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  • Author or Editor: Jim Syvertsen x
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Multiple stresses almost always have synergistic effects on plants. In citrus, there are direct and indirect interactions between salinity and other physical abiotic stresses like poor soil drainage, drought, irradiance, leaf temperature, and atmospheric evaporative demand. In addition, salinity interacts with biotic pests and diseases including root rot (Phytophthora spp.), nematodes, and mycorrhizae. Improving tree water relations through optimum irrigation/drainage management, maintaining nutrient balances, and decreasing evaporative demand can alleviate salt injury and decrease toxic ion accumulation. Irrigation with high salinity water not only can have direct effects on root pathogens, but salinity can also predispose citrus rootstocks to attack by root rot and nematodes. Rootstocks known to be tolerant to root rot and nematode pests can become more susceptible when irrigated with high salinity water. In addition, nematodes and mycorrhizae can affect the salt tolerance of citrus roots and may increase chloride (Cl-) uptake. Not all effects of salinity are negative, however, as moderate salinity stress can reduce physiological activity and growth, allowing citrus seedlings to survive cold stress, and can even enhance flowering after the salinity stress is relieved.

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Young citrus trees and seedlings in Florida's commercial nurseries are often grown under shade cloth netting to avoid high light and temperature. To investigate the potential benefit of altering radiation by colored shade nets, `Cleopatra' mandarin (Cleo, C. reticulata Blanco) seedlings and potted `Valencia' trees [Citrus sinensis (L.) Osbeck] on Cleo or Carrizo [Carr, C. sinensis × Poncirus trifoliate (L.) Raf.] rootstocks were grown in full sun or under 50% shade from blue, black, silver, grey, and red colored shade nets. Changes in photosynthetically active radiation (PAR) and temperatures under the shade were monitored. Leaf function and leaf chlorophyll contents were measured, and plants were harvested by the end of the experiment for shoot and root growth measurements. Plants under the shade received an average of 45% PAR and had lower mid-day leaf temperature than plants in full sun. Plants under blue nets had greatest leaf chlorophyll a, b, and total chlorophyll content, whereas those under red nets had the lowest. However, shading improved photosystem II efficiency from measurements of leaf chlorophyll fluorescence (Fv/Fm) regardless of the color of shade nets. Shading increased shoot growth, shoot to root ratio, and total plant dry weight of Cleo seedlings, especially those under silver nets.

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Effects of nitrogen (N) rate and rootstock on tree growth, fruit yield, evapotranspiration, N uptake, and N leaching were measured over a 2-year period. Four-year-old `Redblush' grapefruit trees on either sour orange (SO), a relatively slow-growing rootstock, or `Volkamer' lemon (VL), a more-vigorous rootstock, were transplanted into 7.9-m3 drainage lysimeter tanks filled with native sand and fertilized at three N rates. N rates averaged from about 14% to 136% of the recommended rate when trees were 5 and 6 years old. More N leached below trees on SO as trees on VL had greater N uptake efficiency. Canopy volume and leaf N concentration increased with N rate, but rootstock had no effect on leaf N. Fruit yield of trees on SO was not affected by N rate, but high N increased water use and yield for larger trees on VL. Canopy growth or yield per volume of water used (water use efficiency) was lowest at low N, but N use efficiency was highest at the low N rates.

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Six-year-old `Spring' navel [Citrus sinensis (L.). Osb.] orange trees were either totally defruited, 50% defruited or left fully cropped to study effects of fruit load on growth net gas exchange characteristics of mature leaves on seven selected clear days from Nov. 2001 through July 2002. Near harvest time, defruited trees had more shoot flushes, greater leaf dry wt per area (LDW/A) but lower net assimilation of CO2 (Ac) and stomatal conductance (gs) at midday than leaves on trees with fruit. Defruited trees had a higher ratio of internal to ambient CO2 (Ci/Ca) concentration in leaves implying internal limitations were dominant over stomatal limitation on Ac. Removal of half the crop increased individual fruit mass but reduced fruit color development. Half the trees were also shaded for four months prior to harvest with reflective 50% shade cloth to determine effects of lower leaf temperature (Tl) and leaf-to-air vapor pressure difference (D) on leaf responses. On selected clear days throughout the season, shade increased midday Ac and gs but decreased Ci/Ca compared to trees in the open implying that high mesophyll temperatures in sunlit leaves were more important than gs in limiting Ac. There were no effects of the shade treatment on canopy volume, yield or fruit size. Shaded fruit developed better external color but lower Brix than sun-exposed fruit. Thus, the presence of mature fruit maintained higher Ac than in leaves on defruited trees but high leaf temperatures and D reduced gs and Ac on warm days throughout the season.

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Maximum CO2 assimilation rates (ACO2) in citrus are not realized in environments with high irradiance, high temperatures, and high leaf-to-air vapor pressure differences (D). We hypothesized that moderate shading would reduce leaf temperature and D, thereby increasing stomatal conductance (g s) and ACO2. A 61% reduction in irradiance under aluminum net shade screens reduced midday leaf temperatures by 8 °C and D by 62%. This effect was prominent on clear days when average midday air temperature and vapor pressure deficits exceeded 30 °C and 3 kPa. ACO2 and gs increased 42% and 104%, respectively, in response to shading. Although shaded leaves had higher gs, their transpiration rates were only 7% higher and not significantly different from sunlit leaves. Leaf water use efficiency (WUE) was significantly improved in shaded leaves (39%) compared to sunlit leaves due to the increase in ACO2. Early in the morning and late afternoon when irradiance and air temperatures were low, shading had no beneficial effect on ACO2 or other gas exchange characteristics. On cloudy days or when the maximum daytime temperature and atmospheric vapor pressure deficits were less than 30 °C and 2 kPa, respectively, shading had little effect on leaf gas exchange properties. The results are consistent with the hypothesis that the beneficial effect of radiation load reduction on ACO2 is related to improved stomatal conductance in response to lowered D.

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Two-month-old citrus rootstock seedlings of Cleopatra mandarin (CM) and Carrizo citrange (CC) were fertilized with nutrient solution, with or without additional 50 mM NaCl, and grown under either ambient CO2 (aCO2, 360 ppm) or elevated CO2 (eCO2, 720 ppm) for 8 weeks. Elevated CO2 increased plant growth, shoot: root ratio, net assimilation of CO2, leaf chlorophyll, and water use efficiency (WUE), but decreased plant water use. Salinity decreased growth, shoot: root ratio, net gas exchange and water use. Neither salinity nor eCO2 affected leaf chlorophyll fluorescence (Fv/Fm), but CC had higher Fv/Fm, leaf gas exchange, chlorophyll, N and Ca than CM. Although salinity increased leaf Cl and Na in both genotypes, CC had higher leaf Cl, but lower Na than CM. Salinity-induced decreases in leaf osmotic potential increased leaf turgor, especially at eCO2. There were no interacting effects of eCO2 and salinity on plant growth, but salinity decreased WUE more at eCO2 than at aCO2 in CM; but not in CC. Elevated CO2 decreased leaf Cl and Na in CC, but tended to increase both ions in CM leaves. Patterns of Cl and Na responses in roots generally were in opposite direction to their respective responses in leaves. Thus, the modifications of citrus seedling responses to salinity by higher growth and lower water use at eCO2 were not only species dependent, but also involved whole plant allocations of Na and Cl.

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The shedding of leaves, branches, flowers, and young fruit; scuffing of bark; and exposed roots that are caused by trunk or canopy shakers during harvest appears to be unavoidable, but generally does not reduce long-term yields. Nonetheless, such visible injuries have limited the widespread adoption of mechanical harvesting in Florida's citrus industry. We determined if such physical injuries caused by a properly operated trunk shaker resulted in any physiological injures or any consequent decline in vigor and productivity of well-managed, healthy citrus trees. We continuously monitored various physiological indexes in mature `Hamlin' and `Valencia' orange trees annually harvested by hand or by a linear-type trunk shaker with various shaking durations. Trunk shaking did not reduce return bloom, fruit set, young fruit growth, or canopy and root growth. There was a correlation between the seasonal timing of a simulated bark injury and recovery from the injury. Although some root exposure was frequently observed during trunk shaking, leaf water relations and fine root growth were unaffected. There was no difference in leaf dry weight per area and leaf nitrogen among treatments. Mechanical and hand harvesting in late season `Valencia' during full bloom removed similar amounts of flowers. However, immature fruit were removed by trunk shaking when `Valencia' were harvested after mid-May, and the number of young fruit removal increased with shaking duration and fruit size. The loss of young fruit for the next crop remains a major problem of mechanical harvesting in late harvest `Valencia'.

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Mechanical harvesting using trunk shakers on late-season `Valencia' sweet orange [Citrus sinensis (L.) Osb.] trees can remove young fruit for the next crop and occasionally cause root exposure or severe bark scuffing on the trunk. To evaluate the effects of these physical injuries on fine root growth and lifespan, we installed minirhizotrons in the root zone of 15-year-old fruiting `Valencia' trees on Swingle citrumelo [C. paradise Macf. × Poncirus trifoliate (L.) Raf.] rootstocks. Images of roots against the minirhizotron tubes were captured biweekly with a custom-made video-DVD recorder system. Trees were harvested in early June by hand or with a linear-type trunk shaker in two consecutive years. Bark injury after trunk shaking was mimicked by removing part (42%) of the bark tissue from the main trunk with a sharp knife. Numbers of fine roots, root activity and lifespan as indexed by the color of the root, and the distribution of new fine roots after harvest were analyzed. Although root exposure was common with the normal operations during mechanical harvesting, few disturbances reached the major fine root zone. There was no clear correlation between root growth and trunk shaking with or without bark injury. The root system might benefit from less competition after the loss of young fruit from mechanical harvesting, as a greater availability of carbohydrates or other resources may compensate for any potential damage due to mechanical harvesting.

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Recent interest in reducing nitrate levels in ground water has stimulated the re-examination of foliar application of urea on citrus trees. Because the cuticle is the primary barrier to foliar uptake, we examined the diffusion of 14C-urea through isolated citrus leaf cuticles. Cuticles were enzymatically isolated from leaves of the four youngest nodes (1 month to 1 year old) of pesticide-free grapefruit trees. The diffusion system consisted of a cuticle mounted on a receiver cell containing stirred buffer solution. Urea (1 μL) was pipetted onto the cuticular surface, and buffer solution was sampled periodically through the side portal of the receiver cell. The time course of urea diffusion was characterized by lag (time to initial penetration), quasi-linear (maximum penetration rate), and plateau (total penetration) phases. Apparent drying time was less than 30 min. Average lag time was about 10 min. The maximum penetration rate occurred about 40 min after droplet application and was about 2% of the amount applied per hour. Rewetting stimulated further penetration. The total penetration averaged about 35% and tended to decrease with leaf age. Dewaxing the second node cuticles by solvent extraction significantly increased maximum penetration rates (30% of the amount applied per hour) and total penetration (64%).

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Four to six-yr-old `Red Ruby' grapefruit trees on either `Volkamer' lemon (VL) or sour orange (SO) rootstocks were fertilized with 3 rates of nitrogen (N) over a 3 year period. We studied the effects of leaf N concentration on stomatal conductance (gs), net assimilation (A) of CO2 (Li-Cor portable gas exchange system), carbon isotope discrimination (δ 13C) of tree tissues, root growth, canopy development and fruit yield. Using springtime measurements of net gas exchange during the fifth year, gs, A and leaf tissue δ 13C were positively correlated with leaf N. The faster growing trees on VL had larger canopy volumes and fruit yields but lower leaf N, A and δ 13C than those on SO. Thus δ 13C was positively correlated with A but negatively related to tree size and yield. By the sixth year, δ 13C was still related to N but tree growth had apparently obscured any rootstock effects on leaf N, water use efficiency, A and δ 13C. Leaf and trunk bark tissue δ 13C did not differ but root bark had lowest δ 13C regardless of rootstock species.

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