GROWTH AND FLOWERING OF ORNITHOGALUM DUBIUM

Ornithogalum dubium is a frost-tender bulbous plant native to South Africa. It is mainly grown for cut flower and flowering pot-plant production. It generally produces 10-25 cm-long flowering-stems, bearing 5-25 yellow to orange flowers with a dark green/brown center. It is in great demand in Europe and North America as a cut flower and a flowering-house pot plant. O. dubium was introduced as a new cut-flower crop less than a decade ago and is still only grown on a small scale due to its variable flower quality. The main objective of the present study was to improve flowering-stem length. A three week preplanting temperarture treatment of 13C resulted in significantly longer flowering-stems than treatments at 2, 9 or 25C. Controlled growth-temperature and day length experiments indicated that warm day/night tempertures of 27/22C induce early anthesis. Under this temperature combination, earliest flowering occurred under long day conditions. Stem length was maximal at moderately-low production temperatures, and long days further increased length. The number of florets per inflorescence depended on temperature. Under the two lower temperature regimes, many flowers developed per inflorescence and, again, long days enhanced this number. However under the highest temperature regime tried, this effect was reversed. Bulbs of different size were planted and even the smallest, flowered during the first year. Leaf production was proportional to planted bulb size. The rate of growth, time to flowering and total yield of daughter bulbs also depended on size of the planted bulb. Gibberellin, applied either as a preplanting dip or as foliar sprays, moderate shading (20%), as well as, a 24-h pre-planting exposure of bulbs to 10 ppm ethylene all increased flowering-stem length. INTRODUCTION Ornithogalum (Family Hyacinthaceae, formerly Liliaceae), a genus of some 150 species of bulbous plants mainly found in the Mediterranean (and also in the temperate) regions of Africa, Asia and Europe (Bryan, 1993). Ornithogalum dubium Houtt. is native to South Africa. It is a frost tender plant grown mainly for cut flower and flowering pot-plant production. The inflorescence is generally a 10-25 cm long raceme, bearing 5-25 yellow to orange flowers with a dark green/brown center (Du Plesis and Duncan 1989). These flowering-stems are in great demand in Europe and North America as cut flowers. The plants are also gaining popularity as a potted flowering-plant. O. dubium was introduced as a new cut-flower crop less than a decade ago and it is still grown on a small scale due to several limitations. Some of these are related to the availability of high-quality planting material. Propagation from seed introduces wide genetic variability, whereas propagation from bulbs is more expensive and results in fast accumulation of virus disease. The main problem however is the often poor flower quality characterized by short flower stems which are unacceptable on the cut flower market. Proc. 8th Int. Symp. on Flowerbulbs Eds. G. Littlejohn et al. Acta Hort. 570, ISHS 2002 114 Growth and flowering of all commercially grown geophytes are significantly affected by environmental conditions at all stages of their development. These can be manipulated for controlled and predictable production (De Hertogh and Le Nard, 1993). Bulb storage, forcing treatments, temperature, daylength and light intensity during growth, as well as, the use of plant growth regulators and shading, have all been used to improve flower quality which also includes elongation of the floral stem (De Hertogh and Le Nard 1993, Halevy 1990). Since little has so far been published for O. dubium (De Hertogh and Gillitano 1997), the objective of the present study was to examine the effects of production temperature, daylength and light intensity, gibberellin and ethylene treatments, as well as bulb size, on growth and flowering of O. dubium. The work was conducted over 4 years. MATERIALS AND METHODS All bulbs used in the following experiments were lifted in July after one season's growth from seed. They were stored in a well ventilated shed for 150 days, under ambient conditions with a daily average temperatures ranging between 25 and 30°C. Bulbs were planted in early November. 1. Precooling Temperature. Bulbs sized 2-3 cm in circumference, were precooled for three weeks at 2, 9, 13 or 25°C. Twenty five bulbs per treatment replicated four times were planted in sandy loam, in unraised beds in an unheated, plastic clad greenhouse. Flowering-stem length was measured at anthesis of the first floret. 2. Precooling duration. In the following year, the effect of precooling duration, ranging from 0 to 8 weeks, was tested at 13°C, as above. 3. Effect of Growth Temperature. Bulbs stored for 150 days and precooled at 13°C were used for this experiment which was conducted in The Hebrew University Phytotron at Rehovot. The temperatures in the different units were:32/27, 27/22, 22/17 and 17/12C (day/night) with a deviation of up to 0.5°C. Light conditions in the units were either 8 h of daylight (short day) or 8 h of daylight plus 8 h of incandescent light at 5 mmol m s (long day). Twenty bulbs (2-3 cm in circumference) were planted individually in 15 cm pots for each treatment. Days to flowering, floweringstem length and the number of florets per stem were recorded. Growth Regulator Treatments 1.Gibberellin and Shading. Stored and precooled bulbs were planted in a greenhouse as described above. The treatments were:a 20 min preplanting bulb dip in a 100 ppm GA3 solution, a single canopy spray with a 100 or 200 ppm GA3 solution when plants were either at the rosette stage (~30 days after planting), or, when the flowering-stem had reached a length of 5 cm (~ 60 days after planting). Shading treatments to reduce natural light intensity by 20 to 40% were combined with the GA treatments. Twenty bulbs were replicated four times per treatment. 2.Ethylene. Twenty bulbs weighing 2g each which were stored for 150 days were held in the presence of 10 ppm ethylene for 24 h at room temperature prior to planting. Sprouting date, leaf length at flowering and flowering stem length were recorded. 3. Effect of Bulb Size on Growth and Flowering. Eighty stored and precooled bulbs were weighed individually and planted at random in a large polystyrene box placed in an unheated greenhouse.To facilitate recording of data, each bulb position in the box was marked. The number of leaves per plant 65 days after planting, days to flowering, and the total weight of daughter bulblets per plant, were recorded at lifting. RESULTS Following 150 days of storage and a three week dry, precooling treatment at 2, 9, 13 or 25°C, all planted bulbs flowered well. Stem length varied with temperature, the 13°C treatment resulting in significantly longer flower stems than in all other temperatures tried (Table 1). The duration of precooling at 13°C did not affect the time from planting to flowering which was 20 ± 1 weeks. However, just a short two week precooling treatment resulted in. significantly longer flowering-stems over those produced by uncooled bulbs. Maximum stem lengths were obtained