The demand for potted plants is increasing worldwide and new species, such as Brunonia australis (Goodeniaceae) and Calandrinia sp. (Portulacaceae), are constantly being sought. Many ornamental plants are grown outside their natural flowering period to align flowering with peak market demand, which requires the capacity to predict flowering date under changing or different environments. The present study aimed to quantify temperature, daylength and vernalisation responses of B. australis and Calandrinia sp. in relation to development rate, flowering and plant morphology. Modelling development rate as a function of environmental factors was investigated.
Cardinal temperatures for seed germination were defined using a thermogradient plate. Reciprocal time to median germination (1/t50) and percentage germination per day were calculated and regressed against temperature. Base temperature estimates for B. australis were 4.9 and 5.5°C and optimum temperatures were 21.4 and 21.9°C, while maximum temperatures were 35.9 and 103.5°C, with the latter being clearly overestimated using the 1/t50 index. Base temperatures for Calandrinia sp. were 5.8 and 7.9°C, while optimum and maximum temperature estimates of 22.5 and 42.7°C, respectively, were reported using the percentage germination per day index.
Floral initiation and development were characterised at 25/10 and 35/20°C (day/night) under long day (LD) conditions using scanning electron microscopy. Floral initiation of B. australis commenced 28 d after seed germination at 25/10 and 35/20°C. However, inflorescence development rate was highly variable at 35/20°C, suggesting genetic variability in heat tolerance among the population. Leaf number at floral initiation reflected differences in accumulated thermal time between treatments, so that about double the number of leaves formed at 20/35°C. Calandrinia sp. commenced reproductive growth 47 d after germination at 25/10°C, whereas flowering was inhibited at 35/20°C for the duration of the 90 d experiment.
The juvenile phase length was determined by transferring plants from less-inductive conditions (short day; 11 h and 30/20°C) to inductive conditions (long day; 16 h or vernalisation; 21 d at 9°C) at six plant ages ranging from 4 to 35 d after seed germination. The effect of plant age, at time of transfer to inductive conditions, on flowering time and floral bud and branch number was investigated. B. australis perceived the LD floral stimulus about 18 to 22 d after germination, whereas both species were vernalisation sensitive at 4 to 35 d. These results suggested that some Australian native ephemeral species show a distinct juvenile phase. Overall, transferring plants of B. australis from short days (SDs) to LDs reduced the time to anthesis compared to SDs plus vernalisation or constant SDs. Calandrinia sp. showed a facultative requirement for cold and an insensitive phase was not detected.
Development rate responses to temperature and photoperiod or vernalisation were modelled from planting to first visible floral bud (VFB) and VFB to anthesis using data collected from a serially sown field experiment (2009–2010). The influence of planting date on floral bud and branch number was investigated. A multiplicative approach, which included a curvilinear temperature function and a triple broken linear daylength function, was used to model development rate from planting to VFB for B. australis. An exponential vernalisation function was combined with a curvilinear temperature function to predict development rate from planting to VFB for Calandrinia sp. The vernalisation function allowed for autonomous flowering and included a variable for the vernalisation ceiling temperature. The present study appears to be the first to incorporate a natural vernalisation function into development rate models for an herbaceous ornamental plant. Development rate from VFB to anthesis was explained using a linear thermal model for both species. Base temperature estimates were 5 and 10.2°C for B. australis, and 13 and 10.5°C for Calandrinia sp. for planting to VFB and VFB to anthesis, respectively. The upper critical daylength for B. australis was estimated at 12 h, with no further increases in development rate beyond 12 h. The ceiling vernalisation temperature for Calandrinia sp. was estimated at 15°C and plants appeared fully vernalised after 26.8 d.
Overall, the research revealed that B. australis is a facultative LD plant and that Calandrinia sp. has a facultative vernalisation requirement. The LD floral stimulus was perceived by B. australis after a period of vegetative growth, whereas Calandrinia sp. was vernalisation-sensitive from a very young age. Plants of both species appeared well adapted to cooler climates. High temperatures caused seed death, inhibited floral initiation in Calandrinia sp. and reduced floral bud and branch number in B. australis. The duration from planting to VFB and VFB to anthesis could be predicted using development rate models that included functions for temperature and photoperiod or vernalisation responses. The present study appears to be the first to model development rate of Australian native species and a more comprehensive approach was used when compared to previous modelling efforts for traditional herbaceous ornamental plants. In addition to crop scheduling and predicting production times, these models could be used to predict plant responses to rising temperatures associated with climate change.