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Madhulika Sagaram and Jacqueline K. Burns

Chlorophyll fluorescence and photochemical and nonphotochemical quenching parameters were measured in 20 genotypes of Citrus spp. or relatives grown in the greenhouse and commercial ‘Valencia’ sweet orange (Citrus sinensis) trees at two Florida locations. The purpose was to determine the utility of measurements for early huanglongbing [HLB (Candidatus Liberibacter asiaticus)] detection in asymptomatic trees and to examine the leaf response to HLB infection. Polymerase chain reaction (PCR)-negative healthy and PCR-positive symptomatic, asymptomatic, and distant asymptomatic leaves were used for fluorescence analysis using a portable chlorophyll fluorometer. Greenhouse-grown genotypes were separated into mild, moderate, and severe symptom groups based on leaf mottling, color, and size. In general, mild symptom genotypes were characterized by increased photosystem II (PSII) excitation pressure and unregulated heat dissipation and decreased regulated heat dissipation, whereas moderate and severe symptom genotypes increased loss of photosynthetic efficiency and increased unregulated and regulated heat dissipation. Distant asymptomatic leaves could be distinguished from healthy ones in moderate and severe symptom genotypes by increased total and regulated heat dissipation measurements. In the field, overall photosynthetic efficiency and total regulated heat dissipation measurements could distinguish between healthy and asymptomatic ‘Valencia’ sweet orange leaves at the location with slow or more recent infection, but not at the location where infection appeared to progress faster or was of longer duration. Starch content followed a similar pattern. The results indicate that no single measurement uniquely described the relationship between HLB and the host in asymptomatic and healthy leaves, but accuracy of field-based detection could be strengthened by a combination of total nonphotochemical quenching, overall photosynthetic efficiency, starch content, and PCR analyses. Chlorophyll fluorescence and quenching measurements suggest a PSII-based explanation for, and temperature dependency of, leaf symptom development.

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Jean-Pierre Goffart, Marguerite Olivier, and Marc Frankinet

al. (2004) . Leaf chlorophyll fluorescence-based methods are also being investigated for crop N monitoring ( Tremblay, 2004 ). These methods are based on leaf chlorophyll fluorescence induced by ultraviolet radiation and visible radiation and on

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Kuo-Tan Li, Jacqueline K. Burns, and James P. Syvertsen

middle section of the canopy. Leaf and fruit retention was measured in all ‘Hamlin’ orange field trials. Leaf water content, leaf dry weight, midday leaf water potential, and leaf chlorophyll fluorescence and chlorophyll content were measured as described

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Diane Feliciano Cayanan, Youbin Zheng, Ping Zhang, Tom Graham, Mike Dixon, Calvin Chong, and Jennifer Llewellyn

, leaf discoloration) of plant leaves was observed and recorded daily. Leaf chlorophyll content index (CCI) was measured with a chlorophyll meter (CCM-200; Opti-Sciences, Tyngboro, MA) and leaf chlorophyll fluorescence (F v /F m ) was measured with a

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Jens N. Wünsche, John W. Palmer, and Dennis H. Greer

Effect of crop load on tree growth, leaf characteristics, photosynthesis, and fruit quality of 5-year-old `Braeburn' apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] trees on Malling 26 (M.26) rootstock was examined during the 1994-95 growing season. Crop loads ranged from 0 to 57 kg/tree [0 to 1.6 kg fruit/cm2 trunk cross sectional area (TCA) or 0 to 8.7 fruit/cm2 TCA]. Fruit maturity as indicated by background color, starch/iodine score, and soluble solids was advanced significantly on low-cropping trees compared to high-cropping trees. Whole-canopy leaf area and percentage tree light interception increased linearly with a significant trend as crop load decreased. From midseason until fruit harvest, leaf photosynthesis decreased significantly on lighter cropping trees and similarly, a positive linear trend was found between whole-canopy gas exchange per unit area of leaf and crop load. Leaf starch concentration in midseason increased linearly as crop load decreased, providing some explanation for the increased down-regulation of photosynthesis on trees with lower crop loads. After fruit harvest, the previous crop loads had no effect on leaf photosynthesis and preharvest differences in whole-canopy gas exchange per unit area of leaf were less pronounced. At each measurement date, daily whole-canopy net carbon exchange and transpiration closely followed the diurnal pattern of incident photosynthetic photon flux. The photochemical yield and electron transport capacity depended on crop load. This was due mostly to reaction center closure before harvest and an increased nonphotochemical quenching after harvest.

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Kuo-Tan Li and Jim Syvertsen

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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Kuo-Tan Li, Jackie Burns, Luis Pozo, and Jim Syvertsen

To determine the effects of abscission compounds 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) and ethephon on citrus leaf function and water relations, we applied CMNP at 0, 200, 500, 1000, or 2000 ppm, or ethephon at 400 or 800 ppm, to canopies of fruiting potted and field citrus trees during the harvest season. Both compounds induced fruit and leaf drop after 3 days of application, especially at high concentrations. Low concentrations of CMNP (0, 200, or 500 ppm) or either ethephon treatments did not affect leaf photosystem II efficiency, as indicated by leaf chlorophyll fluorescence (Fv/Fm). High concentrations of CMNP (1000 or 2000 ppm) immediately reduced photosystem II efficiency in leaves and fruit peel. However, Fv/Fm of leaves remaining on the trees was gradually restored and close to the level of control after 4 days of treatment. Both compounds had little effect on chlorophyll content, ratio of chlorophyll a to chlorophyll b, leaf water content, and mid-day leaf water potential. The results suggest that CMNP at recommended concentrations (200 to 500 ppm) effectively reduced fruit attachment force with little herbicidal effect on leaves.

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Youbin Zheng, Linping Wang, and Mike A. Dixon

Electrolytically generated copper is increasingly used to control diseases and algae in the greenhouse industry. However, there is a shortage of information regarding appropriate management strategies for copper in ornamental crop production. The objectives of this study were to characterize the response of three ornamental crops (Dendranthema ×grandiflorum L. `Fina', Rosa ×hybrida L. `Lavlinger', Pelargonium ×hortorum L. `Evening Glow') to different solution levels of Cu2+ (ranging from 0.4 to 40 μm) and to determine the critical levels above which toxic responses became apparent. The following measurements were used to assess the treatments: leaf chlorophyll fluorescence (Fv/Fm), leaf chlorophyll content, and visible injury of leaf and root. Excessive copper reduced plant root length, root dry weight, total dry weight, root to shoot ratio, leaf area, and specific leaf area in all three species. The critical solution level of Cu2+ that resulted in significantly reduced plant dry weight for chrysanthemum was 5 μm; for miniature rose, 2.4; and for geranium, 8 μm. Plant visible root injury was a more sensitive and reliable copper toxicity indicator than visible leaf injury, leaf chlorophyll content, Fv/Fm, or leaf and stem copper content. Generally, all the species exhibited some sensitivity to Cu2+ in solution culture, with chrysanthemum and miniature rose being most sensitive and geranium being least sensitive. Caution should be taken when applying copper in solution culture production systems.

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Jim Syvertsen and Francisco Garcia-Sanchez

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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Zhongchun Wang and Bruno Quebedeaux

Chlorophyll fluorescence measurements are providing insights into Photosystem II (PSII) quantum efficiency and hence are able to provide a good estimation of carbon assimilation under field conditions. A F2 generation of sibcross seedlings from a cross of `Goldspur' × `Redspur' were selected to identify genetic variations and the relationships among fluorescence parameters, carbon assimilation, and carbon partitioning in apple leaves. Mature leaves from extension shoots were analyzed for chlorophyll fluorescence with a CF-1000 chlorophyll fluorescence measurement system, photosynthetic rate with a LI-6200 portable photosynthesis system, and carbohydrates with a Shimadzu HPLC. Significant variations in leaf chlorophyll fluorescence parameters and photosynthetic rates were found. The ratio of Fv: Fm, an estimation of photochemical efficiency of PSII, decreased from ≈0.90 in June to ≈0.75 in September while the photosynthetic rates decreased from ≈8.5 in June to ≈4.5 μmol·m–2·s–l in September. The relationships between fluorescence parameters, photosynthesis, and carbohydrate partitioning were analyzed and the ratio of sorbitol to sucrose in relation to the efficiency of PSII and NADPH production will be discussed.