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P.R. Fisher, J.H. Lieth, and R.D. Heins

Stem elongation of commercially produced flowering poinsettia (Euphorbia pulcherrima L.) is often sigmoid. However, sigmoid mathematical functions traditionally used for representing plant growth fail to adequately describe poinsettia stem elongation when a shoot has a long vegetative growth period. A model was developed that explicitly described three phases of poinsettia stem elongation: 1) the initial lag phase, where stem length increases approximately exponentially; 2) a period when elongation is linear; and 3) a plateau phase, where elongation rate declines to zero and stem length reaches an asymptotic maximum length. The timing of the plateau phase was linked to flower initiation date. Fit of the resulting model to data from single stem `Freedom' poinsettia grown with different periods between transplant and flower initiation had an R2 of 0.99. Model parameters had clear biological meaning, and the poinsettia model has horticultural application for simulation and graphical tracking of crop height.

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J.H. Lieth, P.R. Fisher, and R.D. Heins

A growth function was developed for describing the progression of shoot elongation over time. While existing functions, such as the logistic function or Richards function, can be fitted to most sigmoid data, we observed situations where distinct lag, linear, and saturation phases were observed but not well represented by these traditional functions. A function was developed that explicitly models three phases of growth as a curvilinear (exponential) phase, followed by a linear phase, and terminating in a saturation phase. This function was found to be as flexible as the Richards function and can be used for virtually any sigmoid data. The model behavior was an improvement over the Richards function in cases where distinct transitions between the three growth phases are evident. The model also lends itself well to simulation of growth using the differential equation approximation for the function.

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P.R. Fisher, J.H. Lieth, and R.D. Heins

The objective was to predict the distribution (mean and variance) of flower opening for an Easter lily (Lilium longiflorum Thunb.) population based on the variability in an earlier phenological stage and the expected average temperature from that state until flowering. The thermal time from the visible bud stage until anthesis was calculated using published data. `Nellie White' grade 8/9 Easter lilies were grown in five research and commercial greenhouse locations during 1995, 1996, and 1997 under a variety of temperature and bulb-cooling regimes. Distributions of visible bud and anthesis were normally distributed for a population growing in a greenhouse with spatially homogenous temperatures. The variance at anthesis was positively correlated with variance at visible bud. The mean and variance at visible bud could therefore be used to predict the distribution of the occurrence of anthesis in the crop. The relationship between bud elongation, harvest, and temperature was also incorporated into the model. After visible bud, flower bud length measurements from a random sample of plants could be used to predict the harvest distribution. A computer decision-support system was developed to package the model for grower use.

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L-Y. Li, J.H. Lieth, R.H. Merritt, and H.C. Kohl

A heat-unit model was established for tracking the development of geranium, based on experimental data collected at UC Davis and Rutgers Univ. The temperature thresholds for initiating development and heat-unit benchmarks needed to accomplish each phenostage are parameters in this model. The methods of estimating these parameters were proposed and tested with the observed data. The model worked well during either vegetative or reproductive stages, but failed to predict the initiation of flowers, suggesting that factors other than only temperature drive the flower initiation process. With this model crop development characterized by a series of specific morphological events can be tracked and predicted under various temperature regimes, so that crop timing can be more precise.

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J.H. Lieth, P.A. Kiehl, S.A. Tjosvold, G.B. Vogel, and D.W. Burger

An irrigation system was developed to water container-grown ornamental plants so that soil moisture tensions could be continuously monitored and controlled. Operation of such a system has been shown to significantly reduce the amount of water which must be applied to produce high-quality potted chrysanthemums. This presentation will focus on modification of drip irrigation systems in commercial production environments to irrigate based on soil moisture tension. High-quality plants were produced in commercial trials with such systems. In all cases significant economic savings due to reduced fertilizer and water application were observed. Furthermore, the amounts of irrigation water run-off were significantly lower than in systems where irrigation was controlled manually or with timers.

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C. M. Grieve, J.A. Poss, S.R. Grattan, P.J. Shouse, J.H. Lieth, and L. Zeng

To explore the possibility that saline wastewaters may be used to grow commercially acceptable floriculture crops, a study was initiated to determine the effects of salinity on two statice cultivars. Limonium perezii (Stapf) F. T. Hubb. `Blue Seas' and L. sinuatum (L.) Mill `American Beauty' were grown in greenhouse sand cultures irrigated with waters prepared to simulate saline drainage waters typically present in the western San Joaquin Valley (SJV) of California. Seven salinity treatments were imposed on 3-week-old seedlings. Electrical conductivities of the irrigation waters (EC) were 2.5 (control), 7, 11, 15, 20, 25, and 30 dS·m–1. Vegetative shoots were sampled for biomass production and ion analysis ten weeks after application of stress. Flower stem numbers, length, and weight were determined at harvest. Stem length of L. perezii was significantly reduced when irrigation water salinity exceeded a threshold of 2.5 dS·m–1. Salt tolerance threshold based on stem length for L. sinuatum was 7 dS m-1. The species exhibited significant differences in shoot-ion relations which appear to be related to differences in salt tolerance. Sodium, K+, Mg2+, and total-P were more strongly accumulated in the leaves of L. sinuatum than L. perezii. Both species accumulated K+ in preference to Na+, but selectivity for K+ over Na+ was significantly higher in L. sinuatum than in the more salt-sensitive L. perezii. Chloride concentration in L. sinuatum leaves increased significantly as salinity increased, whereas the 20-fold increase in substrate-Cl had no effect on leaf-Cl in L. perezii. Both Limonium species completed their life cycles at salt concentrations exceeding 30 dS·m–1, a character associated with halophytic plants. Maximum growth of each species, however, occurred under relatively low salt stress, and steadily declined as external salinity increased. Based on this crop productivity response, L. perezii should be rated as sensitive and L sinuatum as moderately tolerant.

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J.H. Lieth, L.R. Oki, P. Ng, M.C. Garcia-Navarro, S.H. Kim, and L.-Y. Li

Daminozide is a growth retardant used in potted plant production as a foliar spray to inhibit shoot elongation. It has its greatest inhibitory effect immediately after application, becoming less pronounced thereafter; continued retardation is accomplished by reapplication at 7to 14-day intervals. A model for this retardation effect is useful in developing decision support tools, as well as in optimizing (perhaps minimizing) the use of this growth retardant. Such a model, as developed and described earlier, simulates the effect of a foliar spray application of daminozide at various concentrations on various days during the production cycle. The objective of this work was to validate this model for various varieties of chrysanthemum. Using the model to simulate the effect of one application of daminozide resulted in predicted plant heights very close to the observed heights for most of the varieties tested. Of four methods used to implement the multiple-application effect, two resulted in very good simulation of the observed plant heights. In summary, the model was shown to be valid for all the varieties of chrysanthemum tested.