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Gil Nir, Kira Ratner, Eugene E. Gussakovsky and Yosepha Shahak

Photoinhibition (PI; light stress) was studied in leaves of 3-year-old, potted, mango plants, following changes in chlorophyll fluorescence characteristics. Iron deficiency was induced by calcareous soil mix. It was found that during summer days, the photochemical quantum yield (measured as Fv: Fm) was reduced by 15% to 30% in sun-exposed leaves by noon (under 2000 μmol·m–2·s–1). This apparent PI was mostly recovered by the next day. In Fe-deficient plants, the reversible PI was enhanced further. In addition, Fe-deficiency increased the “I” phase (i.e., the QB-nonreducing centers) and the non-photochemical quenching (qN) and reduced the photochemical quenching (qP). Photosynthesis rates in the deficient plants were half the control rates. Overnight chilling at 7C, which by itself did not affect the Fv: Fm ratio much, dramatically increased the susceptibility of the plants to photodamage on the following day (indicated by fluorescence and gas exchange assays). This postchilling PI was mostly irreversible but could slowly (within about 1 week) be recovered in a thermostated greenhouse. These results agree with our observations of field-grown Fe-deficient mango trees that develop severe necrosis and leaf-drop each autumn in the Jordan Valley when the days are still sunny and warm and the nights turn cold.

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Marc W. van Iersel, Geoffrey Weaver, Michael T. Martin, Rhuanito S. Ferrarezi, Erico Mattos and Mark Haidekker

-induced damage (photoinhibition), which decreases quantum yield of PSII. Higher plants have developed a complex set of responses to excess light, which allows them to safely dissipate excess light energy as heat (resulting in NPQ) and minimize photoinhibition

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Xiang Wang, Rajeev Arora, Harry T. Horner and Stephen L. Krebs

particularly vulnerable to photoinhibition or photo-oxidative damage ( Peng et al., 2008 ). Many plants have evolved mechanisms to dissipate excess absorbed light safely as thermal energy through the xanthophyll cycle ( Adams et al., 2004 ) and/or protect

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Ryan N. Contreras, John M. Ruter, James S. Owen Jr. and Andy Hoegh

. Winter browning in japanese-cedar occurs through the conversion of chloroplasts to chromoplasts during winter ( Ida, 1981 ). This transition takes place only in sun-exposed leaves during periods of low temperature, indicating that photoinhibition likely

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Mack Moyo, Manoj G. Kulkarni, Jeffrey F. Finnie and Johannes Van Staden

-h photoperiod at a temperature of 25 °C were moved to continuous dark conditions at 25 °C resulting in the reversal of the photoinhibition effect ( Fig. 4 ). Similarly, 9-month-old seeds that did not germinate at 10, 15, and 20 °C showed

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Jason J. Griffin, Thomas G. Ranney and D. Mason Pharr

Tolerance to high solar irradiation is an important aspect of stress tolerance for landscape plants, particularly for species native to understory conditions. The objective of this study was to evaluate differential tolerance to high solar irradiation and underlying photosynthetic characteristics of diverse taxa of Illicium L. grown under full sun or 50% shade. Eleven commercially available taxa of Illicium were evaluated for light tolerance by measuring light-saturated photosynthetic capacity (Amax), dark-adapted quantum efficiency of photosystem II (Fv/Fm), and relative chlorophyll content using a SPAD chlorophyll meter. Comparisons of Amax indicated that three of the 11 taxa (I. anisatum L., I. parviflorum Michx. ex Vent., and I. parviflorum `Forest Green') maintained similar rates of light-saturated carbon assimilation when grown in either shade or full sun. All other taxa experienced a significant reduction in Amax when grown in full sun. Chlorophyll fluorescence analysis demonstrated that Fv/Fm was similar between sun and shade plants for the same three taxa that were able to maintain Amax. These taxa appeared to experience less photoinhibition than the others and maintained greater maximum photochemical efficiency of absorbed light. SPAD readings were not significantly reduced in these three taxa either, whereas most other taxa experienced a significant reduction. In fact, SPAD readings were significantly higher in I. parviflorum `Forest Green' when grown under full sun, which also maintained the highest Amax of all the taxa. These results suggest that there is considerable variation in light tolerance among these taxa, with I. parviflorum `Forest Green' demonstrating superior tolerance to high light among the plants compared. A more rigorous examination of I. parviflorum `Forest Green' (high light tolerance) and I. floridanum Ellis (low-light tolerance) demonstrated that I. parviflorum `Forest Green' had a considerably higher Amax, a higher light saturation point, greater potential photosynthetic capacity, reduced susceptibility to photoinhibition as indicated by superior PSII efficiency following light exposure, greater capacity for thermal de-excitation as indicated by a higher rate of nonphotochemical quenching (NPQ) under full sun, greater apparent electron transport rate (ETR) at mid-day, and higher concentrations of the free-radical scavenger myo-inositol. All of these factors contribute potentially to a greater capacity to use light energy for carbon fixation while minimizing photodamage.

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Anthony W. Whiley, Christopher Searle, Bruce Schaffer and B. Nigel Wolstenholme

Leaf gas exchange of avocado (Persea americana Mill.) and mango (Mangifera indica L.) trees in containers and in an orchard (field-grown trees) was measured over a range of photosynthetic photon fluxes (PPF) and ambient CO2 concentrations (Ca). Net CO2 assimilation (A) and intercellular partial pressure of CO2 (Ci) were determined for all trees in early autumn (noncold-stressed leaves) when minimum daily temperatures were ≥14 °C, and for field-grown trees in winter (cold-stressed leaves) when minimum daily temperatures were ≤10 °C. Cold-stressed trees of both species had lower maximum CO2 assimilation rates (Amax), light saturation points (QA), CO2 saturation points (CaSAT) and quantum yields than leaves of noncold-stressed, field-grown trees. The ratio of variable to maximum fluorescence (Fv/Fm) was ≈50% lower for leaves of cold-stressed, field-grown trees than for leaves of nonstressed, field-grown trees, indicating chill-induced photoinhibition of leaves had occurred in winter. The data indicate that chill-induced photoinhibition of A and/or sink limitations caused by root restriction in container-grown trees can limit carbon assimilation in avocado and mango trees.

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Ryan N. Contreras, Ron Determann and Mara Friddle

screens or specimens in landscapes. However, winter browning caused by photoinhibition ( Ida, 1981 ) is unsightly and likely has reduced the use of japanese-cedars. Winter browning occurs as a result of the accumulation of the pigment rhodoxanthin during

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Madhoolika Agrawal, Donald T. Krizek, Shashi B. Agrawal, George F. Kramer, Edward H. Lee, Roman M. Mirecki and Randy A. Rowland

Cucumis sativus L. (cvs. Poinsett and Ashley) plants were grown from seed in a growth chamber at a +10C (28/18) or a -10C (18/28) difference (DIF) between day temperature (DT) and night temperature (NT) on a 12-hour photoperiod for 24 days prior to ozone (O3) fumigation (3 hours at 0.5 umol·mol-1). Negative DIF, compared to +DIF, reduced plant height, node count, fresh weight, dry weight, and leaf area in both cultivars. Photosynthetic rate (Pn), chlorophyll concentration, and variable chlorophyll fluorescence (Fv) were lower and O3 injury and polyamine concentrations were higher at -DIF than at +DIF. Ozone fumigation generally increased leaf concentration of polyamines and reduced Pn, stomatal conductance, and chlorophyll fluorescence. `Poinsett' generally had a higher specific leaf mass and higher concentrations of chlorophyll a and polyamines than did `Ashley', but there was no cultivar difference in O3 injury, growth response, Pn, or stomatal conductance.

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Rufaro Madakadze, Ellen M. Chirco and Anwar A. Khan

The effect of preplant conditioning on germination of three flower seeds, Bupleurum griffithii (Tourn.) L. (thorough-wax), Ammi majus L. (greater ammi), and Cirsium japonicum DC. Per. (Japanese thistle), were studied. Seeds were osmoconditioned with -1.2 MPa polyethylene glycol 8000 (PEG) solution and matriconditioned with moist Micro-Cel E (ratio of 2 seed: 0.6 carrier: 3 water by weight for Bupleurum and Cirsium; for Ammi the ratio was 2:1.4:6) and moist expanded vermiculite #5 (the ratio was of 2 seed: 0.6 carrier: 2 water for Bupleurum). In some treatments, water in the matriconditioning mixture was replaced with 1 mm gibberellin A4+7 (GA) or 0.2 % KNO3. In Bupleurum, matriconditioning with Micro-Cel E was generally superior to matriconditioning with vermiculite or osmoconditioning with PEG. A 4-day matriconditioning with Micro-Cel E and germination in the dark reduced the period required for 50% (T50 of final germination by 4 days and improved the percentage germination at 20C (73 % vs. 95%), compared to nonconditioned seeds germinated in the dark. The treatment also improved the percentage of germination at 15C (68% vs. 95%) and effectively removed the thermoinhibition of germination at 25 and 30C. Germination was inhibited to a greater extent for seeds kept in the light during matriconditioning and germination than for seeds conditioned in darkness and germinated in light or conditioned in light and germinated in darkness. Nitrate added during conditioning in light prevented inhibition of germination, provided seeds were kept in darkness during germination. In A. majus, germination in light after 4-day matriconditioning reduced the T50 by ≈2 days, but had little effect on percentage germination. Both GA and irradiance equally promoted germination when added during osmoconditioning, with nitrate having no effect. In C. japonicum, a 4-day matriconditioning or a 7-day osmoconditioning reduced the T50 of germination by -2 days and improved the percentage germination to some extent. Neither irradiance nor nitrate had any significant effect.