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- Author or Editor: J.P. Syvertsen x
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.
Abstract
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.
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.
Abstract
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.
Abstract
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.
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.
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.
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.
The effects of elevated levels of ozone on growth, mineral nutrition and freeze resistance were studied using broadleaf-evergreen citrus and avocado trees. `Ruby Red' grapefruit (Citrus paradisi L.) trees on either Volkamer lemon (Citrus volkameriana Ten. & Pasq.) or sour orange (Citrus aurantium L.) rootstock and `Simmonds' or `Pancho' avocado trees (Persea americana Mill.) on the rootstock `Waldin' were exposed to ozone in open-top chambers for 4 mo in 1988 and in a second experiment in 1989 for 8 mo. Citrus tree growth, estimated by total leaf mass, was unaffected by ozone concentrations of 3 times ambient in either year but avocado growth was reduced by ozone concentration at 2 times ambient in 1989. All trees were well-fertilized and ozone had little effect on mineral nutrient concentrations in leaves. Freeze resistance, estimated by electrolyte leakage from leaf disks and survival of leaves, stems, and whole-plants following exposure to freezing temperatures, was often diminished in avocado and citrus at 3 times ambient ozone, but occasionally was increased at 2 times ambient. Thus, ozone can be related to shifts in freeze resistance that can occur prior to discernible growth effects.
The effects of three highly refined petroleum spray oils and of ambient vapor pressure on net CO2 assimilation (A) and stomatal conductance of water vapor (gs) of single grapefruit (Citrus paradisi Macf.) leaves were investigated. Overall, gs of various-aged leaves was decreased by a large leaf-to-air vapor pressure difference (VPD). In the first experiment, oils with midpoint distillation temperatures (50% DT) of 224, 235, and 247C were applied with a hand atomizer at concentrations of 0, 1%, and 4% oil emulsions in water and 100% oil, all with 0.82% surfactant (by volume). There was a tendency for oils of the two higher DT to decrease net gas exchange during a subsequent 12 days, but significant differences could not be attributed to oil DT. Both A and gs were reduced by the two higher concentrations of oil mixtures. In the second experiment, a commercial airblast sprayer was used to apply the 224C oil at 4% or the 235C oil at 2% and 4% mixtures plus surfactant under field conditions. There were no significant effects of oil treatments on net gas exchange of leaves either measured under moderate VPD outdoors 1 day after spraying or under low VPD in the laboratory 2 days after spraying. No visible phytotoxic symptoms were observed in either experiment.