James P. Syvertsen

Gas exchange, chlorophyll and nutrient contents in relation to Na+ and Cl- accumulation in 'Sunburst' mandarin grafted on different rootstocks

Abstract We investigated effects of salinity on the growth and net gas exchange of CO2 and water vapour in leaves of 2-year-old ‘Sunburst’ mandarin [(Citrus reticulata Blanco)×(Citrus paradisi Macf.×C. reticulata)] trees grafted on either Cleopatra mandarin (C. reticulata) or Carrizo citrange (Citrus sinensis L. Osb.×Poncirus trifoliata L.) rootstocks. Trees were grown in a greenhouse and watered with a complete nutrient solution containing 0, 30, 60 or 90 mM NaCl. After 6 weeks of treatment, tree growth, net gas exchange of leaves, leaf chlorophyll and mineral nutrient concentration were measured. Salinity decreased growth and net gas exchange of leaves on all trees. Cleopatra roots accumulated higher concentrations of Cl− and Na+ than Carrizo roots but ‘Sunburst’ leaves on Cleopatra accumulated less Cl− and Na+ and had higher CO2 exchange rates than those on Carrizo. This corresponded to higher concentrations of leaf N and chlorophyll in leaves of trees on Cleopatra than on Carrizo. Salinity increased N and decreased K+ contents in roots of both rootstocks. Salinity increased N and decreased K+ concentrations in leaves of trees on Cleopatra but not in trees on Carrizo. Salinity also increased Ca2+ concentration and reduced Mg2+ in leaves on Cleopatra. The lower Cl− and Na+ concentration in leaves of ‘Sunburst’ grafted on Cleopatra than on Carrizo, suggests that the salinity tolerance of Cleopatra is associated with ion sequestration in roots with less transport to leaves.

Soil temperature and flooding effects on two species of citrus

SummaryRough lemon (Citrus jambhiri Lush.) and sour orange (C. aurantium L.) seedlings were grown at constant soil temperatures of 16, 24, and 33 C for 3 months. Shoot and root growth of rough lemon was greatest at 33 C while growth of sour orange was greatest at 24 C. There were no significant effects of soil temperature on shoot: root ratio, leaf water potential or stomatal conductance. The hydraulic conductivity of intact root systems of both species was highest when seedlings were grown at 16 C. Thus, acclimation through greater root conductivity at low soil temperature may have compensated for decreased root growth at 16 C and negated effects of soil temperature on plant water relations. Half the plants growing at each soil temperature were subsequently flooded. Within 1 week, the soil redox potential (Eh) dropped below zero mV, reaching a minimum Eh of −250mV after 3 weeks of flooded conditions. Flooded plants exhibited lower root conductivity, a cessation of shoot growth, lower leaf water potentials, lower stomatal conductances, and visual sloughing of fibrous roots. Decreases in root conductivity in response to flooding were large enough to account for the observed decreases in stomatal conductance.

Salinity and flooding stress effects on mycorrhizal and non-mycorrhizal citrus rootstock seedlings

Seedlings of the rootstocks Pineapple sweet orange (SwO), Carrizo citrange (CC), and sour orange (SO) were grown in low phosphorus (P) sandy soil and either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus,Glomus intraradices, or were non-mycorrhizal (NM) and fertilized with P. VAM and NM seedings of similar shoot size and adequate P-status were selected for study of salinity and flooding stress. One-third of each of the VAM and NM plants were given 150 mM NaCl for a period of 24 days. One-third of the plants were placed into plastic bags and flooded for 21 days while the remaining third were non-stressed controls. In general, neither stress treatment affected mycorrhizal colonization. Salinity stress reduced the hydraulic conductivity of roots, leaf water potential, stomatal conductance and net assimilation of CO2 (ACO2) of mycorrhizal and non-mycorrhizal seedlings to a similar extent. VAM plants of CC and SO accumulated more Cl in leaves than NM plants. Cl was higher in non-mycorrhizal roots of SwO and CC than in mycorrhizal roots. Flooding the root zone for 3 weeks did not produce visible symptoms in the shoot but did influence plant water relations and reduce ACO2 of all 3 rootstocks. VAM and NM plants of each rootstock were affected similarly by flooding. Comparable reduction in nitrogen and P content of both mycorrhizal and non-mycorrhizal plants suggested that flooding stress was primarily affecting root rather than hyphal nutrient uptake.

Phosphorus supply and arbuscular mycorrhizas increase growth and net gas exchange responses of two Citrus spp. grown at elevated [CO2]

We hypothesized that greater photosynthate supply at elevated [CO2] could compensate for increased below-ground C demands of arbuscular mycorrhizas. Therefore, we investigated plant growth, mineral nutrition, starch, and net gas exchange responses of two Citrus spp. to phosphorus (P) nutrition and mycorrhizas at elevated atmospheric [CO2]. Half of the seedlings of sour orange (C. aurantium L.) and ‘Ridge Pineapple’ sweet orange (C. sinensis L. Osbeck) were inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith and half were non-mycorrhizal (NM). Plants were grown at ambient or 2X ambient [CO2] in unshaded greenhouses for 11 weeks and fertilized daily with nutrient solution either without added P or with 2 mM P in a low-P soil. High P supply reduced AM colonization whereas elevated [CO2] counteracted the depressive effect of P on intraradical colonization and vesicle development. Seedlings grown at either elevated [CO2], high P or with G. intraradices had greater growth, net assimilation of CO2 (ACO2) in leaves, leaf water-use efficiency, leaf dry wt/area, leaf starch and carbon/nitrogen (C/N) ratio. Root/whole plant dry wt ratio was decreased by elevated [CO2], P, and AM colonization. Mycorrhizal seedlings had higher leaf-P status but lower leaf N and K concentrations than nonmycorrhizal seedlings which was due to growth dilution effects. Starch in fibrous roots was increased by elevated [CO2] but reduced by G. intraradices, especially at low-P supply. In fibrous roots, elevated [CO2] had no effect on C/N, but AM colonization decreased C/N in both Citrus spp. grown at low-P supply. Overall, there were no species differences in growth or ACO2. Mycorrhizas did not increase plant growth at ambient [CO2]. At elevated [CO2], however, mycorrhizas stimulated growth at both P levels in sour orange, the more mycorrhiza-dependent species, but only at low-P in sweet orange, the less dependent species. At low-P and elevated [CO2], colonization by the AM fungus increased ACO2 in both species but more so in sour orange than in sweet orange. Leaf P and root N concentrations were increased more and root starch level was decreased less by AM in sour orange than in sweet orange. Thus, the additional [CO2] availability to mycorrhizal plants increased CO2 assimilation, growth and nutrient uptake over that of NM plants especially in sour orange under P limitation.

Irrigation water salinity affects soil nutrient distribution, root density, and leaf nutrient levels of citrus under drip fertigation 1

Abstract The purpose of this study was to determine the effects of irrigation water salinity on soil nutrient distribution, citrus leaf nutrition and root density. Irrigation water, salinized to an EC of about 0.3,1.6, or 2.5 dS/m using a 3:1 ratio of NaCl:CaCl2 plus uniform weekly applications of liquid fertilizer, was applied through a drip system. Soil samples were taken at depths of 0–15 and 15–30 cm, both directly under the drippers and 45 cm outward from the drippers, near 8‐year old ‘Valencia’ orange trees on either Carrizo citrange or Sour orange rootstocks growing in a Candler fine sand in lysimeter tanks. In both undisturbed and uniformly mixed soil profiles, soil pH and concentrations of Na, Ca, and P were higher under the dripper than 45 cm outward from the dripper at both depths regardless of salinity level. Soil N and Cl tended to be higher outward from the drippers than near the drippers, except in undisturbed soil at the 0–15 cm depth. Increasing salinity levels in the mixed soil profile n...

Salinity reduces growth, gas exchange, chlorophyll and nutrient concentrations in diploid sour orange and related allotetraploid somatic hybrids

Summary Diploid seedlings of sour orange (SO) (Citrus aurantium (L.)) (2×) and allotetraploid (4×) somatic hybrid plants of SO with Carrizo citrange (SO+CC) (Citrus sinsensis × Poncirus trifoliata), Palestine sweet lime (SO+PSL) (Citrus aurantifolia. (Christm.) Swing) and Flying dragon (SO+FD) (Poncirus trifoliata), were grown under greenhouse conditions and irrigated with nutrient solution containing 0, 60, or 90 mM NaCl. Net gas exchange and leaf CI concentration were measured 21,28 and 35 d after beginning the salinity treatment. At the end of the experiment (day 35), Ca, Mg, K, total N and P concentrations in leaves and roots, growth parameters and chlorophyll concentrations were measured. Diploid plants of sour orange (SO) accumulated less CI and Na in leaves than did the tetraploid plants. However, CI and Na concentrations in roots were similar or higher in SO than in tetraploid plants. At day 35 in the 60 mM treatment, accumulation of salts in leaves was negatively correlated with growth. Thus, SO was more salt tolerant than the tetraploids and SO+PSL was the least tolerant to salinity. After 35 d, salinity reduced leaf chlorophyll, photosynthesis, stomatal conductance and whole plant transpiration in all selections. Salinity decreased the concentration of K in leaves and roots but increased the concentration of tissue N. At 90 mM NaCl, there was no correlation between CI and Na accumulations in leaves and shoot fresh weight reduction.

Citrus Seedling Growth and Susceptibility to Root Rot as Affected by Phosphite and Phosphate

ABSTRACT The growth of ‘Ridge Pineapple’ sweet orange [Citrus sinensis (L.) Osbeck] seedlings and their susceptibility to Phytophthora root rot were studied under contrasting supplies of phosphate (Pi) or Phosphite (Phi). After 10 weeks of repeated applications of nutrient solutions, Phi concentrations were barely detectable in soil. Soil Pi was higher in Phi treatments than in pots that received Pi alone. Seedling growth was greatest when supplied with Pi or Phi separately, but when Pi and Phi were combined, growth was reduced to levels comparable to plants that received no P. Phi was found in both stems and leaves after it was applied to soil supporting the mobility of Phi within the plant. In addition, a small amount of Phi was found in roots after applications of Phi in foliar sprays. Different sources of soil-applied P did not affect the amount of Pi in roots, while the amounts of Pi in leaves were higher in plants that received Phi and Pi combined. Root resistance to Phytophthora root rot of citrus seedlings treated with Phi alone or in combination with Pi was greater than in plants treated with Pi alone, confirming the antifungal effect of Phi.

Water stress and root injury from simulated flooding and Diaprepes abbreviatus root weevil larval feeding in citrus

Environmental stress from flooding can occur simultaneously with root weevil infestation to damage plant root systems. We conducted two factorial studies of flooding duration and Diaprepes abbreviatus (L.) root weevil larval feeding injury on citrus in the greenhouse in 2002 and 2003. Our objectives were to investigate the effect of soil anoxia by simulated flooding on plant water stress and the impact of prior flooding on root susceptibility to subsequent larval weevil feeding. The treatments consisted of two rootstock varieties, Swingle citrumelo [SWI; Citrus paradisi Macfad × Poncirus trifoliata (L.) Raf.] and Smooth Flat Seville (SFS; Citrus aurantium L.), flooding durations of 0, 10, 20, 30, or 40 days, and Diaprepes larval infestations of 0 and 5 neonates per seedling for 40 days. We used a Candler sand with 8 replicates in Experiment I and a Floridana loam with 15 replicates in Experiment II. Treatments were arranged in a completely ramdomized design. Plants were flooded, drained for a week, and then 1-day-old neonate larvae were introduced onto the soil surface of each seedling. Flooding significantly reduced soil redox potential (Eh), leaf stomatal conductance (gs), and shoot growth (P < 0.05). Soil Eh decreased from +220 to −0;100 mV within 1-3 days after flooding, and leaf gs declined from 260 to 80 mmol m−2 s−1 within 20 days of flooding. Flood-injured and larval-injured roots had little growth. With equal previous flooding durations (20 days), the larval survival was on average 25% higher in sandy soil than in loamy soil. Twenty-day prior flooded roots were more water stressed and also more susceptible to Diaprepes larval feeding injury. It is suggested that limited soil waterlogging and early root weevil larval control would be useful for plant protection.

Soil and Diaprepes abbreviatus root weevil spatial variability in a poorly drained citrus grove

Soil and water variability in space and time could be important for management of the citrus root weevil, Diaprepes abbreviatus (L.). We conducted a study of soil, tree, and root weevil relationships in a poorly drained grove of Hamlin orange on Swingle citrumelo rootstock (Citrus paradisi Macfad. x Poncirus trifoliata (L) Raf.). in central Florida in 2002. We hypothesized that spatial soil and water variability might influence tree health and root weevil patterns. The objectives were to assess the spatial variability of soil, water, tree health, and Diaprepes root weevil (DRW) and to determine DRW management zones based on spatial correlations. Adult weevils were monitored using Tedders traps arranged in a 34 × 25-m grid across the grove. Soil electrical conductivity (EC) was assessed using EM38, and water table, soil texture, water content, organic matter, pH, P, K, Ca, Mg, B, Zn, Mn, Fe, and Cu were measured at each trap. The weevil population peaked in June (P < 0.001), and weevil density was high in areas that were low in Mg and Ca concentrations (P < 0.05). Semivariograms, a spatial structure function, for DRW, Mg, Ca, and EC, ranged within 75 to 100 m, which matched the limits of DRW management zones delineated using DRW and EC underlying patterns. Soil EC, Mg, Ca, and Fe were correlated, and tree decline was associated with high levels of Fe and soil flooding because plants were more water stressed in flooded areas than in non-flooded areas (P < 0.01). We suggest that a management unit approach might be an option for DRW control, and that flooding events and soil Fe, Mg and Ca levels might be related to tree decline and DRW distribution patterns. (Soil Science 2004; 169:650–662)

Contribution of phosphorus ( 32 P) absorption and remobilization for citrus growth

Background and aimsPhosphorus (P) is a mobile nutrient in the plant so growth depends on its internal remobilization and a plant’s ability to respond to its availability in the growing media. This study was conducted to evaluate the influence of P status and rootstocks on the patterns of P uptake and remobilization in orange trees.MethodsSweet orange trees on Cleopatra mandarin (CM) or Rangpur lime (RL) rootstocks were grown for nine months in nutrient solution (NS) that was either P-deficient (DNS) or was P-sufficient (SNS). After this period, half of the trees were reciprocally transferred between DNS and SNS (from D to S and S to D), while the others remained in their initial P availability.ResultsTrees on RL had more shoot and root growth, accumulated more P and had greater efficiency of P absorption and transport to the shoot (PAE) than those on CM. The major source of P for growth was previously stored P even with an adequate current P supply to the roots. This suggested the dominance of P remobilization over P uptake and the requirement that trees had sufficient stored P to meet P demand of new growth. Trees on CM had greater concentrations of remobilized P in new shoots than trees on RL.ConclusionTrees grafted on rootstocks less able to take up P (CM) were more dependent on the internal reserves of P for new growth than rootstocks with higher PAE (RL).