A series of interspecific and back-cross hybrids have been developed for Limonium sinuatum (L.) Mill. × Limonium perezii (Stapf) Hubb. (Morgan et al., 2009) for commercial release. Production of many flower crops is scheduled and monitored to ensure their harvest times target specific market windows such as holidays. In contrast, cut flower crops like Limonium are produced and marketed over an extended season. Multiple planting dates through the year provide an opportunity to spread the harvest period to meet this market demand, but typically means crops are planted into different environments of light and temperature. Although it is frequently cost-effective to maintain temperature in a suitable range for production, this is less frequently so for light (Faust, 2003). Development of reliable models for scheduling flowering requires an understanding of the environmental requirements for flower induction and development through to anthesis (Funnell, 2008; Oh et al., 2009).
Limonium sinuatum has been reported as a facultative, long-day plant with a critical photoperiod of 13 h (Semeniuk and Krizek, 1972; Shillo and Zamski, 1985). Long days are also likely to promote flowering of L. perezii (Armitage, 2003), albeit this response has not been quantified.
Daily light integral (DLI), being the product of photosynthetic photon flux (PPF) and photoperiod, has been used successfully as a light variable in models developed for crop scheduling (Oh et al., 2009; Warner and Erwin, 2003). Although L. sinuatum has been reported as being indifferent to DLI (Mattson and Erwin, 2005), researchers examined DLI values between 15.3 and 27.6 mol·m−2·d−1 in which DLI may not have been limiting. Other workers have reported a response to DLI during periods of naturally low light intensity (Shillo and Zamski, 1985). In mainland United States, crops can be exposed to DLI values ranging from 4 to 60 mol·m−2·d−1 (Korczynski et al., 2002). A similar range is encountered within New Zealand. At the lower extreme of this range, these DLI values can limit commercial production of numerous greenhouse crops (Faust, 2003).
A facultative vernalization response is evident for cultivars of L. sinuatum (Semeniuk and Krizek, 1973) with subsequent growth rate increased at higher temperatures of 22 to 27 °C day and 12 to 16 °C night. In contrast, L. perezii is reported as being “free-flowering” (Harada, 1992); we interpret this to mean that vernalization is not a requirement for flowering in this species. As used in the current experiment, hybrids between these two species have been successfully grown to flower in greenhouses maintained at temperatures between 15 and 20 °C without vernalization (unpublished data).
Plant growth variables have been successfully incorporated into models for predicting timing of flowering, acknowledging the potential for differences in leaf number and dry matter accumulation in response to DLI (Heins et al., 2000; Liu and Heins, 2002; Warner and Erwin, 2003). Growth variables such as leaf number below the inflorescence are particularly useful in interpreting the environmental response of plants with a determinate shoot growth habit (like Limonium), because it can be used as an indicator of the transition from vegetative to floral growth. Furthermore, the flowering of plants has been proposed to occur when photosynthetic leaf area reaches a critical threshold with leaf area and flowering time being significantly correlated (Armitage, 1984; Ramina et al., 1979). Thus, in the current study, attempts were made to examine the potential for combinations of plant growth variables and environmental factors to predict flowering.
The objective of this research was to develop and validate a model to predict time to flower of a novel Limonium hybrid based on environmental factors (primarily DLI) and/or plant growth variables (e.g., leaf number and groundcover index).
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