Abstract Grete Waaseth

Eija Bakken

The transition from vegetative to reproductive development in herbaceous plants relies on both environmental and endogenous cues, once the shoot apical meristem has reached maturity. After completing juvenile development and the vegetative phase change to adult growth, the meristem is competent to respond to a floral stimulus with floral evocation followed by differentiation and development of flower primordia. Influence of environmental factors on floral evocation through photoperiodic responses and vernalization is well documented in a number of herbaceous plant species, and some of its genetic basis is clarified in the model species Arabidopsis thaliana (L.) Heynh.

In floriculture crops, information on how to control the floral transition is essential in crop scheduling. In this thesis, effects of varying photosynthetic photon flux (PPF) and vernalization periods on floral evocation have been investigated in selected herbaceous ornamental perennial crops.

The experiments were conducted in growth chambers and greenhouses to facilitate control over temperature, photoperiod and PPF. Seed propagated plants of Coreopsis grandiflora Hogg ex Sweet. ‘Sunray’, Lavandula angustifolia Mill. ‘Hidcote Blue’, Lythrum salicaria L. ‘Red Hybrids’ Veronica longifolia L. ‘Rosa Töne’, and Salvia x superba Stapf ‘Blaukönigin’ were given low temperature vernalization for up to 15 weeks and PPF treatments ranging from 50 to 200 µmol m^-2 s^-1 for 20-hour long days, equal to daily light integrals (DLI) from 3.6 to 14.4 mol m-2 s^-1 respectively. The timing of floral evocation was determined by counting the number of leaves developed prior to the initiation of floral organs. In addition, number of days to anthesis, lateral branching and plant size was recorded.

High PPF generally increased the ornamental quality irrespective of species. Only Lavandula and Salvia flowered without vernalization and floral evocation was promoted by high PPF in both species. In Salvia, the stage of floral evocation was promoted by high PPF and further experiments were designed using Salvia as a model plant to examine what seems to be a novel flowering promotion pathway. In addition to PPF, evocation in Salvia was also promoted by both long photoperiods and vernalization, although plants flowered autonomously in short days without cold. Floral evocation occurred at an earlier developmental stage in unvernalized Salvia in response to increasing PPF irrespective of photoperiod, although the response was more prominent in 20-hour days compared with short days of 10 hours. After a saturating vernalization treatment, the response to PPF was bypassed when plants were grown in long photoperiods.

Investigations of assimilate accumulation and partitioning, regarded as the most probable mechanisms underlying the PPF response, proved to be chiefly unproductive. Manipulating the growing temperature was conducted prior to vernalization and flower induction in order to affect the general carbohydrate status. Defoliations and apex darkening which should influence the amount of assimilates transported to the shoot apex were performed in addition to exogenous sucrose applications. After reviewing the data, the mechanism behind the PPF dependent flowering promotion pathway is still an open question, which needs further examination. Since floral evocation in Salvia bears a clear resemblance to late flowering, winter annual strains of Arabidopsis thaliana, Arabidopsis might be utilized as a model species in future investigations.



Oppdatert: 17.11.04
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