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Bernard B. Bible and Richard J. McAvoy

Forty-two poinsettia cultivars were grown as a 15-cm single-plant pinched crop at 21/16.5°C (day/night) temperatures during Fall 1995 with standard commercial practices for irrigating, fertilizing, and pest control. On 7 Dec., 156 consumers rated the cultivars for their overall appeal. On 11 Dec., color coordinate (CIELAB) readings for bracts and leaves were taken with a Minolta 200b colorimeter. The colorimeter was set to illuminate C and has a 8-mm aperture. Bracts and leaves were placed on a white tile background for colorimetric readings. In 1996, a similar evaluation was conducted with 55 poinsettia cultivars. Using the L-value of leaves as a criterion, cultivars were separated into medium green-leafed and dark green-leafed groupings. For bracts among the red types, hue angle values were used to separate cultivars into cool red types (hue angle ≈20–22°) and warm red types (hue angle ≈24–25°). Based on the 1995 study, cultivars within the cool red bracts and dark green foliage group—those that were darker, duller red (lower L and chroma)—were less attractive (lower consumer ratings) than lighter, more-vivid red cultivars. For cultivars within the cool red bracts and medium green foliage group, consumers preferred the darker duller red cultivars. Perhaps dark foliage gives a more pleasing contrast with the more vivid cool reds than does the medium green foliage. In general, consumers rated red cultivars hire than non-red cultivars.

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Harry W. Janes and Richard J. McAvoy

In this paper we review our research of light effects on tomato production. It was demonstrated that, during the production of greenhouse tomatoes, the total fruit yield, as well as time of harvest, was related to light. The date of harvest was inversely correlated with the amount of light the crop received during the seedling phase of growth, while fruit weight was positively correlated with light during the production phase. Additionally, we present information that shows that light was most effective in promoting fruit development between 15 and 45 days after flowering. Some of these relationships were quantified and used to develop a predictive model to help a grower plan a tomato crop to meet market demand. The concept of the Single-cluster Tomato Production System was developed, and the rewards of using our understanding of plant-environment interactions to control plant growth and, therefore maxim&profits were shown. Furthermore, the need to create a more dynamic model and the methods for doing so were discussed.

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Richard J. McAvoy*, Mariya V. Khodakovskaya, Hong Liu, and Yi Li

Cytokinins play an important role in regulating plant growth and development. The cytokinin gene, isopentenyl transferase (ipt), was placed under the control of the ACC oxidase promoter from the LEACO1 gene from Lycopersicon esculentum and introduced into Nicotiana tabacum (cv. Havana) and chrysanthemum (Dendranthema × grandiflorum `Iridon'). Transformants were confirmed by PCR reaction and Southern blot and analyzed for phenotypical changes under both greenhouse and growth chamber conditions. With both species, LEACO1-ipt transgenic plants displayed a wide range of vegetative and generative phenotypes. With plants growing in the vegetative state, some LEACO1-ipt transgenic lines appeared similar to the non-transgenic wild-type cultivars while other lines showed excessive lateral branch development and short internodes. With plants grown under generative conditions, several LEACO1-ipt transgenic lines showed a 2 to 10-fold increase in the number of flower buds relative to the wild-type cultivars. With chrysanthemum, dramatic increases in bud count were observed on transgenic lines that otherwise displayed a morphology similar to the non-transgenic lines. Analysis of ipt expression indicated a marked change in gene expression between the most extreme phenotypes observed in this study. LEACO1-ipt lines that express normal vegetative development but increased flower bud counts appear to have great potential for ornamental crop improvement.

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Mariya V. Khodakovskaya, Richard J. McAvoy*, Hao Wu, and Yi Li

It has been reported that constitutive expression of the fatty acid desaturase enzyme increased the trienoic fatty acid content of thylakoid membranes in transgenic tobacco, allowing the membranes to remain fluid under cold conditions. While increased cold tolerance resulted from this genetic modification, plants with a constitutively expressed desaturase enzyme would not be particularly well suited for growth under warm temperatures. To increase the ability of plants to tolerate prolonged cold-storage and still perform under greenhouse production conditions (25 °C), a unique cold-inducible genetic construct was cloned and tested. The FAD7 gene, which encodes an omega-3-fatty acid desaturase enzyme, was put under the control of a cold-inducible promoter (cor15a) from Arabidopsis thaliana. Transgenic petunia plants (cv, Marco Polo Odyssey) harboring cor15a:FAD7 were established and conformed by PCR and Southern analysis. Therefore in our study, FAD7 gene expression was induced by exposure to cold temperatures and down regulated under normal growing conditions. RT-PCR indicated a marked increase in FAD7 expression between transgenic plants exposed to a short (3 days) cold treatment prior to long-term cold storage and those that did not receive a cold induction treatment. Transgenic and wild-type plants were induced in cold (3 °C) for 3 days, returned for normal greenhouse conditions for 5 days and then subjected 3 weeks of continuous cold storage. It was observed that two out of eight transgenic lines showed superior cold tolerance relative to wild-type petunia plants. Additionally, plants that showed cold tolerance completely recovered; growing and flowering normally when returned to the 25 °C greenhouse conditions.

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Mariya V. Khodakovskaya, Richard J. McAvoy*, Keming Luo, and Yi Li

The bacterial isopentenyl transferase (ipt) gene involved in cytokinin biosynthesis was fused with a promoter from the serine proteinase inhibiter (win3.12) gene of Populus x generosa and introduced into Nicotiana tabacum (cv. Havana). Transformants were confirmed by PCR reaction and Southern blot analysis, and then analyzed for phenotypic characteristics. RT-PCR analysis detected transcripts of the ipt gene following the wounding of win3.12:ipt transgenic plants. In win3.12: ipt transgenic plants, lateral shoot number and the diameter of lateral branches that developed following apical shoot removal increased relative to wild-type plants. However, the increase in cytokinin production following wounding appeared to be short lived. The potential utility of this construct in enhancing lateral branching in ornamental crops will be discussed.

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Richard J. McAvoy, Bernard B. Bible, and Michael R. Evans

The early onset of bract necrosis in poinsettia (Euphorbia pulcherrima Willd. ex. Klotzch) is characterized by small dark-stained spots that precede the development of enlarged necrotic lesions. Electron micrographs of adaxial epidermal and subepidermal tissues with early symptoms of necrosis revealed large, electron-dense deposits in cell vacuoles. These spherical bodies resembled condensed tannins observed in the epidermal tissues of peach and apple fruit. Chemical analysis of bract tissues confirmed the presence of condensed tannins. Furthermore, there were higher concentrations of condensed tannin in bract samples with 2-mm-diameter lesions than in samples with lesions <0.5 mm (equivalent to catechin concentrations of 59 and 13 mg·g-1 fresh mass, respectively). No tannin bodies were observed in parallel samples of healthy-appearing bracts in which only trace concentrations of condensed tannins were measured (0.2 mg·g-1 fresh mass). The evidence suggests an association between condensed tannin accumulation in localized areas of the bract and the early appearance of bract necrosis symptoms.

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Richard J. McAvoy, Bernard B. Bible, and Xiusheng Yang

Poinsettias were grown in a closed hydroponic system with a modified Hoagland's solution concentration of either 1 or 3 mS·cm-1. Water use and whole plant fresh mass were measured gravimetrically at 2 to 3 day intervals over an eleven week period (initial break development through full bract development). At two week intervals, poinsettias were harvested and the fresh and dry mass of leaves, bracts stems, and roots were determined, and total laminar surface area was measured. Leaf temperature (LT), root-zone solution temperature (RZT), and at canopy level, air temperature (CAT), VPD, and photosynthetic photon flux (PPF) were monitored at 1 mm intervals and mean values recorded each 30 mm using a 21X micrologger (Campbell Scientific, Logan Utah). Water use (ml·dm-2·day-1) averaged 15% higher for poinsettias grown in the 1 mS·cm-1 solution than in the 3 mS·cm-1 nutrient solution. Simple linear regression of daily water use with PPF, or VPD, or CAT, while significant, accounted for less than half of the daily fluctuation in water use (r2; PPF= 0.47, VPD=0.21, CAT=0.30). However, multiple regression involving daily PPF, VPD, CAT, RZT and LT accounted for up to 82% of the variation in daily water use.

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Usha R. Palaniswamy, Richard J. McAvoy, and Bernard B. Bible

Purslane (Portulaca oleracea L.) is an excellent source of the essential fatty acid α-linolenic acid (LNA) but little is known of the effects of cultural conditions on LNA concentration. Purslane seedlings were grown under an instantaneous photosynthetic photon flux [PPF (400 to 700 nm)] of 299 or 455 μmol·m-2·s-1 for a daily duration of either 8, 12, 16, or 20 hours. Thus, plants were exposed to a daily PPF of 8.6, 12.9, 17.2, or 21.5 mol·m-2·d-1 in the low PPF treatment (299 μmol.m-2.s-1) and 13.1, 19.7, 26.2, or 32.8 mol·m-2·d-1 in the high PPF treatment (455 μmol·m-2·s-1). Plants in all treatments received a 20-hour photoperiod by providing ≈5 μmol·m-2·s-1 from incandescent lamps starting at the end of the photosynthetic light period. At low PPF, purslane grown under a 16 hour PPF duration produced the highest concentrations of total fatty acids (TFA) and LNA per unit leaf dry weight (DW), but at high PPF, concentrations of these compounds were highest under 8 and 12 hour PPF duration. Trend analysis indicated that maximum TFA and LNA concentrations occurred with a daily PPF of 14.1 and 17.2 mol·m-2·d-1, respectively; and in the thylakoids, protein, chlorophyll, and LNA concentrations peaked at a PPF of 21.8, 19.9, and 16.1 mol·m-2·d-1, respectively. LNA as a percentage of TFA was unaffected by treatment. Shoot DW increased with PPF up to the highest PPF exposure of 32.8 mol·m-2·d-1.

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Usha Rani Palaniswamy, Richard J. McAvoy, and Bernard B. Bible

Purslane (Portulaca oleracea L.) has been identified as an exceptionally rich source of α-linolenic acid (LNA), an essential fatty acid that is beneficial in reducing the incidence of coronary heart disease and certain cancers. In general, about two thirds of the LNA in terrestrial plants is associated with chloroplasts. A green-leafed unnamed cultivar of purslane and a golden-leafed cultivar `Goldberg' were grown hydroponically in a complete nutrient solution with 14.3 mm nitrogen provided as nitrate (NO3 -) and ammonium (NH4 +) forms to yield NO3 --N: NH4 +-N ratios of 1:0, 0.75:0.25, 0.5:0.5, and 0.25:0.75. Young leaves, harvested 18 days after treatment initiation, were analyzed to determine the fatty acid composition and concentrations, and thylakoid protein and chlorophyll concentrations. Although the leaves of plants grown with a NO3 --N: NH4 +-N ratio of 0.5:0.5 contained 239% and 114% more LNA than plants grown with ratios 1:0 and 0.75:0.25, respectively, they contained only 41% and 26% more chlorophyll. The green-leafed cultivar had higher (39%) chlorophyll concentrations than `Goldberg', but both cultivars had similar LNA concentrations [per g dry weight (DW)]. These results suggest that the LNA concentration in the fatty acid-rich species P. oleracea may not be as closely associated with chlorophyll concentration as reported earlier for other plants. Leaves of plants grown in solutions with 0.25:0.75 NO3 --N: NH4 +-N ratio contained 35% less LNA per g leaf DW than the leaves of plants grown in nutrient solutions with a 0.5:0.5 ratio. Although total DW production was not affected by the NO3 --N: NH 4 +-N ratios in the nutrient solutions, the green-leafed cultivar produced higher fresh weight, leaf area, total DW, and number of shoots than `Goldberg'.

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Martin P.N. Gent, Wade H. Elmer, Kranti Macherla, and Richard J. McAvoy

Can regulated deficit irrigation in an ebb and flow system alleviate the effects of salinity stress on poinsettia? Two cultivars of poinsettia (Euphorbia pulcherrima Willd ex Klotzsch) were grown under partial- or full-saturation irrigation using a standard fertilizer solution, with or without the addition of 0.5 g·L−1 NaCl. The volumetric water content of the medium averaged 0.25 and 0.33 L·L−1 before irrigation, and 0.5 and 0.67 L·L−1 following irrigation, for partial- or full-saturation regimes, respectively. Plants had lower fresh weight with partial than full saturation. Sodium concentrations in bract, leaf, and stem tissues were higher (P ≤ 0.05) in plants exposed to salinity, and these plants accumulated less K in stems and less P in bracts. Eight cultivars were grown in a second study with or without salinity of 1.2 g·L−1 NaCl under drip or ebb and flow watering. Cultivar and watering had effects on plant fresh weight, but salinity did not. Of the cultivars tested, ‘DaVinci’, ‘Premium Picasso’, and ‘Prestige Red’ had the highest sodium in bracts under salinity with drip irrigation, whereas ‘Snowcap’ had the least. ‘Ruby Frost’ had the most sodium in stems, whereas ‘Snowcap’ had the least. For all cultivars, added salinity resulted in lower K in leaves and stem. Snowcap was the cultivar with the least sodium in stems and bracts under saline irrigation, with either drip or ebb and flow. Our research demonstrates that regulated deficit irrigation resulting in partial saturation of the growing medium is an effective water management option, when control of plant height and overall crop growth are desirable, and it limits the accumulation of sodium when raw water contains elevated salinity.