Bloemproduktie bij Alstroemeria 'Walter Fleming'

Alstroemeria is a relatively new crop in floriculture and is rapidly increasing in popularity. The plant is a herbaceous perennial and produces flowers that are attractive and long lasting. Some authors refer this genus to the family Amaryllidaceae, other to the Liliaceae, and same prefer a separate family Alstroemeriaceae.

The erect, unbranched shoots originate from a subterranean sympodial rhizome. Alstroemeria and some closely related genera are characterized by leaves which are twisted 180°, so that the morphological upper side is turned downwards. Under favourable conditions the shoots form terminal umbelliform clusters of flowers. During the growth period the roots thicken into tubers. The research described in this paper has been performed with the hybrid 'Walter Fleming', a cultivar of English origin, general grown in Holland under the name 'Orchid'.

The aim of this research was to investigate the structure of the plant and its ecological reactions in order to realize optima production conditions and a better timing of the crop.

SHOOT FORMATION

Shoots are initiated on a subterranean rhizome and its branches. Under greenhouse conditions, the number of shoots increases rapidly after the winter period. At that tine many rhizomes are active and the shoots grow fast. During flowering and shortly after, however, there is a rest period following which shoot formation is resumed. Until the beginning of the flowering period there is no significant influence of temperature (minimum 9°C) on shoot formation.

Comparing growth at 17, 21 and 25°C shoot formation was found to increase with. increasing temperature. At 13 and 9°C shoot formation stops in long photoperiod and the plants enter a rest stage, firstly at 13°C and afterwards at 9°C. In the early stage of plant development shoot formation in plants grown at high temperature was stimulated by lowering the temperature and vice versa. Lowering the temperature caused the rhizomes to branch more, while raising the temperature caused more lateral shoots to develop. Rest started in the lateral rhizomes. Dormancy is prevented or broken by high temperature.

High temperature stimulated shoot formation; there was no evidence of a specific diurnal effect however. High soil temperature also promoted shoot formation. At a temperature of 25°C both flowering and growth were completely inhibited.

Low light intensity decreased the number of shoots produced. It was difficult however to determine the precise effect of light intensity because of the interaction with temperature.

Shoot formation was inversely proportional to daylength. LDs inhibited growth of young shoots as well as the branching of rhizomes and SDs reversed the effect.

FLOWER INITIATION

Low temperature promoted flowering of Alstroemeria; it decreased the number of nodes below the inflorescence and shoots of more proximal origin flowered earlier. At 25°C and above no flowers were initiated. The number and percentage of generative shoots were not a useful-measure of induction as the number of shoots was also affected by temperature and daylength. The length of the temperature treatment during the 24 hour period had-more influence m flower initiation than either the day or night temperature. High soil temperature had a negative influence, this was more pronounced when the air temperature was high.

The number and percentage of generative shoots was reduced by very low light intensity. Increasing daylength decreased the number of nodes under the inflorescence and the number of vegetative shoots per rhizome.

The highest number of flowering shoots were produced at a daylength of 12 hours, which permitted both shoot formation and flower initiation to proceed.

At a given daylength or temperature the further the shoots were from the proximal end of the rhizome, the earlier they flowered.

FLOWERING

In spring when daylength and temperature are favourable for flower initiation Alstroemeria flowers abundantly. The reduction in the number of nodes below the inflorescence is accompanied by a decrease in the number of days to flowering. The flowering period is markedly compressed by the fact that the youngest shoots i.e. those arising at the distal end of the rhizome, form less nodes below the inflorescence than proximal shoots. In addition, the increasing daylength induces earlier flowering. The influence of a given temperature on the flowering time is the result of its effect on flower initiation and rate of development. High temperature inhibits initiation but promotes development.

The number of branches per umbel showed an unequal frequency distribution with a clear preference for 3, 5 and 8. This agrees with the phyllotaxis 3/8, which indicates that flower bud differentiation stops more frequently after initiation of a whole spiral than after a partial one. Numbers of branches per umbel are correlated with thickness of the main stem. The number of branches is influenced by daylength but not by temperature. Numbers of flowers per stalk increased with temperature to 21°C, but the differences were not statistically significant. In the greenhouse during winter most flower buds abort as a result of the poor light conditions.

TUBER FORMATION

Low temperature and LDs promote both flowering and thickening of the roots. During and just after flowering there was a strong increase in tuber growth. This increase of tuber weight can be attributed to resorption of substrates from the old shoots and their storage in the tubers. The nearly simultaneous swelling of the young, short roots just behind the growing points of the rhizome and the retardation of the growth of the young shoots indicate a common factor controlling both effects.

FLOWERING TIME

The main flowering of 'Walter Fleming' grown in greenhouses in the Netherlands occurs in the months of April, May and June, independent of the planting date. Flowering can be advanced by means of artificial lighting, but this should not be continued longer than necessary as shoot formation is retarded. A 4 hour nightbreak of cyclic lighting at an intensity of 100-150 mW over two weeks is suitable. Another method is to extend the daylength to 12 hours, this has only a small adverse effect m shoot formation.

The high greenhouse temperatures in summer are unfavourable for flowering. A good second crop in late summer may be realized using mobile greenhouses, leaving the plants temporarily outdoors where the temperature is lower. In the greenhouses flower initiation occurs by the natural lowering of temperature during late summer and autumn. The success of this late flush however depends on the time of initiation as decreasing light intensity results in flower bud blasting. For this reason it is not desirable to induce flowering in autumn by extending the daylength.

REST

In their native habitat many Alstroemeria species enter a stage of rest during summer, this also occurs in the Netherlands. In 'Walter Fleming' the symptom are highly dependent on temperature and daylength. Low temperature and LD not only promote flowering and tuber growth, but also rest. At high temperature LDs cause only a reduction of shoot growth which disappears in SDs. When plants grow at low temperatures (9 or 13°C) and LDs they enter a state of complete rest, with shoots dying off after flowering. This Test can be broken by a long period of high temperature.

Removing flower buds or shoots and harvesting flowers diminishes the rest period. When old shoots are left on the plants they promote rest.

In the discussion this pattern of reactions of 'Walter Fleming' is compared with that of other crops. The reactions are discussed in relation to the climate in the native habitat of the supposed parent species of this hybrid. Since Alstroemeria presents a wide variation of species growing under extremely different climatic conditions, it is to be expected that hybrids my be bred in which flower production and the flowering period can be controlled more easily.