枳柚以及枳作砧木的"国庆4号"温州蜜柑、脐橙(无枳砧)和不嫁接的砧木幼树为试材,通过两年盆栽试验研究接穗对砧木生长、根系活力和根系抗氧化酶活性的影响。结果表明,Swingle枳柚和红桔+粗柠檬的砧粗易受接穗影响,接穗显著地影响砧木根系体积。在一般情况下,同一种砧木嫁接后砧木根系活力、超氧化物歧化酶(SOD)和过氧化物酶(POD)活性低于未嫁接砧木,或与其差异不显著;同时存在接穗促进根系过氧化氢酶(CAT)活性升高的现象。     

茄子/番茄嫁接植株的生理特性及其对黄萎病的抗性

以能降低茄子植株黄萎病病情指数的番茄为砧木嫁接茄子,结果表明,嫁接后的茄子植株光合作用速率和根系活跃吸收面积增大;叶片中束缚水／自由水比值增大,水势降低;根系中苯丙氨酸解氨酶(PAL)、根系和叶片中过氧化物酶(POD)活性增强。     

Influence of rootstock on antioxidant system in leaves and roots of young apple trees in response to drought stress

Grafting rootstocks are widely used to enhance plants resistance to various biologic and abiotic stresses. We determined how the rootstock genotype might influence plant responses to drought, using 2-year-old ‘Gale Gala’ apple trees grafted onto Malus sieversii and M. hupehensis. Under water stress, trees with the former as their rootstock had smaller reductions in rates of relative growth and photosynthesis, total biomass, leaf area, levels of leaf chlorophyll, and relative water content compared with those grafted onto the latter. They also had greater maximum photochemical efficiency and water-use efficiency. On the other hand, trees growing on M. sieversii rootstock had less production of superoxide radicals and hydrogen peroxide in both leaves and roots than those growing on M. hupehensis in response to drought stress. Furthermore, under drought conditions, leaves and roots from trees grafted onto M. sieversii had greater synthesis of ascorbic acid and glutathione, as well as higher activities of superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase. These results suggest that the choice of grafting rootstock can enhance drought resistance by improving the antioxidant system in a plant. Here, ‘Gale Gala’ trees grafted onto M. sieversii were more drought-resistant than those on M. hupehensis rootstock.     

Hydraulic conductance and rootstock effects in grafted vines of kiwifruit

Whole-plant hydraulic conductance, shoot growth, and leaf photosynthetic properties were measured on kiwifruit vines with four clonal rootstocks to examine the relationship between plant hydraulic conductance and leaf stomatal conductance (gs) and to test the hypothesis that reduced hydraulic conductance can provide an explanation for reductions in plant vigour caused by rootstocks. The rootstocks were selected from four species of Actinidia and grafted with Actinidia chinensis var. chinensis 'Hort16A' (yellow kiwifruit) as the scion. Total leaf area of the scion on the least vigorous Actinidia rootstock, A. kolomikta, was 25% of the most vigorous, A. hemsleyana. Based on shoot growth and leaf area, the selections of A. kolomikta and A. polygama are low-vigour rootstocks, and A. macrosperma and A. hemsleyana are high-vigour rootstocks for A. chinensis. Whole-plant hydraulic conductance, the ratio of xylem sap flux to xylem water potential, was lower in the low-vigour rootstocks, reflecting their smaller size. However, leaf-area-specific conductance (Kl) and gs were both higher in the low-vigour rootstocks, the opposite of the expected pattern. Differences in Kl were found in the compartment from the roots to the scion stem, with no difference between rootstocks in the conductance of stems or leaves of the scion. There was no evidence that the graft union caused a significant reduction in hydraulic conductance of vines with low-vigour rootstocks. Leaf photosynthetic capacity did not vary between rootstocks, but photosynthesis and carbon isotope discrimination (Delta13C) under ambient conditions were higher in the low-vigour rootstocks because gs was higher. gs and Delta13C were positively correlated with Kl, although the mechanism for this relationship was not based on stomatal regulation of a similar xylem water potential because water potential varied between rootstocks. For Actinidia rootstocks, changes in Kl do not provide a direct explanation for changes in vigour of the scion. However, depending on the rootstock in question, changes in hydraulic conductance, biomass partitioning, and crown structure are involved in the response.     

The physiological and nutritional responses to an excess of boron by Verna lemon trees that were grafted on four contrasting rootstocks

Citrus species are sensitive to an excess of boron (B). Currently, this toxicity is becoming a serious problem in the soils of arid and semi-arid environments throughout the world, where high concentrations of B may occur due to the agricultural use of wastewater. Citrus rootstocks can greatly influence the tolerance of citrus trees to different abiotic stresses. However, little is known about how the rootstock influences the tolerance of these trees to an excess of B. In this study, the effects of the nutrient solution’s B concentration (0.25, 2, 4.5 or 7?mg?l^?1) on the growth and other physiological, nutritional and biochemical parameters of Verna lemon trees that were grafted on four contrasting rootstocks [Carrizo citrange (CC), Cleopatra mandarin (CL), Citrus macrophylla (CM) and sour orange (SO)] were investigated. The plants were grown in a greenhouse in pots containing a universal substrate media and were watered daily with a Hoagland nutrient solution containing different concentrations of B. The results showed that the plant growth was progressively inhibited with an increasing concentration of B in the nutrient solution. However, the shoot was more sensitive to the B toxicity than were the roots. In addition, the growth inhibition was reduced in trees that were grafted on CL and CM when compared with those that were grafted on CC and SO. The concentration of B in the leaves, stems and roots also increased with an increase in the concentration of external B in the following order: leaves?>?roots?>?stem. The rootstock also had an influence on the B concentration in the different plant tissues. In the leaves, the B concentration was lowest in the plants that were grafted on the SO rootstock followed by the plants that were grafted on either the CM or CL rootstock and highest in the plants that were grafted on the CC rootstock. The net assimilation of CO₂ ( $ A_{{{\text{CO}}_{2} }} $ ) and the stomatal conductance (g _s) leaf gas exchange parameters were reduced with an excess of B in the leaves, and this reduction was less pronounced for trees on CM and CL. The intercellular CO₂ concentration (C _i) and the chlorophyll fluorescence indicated that the reduction of $ A_{{{\text{CO}}_{2} }} $ that was found with an excess of B was mainly due to non-stomatal factors. The mineral nutrition and organic solute data are also shown in this study. All of the data indicate that the tolerance to an excess of B is not related to the concentration of B that has accumulated in the leaves, which indicates that a combination of rootstock-dependent physiological, biochemical and anatomical responses determine the tolerance to an excess of B in citrus plants.     

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.     

肉桂酸和香草醛对嫁接茄子根系生长及生理特性的影响

以“托鲁巴姆”茄子为砧木,“西安绿”茄子为接穗,采用盆栽试验研究了自毒物质肉桂酸和香草醛对嫁接、自根和砧木茄子根系生长及抗氧化酶活性、渗透调节物质含量的影响.结果表明：肉桂酸、香草醛对茄子根系生长及生理代谢的化感作用基本表现为“低促高抑”,两物质对自根茄子根系表现促进、抑制效应的临界浓度点分别为0.1和0.5 mmol·kg^-1;对嫁接、砧木茄子表现促进、抑制效应的临界浓度点分别为0.5和1 mmo·kg^-1.茄子根系对自毒物质耐性大小依次为：砧木茄>嫁接茄>自根茄;在较高浓度肉桂酸(0.5～4 mmol·kg^-1)和香草醛(1～4mmol·kg^-1)作用下,与自根茄子相比,嫁接茄子根系SOD活性提高了8.50%～24.50%,脯氨酸含量增加了9.39%～27.64%,可溶性糖含量增加了12.77%～81.81%,可溶性蛋白含量、根系鲜、干物质量和根系活力显著高于自根茄.表明嫁接换根使茄子根系具有了砧木抗自毒物质的特性,缓解了自毒物质给茄子根系生长带来的不良影响.     

Control of plant growth resides in the shoot,and not in the root,in reciprocal grafts of flacca and wild-type tomato (Lysopersicon esculentum), in the presence and absence of salinity stress

Grafted plants of flacca, an ABA-deficient mutant of tomato (Lycopersicon esculentum), and the wild-type variety Rheinlands Ruhm were grown with and without salinity stress to test the roles of roots and shoots in the regulation of plant growth. Fourteen days after exposure to 200 mM NaCl, shoot and root fresh weight, endogenous ABA concentrations, nitrate concentration, activities of selected enzymes related to nitrogen assimilation, and cation accumulation were determined. Rootstock genotype had little influence on the growth of the grafted plants, whereas grafted plants having wild-type shoots (Ws) produced more biomass than those having flacca shoots (Fs), irrespective of the salinity level. Growth of flacca shoots grafted onto wild-type rootstock (Fs/Wr) was superior to that of flacca shoots grafted onto flacca rootstock (Fs/Fr). The improved growth correlated with enhanced levels of ABA in the flaccashoots of Fs/Wr. In all the graft combinations, ABA content was higher in wild-type shoots than in flacca shoots, with or without salinity. There were no significant differences in root ABA concentrations among the different grafted types. Enhanced growth correlated with higher nitrate levels and higher nitrate reductase activity in the roots and shoots of plants with wild-type shoots and with higher shoot concentrations of ABA in plants with wild-type shoots. There were no significant differences in glutamine synthetase and phosphoenol pyruvate carboxylase activities in the shoots and roots of all the grafted plants, regardless of the salinity level. While shoot genotype determined the accumulation of K⁺ and Na⁺ in grafted plants regardless of salinity, it had no influence on Ca²⁺ concentrations. Regardless of the salinity, the total concentration of cations was the same in all the plants, while salinity decreased Mg²⁺ concentration in roots and shoots of all grafts, with the exception of flacca grafted shoots. The scion genotype – and its ABA level – thus played the major role in the growth of grafted plants, regardless of the rootstock genotype and the salinity of the growth medium.     

Rootstock effects on xylem conduit dimensions and vulnerability to cavitation of <Emphasis Type="Italic">Olea europaea</Emphasis> L.

Two clones of Olea europaea L. were studied for their potential impact on hydraulic architecture and vulnerability to xylem cavitation, when used as rootstocks. The clones used were “Leccino Minerva” (LM), showing vigorous growth and “Leccino Dwarf” (LD) with strongly reduced growth. Self-rooted LM and LD plants as well as their grafting combinations were compared, namely, LM/LD (Leccino Minerva grafted onto Leccino Dwarf rootstock) and LD/LM (Leccino Dwarf grafted onto Leccino Minerva rootstocks). Plants with LD roots (LD and LM/LD) showed significantly reduced leaf surface area compared with plants with LM roots. Xylem conduits of LD shoots were 25% more numerous than in LM shoots. When grafted onto LM rootstocks, however, LD shoots produced consistently wider and longer vessels than measured in LD self-rooted plants. This caused LD/LM plants to increase stem vulnerability to cavitation with threshold pressures for cavitation (P _c) of less than 0.5 MPa compared with LD self-rooted plants that had P _c of over 2.0 MPa. By contrast, although LD rootstocks caused some reduction of vessel diameter and length of LM scions, their influence on LM hydraulic architecture was too small to reduce vulnerability to cavitation of LM scions with respect to that measured for LM self-rooted plants. Our conclusion is that although dwarfing rootstocks effectively reduce grafted plant size, they do not necessarily confer higher resistance to xylem cavitation to scions which would improve plant resistance to drought.     

Scion and Rootstock Effects on ABA-mediated Plant Growth Regulation and Salt Tolerance of Acclimated and Unacclimated Potato Genotypes

Tolerance of salt stress in potato (Solanum tuberosum L.) increased when the plants were pre-exposed to low concentrations of salt (salt acclimation). This acclimation was accompanied by increased levels of abscisic acid (ABA) in the shoot. To further study the role of roots and shoots in this acclimation process, reciprocal grafts were made between a salt-tolerant (9506) and salt-sensitive ABA(−) mutant and its ABA(+) normal sibling potato genotype. The grafted plants were acclimated with 75 or 100 mM NaCl for 3 weeks and then exposed to 150–180 mM NaCl, depending on the salt tolerance of the rootstock. After 2 weeks of exposure to the salt stress, the acclimated and unacclimated plants were compared for physiologic and morphologic parameters. The response to the salt stress was strongly influenced by the rootstock. The salt-tolerant 9506 rootstock increased the salt tolerance of scions of both the ABA-deficient mutant and its ABA(+) sibling. This salt tolerance induced by the rootstock was primarily modulated by salt acclimation and manifested in the scion via increased plant water content, stem diameter, dry matter accumulation, stomatal conductivity, and osmotic potential, and is associated with a reduction in leaf necrosis. There was also a pronounced scion effect on the rootstock. Using 9506 as a scion significantly increased root fresh and dry weights, stem diameter, and root water content of ABA(−) mutant rootstocks. Specific evidence was found of the role of exogenous ABA in the enhancement of water status in grafted plants under salt stress beyond that of grafting alone. This was verified by more positive stomatal conductivity and upward water flow in ABA-treated grafted and nongrafted plants and the absence of upward water flow in nontreated grafted plants through NMR imaging. Grafting using either salt-tolerant scions or rootstocks with inherently high ABA levels may positively modify subsequent responses of the plant under salt stress.     

The relationship of hydraulic conductance to root system characteristics of peach (Prunus persica) rootstocks

Specific rootstocks can differentially influence the vegetative growth and development of fruit trees. However, the physiological mechanism involved in this phenomenon has been elusive. Recent research comparing different peach ( Prunus persica L. Batsch) rootstocks suggests that the rootstock effect on vegetative growth in peach trees is associated to water relations and more specifically to differences in rootstock hydraulic conductance. This study was intended to confirm differences in hydraulic characteristics of similar size peach trees grafted on different rootstocks and to examine root system characteristics that could be associated with rootstock hydraulic limitations. Trees on rootstocks that were known to have a size-controlling effect when grown under field conditions had lower rootstock conductance than trees on the vigorous (control) rootstock when rootstock hydraulic conductance was measured with both the high-pressure and evaporative flow methods. Rootstocks with the lowest hydraulic conductance had less fine root surface area and length per unit root dry weight than the more vigorous (control) rootstock. However, contrary to previous field studies, in this study there were no significant differences in dry matter production and distribution among trees on the different rootstocks suggesting that whatever the normal growth control mechanism was, it did not differentially influence growth under the specific conditions of this study. This research confirmed that peach rootstocks exhibiting size-controlling behavior under field conditions differed in their hydraulic and morphological characteristics under controlled growth conditions even when those growth conditions negated the expression of the size-controlling behavior.     

Variation in xylem structure and function in roots and stems of scion–rootstock combinations of sweet cherry tree (<Emphasis Type="Italic">Prunus avium</Emphasis> L.)

To assess hydraulic architecture and limitations to water transport across scion–rootstock combinations (Prunus avium L. cultivar Van grafted on five differing size-controlling rootstocks: P. avium (vigorous) > CAB 11E > Maxma 14 > Gisela 5 > Edabriz (dwarfing)), we compared xylem anatomy, and calculated relative hydraulic conductivity (RC) and vulnerability index (VI) of roots (small, medium and large diameter) and stems. Water relations, leaf gas exchange and variations in growth were also determined. Roots exhibited larger-diameter xylem conduits (VD), greater RC and VI than stems in all Van–rootstock combinations. Moreover, there was a significantly higher vessel frequency (VF), lower VD, RC and VI in dwarfed trees, especially grafted on Gisela 5 than trees on the invigorating rootstocks, P. avium L., CAB 11E and Maxma 14. Anatomical constraints on water status imposed by the smaller VD (and/or in lower xylem thickness and root system length) of dwarfed trees imply a series of negative feedbacks, like a decrease in RC, stem water potential, leaf gas exchange and growth. On the other hand, Van grafted on CAB 11E and Maxma 14, with wide vessels and high VI, could be more susceptible to embolism, especially during periods of severe water stress, than trees on dwarfing rootstocks, which had small vessels and low VI.     

Modification of gibberellin biosynthesis in the grafted apple scion allows control of tree height independent of the rootstock

The availability of short stature apple scions that required minimal applications of chemical growth retardants and could be used with a range of rootstocks would be of considerable benefit to fruit growers. We have suppressed the expression of a gene encoding the gibberellin (GA) biosynthetic enzyme GA 20-oxidase to reduce the levels of bioactive GAs in a scion variety, resulting in significant reductions in stem height. Application of GA3 reversed the effect. The scion remained dwarfed after grafting on to normally invigorating rootstocks, whilst control plants of the same cultivar displayed the expected vigour when grafted on to these rootstocks. This approach could be applicable to any perennial crop variety, allowing dwarf trees to be obtained on any available rootstock or on their own roots without the need for chemical growth retardant application. In effect, seedlings that are well suited to local conditions (drought, salinity) could be employed as tree rootstocks, as could existing rootstocks valued for characters other than vigour control, such as pest and disease resistance.     

Seasonal uptake of 15N-nitrate and distribution of absorbed nitrogen in peach trees

The absorption and distribution of N was measured monthly throught a calendar year in 3-year old peach trees (Prunus persica (L) c.v. Maycrest) grafted on Nemaguard rootstock. Plants were grown on siliceous sand in 500-L pots and fertilized with a solution containing ¹⁵N enriched KNO₃. During flowering and fruit set (March) approximately 7% of N found in new growth came from the fertilizer and the remainder came from the N stored in the old organs. Maximum N absorption took place during the periods of fruit ripening and maximal vegetative growth (May to August). This nitrogen was relocated from leaves to woody tissues and stored as reserve-N before leaf fall. In the following growth season reserve-N was used for flower development and new shoot growth. The N absorbed during plant dormancy was quite low and remained in the stem bark and roots mainly as soluble-N.     

Na2CO3处理对黄瓜嫁接苗与自根苗耐盐性的影响

通过Na2CO3,胁迫对黄瓜嫁接苗和自根苗耐盐性差异的研究,揭示黄瓜幼苗的耐盐机制,为设施黄瓜生产提供理论依据。实验以孝感早瓠瓜为砧木,津春四号黄瓜为接穗,采用不同浓度Na2CO3,溶液对黄瓜嫁接苗和自根苗进行处理,研究其对黄瓜幼苗的生理胁迫效应。结果表明：在0—7000mg／L范围内,随着Na2CO3,处理浓度的增加,黄瓜嫁接苗和自根苗的株高、茎粗、叶面积、地上部鲜重、根鲜重、叶绿素和根系活力均下降,但嫁接苗受抑制的程度显著低于自根苗;脯氨酸、丙二醛和根冠比均呈上升趋势,且这三项指标均表现为嫁接苗显著高于自根苗;叶片SOD酶活性均呈先上升后下降趋势,均在Na2CO3,处理浓度为1000mg／L时达到最大值,且嫁接苗的活性显著高于自根苗。嫁接苗的耐盐性优于自根苗,Na2CO3胁迫条件下,嫁接苗和自根苗的生长条件盐浓度以不超过3000mg／L为宜。     

Grafting raises the salt tolerance of tomato through limiting the transport of sodium and chloride to the shoot

With the aim of determining whether grafting could improve salinity tolerance of tomato (Lycopersicon esculentum Mill.), and what characteristics of the rootstock were required to increase the salt tolerance of the shoot, a commercial tomato hybrid (cv. Jaguar) was grafted onto the roots of several tomato genotypes with different potentials to exclude saline ions. The rootstock effect was assessed by growing plants at different NaCl concentrations (0, 25, 50, and 75 mM NaCl) under greenhouse conditions, and by determining the fruit yield and the leaf physiological changes induced by the rootstock after 60 d and 90 d of salt treatment. The grafting process itself did not affect the fruit yield, as non-grafted plants of cv. Jaguar and those grafted onto their own root showed the same yield over time under non-saline conditions. However, grafting raised fruit yield in Jaguar on most rootstocks, although the positive effect induced by the rootstock was lower at 25 mM NaCl than at 50 and 75 mM NaCl. At these higher levels, the plants grafted onto Radja, Pera and the hybrid VolgogradskijxPera increased their yields by approximately 80%, with respect to the Jaguar plants. The tolerance induced by the rootstock in the shoot was related to ionic rather than osmotic stress caused by salinity, as the differential fruit yield responses among graft combinations were mainly related to the different abilities of rootstocks to regulate the transport of saline ions. This was corroborated by the high negative correlation found between fruit yield and the leaf Na(+) or Cl(-) concentrations in salt-treated plants after 90 d of salt treatment. In conclusion, grafting provides an alternative way to enhance salt tolerance, determined as fruit yield, in the tomato, and evidence is reported that the rootstock is able to reduce ionic stress.     

Volkamer Lemon Tetraploid Rootstock Transmits the Salt Tolerance When Grafted with Diploid Kinnow Mandarin by Strong Antioxidant Defense Mechanism and Efficient Osmotic Adjustment

Salinity restricts plant growth and production by specific ions toxicity to particular plants. Cl ion is exceptionally toxic to citrus. Citrus rootstock and scion has a significant effect on each other under unfavourable conditions. Nevertheless, their specific response can be different depending on the way to translocate and compartment the toxic ions, or to induce antioxidant systems. In this paper, we studied the behaviour of diploid (2x) and tetraploid (4x) Volkamer lemon rootstocks grafted with commercial cultivar Kinnow mandarin (KM/VM2x and KM/VM4x, respectively) when exposed to moderate (75 mM) and high salt stress (150 mM). Both genotypes showed a decrease in their photosynthetic variables (Pn, gs, E, Fv/Fm, Fv′/Fm′, NPQ), and the decline was more significant in KM/VM2x plants as compared to KM/VM4x. The highest increase in the concentration of stress indicators (MDA and H₂O₂) was observed in leaves and roots of KM/VM2x at 75 and 150 mM of salt stress. The KM/VM4× showed the maximum increase in antioxidative enzymes (SOD, CAT, POD, APx, GR) and osmolytes (PRO, GB) in leaves and roots at 75 and 150 mM. Minerals (Cl ion, Na, K, P, N, Ca) accumulation was also significantly affected in leaves and roots of KM/VM2x and KM/VM4x under moderate and high NaCl stress. Overall, our results showed that Cl ion accumulation presents a robust correlation with stress indicators and their scavenging enzymes in leaves and roots. Moreover, 2x scion significantly mitigated by the 4x rootstock and showed more tolerance as compared to grafted on 2x rootstock.

     

Growth and inorganic composition of ‘Nova’ mandarin plants grafted on two commercial rootstocks in response to salinity and silicon

The effects of salinity and its combination with silicon (Si) were studied in ‘Nova’ mandarin plants grafted on Citrus aurantium L. or Swingle Citrumelo to determine: (1) which combination is more tolerant to salt stress and (2) the impact of Si in limiting the harmful effects of salinity. Six groups of plants were grown in a greenhouse for 120 days and irrigated with: (1) 50 % Hoagland’s solution (Control), (2) 50 % Hoagland’s solution plus 80 mM NaCl (NaCl), and (3) 50 % Hoagland’s solution plus 80 mM NaCl plus 0.5 mM Si (NaCl + Si). Grafted plants exhibited accumulation of Na and Cl in their tissues following exposure to salinity. The ability of S. Citrumelo to retain the toxic ions in the roots in corroboration with the observation that the dry weights (DWs) of S. Citrumelo tissues were not influenced by NaCl treatment indicates that this rootstock is more tolerant to salinity. Silicon supplementation into the saline medium promoted the accumulation of toxic ions, whereas, when compared to NaCl treatment, it increased the DW of S. Citrumelo roots. Mineral concentrations were significantly affected by rootstock, treatment, and their interaction with S. Citrumelo, which presented better nutrient status than Sour Orange; and Si which differed depending on citrus tissue. It appears that S. Citrumelo rootstock is the most tolerant for ‘Nova’ mandarin plants under salinity, whereas salt tolerance in grafted citrus plants is not improved by Si application, indicating that the beneficial role of Si depends on the cultivar or rootstock–scion combinations.