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Eugenio Pérez-Molphe-Balch and Neftalí Ochoa-Alejo

An efficient system for in vitro regeneration by organogenesis starting from internodal stem segments from seedlings of Mexican lime (Citrus aurantifolia Christm. Swing.) and mandarin (C. reticulata Blanco cv. Monica) was developed. The best results were obtained when the wounded edges of internodal stem segments cut longitudinally were placed downward on the surface of the culture medium. The optimal culture medium from both species was Murashige and Skoog with vitamins from B5 medium, 5% sucrose, 33.3 μm BA and 5.4 μm NAA. The best response was obtained when the segments were incubated at 25 ± 2 °C for 21 d in darkness, followed by 29 d on a 16/8-h light/dark cycle (fluorescent light, 54 μmol·m-2·s-1). The best regeneration system tested allowed the attainment of adventitious shoots from 96% and 88% of the explants in Mexican lime and mandarin, respectively. In Mexican lime an average of 7.8 well-differentiated shoots per explant was obtained, and in mandarin the yield was 5.1. Rooting of 70% of the shoots was achieved in culture medium with NAA (2.7–5.4 μm) or IBA (2.5–4.9 μm). Of the rooted plants, 85% adapted well to soil conditions. Chemical names used: 6-benzylaminopurine (BA), α-naphthaleneacetic acid (NAA), indole-3-butyric acid (IBA).

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Issam A. Hassaballa, M.G. Moughieth, N.A. Hagagy, and N.S. Zayed

Shoot tip and single-node cutting explants of `Hamawy' and `El-Amar' apricot cultivars were initiated from forced shoots of field-grown, virus-free trees. Explants were cultured on Murashige & Skoog (MS) Nitsch & Nitsch and Anderson media. Different modifications of MS medium were also evaluated. Antioxidant pretreatment reduced phenolic compounds and decreased necrosis. Modified MS was the best medium for plantlets regeneration, with positive effectiveness of adenine sulfate addition to the modified MS. Shoot multiplication was best on 2.0 mg·L–1 BAP and 1.0 mg·L–1 thidiazuron (TDZ). Also, half-strength MS medium was superior for shoot elongation Surface coverage, 16 hours light/8 hours dark cycle, and 2.0 mg·L–1 IBA induced good rooting. Rooted plantlets were successfully acclimated ex vitro.

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Mark H. Brand

The effect of shading during nursery production on the growth, foliage color, and foliar chlorophyll content of container-grown Kalmia latifolia cultivars was investigated. Five cultivars were grown under 40% shade, 60% shade, or full sunlight for a 2-year production cycle. During the first year of production, there were no significant differences in measured growth characteristics for most cultivars in response to light treatment. Shade improved foliar color by decreasing lightness (L*), decreasing chroma, and changing hue angle from a yellow-green to a darker green. Foliar chlorophyll concentration increased under shade. In the second year of the production cycle, the response of foliar color and chlorophyll concentration to shade was similar to that observed in year 1. Plant size, number of branches, leaf area, leaf dry mass, and stem dry mass decreased linearly with increasing shade in year 2. Although shading improves foliar color, it probably should not be employed for container production of Kalmia latifolia in cool, northern production areas due to reduced plant growth during year 2. Shade may be useful in the first year of production to enhance foliar color without reducing shoot growth.

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Paul H. Jennings, N. Ishii, and R. Rufner

Chlorotic bands across sugarcane leaves were first described as symptoms of cold chlorosis in 1926 and later described in sorghum and maize. The injury develops after exposure of seedlings to temperatures in the 0°C to 12°C range. The severity of injury in maize seedlings may be reduced by high relative humidity during the post-chilling period suggesting a temperature induced water stress. An early visible chilling response is the appearance of a glazed area in the region in which the chlorotic band will develop. This area of the young expanding maize leaf was studied with scanning electron microscopy(SEM). Maize seedlings were grown for 6 days at 24°C with a 15/9 h light/dark cycle. Plants were chilled at 10°C for 9 h during the 7th dark period and leaves sampled 39 h after the end of chilling. SEM photomicrographs revealed a gradient of epicuticular wax deposition from the tip to the base of the leaf. In the region of chill-induced chlorotic band formation, the control leaves exhibited a greater amount of wax deposition than the chilled leaves. It is suggested that the reduced epicuticular wax in a band across the chilled leaves might lead to a water stress resulting in chlorosis and eventually developing into the typical necrotic band.

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Thomas G. Ranney and Mary M. Peet

Leaf gas-exchange and chlorophyll fluorescence measurements were used as indexes for evaluating heat tolerance among five taxa of birch: paper (Betula papyrifera Marsh.), European (B. pendula Roth.), Japanese (B. platyphylla var. japonica Hara. cv. Whitespire), Himalayan (B. jacquemontii Spach.), and river (B. nigra L. cv. Heritage). Gas-exchange measurements were conducted on individual leaves at temperatures ranging from 25 to 40C. River birch maintained the highest net photosynthetic rates (Pn) at high temperatures, while Pn of paper birch was reduced the most. Further study of river and paper birch indicated that the reduced Pn at high temperatures and the differential sensitivity between taxa resulted from several factors. Inhibition of Pn at higher temperatures was due largely to nonstomatal limitations for both taxa. Increases in respiration rates, decreases in maximal photochemical efficiency of photosystem (PS) II (F V/F M), and possible reductions in light energy directed to PS II (F 0 quenching) were apparent for both taxa. The capacity of river birch to maintain greater Pn at higher temperatures seemed to result from a lower Q10 for dark respiration and possibly greater thermotolerance of the Calvin cycle as indicated by a lack of nonphotochemical fluorescence quenching with increasing temperatures. Thermal injury, as indicated by a rapid increase in minimal, dark-acclimated (F 0) fluorescence, was not evident for either paper or river birch until temperatures reached ≈49C and was similar for both taxa.

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G.W. Stutte and N.C. Yorio

The relationship between whole canopy and single leaf measurements of gas exchange has not been well documented. Two experiments were conducted in the Biomass Production Chamber at Kennedy Space Center (20-m2 growing area) to compare whole canopy versus single leaf net carbon assimilation rate (Anet) measurement of a stand of tomato (Lycopersicon esculentum Mill. cv. Reimann Philipp) and soybean [Glycine max (L.) Merr. cv. Hoyt]. Both crops were grown under a 12/12 hour photoperiod under HPS lamps at PPF of 800 (mol·m–2·s–1, at 26/22°C (light/dark), and constant 65% RH for 90 days. CO2 concentration was controlled to 1200 (mol·mol–1 during the light cycle. Midday measurements of Anet of single leaves were obtained weekly from upper canopy leaves using a portable photosynthesis system. Whole canopy measurements of Anet were calculated daily from CO2 addition data obtained at 5-minute intervals by the BPC monitoring and control system. Single leaf rates exceeded whole canopy rates prior to full canopy coverage then averaged 0.63 of whole canopy for both species during the period of full canopy coverage. Results suggest that reliable estimates of canopy gas exchange can be obtained from single leaf measurements under relatively constant environment conditions.

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Yu Sung, Daniel J. Cantliffe, and Russell T. Nagata

Lettuce (Lactuca sativa L.) seeds can fail to germinate at temperatures above 24 °C. The degree of thermotolerance is thought to be at least partly related to the environment under which the seed developed. In order to study the effects of temperature during seed development on subsequent germination, various lettuce genotypes were screened for their ability to germinate at temperatures ranging from 20 to 38 °C. Seeds of the selected genotypes `Dark Green Boston' and `Valmaine' (thermosensitive), `Floricos 83', `Everglades', and PI 251245 (thermotolerant) were produced at 20/10, 25/15, 30/20, and 35/25 °C day/night temperature regimes in plant growth chambers. Seeds were germinated on a thermogradient bar from 24 to 36 °C under 12 h light/dark cycles. As germination temperature increased, the number of seeds that failed to germinate increased. Above 27 °C, seeds matured at 20/10 or 25/15 °C exhibited a lower percent germination than seeds that matured at 30/20 or 35/25 °C. Seeds of `Dark Green Boston' and `Everglades' that matured at 30/20 °C exhibited improved thermotolerance over those that matured at lower temperatures. Seeds of `Valmaine' produced at 20/10 °C exhibited 40% germination at 30 °C, but seeds that matured at higher temperatures exhibited over 95% germination. Germination of `Valmaine' at temperatures above 30 °C was not affected by seed maturation temperature. The upper temperature limit for germination of lettuce seed could thus be modified by manipulating the temperature during seed production. The potential thermotolerance of seed thereby increased, wherein thermosensitive genotypes became thermotolerant and thermotolerant genotypes (e.g., PI251245) germinated fully at 36 °C. This information is useful for improving lettuce seed germination during periods of high soil temperature, and can be used to study the biology of thermotolerance in lettuce.

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Yin-Tung Wang

Blooming Phalaenopsis orchids have become a popular pot plant in recent years. Plants start producing spikes after experiencing cool air in early fall, bloom in early spring, and become limited in supply after April when market demand is strong. Deferring spiking and flowering by maintaining the greenhouse air constantly above 28°C is cost prohibitive. Previous research has discovered that plants must be given light while being exposed to cool air to induce spiking. In Fall 1994, 2-year old Phalaenopsis TAM Butterfly plants were exposed to repeated cycles of 1 day in darkness and another day in light (1D/1L), 4D/3L, 7D/7L, or 0D/7L (continuous lighted control) between 15 Sept. and 16 Dec. Each plant was removed from the treatment once it had started spiking. The control plants bloomed on 20 Jan. 1995, whereas the 4D/3L plants did not reach anthesis until April 17, nearly three months later. Flowering of the 1D/1L and 7D/7L plants was also deferred until early April. The treatments had no adverse effect on flower count or size. In 1995, 3-year old plants were exposed to 0D/7L (control), 2D/5L, 3D/4L, 4D/3L, or 5D/2L from 15 Sept. to 22 Jan. 1996. The control plants spiked on 17 Oct. and bloomed on 8 Feb. 1996 when spikes had just emerged from plants in the 5D/2L treatment. The 5D/2L plants are expected to bloom in late May or early June. The other treatments were not as effective as that in 1994 and resulted in blooming only 2–3 weeks after the untreated control. The results of this research will help producers to stagger or precisely program the time of flowering to meet the market demand.

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Andrew Frève and M. Auger

A multiplication technique by sprout cutting (spc) was developed at La Pocatière from 1988 to 1992. Tubers (t) or mini-tubers (mt) were warmed and, if necessary, treated to break dormancy. After 2 to 4 weeks of presprouting, they were planted 50 to 100 t per flat (28 × 53 cm) half filled with a soil mixture. The t or mt were planted partially in soil, leaving the bud end out. They produced roots in 3 to 8 days under 16 hours of light at 22 to 24C. When 50% of the sprouts have 3 to 4 cm, they were placed in the dark for 3 days and under the initial lighting condition on day 4. The sprouts were cut on day 5. The mother plants were maintained in the same conditions for another 72 hours before repeating the cycle 6 to 10 times (with the same t or mt). The multiplication rate depended on tuber size, storage period, and cultivar. The t (30 mm) and mt (15 mm) of `Novachip' with 14 and 7 months of storage, respectively, produced 55 and 4.3 spc/t or mt in four multiplications over 28 days. The mt (12 mm) of `Tolaas' with 6 months of storage and without breaking dormancy produced 10.3 spc/mt in eight multiplications over 87 days (it took 4 weeks to grow). An equation considering weeks of storage and t size has been developed with six cultivars and four seedlings to calculate the number of plants multiplied in 1 to 7 weeks.

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William L. Bauerle, Jerry B. Dudley, and Lawrence W. Grimes

Cultivars of red (Acer rubrum L.) and Freeman maple (Acer ×freemanii E. Murray) are popular ornamental plants which are commonly placed in a variety of landscapes. To date, little information quantifies the capacity to tolerate and recover from drought among cultivars of red and Freeman maple. The objective of this study was to compare the effects of water stress on the physiology of five different maple cultivars of marketable size including four red maple genotypes, `Summer Red', `October Glory' (October Glory), `Autumn Flame', and `Franksred' (Red Sunset), as well as one hybridized Freeman maple genotype, `Jeffersred' (Autumn Blaze). Two-year-old cloned genotypes of red and Freeman maple were subjected to two treatments: irrigated daily to container capacity or irrigation withheld for one drought and recovery cycle. Light absorption, gas exchange, and chlorophyll fluorescence measurements were conducted under well-watered and drought stress conditions that approached 0.070 m3·m-3. Compared to well-watered conditions, drought stress conditions of 0.090 m3·m-3 had a significant main effect that reduced the amount of light absorption in four of the five genotypes. Additionally, absorption among genotypes was different under both well-watered and water stress conditions. Over the course of drought stress and a recovery phase, net photosynthesis and stomatal conductance were different among genotypes. Maximum photosystem II (PSII) efficiency of dark-adapted leaves (Fv/Fm) was lowered by the water stress condition. The efficiency of excitation capture by open PSII reaction centers (Fv`/Fm') was variable among genotypes. Photochemical quenching was higher in Autumn Blaze, October Glory, and `Summer Red' under drought conditions, which corresponded with a low degree of closure of PSII centers. Additionally, the fraction of excess excitation energy was also lower. Lastly, water deficit caused an increase in PSII efficiency in all genotypes except Autumn Blaze. This research demonstrated physiological variation among commercially available red and Freeman maple genotypes that may be selected for drought tolerance based on site moisture characteristics.