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James E. Faust and Royal D. Heins

The effects of temperature and daily-integrated photosynthetic photon flux (PPFDI) on African violet (Saintpaulia ionantha Wendl.) flower initiation and development were quantified to provide the basis for an inflorescence development model. The percentage of leaf axils in which an inflorescence initiated and continued development increased as the PPFDI increased from 1 to 4 mol·m-2·day-1, while the rate of inflorescence development was a function of the average daily temperature (ADT). The appearance of a visible flower bud (VB) in a leaf axil was related to the growth of the subtending leaf blade. A polynomial model based on ADT and PPFDI was used to describe leaf blade length at visible bud (LBLVB). A nonlinear model was used to describe the influence of ADT on leaf expansion rate (LER). Inflorescence appearance in the leaf axil was predicted by measuring LBL and estimating the time for the leaf blade to develop to the length required for VB. A phasic-development scale was developed to quantify inflorescence development. Days required for an inflorescence to develop from VB to first open flower was described as a function of ADT and either inflorescence height or inflorescence development stage (IDS). Days from leaf emergence to first open flower for the inflorescence initiated in that leaf axil decreased from 86 to 55 as ADT increased from 18 to 26C.

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Terence L. Robinson and Bruce H. Barritt

In unstressed apple seedlings (Malus domestics Borkh.), concentrations of free abscisic acid (ABA) decreased in order from apical stem sections, immature expanding leaves, mature stem sections, and mature leaves. PEG-induced water stress stimulated a 2- to 10-fold increase in free ABA concentrations 1 day after treatment, depending on the amount of stress and the tissue. By the 3rd day of stress, free ABA concentrations were nearly the same as the unstressed treatment and remained low for the remainder of the 21-day stress period. Bound ABA concentrations were an order of magnitude lower than free ABA and were not influenced dramatically by water stress. Shoot growth rate, leaf expansion rate, and leaf emergence rate were reduced by water stress in relation to the severity of the stress; this reduction was associated with the initial increase in ABA. However, there was no increase in shoot or leaf growth rates associated with the decline in ABA concentrations by day 3 as growth rates remained depressed on water-stressed plants throughout the 21-day stress period. Water stress reduced evapotranspiration rate and midshoot leaf water potential (ψW)after 1 day, but leaf osmotic potential (ψS) adjusted more slowly, resulting in a loss of leaf turgor. The reduction in leaf turgor pressure (ψP) was highly correlated with decreased shoot growth rate and increased ABA concentrations on day 1 after treatment. By the 3rd day of water stress, ψP bad recovered even in the most severe treatment, and the recovery of turgor was associated with the drop in ABA concentrations. However, the increase in midshoot ψP and the decline in ABA were not associated with any increase in shoot growth rate. The continued inhibition of shoot growth was probably not related to ABA or turgor pressure of mature leaves but may have been related to turgor pressure in the growing tip.

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Thomas E. Marler

of wind-exposed plants was approximately half of that for protected plants by Week 3 ( Fig. 1C ). Leaf area, leaf expansion rate, and root extension rate were not influenced by cultivar, wind, or their interaction ( Table 1 ). Table 1. Single leaf

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A. Abu El-Kashab, A.F. El-Sammak, A.A. Elaidy, M.l. Salama, and M. Rieger

We studied the effect of a 200-mg·liter–1 foliar application of paclobutrazol (PBZ) on growth and physiological responses of Prunus persica `Nemaguard' (salt-sensitive) and Olea europea `Manzanillo' (salt-tolerant) to salt stress. One-year-old trees were grown in 3 sand: 3 field soil: 4 pine bark media in 20-cm pots in a greenhouse and were irrigated with nutrient solutions adjusted with 0, 9, 18, or 36 mmol NaCl for peach and 0, 36, 72, 108 mmol NaCI for olive. Dry weight, photosynthesis, and leaf conductance decreased with increasing salinity for both species. However, leaf expansion rate was unaffected by NaCl. PBZ reduced dry weight for peach only, but PBZ increased photosynthesis and reduced leaf expansion rate for both species. Relative water content was decreased by salt but increased by PBZ. PBZ reduced the foliar Na and Cl content in peach but not olive. Olive had less Na in leaves than peach at 36 mmol NaCI, accumulated less C in leaves in all salt treatments, and had higher foliar Na without symptom expression. PBZ may reduce salt stress in sensitive species like peach by reducing foliar Na and Cl accumulation but has less influence on the salinity response of the more salt-tolerant olive.

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Joanna Hubbard and Ted Whitwell

Twelve ornamental grasses from the genera Calamagrostis, Cortaderia, Eragrostis, Erianthus, Miscanthus, Sorghastrum, Spartina, Panicum, and Pennisetum were evaluated for tolerance to the postemergence herbicides fenoxaprop-ethyl, fluazifop-P, and sethoxydim at 0.4 kg a.i./ha. Calamagrostis was uninjured by fenoxaprop-ethyl as measured by visual injury ratings, height, and foliage dry weight. Greenhouse studies evaluated the tolerance of three Calamagrostis cultivars to fenoxaprop-ethyl rates of 0.4 to 3.2 kg a.i./ha with no observed visual injury from any treatment. However, the expansion rate of the youngest Calamagrostis leaf was reduced linearly with increasing herbicide rates each day after application. The highest rate (3.2 kg a.i./ha) reduced the leaf expansion rate by 1 day and all other rates by 3 days after treatment. Leaf expansion rate differed between Calamagrostis cultivars at different times after herbicide treatment. Dry weight of Calamagrostis arundinacea `Karl Foerster' was reduced at 4 weeks after treatment but not at 10 weeks after treatment. Chemical names used: (±)-ethyl 2-[4-[(6-chloro-2-benzoxazolyl)oxy)phenoxy]propanoate (fenoxaprop-ethyl); (R)-2-[4-[[5-trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid (fluazifop-P); 2[1-(ethoxy imino)butyl]-5[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one (sethoxydim).

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Philip A. Loretan, Fred A. Avicki, Desmond G. Mortley, and Wiletha Horton

A cooling system using the principles of heat transfer was designed to provide a temperature difference of 6C between root and shoot zones and to study the effect of this difference on growth, yield, and phenology of `TI-155' sweetpotato [Ipomoea batatas (L.) Lam.] grown using the nutrient film technique in a greenhouse. Treatments were temperature control (20C) and variable temperature (26C) in a randomized complete-block design with two replications. A modified half Hoagland's nutrient solution with a 1 N: 2.4 K ratio was used and was changed every 2 weeks. Nutrient solution pH was maintained between 5.5 and 6, and electrical conductivity, salinity, and solution temperature were monitored at regular intervals. Storage root fresh and dry weights (except for fibrous root dry weight) and foliage fresh and dry weights were not significantly influenced by root zone temperature. Leaf expansion rate and vine length were lower for root zone temperature control plants; stomatal conductance, transpiration, and leaf unfolding rates were similar for both treatments.

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Milton E. McGiffen Jr., E.J. Ogbuchiekwe, and B.S. Saharan

While there are published reports of varietal differences in competitiveness with weeds, no crop varieties have been specifically developed for tolerance to weed interference. We explored several methods that mechanistically compare potential sources of tomato varietal tolerance to purslane, velvetleaf, and black night-shade: 1) The influence of canopy structure and development was studied with a wide range of crop and weed germplasm with different growth habits. Leaf expansion rate and other morphological characters were used to select crop genotypes for more-detailed study. 2) Replacement series experiments with selected cultivars found that purslane and other species can adapt to avoid competition. The greatest varietal differences in competitiveness were with nightshade species that had a canopy structure similar to tomatoes. 3) Field measurements of canopy development and light interception found that competitive advantage shifted over time as height and leaf area of weeds and crops changed. 4) A systems analysis method, sensitivity analysis, found that changes in plant architecture over time were more important than initial or final crop characteristics in determining competitive outcomes.

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Carleton B Wood, Timothy J. Smalley, and Mark Rieger

Container-grown Viburnum plicatum var. tomentosum `Mariesii' were planted in tilled beds and tilled beds amended with aged pine bark. After transplanting, plants were fertilized at three different rates: no fertilizer, 18.4 g of N m-2, and 36.8 g of N m-2. A 31 day drought was begun 73 days after planting. Fertilization of tilled plots induced ammonium toxicity, which caused a linear reduction in leaf area, shoot dry weight, and root dry weight. Fertilization of amended plots had no effect on shoot growth but reduced mot growth by 54%; thus, amendments ameliorated ammonium toxicity. Between 10 and 28 days after beginning the drought, plants in unfertilized-amended plots maintained higher relative leaf water contents (RLWC) and relative leaf expansion rates (RLER) than plants in unfertilized-tilled plots. Amendment induced nitrogen deficiencies contributed to the increased drought tolerance of plants from unfertilized-amended plots. Since fertilized plants developed symptoms of ammonium toxicity, we were unable to determine if increasing fertility would counteract the drought tolerance conferred by pine bark soil amendments.

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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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Desmond G. Mortley, P.A. Loretan, C.K. Bonsi, and W.A. Hill

Growth chamber studies were conducted to evaluate the effect of four diurnal temperatures (24/18C, 26/20C, 28/22C, and 30/24C) on yield, leaf expansion and unfolding, and vine length of sweetpotatoes [Ipomoea batatas (L.) Lam]. Four vine cuttings (15 cm in length) of `TI-155' and `Georgia Jet' were grown for 120 days using a modified half-Hoagland nutrient solution with a 1:2.4 N:K ratio. Irradiance at canopy level averaged 600 μmol·m–2·s–1 at an 18/6 photoperiod, and RH of 70%. Storage root number/plant for both cultivars decreased with increased temperature. Storage root fresh and dry weights for both cultivars increased with temperatures up to 28/22C and declined at 30/24C. Foliage fresh and dry weights were not influenced by temperature for either cultivar. Leaf expansion rate and vine length were highest at 26/20C and lowest at 24/18C for both cultivars. Leaf unfolding rate was not affected by temperature foe either cultivar, but was more influenced by time of measurements.