Annelies Christiaens

Suboptimal Light Conditions Influence Source-Sink Metabolism during Flowering.

Reliance on carbohydrates during flower forcing was investigated in one early and one late flowering cultivar of azalea (Rhododendron simsii hybrids). Carbohydrate accumulation, invertase activity, and expression of a purported sucrose synthase gene (RsSUS) was monitored during flower forcing under suboptimal (natural) and optimal (supplemental light) light conditions, after a cold treatment (7°C + dark) to break flower bud dormancy. Post-production sucrose metabolism and flowering quality was also assessed. Glucose and fructose concentrations and invertase activity increased in petals during flowering, while sucrose decreased. In suboptimal light conditions RsSUS expression in leaves increased as compared to optimal light conditions, indicating that plants in suboptimal light conditions have a strong demand for carbohydrates. However, carbohydrates in leaves were markedly lower in suboptimal light conditions compared to optimal light conditions. This resulted in poor flowering of plants in suboptimal light conditions. Post-production flowering relied on the stored leaf carbon, which could be accumulated under optimal light conditions in the greenhouse. These results show that flower opening in azalea relies on carbohydrates imported from leaves and is source-limiting under suboptimal light conditions.

Cold Treatment Breaks Dormancy but Jeopardizes Flower Quality in Camellia japonica L.

Camellia japonica L. is an evergreen shrub whose cultivars are of great ornamental value. In autumn, after flower bud differentiation, dormancy is initiated. As in many other spring flowering woody ornamentals, winter low temperatures promote dormancy release of both flower and vegetative buds. However, warm spells during late autumn and winter can lead to unfulfilled chilling requirements leading to erratic and delayed flowering. We hypothesized that storing plants at no light and low temperature could favor dormancy breaking and lead to early and synchronized flowering in response to forcing conditions in C. japonica ‘Nuccio’s Pearl’. Plants with fully developed floral primordia were stored at dark, 7°C, and RH > 90% for up to 8 weeks. To monitor endodormancy release during the storage, we evaluated the content of abscisic acid (ABA) in flower buds and the expression profiles of five putative genes related to dormancy and cold acclimation metabolism in leaves and flower buds. In addition, the expression of four anthocyanin biosynthesis pathway genes was profiled in flower buds to assess the effect of the treatment on flower pigment biosynthesis. At 0, 4, 6, and 8 weeks of cold treatment, 10 plants were transferred to the greenhouse and forced to flower. Forced plant flower qualities and growth were observed. The ABA content and the expression profiles of two dormancy-related genes (CjARP and CjDEH) suggested that dormancy breaking occurred after 6–8 weeks of cold treatment. Overall, plants treated for 6–8 weeks showed earlier vegetative sprouting, enhanced, and homogeneous flowering with reduced forcing time. Prolonged cold treatments also reduced flower size and longevity, anthocyanin content, and pigment biosynthesis-related gene transcripts. In conclusion, the cold treatment had a promotive effect on dormancy breaking but caused severe drawbacks on flower quality.

Rationalization of Camellia japonica L. pot cultivation: a multidisciplinary approach

Camellia japonica L. is an evergreen flowering perennial with more than 3,000 named cultivars of great ornamental value. These are nowadays traded worldwide as containerized small sized plants. Cultivation of ornamental plants is strongly oriented to sustainable production on the one hand and on a consumer-oriented high quality product on the other hand. In this context, the present work concerned a series of critical points of the cultivation cycle of potted C. japonica: the reduction of fertilization; the choice of a peat alternative substrate; the regulation of plant growth for size and flowering control; the control of the dormancy release of flower buds. Experiment 1 concerned the application of commercial biofertilizer inocula which included a specific arbuscular mycorrhizal fungus (AMF) isolate or a consortium of microorganisms (AMF, saprophytic fungi and helper bacteria) as alternatives to inorganic fertilization of pot cultivated C. japonica. To deepen AMF role, Experiment 2 evaluated potential specific isolates associated with camellia roots (active population) or surrounding soil (potential and soil exploring population) of centennial specimens found in natural or semi-natural ecosystems. Experiment 3 assessed the efficacy of flurprimidol and three peat alternatives (nutshells, rice husk, coconut fiber) on camellia growth control. Finally, Experiment 4 used a multidisciplinary approach to describe and characterize the effects of cold treatments on the dormancy release of flower buds. Overall, the results highlighted that C. japonica is susceptible to AMF inoculation and that a series of benefits can be achieved by inoculating pots with specific symbiotic fungi. Moreover, two newly tested materials (nutshells and rice husk) were suitable as partial peat alternatives (30% v/v), and the efficacy of very low concentrations of flurprimidol to control growth and enhance flowering was underlined. Lastly, the use of cold treatment allowed the achievement of an earlier, uniform and enhanced flowering.

An in vitro bioassay for the evaluation of cold treatment on flower bud dormancy in Camellia

Camellia japonica L. is an evergreen shrub whose cultivars are of great ornamental value. Initiation and early differentiation of flower buds in C. japonica starts from late spring on while flower bud development and visible bud enlargement sequel until autumn. In many temperate woody ornamentals dormancy is installed after flower bud differentiation. The exposure of floral buds to cold temperatures (between 2-7°C) is supposed to stimulate the initiation of normal growth and anthesis during the next spring. As an attempt to quicken the fulfillment of the cold need, as well as to homogenize flowering, five low temperature regimes, consisting of darkness and a constant temperature of 7°C applied for 0, 2, 4, 6 and 8 weeks, were tested on budded plants of cultivar ‘Nuccio’s Pearl’. As indicator of dormancy release, an inexpensive and easygoing test was conducted by forcing excised flower buds with an in vitro bioassay. To infer about the reliability of this test, a possible hormonal basis was assessed by measuring the concentration of abscisic acid in floral buds. Four groups of camellias of ten specimens each, treated respectively for 0, 4, 6 and 8 weeks, were then forced in the greenhouse with supplementary lighting and semi-controlled temperature for qualitative and quantitative evaluations of flowering. Our results indicated that the bioassay is a suitable indicator of the moment of dormancy release as it was able to highlight a reduced amount of dormancy when buds stayed longer at cold. Abscisic acid content in floral buds showed to be reduced and homogenized after 6 and 8 weeks of cold, thereby promoting a more uniform flowering. Hence, the bioassay appears as a costeffective tool that could be of interest for breeders, to characterize the chilling requirements of flowers of parental plants or their offspring.