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  • Author or Editor: James P. Syvertsen x
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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).

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We examined how N supply affected plant growth and N uptake, allocation and leaching losses from a fine sandy soil with four Citrus rootstock species. Seedlings of `Cleopatra' mandarin (Citrus reticulata Blanco) and `Swingle' citrumelo (C. paradisi × P. trifoliata) were grown in a glasshouse in 2.3-liter pots of Candler fine sand and fertilized weekly with a complete nutrient solution containing 200 mg N/liter (20 mg N/week). A single application of 15NH4 15NO3(17.8% atom excess 15N) was substituted for a normal weekly N application when the seedlings were 22 weeks old (day O). Six replicate plants of each species were harvested at 0.5, 1.5, 3.5, 7, 11, and 30 days after 15N application. In a second experiment, NH4 NO3 was supplied at 18,53, and 105 mg N/week to 14-week-old `Volkamer' lemon (C. volkameriana Ten. & Pasq.) and sour orange (C. aurantium L.) seedlings in a complete nutrient solution for 8 weeks. A single application of 15NH4 15NO3 (23.0% 15N) was substituted at 22 weeks (day 0), as in the first experiment, and seedlings harvested 3,7, and 31 days after 15N application. Nitrogen uptake and partitioning were similar among species within each rate, but were strongly influenced by total N supply and the N demand by new growth. There was no 15N retranslocation to new tissue at the highest (105 mg N/week) rate, but N supplies below this rate limited plant growth without short-term 15N reallocation from other tissues. Leaf N concentration increased linearly with N supply up to the highest rate, while leaf chlorophyll concentration did not increase above that at 53 mg N/week. Photosynthetic CO2 assimilation was not limited by N in this study; leaf N concentration exceeded 100 mmol·m-2 in all treatments. Thus, differences in net productivity at the higher N rates appeared to be a function of increased leaf area, but not of leaf N concentration. Hence, N use efficiency decreased significantly over the range of N supply, whether expressed either on a gas-exchange or dry weight basis. Mean plant 15N uptake efficiencies after 31 days decreased from 60% to 47% of the 15N applied at the 18,20, and 53 mg N/week rates to less than 33% at the 105 mg N/week rate. Leaching losses increased with N rate, with plant growth rates and the subsequent N requirements of these Citrus species interacting with residual soil N and potential leaching loss.

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Urea solutions, with or without non-ionic (X-77) and organosilicone (L-77) surfactant, were applied to Citrus leaves and isolated cuticles to examine adjuvant effects on urea uptake and leaf net gas exchange. When compared to X-77, L-77 exhibited superior features as a surfactant, resulting in smaller contact angles of droplets deposited on teflon slide. Both L-77 and X-77 had a strong effect on penetration rate of urea within first 20 min of experiment. Effect of L-77 on urea penetration rate decreased quickly within next 20 min, whereas the effect of X-77 was sustained over a 24-h period following application. When compared to solution of urea alone, addition of X-77 to urea resulted in significant increase of the total amount of urea that penetrated the cuticles. The effect of L-77 was smaller, although the total amount of urea that penetrated the cuticles within a 4-day period was similar for both surfactants. Solutions of either urea alone, urea+L-77 and urea+X-77, or L-77 alone, induced a negative effect on net CO2 assimilation (ACO2) for 4 to 24 h after they were sprayed onto leaves. X-77, when applied alone, had no effect on ACO2. Scanning electron microscopy revealed that 1 h after application, leaf surfaces treated with X-77 appeared to be heavily coated, as opposed to those treated with L-77, which appeared similar to untreated control leaves.

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We examined the effects of air temperature, relative humidity (RH), leaf age, and solution pH on penetration of urea through isolated cuticles of citrus leaves. Intact cuticles were obtained from adaxial surfaces of different aged grapefruit leaves. A finite dose diffusion system was used to follow movement of 14C-labeled-urea from solution droplets across cuticles throughout a 4-day period. The rate of urea penetration increased as temperature increased from 19 °C to 28 °C, but penetration was not further increased at 38 °C. Increasing RH increased droplet drying time and urea penetration at both 28 °C and 38 °C. Cuticle thickness, weight per area, and the contact angle of urea solution droplets increased as leaves aged. Cuticular permeability to urea decreased as leaf age increased from 3 weeks to 7 weeks, but permeability increased in cuticles from leaves older than 9 weeks. Contact angles decreased with increased urea solution concentration on six 7-week-old leaf surfaces, but solution concentration had no effect on contact angle on cuticles from younger and older leaves. Reducing pH of urea solution from pH 8 to pH 4 accelerated the loss of urea from breakdown, possibly due to hydrolysis.

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The use of abscission compounds to loosen fruit from stems can be accompanied with various levels of phytotoxicity. To determine the effects of a promising abscission compound, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP), and ethephon on sweet orange [Citrus sinensis (L.) Osbeck] leaf function, water relations, and young fruit growth, we sprayed CMNP at 0, 200, 500, 1000, or 2000 mg·L−1 or ethephon at 400 or 800 mg·L−1 to fruiting branches of potted and field-grown sweet orange during the 2005–06 harvest season. Both compounds induced abscission of mature fruit and leaves 3 days after application but had little effect on leaf chlorophyll content, water content, and midday leaf water potential (Ψleaf) of remaining leaves. CMNP sprayed at 200 mg·L−1 or either concentration of ethephon did not affect leaf photosystem II efficiency, as indicated by leaf chlorophyll fluorescence (Fv/Fm). High CMNP concentrations (1000 or 2000 mg·L−1) reduced Fv/Fm 1 day after treatment, but Fv/Fm of leaves remaining on sprayed branches gradually recovered to the level of control leaves by 4 days after treatment. Similarly, high concentrations of CMNP and ethephon temporarily reduced net gas exchange of leaves for about 4 days. Young fruit growth also was temporarily inhibited by CMNP concentrations greater than 200 mg·L−1. We conclude that CMNP sprayed at recommended concentrations (200–500 mg·L−1) caused mature fruit abscission with little long-term phytotoxic effect on leaves or young fruit.

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We determined if frequency of application of irrigation water plus fertilizer in solution (fertigation) could modify root and shoot growth along with growth per unit nitrogen (N) and water uptake of seedlings of the citrus rootstock Swingle citrumelo growing in a greenhouse. In the first experiment, all plants received the same amount of water with sufficient fertilizer N but in three irrigation frequencies applied in 10 1.5-mL pulses per day, one 15-mL application per day, or 45 mL applied every 3 days. Plants irrigated at the highest frequency grew the least total dry weight and had the highest specific root length. Plants with lowest irrigation frequency grew the most and used the least water so had the highest water use efficiency. There were no irrigation frequency effects on relative growth allocation between shoot and roots, net gas exchange of leaves, or on leaf N. A second experiment used identical biweekly irrigation volumes and fertilizer rates, but water and fertilizer were applied using four frequency combinations: 1) daily fertigation; 2) daily irrigation with fertilizer solution applied every 15 days; 3) fertigation every 3 days; or 4) irrigation every 3 days and fertilizer solution applied every 14 days. Total plant growth was unaffected by treatments, but the highest frequency using the lowest fertilizer concentration grew the greatest root dry weight in the uppermost soil depths. Roots grew less and leaf N was highest when N was applied every 15 days, implying that root N uptake efficiency was increased when fertigated with the highest fertilizer concentration. All plants had similar water use efficiencies. A third experiment was conducted with irrigation every 3 days and with four different N application frequencies: every 3, 6, 12, or 24 days using four fertilizer concentrations but resulting in similar total N amounts every 24 days. There were no differences in growth, gas exchange, or water use efficiency. Given the fact that all treatments received adequate and equal amounts of water and fertilizer, fertigation frequency had only small effects on plant growth, although very high frequency fertigation decreased N uptake efficiency.

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Oleocellosis or oil spotting on the peel of citrus fruit is a common post-harvest injury caused by improper handling. Mechanical injury allows phytotoxic oil to leak out of oil glands and cause injury to surrounding flavedo cells, resulting in oleocellosis. Mechanical harvesting (MH) of ‘Valencia’ sweet orange is conducted in late spring, when the next season's fruitlets are in their early stages of development. There is a concern that mechanical injury from harvesting machines can cause oleocellosis and fruit drop of young, green ‘Valencia’ sweet orange fruitlets, especially late in the harvest season when fruitlets are relatively large. We evaluated the effects of winter drought stress and subsequent late-season MH with a canopy shaker on oleocellosis of ‘Valencia’ sweet orange fruitlets. In April, mature fruit size, juice content, total soluble solids, and acidity were unaffected by previous winter drought stress treatments. Mechanical harvesting removed ≈90% to 95% of mature fruit and 20% to 50% of fruitlets depending on previous drought stress treatments and harvesting date. Beginning 1 week after the late harvest (13 June), attached fruitlets were tagged and visually evaluated approximately every other month to determine oleocellosis injury until the late-season harvest 12 months later. Only 12% of the fruitlets had oleocellosis on more than 30% of their surface area. Up to 75% of the fruitlets from the previously drought-stressed trees had less than 10% of their surface area injured after MH and 11% of these fruitlets dropped before harvest. Nonetheless, there was no significant increase in fruit drop with increased surface area injured nor was juice quality affected at harvest. Overall, fruit surface oleocellosis decreased and healed as fruit expanded, but surface blemishes did not completely disappear. Thus, fruitlet oleocellosis in late-season mechanically harvested trees was cosmetic and did not increase fruit drop nor alter internal fruit quality.

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Correlations between extractable leaf chlorophyll (Chl) concentration and portable, nondestructive leaf “greenness” meter readings imply that such estimates can be used as surrogate measurements of leaf nitrogen (N) status. However, few studies have actually found a direct relationship between Chl meter readings and leaf N. We evaluated the utility of two handheld transmittance-based Chl content meters (SPAD-502, Minolta Corp. and CCM-200, Opti-Sciences) and one reflectance-based meter (Observer, Spectrum Technologies), in estimating Chl and N concentrations in intact leaves of several citrus cultivars. Total Chl determined analytically, correlated well with nondestructive Chl meter readings (r 2: 0.72 to 0.97; P < 0.0001), but regression models differed among cultivars using the same meter and also among meters for a given cultivar. The relationships were generally more linear and stronger at low Chl concentrations (<0.5 mmol·m-2) than at higher Chl concentrations, reflecting increased variability in Chl meter readings with increasing leaf Chl. Significant relationships between Chl meter readings and measured leaf N concentrations were also found in all the cultivars tested (r 2: 0.23 to 0.69; P < 0.01), but the data were more variable than those for Chl. Field-grown leaves were significantly thicker and had higher Chl meter readings than greenhouse-grown leaves of similar Chl or N concentrations. The results suggest that nondestructive Chl content meters can overestimate Chl and N in thicker leaves and/or leaves with high Chl concentrations. A single prediction equation derived from a wide range of Chl or N concentrations could be applicable across the range of citrus cultivars when grown in the same environment. Potential limitations associated with leaf thickness as influenced by environmental factors may necessitate the development of more specific calibration equations.

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Although urea can be an effective adjuvant to foliar sprays, we examined effects of additional surfactants on urea penetration through leaf cuticles along with the effect of urea with and without surfactants on net gas exchange of leaves of `Marsh' grapefruit (Citrus paradisi Macf.) trees budded to Carrizo citrange (C. sinensis L. Osbeck × Poncirus trifoliata L. Raf.) rootstock. Various combinations of urea, a nonionic surfactant (X-77), and an organosilicone surfactant (L-77), were applied to grapefruit leaves and also to isolated adaxial cuticles. When compared to X-77, L-77 exhibited superior surfactant features with smaller contact angles of droplets deposited on a teflon slide. Both L-77 and X-77 initially increased penetration rate of urea through cuticles, but the effect of X-77 was sustained for a longer period of time. The total amount of urea which penetrated within a 4-day period, however, was similar after addition of either surfactant. Solutions of either urea, urea + L-77, urea + X-77, or L-77 alone decreased net assimilation of CO2 (ACO2) for 4 to 24 hours after spraying onto grapefruit leaves. A solution of X-77 alone had no effect on ACO2 over the 4-day period. Although reductions in ACO2 were similar following sprays of urea formulated with two different surfactants, the underlying mechanisms may not have been the same. For the urea + X-77 treatment, X-77 increased the inhibitory effects of urea on ACO2 indirectly by increasing penetration of urea into leaves. For the urea + L-77 formulation, effects of L-77 on ACO2 were 2-fold, direct by inhibiting ACO2 and indirect by increasing urea penetration. One hour after application, scanning electron microscopy (SEM) of leaf surfaces treated with X-77 revealed that they were heavily coated with the residue of the surfactant, whereas leaves treated with L-77 looked similar to nontreated leaves with no apparent residues on their surfaces. The amount of X-77 residue on the leaves was lower 24 hours after application than after 1 hour as observed by SEM.

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In three separate experiments, the growth and water use of salinized citrus rootstock seedlings and grafted trees were modified using different growth substrates, elevated CO2, or 50% shade screen under field conditions. By reanalyzing previously published data, we tested the hypothesis that salinity tolerance in citrus can be characterized as the ability to maintain low levels of leaf Cl accumulation through high plant growth and high water use efficiency (WUE) under saline conditions. Well-irrigated salinized seedlings of the relatively salt-sensitive Carrizo citrange [Carr (Citrus sinensis × Poncirus trifoliata)] were grown in sand, clay, or a peat-based soilless media. Salinity stress reduced plant growth and water use. Leaf Cl concentration was negatively related to plant growth, but leaf Cl increased with transpiration rate in low-saline treatments. In a second experiment using salinized seedlings of the relatively salt-tolerant Cleopatra mandarin [Cleo (Citrus reticulata)] grown along with Carr seedlings with or without elevated CO2, leaf Cl was negatively related to growth and to shoot/root dry weight ratio, but was positively related to water use such that leaf Cl was negatively related to leaf WUE. In a third experiment using salinized 2-year-old ‘Valencia’ orange (C. sinensis) trees grafted on Cleo or Carr rootstocks and grown with or without shadecloth, leaf Cl was positively related to leaf transpiration as both were higher in the spring than in the fall, regardless of rootstock or shade treatment. Overall, leaf Cl was positively related to water use and was negatively related to leaf WUE. High growth, low water use, and consequently, high WUE of salinized citrus were related to low leaf Cl. Such relationships can be used as indicators of salinity tolerance.

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