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  • Author or Editor: J. Heinrich Lieth x
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A two-dimensional mathematical model was developed to describe the time course of root growth and its spatial distribution for container-grown plants, using chrysanthemum [Dendranthema ×grandiflorum (Ramat.) Kitamura] as the model system. Potential root growth was considered as consisting of several concurrent processes, including branching, extension, and death. Branching rate was assumed to be related sigmoidally to existing root weight density. Root growth extension rate was assumed to be proportional to the existing root weight density above some threshold root weight density in adjacent cells. The senescence rate of root weight density was assumed to be proportional to existing root mass. The effects of soil matric potential and temperature on root growth were quantified with an exponential function and the modified Arrhenius equation, respectively. The actual root growth rate was limited by the amount of carbohydrate supplied by the canopy to roots. Parameters in the model were estimated by fitting the model to experimental data using nonlinear regression. Required inputs into the model included initial root dry weight density distribution, soil temperature, and soil water potential data. Being a submodel of the whole-plant growth model, the supply of carbohydrates from canopy to roots was required; the total root weight incremental rate was used to represent this factor. Rather than linking to a complex whole-plant C balance model, the total root weight growth over time was described by a logistic equation. The model was validated by comparing the predicted results with independently measured data. The model described root growth dynamics and its spatial distribution well. A sensitivity analysis of modeled root weight density to the estimated parameters indicated that the model was more sensitive to carbohydrate supply parameters than to root growth distribution parameters.

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Conventional irrigation practices of cut-fl ower greenhouse crops may result in application of excess water, resulting in runoff which may pollute the environment and contaminate drinking water supplies. A computerized irrigation control system based on soil moisture tension, originally designed for potted plants, was adapted for use in cut flower production. Tensiometers equipped with a high-fl ow ceramic tip and pressure transducers were effective in monitoring the soil moisture in the root zone of plants grown in ground beds and responded to rapid changes in soil moisture. The irrigation control system using these sensors, a computer, and custom-written software continuously monitored the moisture condition of the soil, initiated irrigation when the soil dried to a specific level, and turned off the water when an adequate amount was applied. When the system was installed in a greenhouse producing roses, water use decreased while productivity (stems harvested/m2) and stem length increased substantially. The observed increases in productivity and quality can result in significant increases in profitability for commercial rose producers.

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Nondestructive dry-weight (DW) estimates of plant parts are important for analyzing production and partitioning patterns of horticultural crops, particularly when repeated measurements of the same plant must be made without affecting growth. Equations were developed for estimating leaf, stem, and flower bud DW (LDW, SDW, and FDW, respectively) from linear measurements of the flowering shoot parts of Rosa hybrida L. `Cara Mia'. We used a stepwise forward polynomial regression to develop a set of equations that represented the data well; from these, we chose equations to make data collection as simple as possible. LDW was computed from leaf length. LDW of the whole shoot was calculated by adding the computed LDW of each leaf on a shoot. Each stem was divided into 30-mm segments and the DW of each segment was correlated with its diameter. SDW was calculated by adding all of the stem segments' DWs. FDW was directly correlated with flower bud diameter. The selected models can be used for rose shoot DW prediction; although in some cases, errors were encountered. Despite these errors, this approach may represent the only feasible method for DW estimation when destructive methods cannot be used.

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Stem elongation of poinsettia (Euphorbia pulcherrima Klotz.) was quantified using an approach that explicitly modelled the three phases of a sigmoidal growth curve: 1) an initial lag phase characterized by an exponentially increasing stem length, 2) a phase in which elongation is nearly linear, and 3) a plateau phase in which elongation rate declines as stem length reaches an asymptotic maximum. For each growth phase, suitable mathematical functions were selected for smooth height and slope transitions between phases. The three growth phases were linked to developmental events, particularly flower initiation and the first observation of a visible flower bud. The model was fit to a data set of single-stemmed poinsettia grown with vegetative periods of 13, 26, or 54 days, resulting in excellent conformance (R 2 = 0.99). The model was validated against two independent data sets, and the elongation pattern was similar to that predicted by the model, particularly during the linear and plateau phases. The model was formulated to allow dynamic simulation or adaptation in a graphical control chart. Model parameters in the three-phase function have clear biological meaning. The function is particularly suited to situations in which identification of growth phases in relation to developmental and horticultural variables is an important objective. Further validation under a range of conditions is required before the model can be applied to horticultural situations.

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Physical characteristics of two media were studied concerning water availability to roots, as reflected in specific transpiration rate, stomatal conductance, and specific growth rate of very young leaflets of `Kardinal' rose (Rosa ×hybrida L.), grafted on Rosa canina L. `Natal Brier'. Plants were grown in UC mix [42% composted fir bark, 33% peat, and 25% sand (by volume)] or in coconut coir. Water release curves of the media were developed and hydraulic conductivities were calculated. Irrigation pulses were actuated according to predetermined media moisture tensions. Transpiration rate of plants was measured gravimetrically using load cells. Specific transpiration rate (STR) was calculated from these data and leaf area. STR and stomatal conductance were also determined using a steady-state porometer. Specific growth rate (RSG) of young leaflets was calculated from the difference between metabolic heat rate and respiration rate, which served as an indicator for growth potential. Low STR values found at tensions between 0 and 1.5 kPa in UC mix suggest this medium has insufficient free air space for proper root activity within this range. Above 2.3 kPa, unsaturated hydraulic conductivity of UC mix was lower than that of coir, possibly lowering STR values of UC mix-grown plants. As a result of these two factors, STR of plants grown in coir was 20% to 30% higher than that of plants grown in UC mix. STR of coir-grown plants started to decline only at tensions around 4.5 kPa. Yield (number of flowers produced) by coir-grown plants was 19% higher than UC mix-grown plants. This study demonstrated the crucial role of reaching sufficient air-filled porosity in the medium shortly after irrigation. It also suggests that hydraulic conductivity is a more representative measure of water availability than tension.

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Shoot-bending has become a standard cultural practice in cut-flower rose (Rosa hybrida L.) production. Physiological effects of shoot-bending on leaf net photosynthesis (A), stomatal conductance (gs ), transpiration rate (E), and stem water potential (ψ) were investigated for rose plants. With saturating light conditions, shoot-bending decreased rates of A, gs , and E in comparison with the rates prior to shoot-bending. A, gs , and E of bent shoots were significantly lower than those of the control shoots that were not bent. The differences in A between bent and control shoots decreased over time, disappearing within 3 weeks after bending. Bent shoots exhibited reduced ψ. Leaves projecting upward on a bent stem were found to have higher A, gs , and E than those projecting downward. This was probably due to the destruction of xylem vessels serving the leaves attached to the lower side (compression side) of the bent stem. Our results support the hypothesis that hydraulic conductivity is reduced in bent shoots probably due to disturbed xylem tissues, and that reduced photosynthetic rates of bent shoots are a function of water status.

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Easter lily bulbs are harvested in fields in northern California and southern Oregon, packed in cases, and shipped to distributors and growers. The greenhouse forcer then cools the bulbs at 40–45°F for 6 weeks. This cold period is needed to vernalize the bulbs and to assure that the plants will later flower uniformly. Bulbs that have been cooled for longer or shorter lengths of time respond differently. The objective of this study was to determine the optimal storage temperature regime for the bulbs dug during the early part of the 3-week bulb-harvest period. Twelve groups of bulbs at various storage temperature regimes were evaluated as to their performance during greenhouse forcing. The variables that were considered were: 1) bud count, 2) variability of flowering date, 3) earliness of flowering date, 4) variability of Visible Bud date, and 5) variability of final plant height. An index was developed to evaluate the degree to which each variable impacted the production during the forcing phase. We found that the best protocol for bulb growers is to dig the bulbs and then hold them at cool (>45°F) ambient temperatures for a week. Temperatures higher than the high 65°F should be avoided. If the bulbs will be stored just 1 more week, then they can stay at this temperature; otherwise, the bulbs should be cooled down to, and held at, 42 to 45°F.

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An irrigation system for monitoring and controlling soil moisture tension in the root zones of potted plants using computer and solid-state tensiometer technologies was evaluated in a commercial greenhouse on 'V-14 Glory' poinsettias over a 10 week period. Replicated benches with separate drip circuits controlled by the computer maintained the soil moisture tension of the potted poinsettia plants between 1 kPa and 5 kPa. The amount of water used by each bench and the amount leached was compared to benches with separate drip circuits that were manually operated by the grower according to standard commercial practice. There was a 65% savings in the total amount of water used for the computer-controlled system and an average weekly reduction of 98.6% in leachate. The differences were significant and there was no measurable reduction in plant quality, even though soil analyses showed slightly elevated EC levels.

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Commercial production of Easter lily (Lilium longiflorum Thunb.) requires precise temperature control to ensure that the crop flowers in time for Easter sales. The objective of this project was to develop and validate a greenhouse decision-support system (DSS) for producing Easter lily to predetermined height and flower date specifications. Existing developmental models were integrated with a knowledge-based system in a DSS to provide temperature recommendations optimized for Easter lily scheduling and height control. Climate data are automatically recorded in real time by linking the DSS to a greenhouse climate control computer. Set point recommendations from the DSS can be manually set or automatically implemented in real time. Potential benefits of the project include improved crop quality and the transfer, validation, and integration of research-based models. The DSS was implemented at several research and commercial locations during the 1994 Easter lily season. DSS recommendations were compared with the strategies of experienced growers. The system design, implementation, production results, quality of recommendations, and potential are discussed.

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