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P. Healey, T.J Ng, and F.A. Hammerschlag

Mesophyll cells are desirable targets for studying responses to pathogens or pathogen-induced toxins. Based on host-pathogen or host-toxin interaction studies at the cellular level it can be determined whether a toxin can be used as a selective agent. Suspension cells are suitable selection units for in vitro selection of potentially useful somaclonal variants. Protocols for the isolation of muskmelon mesophyll and suspension cells were developed in order to study the effects of roridin E, a toxin produced by Myrothecium roridum, on leaf spot tolerant and sensitive muskmelon cultivars. Viable mesophyll cells were obtained by exposing leaf tissue to 1% cellulysin and 5% macerase in B5 medium with 0.4M sucrose for one hour. Viable suspension cells were maintained a medium consisting of MS salts, 3% sucrose, 3 (μM thiamine·HCl, 555 μM myo-Inositol, 28 μM kinetin and 9 μM IAA. Fluorescein diacetate was used to determine viability over time. Membrane stability was monitored by measuring changes in the fluorescence of cells stained with Merocyanine 540 (MC 540), an optical probe for changes in transmembrane electrical potential (PD).

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Michael V. Mickelbart and Thomas E. Marler

Sapodilla [Manilkara zapota (L.) Royen], reportedly tolerant of saline conditions relative to other tropical fruit species, was studied in sand culture under greenhouse conditions to examine the physiology of sapodilla trees exposed to NaCl and to aid in determining the basis for this apparent tolerance. Treatments, consisting of a complete nutrient solution of 1 dS·m–1 (control) or this solution amended to 12 or 20 dS·m–1 with NaCl, were administered from 16 Nov. 1991 until 29 Jan. 1992. Net CO2 assimilation (A) of plants receiving NaCl gradually decreased relative to that of the control plants. At the end of 8 weeks of salinity, A of plants receiving 12 or 20 dS·m–1 was 72% or 31% of control plants, respectively. Substrate NaCl reduced apparent quantum yield, photosynthetic CO2-use efficiency, leaf osmotic potential, and predawn xylem potential of sapodilla leaves. Dark respiration and the variable: maximal chlorophyll fluorescence ratio were not influenced by NaCl. Exposure to NaCl also increased leaf tissue Na+ and Cl concentrations and the Na+: K+ ratio. These results indicate that gas exchange of sapodilla is relatively low for woody evergreen species. Moreover, sapodilla may not be as tolerant of salt stress as previously reported. The responses of sapodilla to root zone NaCl were consistent with other woody perennial glycophyte species. Photochemical efficiency of leaves on plants receiving NaCl was not different from that of leaves on control plants for >8 weeks after NaCl reduced gas exchange.

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Thomas G. Ranney, John M. Ruter, and Clifford D. Ruth

Temperature sensitivity of net photosynthesis (PSN), dark respiration, and chlorophyll fluorescence was evaluated among three taxa of hollies including I. aquifolium, I. cornuta, and I. rugosa. Variations in foliar heat tolerance among these species were expressed as differential temperature responses for PSN. Temperature optima for PSN was 22.0, 26.3 and 27.9 umol·m–2·s–1 for I. rugosa, I. cornuta, and I. aquifolium, respectively. Differences in temperature optima for PSN and thermotolerance of PSN appeared to result from a combination of stomatal and nonstomatal limitations. At 40°C, potential photosynthetic capacity, measured under saturating CO2, was 4.1, 9.4, and 14.8 μmol·m–2·s–1 for I. rugosa, I. aquifolium, and I. cornuta, respectively. Based on these results, I. rugosa was identified as the most heat-sensitive species followed by I. aquifolium then I. cornuta. Comparative tolerance to root-zone inundation was evaluated among 14 holly taxa. Following 8 weeks of flooding, four of the taxa: I. cornuta `Burfordii', I. × `Nellie R. Stevens', I. cassine, and I. × attenuata `Foster's #2' performed remarkably well during and after flooding with photosynthetic rates > 40% of the controls, root ratings >75% of the controls, <5% of the foliage showing deterioration, and 100% survival. Conversely, I. crenata `Convexa', Ilex × meserveae `Blue Princess', I. rugosa and I. aquifolium `Sparkler' did not tolerate flooding well as indicated by severely depressed photosynthetic rates, deterioration of foliage and roots, and decreased survival. The remaining taxa were intermediate.0

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Kuo-Tan Li* and James P. Syvertsen

Mechanical harvesting of citrus trees by trunk or canopy shakers can cause leaf and twig removal, bark injury and root exposure. Such problems have restricted the adoption of mechanical harvesting in Florida citrus. We assessed physiological responses of citrus trees that were mechanically harvested with a linear-type trunk shaker, operating at 4 Hz, 70.8 kg mass weight, and 6.5 cm displacement, for 10 or 20 seconds. We measured fruit recovery efficiency, leaf and shoot removal, mid-day stem water potential, leaf gas exchange, and leaf fluorescence emission of mature `Hamlin' and `Valencia' orange trees under restricted or normal irrigation. Shaking treatments effectively removed 90% to 94% of fruit without bark damage. Compared to harvesting by hand, trunk shaking removed 10% more leaf area and twigs, and caused some visible exposure of fibrous roots at the soil surface. There were no significant treatment differences on mid-day stem water potential, leaf gas exchange, and leaf photosystem efficiency. Excessively shaken trees for 20-30 seconds can temporary induce stress symptoms resembling that in trees without irrigation. Trees may have benefited from the low levels of leaf and twig loss after trunk shaking that compensated for any root loss. Long-term effects of trunk shaking will be assessed by tree growth, return bloom, subsequent yield, and carbohydrate reserves.

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Cathleen Feser, Rolston St. Hilaire, and Dawn VanLeeuwen

Mexican elder (Sambucus mexicana Presl.) is used in arid landscapes of the Southwest, but the plant is known for its unpredictable performance in those landscapes. We studied drought responses of mexican elder plants grown in an arid environment using an in-ground nursery production system. Plants were maintained as well-irrigated controls or exposed to cyclic drought and irrigated based on evapotranspiration. Drought treatment lasted 165 days. Plants exposed to drought had more negative predawn and midday water potentials than well-watered plants. The ratio of variable to maximal fluorescence (Fv/Fm) for the drought group (0.76), was near the optimum value of 0.8, suggesting that chloroplasts of drought-stressed plants maintained high levels of activity. Drought cycle, but not drought treatment affected stomatal conductance. Drought-stressed plants had lower transpiration rates than controls except at drought cycle five when transpiration rates were similar between irrigation treatments. Relative water content was higher in controls (76%) than plants exposed to drought (66%). Leaf area of well-irrigated plants was over four times higher than that of plants exposed to drought. Leaf area to root dry weight ratio of drought-stressed plants was 79% lower than control plants. Severely reduced leaf area of drought-stressed plants might be one reason why landscape personnel conclude that mexican elder plants perform poorly in arid landscapes.

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J.L. Garcia-Hernandez, E. Troyo-Dieguez, H. Nolasco, H.G. Jones, and A. Ortega-Rubio

The phytotoxic effects on the physiology of chili (Capsicum annum L. cv. Ancho San Luis) caused by four different insecticides were evaluated. Three commercial mixes (methyl azinfos, methyl parathion CE720, and metamidophos 600 LM), and an active ingredient alone (methamidophos) were assayed; water was used as the control. The main goal was to evaluate the insecticide effects on chili using four different doses; the mean dose, recommended on the label of the product (R), a half one (1/2R), 1.5 times (1.5R) and twice the recommended dose (2R). Three frequencies of application were applied; once a week, twice a week, and once every other week, for 6 weeks from the beginning of flowering. Phytotoxicity was evaluated measuring the response of some physiological traits, Chlorophyll Fluorescence (CF), Leaf Temperature (LT), Transpiration (Tr), and Stomatal Resistance (SR). CF was measured by means of a portable chorophyll fluorscence meter; LT, Tr, and SR were measured using a LI-Cor Porometer. The doses and frequencies used are all common in commercial chili fields in Mexico. Results showed that phytotoxicity caused by insecticides can be an important damage factor to the plants, something that can cause reduction of yields. CF was shown to be the most sensitive variable to evaluate the phytotoxicity caused by insecticides. Fruit malformation was observed in all treatments. Chlorophyll content was reduced up to 25%, on average. The phosphorate insecticides affected the physiological parameters more drastically than the others. Results evidence the irreversible crop damage caused by excessive insecticide applications.

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Thomas E. Marler, Bruce Schaffer, and Jonathan H. Crane

Growth and leaf physiology responses of container-grown `Arkin' carambola (Averrhoa carambola L.) trees to long-term exposure of ≈25%, ≈50%, or 100% sunlight were studied in four experiments in Guam and Florida. Shading increased rachis length and leaflet area, and decreased leaflet thickness. Shaded trees also had a more horizontal branch orientation. Shading reduced dark respiration (Rd) and light compensation and saturation points but increased chlorophyll concentration and N-use efficiency. Light-saturated net CO2 assimilation (A) was not affected by developmental light level. Trees in full sun had smaller total leaf area, canopy diameter, and shoot: root ratio and exhibited leaflet movement to avoid direct solar radiation. Also, trees grown in 100% sunlight had a more vertical branch orientation and greater stomatal density than shaded trees. The ratio of variable to maximum fluorescence (Fv/Fm) declined during midday in 100% sunlight trees. This pattern was accompanied by a midday suppression of A in 100% sunlight-grown trees in Guam. `Arkin' carambola trees exposed to ≈25%, ≈50%, or 100% sunlight for up to 39 weeks exhibited physiological and morphological adaptations that resulted in similar growth. These results indicate that carambola efficiently adapts to different developmental light intensities.

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Li-Song Chen and Lailiang Cheng

To determine the cause of a characteristic zonal chlorosis of `Honeycrisp' apple (Malus ×domestica Borkh.) leaves, we compared CO2 assimilation, carbohydrate metabolism, the xanthophyll cycle and the antioxidant system between chlorotic leaves and normal leaves. Chlorotic leaves accumulated higher levels of nonstructural carbohydrates, particularly starch, sorbitol, sucrose, and fructose at both dusk and predawn, and no difference was found in total nonstructural carbohydrates between predawn and dusk. This indicates that carbon export was inhibited in chlorotic leaves. CO2 assimilation and the key enzymes in the Calvin cycle, ribulose 1,5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase, and the key enzymes in starch and sorbitol synthesis, ADP-glucose pyrophosphorylase, cytosolic fructose-1,6-bisphosphatase, and aldose 6-phosphate reductase were significantly lower in chlorotic leaves than in normal leaves. However, sucrose phosphate synthase activity was higher in chlorotic leaves. In response to a reduced demand for photosynthetic electron transport, thermal dissipation of excitation energy (measured as nonphotochemical quenching of chlorophyll fluorescence) was enhanced in chlorotic leaves under full sun, lowering the efficiency of excitation energy transfer to PSII reaction centers. This was accompanied by a corresponding increase in both xanthophyll cycle pool size (on a chlorophyll basis) and conversion of violaxanthin to antheraxanthin and zeaxanthin. The antioxidant system, including superoxide dismutase and ascorbate peroxidase and the ascorbate pool and glutathione pool, was up-regulated in chlorotic leaves in response to the increased generation of reactive oxygen species via photoreduction of oxygen. These findings support the hypothesis that phloem loading and/or transport is partially or completely blocked in chlorotic leaves, and that excessive accumulation of nonstructural carbohydrates may cause feedback suppression of CO2 assimilation via direct interference with chloroplast function and/or indirect repression of photosynthetic enzymes.

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Zi Wei, Peter Jeranyama, Fan Zhang, Carolyn DeMoranville, and Harvey J.M. Hou

fluorescence analysis. The Chl fluorescence is a unique tool to study photosynthetic reaction in cyanobacteria, algae, and higher plants in vivo and is widely used to monitor plant response to the environmental stress, including photoinhibition, chilling

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Artemio Z. Tulio Jr., Kazuo Chachin, Yoshinori Ueda, Kimiko Ose, and Hiroyuki Yamanaka

The physiological and physico-chemical responses of jute, a tropical green leafy vegetable, to several temperature conditions were determined during postharvest storage. Jute were sourced from wholesale market and harvested from the university farm, packed in low-density polyethylene bags and stored at 1 to 30 °C and 1 to 20 °C, respectively, before it was analyzed for postharvest quality changes. There was no significant difference in the time-temperature tolerance of both leaves in all treatments. At 1 °C and 8 °C, both jute showed high sensitivity to chilling injury, which manifested by browning of the stems, darkening of young and mature leaves, wilting, and excretion of slimy substances. These symptoms developed within 3 to 9 days at 1 °C and 5 to 13 days at 8 °C, and it seems related with the decrease of ortho-diphenol content. The chlorophyll fluorescence of jute measured in terms of Fv: Fmax ratio decreased before the onset of browning and remained at lower levels during development of chilling injury at 1 and 8 °C. The ethylene concentration decreased after increasing for 1 day at 8 °C and before chilling injury occurred at 1 °C. However, at 15, 20, and 30 °C, both jute were more susceptible to yellowing with chlorophyll degradation and abscission of leaves due to senescing effect of high-temperature storage. Senescent symptoms were evident within 1 to 5 days at 30 °C and 3 to 7 days at 15 and 20 °C. Jute with stems partly immersed in water during storage had lower respiration rates at 1 and 8 °C due to its sensitivity to chilling injury manifested by wilting of leaves within 1 to 4 days of storage.