Growth and development in Achimenes cultivars

Achimenes is a herbaceous perennial of the Gesneriaceae family. Its origin is Central and South America. It has been cultivated as a pot plant since the Victorian era. Extensive hybridization has produced many attractive cultivars which have been Introduced in commercial floriculture both In the United States and Europe. Commercial Achimenes cultivars have short erect stems bearing flowers in the axils of the leaves and form rhizomes at the tips of underground stolons. At the end of the growing period, in late summer, the plants enter a period of senescence and rhizomes undergo a period of dormancy before they sprout for a new plant production in early spring . Thus cultivation of Achimenes is limited to one crop per year.The aim of the present study was to describe the effect of the environment, mainly light and temperature, on plant growth and development In order to realize optimum flower and / or rhizome production . Moreover the possibilities to manipulate plant development and release of rhizomes from dormancy by means of growth regulators are indicated.Vegetative growth and flower developmentGrowth and flowering were strongly affected by treatments involving duration, intensity and quality of light (Chapters 1 through 4). Longer photoperiods increase vegetative growth in most Achimenes cultivars. LD (16 h) increased plant height, shoot weight and number of flowers compared to SD (8 h). Increased duration of illumination, of the same light level, increased shoot dry weight and reduced plant height (Chapter 2). When the same daily light integral was maintained, long day i.e. low light Intensity favoured growth and flowering and increased relative growth rate (Chapter 3). This is due to increase in net assimilation rate but not in leaf area ratio suggesting a better light use efficiency when a low light intensity is combined with longer days. When daylength was extended with low intensity incandescent light (low R/ FR ratio) total leaf area , total dry weight of the plant and leaves Increased . Fluorescent light (high R/ FR ratio) had no effect on total dry weight of the plant. Number of flowers was not influenced by light quality except in 'Rosenelfe' where number of axillary shoots was suppressed by incandescent light and flowering was promoted by fluorescent light . Furthermore, light duration and quality influenced calculated parameters of growth. Daylength extension by either incandescent or fluorescent low intensity light increased relative growth rate , net assimilation rate and lowered leaf area ratio (Chapter 4).Vegetative and generative growth were Influenced by the temperature treatments. Higher temperatures Increased plant height, number of leaf whorls and flowers and reduced time to anthesis (Chapters 2 and 4). At 25 °C , total leaf area and total dry weight of the plant increased compared to 17 °C Application of gibberellic acid (GA 3 ) alone or in combination with benzyladenine (BA) and 1 or naphthalenacetic acid (NAA) increased plant height and weight of aerial plant parts but number of flowers was not affected (Chapters 6 and 7). The growth retardants Ancymidol, Paclobutrazol and 5-3307 suppressed plant height and development of axillary shoots and flowers in 'Viola'. The effects of the growth retardant treatments were more pronounced under shade rather under full sunlight. The growth regulators used in this study do not cause any desirable effects in Achimenes as plants become either too tall (GA 3 ) or stunted (retardants) with subsequent adverse effects on flowering (Chapter 8).Rhizome formation and developmentIn general rhizome development seemed to be correlated to shoot growth. Daylength hardly influences number of rhizomes, however, in some cultivars number and fresh weight of rhizomes increase in SD (Chapter 1). LD or continous ligh tdid not inhibit rhizome formation and development (Chapter 2). Higher irradiance levels, increased number and dry weight of rhizomes, along with increases in dry weight of the aerial parts, indicating that rhizome development depends largely on the accumulation of metabolites provided by increased assimilation of the above ground parts. light quality ( incandescent vs. fluorescent light) affected dry weight of rhizomes depending on the cultivar. Fluorescent light increased dry weight of rhizomes in 'Flamenco' only. Number of rhizomes was not affected by light quality. Temperature had no effect on rhizome development or on rhizome weight ratio (percentage of dry matter incorporated into rhizomes ) as shown in Chapters 2 and 4.Application of GA3 reduced rhizome weight ratio whereas the combination of BA and NAA increased dry weight of rhizomes (Chapter 7). BA also increased fresh weight of rhizomes whereas GA3 reduced it (Chapter 3). The use of growth retardants, however, did not promote rhizome development but in some cases even reduced it. This finding confirms that rhizome development depends on good shoot growth.Sprouting of rhizomesFreshly harvested rhizomes are released from dormancy with either benzyladenine (BA) or giberrellic acid (GA 3). Soaking rhizomes of 'English Waltz' in a BA solution (50 mg.l -1) and keeping them at 25 °C in the dark for two weeks resulted in the highest percentage of sprouting . Soaking rhizomes of 'Viola' in GA 3(50 mg.l -1) for 16 h also promoted sprouting depending on subsequent storage treatment. Continous light during storage inhibited sprouting whereas in continous darkness at 21 or 25 °C resulted in 100% sprouting in three weeks. When rhizomes were cut in half the apical halves always sprouted first and more than the basal ones. Rhizomes which had been treated with GA 3produced taller plants with more leaf whorls, flowering earlier with more flowers. High temperature pretreatment of rhizomes increases the number of flowers produced. This is the result of the promotive effect of temperature on sprouting which subsequently affects flowering. Earlier sprouting ensures earlier flowering.Varietal differencesSeveral cultivars were studied. Plant height, number of axillary shoots, flowers , earliness , number and dry weight of rhizomes are clearly cultivar characteristics usually not influenced by environmental factors. Certain cultivars are slow in forming rhizomes. During the period of the experiments 'Blauer Planet' did not form any rhizomes at all. 'Prima Donna' , 'Viola ' and 'Hilda' formed very few. In 'Hilda' the relative growth rate was lower than in in 'Flamenco ' or 'Rosenelfe'. Responses to temperature also varied among cultivars. 'Rosenelfe' and 'Hilda' were the most responsive and this was due to increased net assimilation rate at higher temperatures (Chapter 4).Leaf morphology was responsible for some of the differences observed. 'Early Arnold' had greater total leaf area and total dry weight of leaves and plant due to a high net assimilation rate which was attributed to its large thin leaves, 'Linda' on the other hand, had a low net assimilation rate because of its many and small leaves which caused more internal shading. It is concluded that differences in the mean values for net assimilation rate and leaf area ratio are responsible for the differences observed among cultivars.Levels of endogenous hormones may be responsible for some differences among cultivars. 'Flamenco' had a low shoot weight whereas rhizome dry weight was high. It also responded to the far red light treatment by decreasing the dry weight of rhizomes. Relative growth rate and fresh weight per rhizome were greater in 'Flamenco' rather than in 'Hilda' or 'Viola'. It was shown that the number of flowers is correlated to height of the main stem and number of leaf whorls. Furthermore, 'Rosenelfe', 'Blau Import' and 'Tetraelfe' had a higher number of flowers per plant as they bear flowers in clusters of 3 to 9 from the leaf axils where the other cultivars bear only one or two. All cultivars tested flowered regardless of daylength thus confirming that Achimenes is a day neutral plant.It is concluded that it is possible to control growth and flowering in Achimenes through temperature and light, the latter being the more crucial factor . Dormancy of rhizomes can be broken and sprouting enhanced with the use of growth substances. Thus the time interval between two growing seasons can be shortened. However, additional light, at least of the same levels and/or duration as in the present experiments, should be given during winter months. The right choice of cultivar, depending on the intended purpose of cultivation (flower or rhizome production) is also an important factor to consider. Growth regulators are not recommended for application to whole plants except for the purpose of increasing rhizome production.