James P. Syvertsen
John D. Lea-Cox and James P. Syvertsen
Eighteen, 4-year-old Grapefruit (Citrus paradisi) cv. `Redblush' trees on either Volkamer lemon (C. volkameriana = VL) or Sour orange (C. aurantium = SO) rootstocks were grown in 7.6 kiloliter drainage lysimeters in a Candler fine sand (Typic Quartzipsamments), and fertilized with nitrogen (N) in 40 split applications at 76, 140 and 336 g N year-1 (= 0.2, 0.4 and 0.9 x the recommended annual rate). Labelled 15N was substituted for the N in a single fertigation at each rate at the time of fruit set the following year, to determine N uptake, allocation and leaching losses. “Nitrogen-uptake and allocation were primarily determined by the sink demand of fruit and vegetative growth, which in turn were strongly influenced by rootstock species. Larger trees on VL required at least 336 g N yr-1 to maintain high growth rates whereas smaller trees on SO of the same age only required 140 g N year-1. Of the 15N applied at the 336 g N rate to the SO trees, 39% still remained in the soil profile after 29 days. With optimally scheduled irrigations, 15N leached below the root zone was less than 3% of that applied after 29 days, regardless of rate. However, 17% of the applied 15N was recovered from a blank (no tree) lysimeter tank. Total 15N recovery ranged from 55-84% of that applied, indicating that a sizeable fraction of the 15N applied may have been lost through denitrification.
Kuo-Tan Li and James P. Syvertsen
Mechanical harvesting of citrus trees can cause physical injuries, such as shedding of leaves, exposing roots, and scuffing bark. Although mechanical harvesting usually has not reduced yield, physiological consequences to the tree from these visible injuries have not been investigated. We hypothesized that physical injuries to tree canopies and root systems from a properly operated trunk shaker would not cause short-term physiological effects. Tree water status and leaf gas exchange of mature `Hamlin' and `Valencia' sweet orange [Citrus sinensis (L.) Osb.] trees that were harvested by a trunk shaker were compared to hand-harvested trees. A trunk shaker was operated with adequate duration to remove >90% of mature fruit or with excessive shaking time under various environmental conditions and drought stress treatments throughout the harvest season. Mid-day stem (Ψstem) and leaf (Ψleaf) water potentials along with leaf gas exchange were measured before and after harvest. Trees harvested by the trunk shaker did not develop altered water status under most conditions. Trees harvested with excessive shaking time and/or with limited soil water supply developed low Ψstem resembling Ψstem of drought-stressed trees. However, water potential of all treatments recovered to values of the well-irrigated, hand-harvested trees after rainfall. In addition, mechanical harvesting did not reduce CO2 assimilation, transpiration, stomatal conductance, water use efficiency, or photosystem II efficiency as measured by chlorophyll fluorescence. Thus, despite visible injuries, a properly operated trunk shaker did not result in any measurable physiological stress.
John D. Lea-Cox and James P. Syvertsen
The objectives of this greenhouse study were to determine the rate of nitrogen (N) uptake over a 30 day period, use efficiency and N partitioning within two citrus rootstock species. Sixteen-week old seedlings of Cleopatra mandarin (C. reticulata Blanco) and Swingle citrumelo (C. paradisi × P. trifoliata) were assigned to treatments (harvest day × rootstock species) in a completely randomized design, grown in a Candler fine sand for 6 weeks and fertilized weekly with a N:P:K (5:1:5) plus minor elements solution at 200 mg N · liter-1. A single application of 15NH4 15NO3 (20% 15N) was substituted for a normal weekly fertigation. Six replicate plants of each rootstock species were harvested at ½, 1½, 3½, 7½, 10½ and 30 days after I5N application. Uptake of 15N was more rapid in SC over the first 7½ days (17% of applied) than in CM (11%), but uptake over 30 days was similar (52-53%) for both species. A higher proportion of 15N was found in the photosynthetic tissues of CM (74%) than in SC (48%), whereas a lower proportion was found in the fibrous roots of CM (9%) than SC (22%).
Vladimir Orbović, Diann Achor, and James P. Syvertsen
Copper (Cu)-based fungicidal sprays are widely used on many crops although Cu sprays can be phytotoxic under some conditions. The mechanism of phytotoxicity is poorly understood but must involve toxic levels of Cu penetrating plant tissues. We studied the effect of different adjuvants on the deposition pattern of droplets and penetration of Cu (in Kocide fungicide) through isolated cuticles of ‘Marsh’ grapefruit leaves and ‘Valencia’ orange fruit. The addition of the silicone-based L-77 surfactant to the Kocide suspension markedly increased the spread of the droplets on cuticles and increased the penetration of Cu through fruit and abaxial leaf cuticles, both with stomatal pores, but not through astomatous adaxial leaf cuticles, which had much lower permeability. Urea and petroleum spray oil adjuvants had no effect on surface area of droplets or the penetration of Cu through leaf and fruit cuticles. Spray tank mixes of Cu fungicides with organosilicone surfactants should be avoided because these surfactants can enhance the penetration of Cu into citrus leaves and fruit thereby leading to phytotoxicity.
L. Gene Albrigo and James P. Syvertsen
In order to evaluate possible reduced nitrate leaching while maintaining yield, `Hamlin' orange and `Flame' grapefruit trees on `Carrizo' or `Swingle Citrumelo' rootstocks were grown from planting using only foliar urea or soil-applied nitrate or ammonium N. An intermediate treatment of foliar and ground N was included also. From the 4th year, yields were recorded for 3 years. As previously reported, canopy growth was greater for the foliar urea treatment for the first 3 years. For 2 of the next 3 bearing years, the grapefruit trees in the foliar urea N treatment produced significantly less yield than the soil-applied treatment and the intermediate treatment was intermediate. The orange trees in the foliar urea treatment produced significantly less fruit than the soil N treatment in only 1 of 3 years, but the yields were numerically less every year. Results for fruit quality and nitrate leaching will be reported also. Foliar urea application alone was more costly and less productive than a soil N program.
Francisco García-Sánchez and James P. Syvertsen
To gain insight into salinity tolerance of citrus, we studied growth, leaf, and root Cl– concentrations and physiological responses of 5-month-old seedlings of the citrus rootstock Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliate L.] grown in a greenhouse in three different substrates: Candler sand soil, Floridana sandy clay soil, or a commercial soilless peat/perlite/vermiculite potting media. Plants were kept well-watered with a complete nutrient solution plus either no salt (control) or 50 mM NaCl for 9 weeks. Without salinity, substrate type did not affect total plant growth although there were differences in shoot/root dry weight ratio and mineral nutrient relationships attributable to substrate. Predawn leaf water potential, midday CO2 assimilation, and leaf water use efficiency were highest in seedlings grown in the soilless peat. The salt treatment decreased leaf and root growth, reduced leaf Ca2+, and increased leaf K+ concentration in all the three substrates. Overall, plant growth was negatively related to leaf Cl–. Leaf growth reductions were least in Candler-grown seedlings and greatest in Floridana soil as Cl– concentrations were lowest in Candler sand and highest in Floridana soil. Leaf Na+ was also highest in Floridana seedlings. In contrast to salt ions in leaves, roots of salinized seedlings in Candler sand had the highest Na+ and Cl– concentration. Salinity reduced net gas exchange of leaves similarly in all three substrates. Salinity reduced both leaf water potential and osmotic potential such that leaf turgor was increased. Thus, salinity-induced reductions in growth and net gas exchange were not the result of loss of turgor but more likely resulting from toxic ion accumulation in leaves. Based on the relative rankings of leaf growth and leaf Cl– concentrations, Carrizo seedlings from Candler sand had the highest salt tolerance and those grown in Floridana soil had the lowest salt tolerance. Substrate type should be considered when characterizing plant growth and physiological responses to salinity.
Kuo-Tan Li* and James P. Syvertsen
Mechanical harvesting of citrus trees by trunk or canopy shakers can cause leaf and twig removal, bark injury and root exposure. Such problems have restricted the adoption of mechanical harvesting in Florida citrus. We assessed physiological responses of citrus trees that were mechanically harvested with a linear-type trunk shaker, operating at 4 Hz, 70.8 kg mass weight, and 6.5 cm displacement, for 10 or 20 seconds. We measured fruit recovery efficiency, leaf and shoot removal, mid-day stem water potential, leaf gas exchange, and leaf fluorescence emission of mature `Hamlin' and `Valencia' orange trees under restricted or normal irrigation. Shaking treatments effectively removed 90% to 94% of fruit without bark damage. Compared to harvesting by hand, trunk shaking removed 10% more leaf area and twigs, and caused some visible exposure of fibrous roots at the soil surface. There were no significant treatment differences on mid-day stem water potential, leaf gas exchange, and leaf photosystem efficiency. Excessively shaken trees for 20-30 seconds can temporary induce stress symptoms resembling that in trees without irrigation. Trees may have benefited from the low levels of leaf and twig loss after trunk shaking that compensated for any root loss. Long-term effects of trunk shaking will be assessed by tree growth, return bloom, subsequent yield, and carbohydrate reserves.
John L. Jifon and James P. Syvertsen
Effects of foliar sprays of a kaolin clay particle film (Surround WP) on leaf temperature (Tlf), net gas exchange, chlorophyll fluorescence and water relations of sun-exposed leaves on field-grown grapefruit trees (Citrus paradisi L.) were studied during Summer and Fall 2001. Trees were sprayed twice a week for 3 weeks with aqueous suspensions of kaolin (Surround) at 60 g·L-1. Physiological effects of kaolin application were most prominent around midday on warm sunny days than in mornings, evenings or cloudy days. Kaolin sprays increased leaf whiteness (62%), reduced midday leaf temperature (Tlf; ≈3 °C) and leaf to air vapor pressure differences (VPD; ≈20%) compared to water-sprayed control leaves. Midday reductions in Tlf and VPD were accompanied by increased stomatal conductance (gs) and net CO2 assimilation rates (ACO2) of kaolin sprayed leaves, suggesting that gs might have limited ACO2 in water-sprayed control leaves. Midday photoinhibition of photosynthesis was 30% lower in kaolin-sprayed leaves than in control leaves. Midday water use efficiency (WUE) of kaolin-sprayed leaves was 25% higher than that of control leaves. However, leaf transpiration and whole-tree water use were not affected by kaolin film sprays. Increased WUE was therefore, due to higher ACO2. Leaf intercellular CO2 partial pressures (Ci) were similar in control and kaolin-sprayed leaves indicating that stomatal conductance was not the major cause of reduced ACO2. These results demonstrate that kaolin sprays could potentially increase grapefruit leaf carbon uptake efficiency under high radiation and temperature stress.
John D. Lea-Cox and James P. Syvertsen
We studied whether foliar-applied N uptake from a single application of low-biuret N-urea or K NO to citrus leaves was affected by N source, leaf age, or whole-shoot N content. In a glasshouse experiment using potted 18-month-old Citrus paradisi (L.) `Redblush' grapefruit trees grown in full sun, 2- and 6-month-old leaves on single shoots were dipped into a 11.2 g N/liter (1.776% atom excess N-urea) solution with 0.1% (v/v) Triton X-77. Two entire trees were harvested 1.5,6,24, and 48 hours after N application. Uptake of N per unit leaf area was 1.6- to 6-fold greater for 2-month-old leaves than for older leaves. The largest proportion of N remained in the treated leaf, although there was some acropetal movement to shoot tips. In a second experiment, 11.2 g N/liter (3.78% atom excess) urea-15N and 3.4 g N/titer (4.92% atom excess) KNO solutions of comparable osmotic potential were applied to 8-week-old leaves on 5-year-old `Redblush' grapefruit field-grown trees of differing N status. Twenty-four percent of the applied N-urea was taken up after 1 hour and 54% after 48 hours. On average, only 3% and 8% of the K NO was taken up after 1 and 48 hours, respectively. Urea increased leaf N concentration by 2.2 mg N/g or 7.5% of total leaf N after 48 hours compared to a 0.5 mg N/g increase (1.8% of total leaf N) for KNO. Foliar uptake of N from urea, however, decreased (P < 0.05) with increasing total shoot N content after 48 hours (r = 0.57).