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Jonathan M. Frantz, Glen Ritchie, Nilton N. Cometti, Justin Robinson, and Bruce Bugbee

The productivity of lettuce in a combination of high light, high temperature, and elevated CO2 has not been commonly studied because rapid growth usually causes a calcium deficiency in meristems called tipburn, which greatly reduces quality and marketability. We eliminated tipburn by blowing air directly onto the meristem, which allowed us to increase the photosynthetic photon flux (PPF) to 1000 μmol·m-2·s-1 (57.6 mol·m-2·d-1); two to three times higher than normally used for lettuce. Eliminating tipburn doubled edible yield at the highest PPF level. In addition to high PPF, CO2 was elevated to 1200 μmol·m-2·mol-1, which increased the temperature optimum from 25 to 30 °C. The higher temperature increased leaf expansion rate, which improved radiation capture and more than doubled yield. Photosynthetic efficiency, measured as canopy quantum yield in a whole-plant gas exchange system, steadily increased up to the highest temperature of 32 °C in high CO2. The highest productivity was 19 g·m-2·d-1 of dry biomass (380 g·d-1 fresh mass) averaged over the 23 days the plants received light. Without the limitation of tipburn, the combination of high PPF, high temperature, and elevated CO2 resulted in a 4-fold increase in growth rate over productivity in conventional environments.

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Samuel Salazar-García, Elizabeth M. Lord, and Carol J. Lovatt

The developmental stage at which the shoot primary axis meristem (PAM) of the `Hass' avocado (Persea americana Mill.) is committed to flowering was determined. Three-year-old trees were subjected to low-temperature (LT) treatments at 10/7 °C day/night with a 10-h photoperiod for 1 to 4 weeks followed by 25/20 °C day/night at the same photoperiod. Before LT treatment, apical buds of mature vegetative shoots consisted of a convex PAM with two lateral secondary axis inflorescence meristems lacking apical bracts each associated with an inflorescence bract. Apical buds did not change anatomically during LT treatment. However, the 3- and 4-week LT treatments resulted in inflorescences at 17% and 83% of apical buds, respectively. Trees receiving 2 weeks or less LT, including controls maintained at 25/20 °C, produced only vegetative shoots. Apical buds of 2-year-old trees receiving 3 weeks at 10/7 °C plus 1 week at 20/15 °C produced 100% inflorescences. GA3(100 mg·L-1) applied to buds 2 or 4 weeks after initiation of this LT treatment did not reduce the number of inflorescences that developed. `Hass' avocado apical buds were fully committed to flowering after 4 weeks of LT, but were not distinguishable anatomically from those that were not committed to flowering.

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Amalia Barzilay, Hanita Zemah, Rina Kamenetsky, and Itzhak Ran

The life cycle and morphogenesis of the floral shoot of Paeonia lactiflora Pallas cv. Sarah Bernhardt were studied under Israeli conditions. The renewal buds for the following year originate on the underground crown, at the base of the annual stems. Bud emergence begins in early spring. Stems elongate rapidly and reach heights of 50-70 cm in 60-70 days. Flowering begins in April and continues until the end of May. After flowering, the leafy stems remain green until September-October, when the leaves senesce, and the peony plant enters the “rest” stage for 3-4 months. The new monocarpic shoot initiated in the renewal bud at the end of June with the formation of the first leaf primordia and continued to increase in size until February. During summer, the renewal buds remain vegetative. The apical meristem ceases leaf formation after senescence of the aboveground shoots in the fall. During September, the apical meristem of the renewal buds reaches the generative stage and achieves the form of a dome, but remains undifferentiated. In October, floral parts become visible. Floral differentiation is terminated at the beginning of December. Floral initiation and differentiation of peony do not require low temperatures. Morphological development and florogenesis were similar to other geophyte species with an annual thermoperiodic life cycle.

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Jeffrey A. Adkins and Michael A. Dirr

Floral induction of 10 Hydrangea macrophylla (Thunb.) Ser. cultivars (`All Summer Beauty', `Dooley', `Endless Summer', `Générale Vicomtesse de Vibraye', `Lilacina', `Mariesii Variegata', `Mme. Emile Mouillère', `Nikko Blue', `Penny Mac', and `Veitchii') was evaluated to determine the remontant flowering potential. Cultivars exhibiting superior flowering potential could facilitate year-round production of florist s hydrangea and expand the geographic adaptation of hydrangeas in the landscape. Plants were tested under either 8-hour inductive short-day (SD) or 24-hour noninductive extended-day (ED) photoperiods at 24 ± 2 °C. Floral induction of H. macrophylla occurred under both SD and ED but was more rapid under SD than under ED. Shoot length and total number of nodes were significantly different for cultivars, photoperiods, and harvest dates. Significant differences in the floral initiation and development were observed among cultivars. `Penny Mac', `Endless Summer', `Lilacina', `Mme. Emile Mouillère', and `Nikko Blue' displayed significantly greater floral meristem development under SD than all other cultivars. Over the nine-week period, only `Dooley', `Endless Summer', and `Penny Mac' floral development was affected significantly greater by SD photoperiods than ED photoperiods. All others cultivars showed similar floral development under SD and ED. Stage of meristem development was highly correlated with number of nodes, but not with shoot length.

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Shailaja Sinha and Madhuri Sharon

Somatic embryogenesis from apical meristem, cell elongation zone, and cell differentiation zone of roots of Punica granatum L. var. Ganesh was obtained. The basal medium used was Gamborg's B5. 2, 4-D induced white globular callus in root tip explants, which on further subculture to medium containing 0.5 mg/L BAP, produced somatic embryos from the outer surface of the callus. Direct somatic embryogenesis occurred from all the three zones of the root in presence of 2 mg/L kinetin. BAP induced embryogenic callus in elongation and cell differentiation zone segments of the roots, which, on further subculture onto the same medium, produced somatic embryos. NAA caused rhizogenesis in all the three root segments. Differentiation of somatic embryos into plantlets took place on B5 medium supplemented with 0.01 mg/L NAA + 0.5 mg/l BAP + 2 mg/L kinetin.

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Ryoichi Nakajima, Shungo Otagaki, Katsuhiro Shiratake, and Shogo Matsumoto

Previously, we showed that a reduction of FaTFL2 (homolog of Arabidopsis thaliana TFL1) expression is a key signal for flowering in domesticated octaploid strawberries (Fragaria ×ananassa). Since FaTFL2 expression seemed to be regulated by temperature rather than by photoperiod, we investigated whether down regulation of FaTFL2 and floral meristem generation occurred at different temperature conditions. In addition to the conditions for a normal super-forcing cultivation system of an 8-hour photoperiod and day/night temperatures of 31.2 or 30/15 °C, flowering also was generated under the same photoperiod and day/night temperatures of day/night-half/night-half temperatures of 30/15/25 °C conditions. We demonstrate that the new super-forcing cultivation system is energy saving based on the reduction of FaTFL2 expression.

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J.W. White, H. Chen, X. Zhang, D.J. Beattie, and H. Grossman

Floral initiation and development of greenhouse and growth room-grown Aquilegia × hybrida Sims cultivars were studied using a scanning electron microscope. All greenhouse-grown cultivars initiated floral buds before cold treatment, ≈ 5 months after sowing. Floral initiation occurred at the apical meristem and proceeded acropetally on an elongated conical axis in the sequence: sepals, petals, stamens, stamenodia, and carpels. In a second experiment, 13 Aquilegia cultivars, three of which had been used in the first experiment, were grown as seedlings in a growth room at 20C under an 8-, 12-, 16-, or 20-hr photoperiod, each totaling 10.2 mol·day-1·m-2 irradiance from cool-white fluorescent lamps. Here, floral initiation was absent even after 7 months from sowing, presumably because there was no diurnal variation in irradiance or temperature.

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Roar Moe, John E Erwin, and Will Carlson

The role of irradiance and/or ethylene in inducing mortality and self-branching disorders in Gerbera jamesonii Bolus. seedlings was studied. Seedling mortality increased from 8% to 57% when seed was covered with vermiculite than left uncovered during germination. Supplemental lighting for 30 days after germination decreased seedling mortality and decreased the time to visible bud compared to seed germinated under natural light only. In subsequent experiments, seeds were germinated and then seedlings were water logged or sprayed with ethephon (0.69, 3.45, or 17.25 mM) at four different stages of seedling development. Half of the ethephon-treated seedlings were sprayed with silver thiosulfate (STS). Seedling mortality was greatest after cotyledon expansion but before expansion of the first tree leaf. The highest ethephon concentration caused reduced seedling dry weight after 42 days. Applying STS did not overcome self-branching or meristem necrosis.

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Michael Parker and Eric Young

Controlling vegetative growth resulting from a long growing season in the southeast is difficult while trying to promote early fruiting. This study was initiated in 1990 to evaluate higher density leader training techniques, cultivar interactions, and the benefits of pre-plant fumigation on apple replant sites. Another objective was to evaluate these management parameters in four regions with very different climates and elevations. The training techniques evaluated were, weak leader renewal, bending of the leader during the growing season, and partial terminal leaf removal every 10 inches of leader growth (without injuring the apical meristem). The cultivars used were Jonagored, Spur Gala, and Red Fuji, all on Mark rootstock. First and second year branching was not significantly different between the various training techniques. Yields during the third year did not appear to differ between the leader training techniques. Pre-plant fumigation appeared to be beneficial in increasing tree growth in only two of the four sites.

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Les Frey, Yehoshua Saranga, and Jules Janick

Somatic embryogenesis was induced from internodal callus of `Scania', `Improved White Sim', and `Sandra' carnation (Dianthus caryophyllus L.). The optimum protocol for the induction of somatic embryogenesis included initiation of callus in liquid basal Murashige and Skoog medium supplemented with 3.0 μm 2,4-D followed by transfer to liquid basal medium lacking 2,4-D for embryo development. Somatic embryos originated from single cells and early embryonic development proceeded conventionally (i.e., via globular, heart-shaped, and torpedo stages), but clearly developed apical or root meristems were not always formed. A few embryos developed into seedlings and were acclimatized to ex vitro conditions. Chemical name used: 2,4-dichlorophenoxyacetic acid (2,4-D).