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  • Author or Editor: J.P. Syvertsen x
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The hydraulic conductivities of intact root systems of 4 commercial citrus rootstocks were estimated using a pressure chamber technique. The rootstocks used were rough lemon (Citrus jambhiri Lush.), sour orange (C. aurantium L.), Carrizo citrange [Poncirus trifoliata (L.) Raf. × C. sinensis (L.) Osb.], and Cleopatra mandarin (C. reshni Hort. ex TAN). Carrizo and rough lemon seedlings had the highest root conductivity, whereas Cleopatra and sour orange had the lowest root conductivity. Although these rootstocks as seedlings produce root systems in pots that differ from those in the field, some of the growth, yield, and drought resistance chartacteristics that have been previously assoicated with these rootstocks may be at least partially explained by the hydraulic conductivity of their roots.

Open Access
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Net CO2 assimilation (A) rates of ‘Duncan’ grapefruit (Citrus paradisi Macf.) and ‘Pineapple’ orange (C. sinensis L.) seedlings grown under 3 different photosynthetic photon flux densities (PPFD), were measured in an open gas exchange system under controlled environmental conditions. Apparent quantum yield (ø), mesophyll conductance to CO2 (Gm), leaf conductance to H2O vapor (G1), transpiration (E) and water use efficiency (WUE) also were examined. Leaves of both species grown under high PPFD (full sunlight) had the greatest maximum rates of A, but the low PPFD (90% shade) leaves had the highest ø. The WUE of low PPFD grapefruit leaves was less than that of the high PPFD leaves but increased within 2 weeks after being moved into full sunlight. Transferring seedlings from low to high PPFD decreased ø of grapefruit but not of orange leaves. Changes in A were more strongly correlated to Gm than to G1. Carbon dioxide assimilation rate was positively correlated to total leaf nitrogen content. Citrus leaf photosynthetic characteristics and resources use efficiency not only acclimate to the light regimes under which they expand and mature, but leaves are capable of acclimating to new light regimes, even after full maturation.

Open Access
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Freeze-damaged ‘Valencia’ oranges [Citrus sinensis (L.) Osbeck] were tightly covered with polyethylene bags while still on the tree, and their dehydration rates were compared to those of nonbagged fruit. The bags significantly reduced water loss from fruit over a period of 11 weeks after the freeze. After fruit were harvested and removed from the bags, previously bagged fruit had higher water loss rates than nonbagged fruit. These data indicate a significant amount of water in the fruit is lost through the peel after freeze damage has occurred. The dryness cut method did not quantitatively determine dehydration of freeze-damaged oranges.

Open Access
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Abstract

Two major contact points exist between terrestrial plants and their external environment. One interface exists between the root and the soil, where water, mineral nutrients, and oxygen enter the plant system while carbohydrates, amino acids, other exudates, and CO2 can leave the plant and enter the rhizosphere. The other interface exists between shoot tissues and their aerial environment, where in leaves, CO2 and oxygen are exchanged and H2O vapor inevitably diffuses out of stomata. These interfaces are critical control points where environmental factors and physiological mechanisms interact to regulate exchanges of energy (heat and radiation) and materials (water, carbon, oxygen, and mineral nutrients). When available soil water is limited, water movement through the plant system is regulated primarily by soil water supply and conductivity of the roots. When soil water is adequate, water movement is controlled by transpiration which is regulated by both stomatal aperture and evaporative demand.

Open Access
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Abstract

Seedlings of orange [Citrus sinensis (L.) Osbeck] and grapefruit (C. paradisi Macf.) were grown under various nutrient regimes to determine the relationships between components of net gas exchange and N and P content. There were no significant differences in photosynthetic characteristics between the two species. Total N content of leaves varied from about 10 to 30 mg·g−1, whereas leaf P was sufficient, varying from about 1 to 2 mg·g−1. Potential rates of single leaf CO2 assimilation, water use efficiency, chlorophyll content, and carboxylation efficiency were all positively correlated with leaf N, whereas these parameters were not related to leaf P content. CO2 compensation point was negatively correlated with leaf N content. Thus, N deficiency stress reduces net CO2 assimilation by affecting several different partial processes of photosynthesis.

Open Access
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The objectives of these greenhouse experiments were to determine the effects of elevated CO2 on growth, mineral nutrition, and gas exchange physiology of seedlings of four commercial Citrus rootstocks. We grew well-watered and fertilized seedlings of `Volkamer' lemon (VL), `Cleopatra' mandarin (CL), `Swingle' citrumelo (SW), and `Troyer' citrange (TC) cultivars (in decreasing order of vigor) in unshaded, air-conditioned greenhouses at ambient CO2 (350 μmol/mol) or 2x ambient CO2 for 5 months. CL was the smallest cultivar, had the lowest root/shoot (r/s) ratio,and lowest rates of CO2 assimilation (A) of leaves, transpiration (E), and water-use efficiency, (A/E). Overall, daily whole-plant water use was correlated with single-leaf E. Elevated CO2 increased both shoot and root growth similarly; therefore, r/s was not affected. Elevated CO2 increased A, leaf dry wt/area, and leaf C, but decreased transpiration and leaf N so that leaf C/N, A/N, and A/E all increased. Although plant size of the four cultivars ranked similarly at both ambient and high CO2, the more-vigorous cultivars grew proportionately more at high CO2 than the less-vigorous cultivars. Growing cultivars at elevated CO2 can yield insights into mechanisms determining vigor and relationships between A and plant growth.

Free access

Abstract

Whole plant transpiration and maximum rates of net gas exchange of CO2 and H2O vapor from single leaves were positively correlated with the hydraulic conductivity of roots of seedlings of 5 citrus rootstock species, [Poncirus trifoliata (L.) Raf. × Citrus sinensis (L.) Osbeck, P. trifoliata, C. autantium L., C. paradisi Macf. × P. trifoliata, and C. reticulata Blanco]. Leaf N and P content and shoot:root ratio also were positively correlated with root conductivity. Differences in soil water depletion and plant water relations of 2 of the rootstocks during drought and recovery cycles apparently were related to their root conductivity. The ranking of root conductivities of these seedlings generally reflects the vigor that these rootstocks impart to trees in the field. Thus, the capability of root systems to conduct water and mineral elements is an important factor in plant growth and physiological activity.

Open Access

We tested the hypothesis that amendments of two hydrophilic gels to a sand soil would reduce N leaching losses and increase growth of citrus seedlings. Three-month-old seedlings of `Swingle' citrumelo [Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] were transplanted into containers of steam-sterilized Candler sand, amended with a linear acrylamide/acrylate copolymer (PAM), and/or a cross-linked copolymer agronomic gel (AGRO). Two rates of each amendment were applied either alone or together and were either mixed into dry sand prior to seedling transplant, used as a root-dip slurry at transplant or applied to the soil surface in a solution after transplant. Seedlings were grown in the greenhouse for 5 months and irrigated to container capacity with a dilute nutrient solution without leaching. Pots were leached every 2 weeks and total N losses from the soil were measured in the leachate. PAM amendments increased N retention in soil slightly but PAM had no effect on plant growth, water use, N uptake, or N leaching relative to unamended control plants. The AGRO amendments increased seedling growth, plant water use and uptake of N from 11% to 45% above that of the unamended control plants depending on application method. AGRO decreased N concentrations in the leachate to as low as 1 to 6 mg·L-1. Only 6% of the total applied N was leached from the AGRO treatments, which was about half that from the untreated control plants. There was no additional benefit of using both amendments together or of an additional AGRO root dip treatment. The largest plants used the most water, required the most N and had the greatest N uptake efficiency. AGRO amendments clearly enhanced seedling growth, increased their N uptake efficiency, and reduced N losses from this sand soil.

Free access
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Four-year-old `Redblush' grapefruit (Citrus paradisi Macf.) trees on either the relatively fast-growing rootstock `Volkamer' lemon (VL) (C. volkameriana Ten. & Pasq.) or on the slower-growing rootstock sour orange (SO) (C. aurantium L.) were transplanted into 7.9-m3 drainage lysimeter tanks filled with native Candler sand, irrigated similarly, and fertilized at three N rates during 2.5 years. After 6 months, effects of N application rate and rootstock on tree growth, evapotranspiration, fruit yield, N uptake, and leaching were measured during the following 2 years. When trees were 5 years old, low, medium, and high N application rates averaged about 79,180, or 543 g N/tree per year and about 126,455, or 868 g N/tree during the following year. Recommended rates average about 558 g N/tree per year. A lysimeter tank with no tree and additional trees growing outside lysimeters received the medium N treatment. Nitrogen concentration in the drainage water increased with N rate and exceeded 10 mg·liter-1 for trees receiving the high rates and also for the no tree tank. Leachate N concentration and total N recovered was greater from trees on SO than from those on VL. Average N uptake efficiency of medium N rate trees on VL was 6870 of the applied N and 61 % for trees on SO. Nitrogen uptake efficiency decreased with increased N application rates. Trees outside lysimeters had lower leaf N and fruit yield than lysimeter trees. Overall, canopy volume and leaf N concentration increased with N rate, but there was no effect of N rate on fibrous root dry weight. Fruit yield of trees on SO was not affected by N rate but higher N resulted in greater yield for trees on VL. Rootstock had no effect on leaf N concentration, but trees on VI. developed larger canopies, had greater fibrous root dry weight, used more water, and yielded more fruit than trees on SO. Based on growth, fruit yield and N leaching losses, currently recommended N rates were appropriate for trees on the more vigorous VL rootstock but were 22% to 69 % too high for trees on SO.

Free access

We compared net gas exchange rates of CO2 and H2O vapor of greenhouse-grown `Duncan' grapefruit (Citrus paradisi Macf.) and `Valencia' orange [C. sinensis (L.) Osbeck] leaves after multiple foliar sprays of urea N with and without NaCl: CaCl2 solutions. Highly saline solutions (3.8 dSm-1) caused necrotic burn symptoms after leaf chloride levels reached 7 mmol·m-2. Grapefruit leaves had higher leaf Cl and more burn symptoms than orange leaves. The remaining green areas of all salt-stressed leaves, however, had similar rates of net CO2 assimilation (ACO2) and stomatal conductance (gs) as water-sprayed control leaves. Total leaf N and chlorophyll increased with repeated foliar applications of urea solutions regardless of salinity levels in the spray solution. Thus, salts in solution did not interfere with foliar absorption of N. High urea N solutions (33.6 g·liter-1) without salts caused foliar burn and leaf abscission after one application. Three sprays of urea-N solution (11.2 g·liter-1) increased N concentration of N-deficient leaves about 60% and increased ACO2 rate about 50%. ACO2 did not increase when nitrogen concentration in leaves exceeded a threshold value of about 200 mmol·m-2 so photosynthetic nitrogen use efficiency (PNUE = ACO2/N) decreased with increasing leaf N concentration. Net gas exchange and PNUE was higher for grapefruit than for orange leaves. Leaf Cl levels from foliar-applied salts may not be as detrimental to leaf gas exchange as Cl from salts in soil-applied irrigation water.

Free access