Flowering Behaviour of Helichrysum obconicum DC.

Transcription

1 Flowering Behaviour of Helichrysum obconicum DC. M.J.Oliveira Dragovic Direcção Regional de Agricultura, Centro de Floricultura, Lugar de Baixo, Ponta do Sol, Madeira Portugal Keywords: Madeira, endemic, ornamental, photoperiod, temperature Abstract Helichrysum obconicum is an endemic plant of Madeira Island (Portugal) that has already shown potential to be used as ornamental plant. It is a small shrub with grey silver leaves and small yellow golden flowers. The mechanisms involved in the flowering of this species are not well known. Under natural conditions, plants of H. obconicum start to flower in July-August, and the reproductive period extends until October-November. In January, plants were subjected to the same temperatures (night temperature of 17ºC and 23ºC as day temperature) occurring in nature during the reproductive period and to a photoperiod of 16 hours of light/8 hours of dark. After 3 months they started to flower, while those under natural conditions stayed in vegetative stage and only started to flower in July. A second experiment was conducted to understand if the flowering induction was due to the day length. At the end of March 1, 122 plants were submitted to natural day length (control) and 122 plants to continuous long days (16 hours of light per day), both of them under the same temperature regime ( ºC). Plants under permanent long day conditions started to flower after 2 months and maintained the reproductive stage until the end of the experiment (March, 3). The control plants started to flower in July 1 and the reproductive period ended by December 1. In the following year, the reproductive period was observed from June 2 to December 2. The results obtained from the above experiments lead us to the conclusion that Helichrysum obconicum is a long day plant. INTRODUCTION Helichrysum obconicum is an endemic plant from Madeira Island (Portugal) that has potential to be used as an ornamental plant of pot and garden. It is a small grey white-tomentose shrub that can reach a height of 6cm, and it can be found on coastal rocks and sea cliffs. The leaves are 25-5 x 8-mm, obovate to elliptic, petiolated and tomentose on both surfaces. The flower is an obconical capitulum, 5-7 x 6-8mm, with golden-brown involucral erect bracts and not scented yellow florets (Vieira, 1992; Press and Short, 1994). Under natural conditions or in cultivation, plants of H. obconicum are in the reproductive period from July to November; during this period plants exhibit yellow golden flowers that add to the silver grey canopy another attractive feature. The average maximum and minimum temperature of the air during this period ranges between ºC and ºC, respectively (Dragovic et al., ). The mechanisms involved in the flowering of this species are not well known. The purpose of this study was to determine the conditions needed for the flowering of H. obconicum plants. MATERIALS AND METHODS The experiments were conducted in the Floriculture Centre (Lugar de Baixo, Ponta do Sol), located at -4m above sea level. Air temperature data were obtained from the Meteorological Station of Lugar de Baixo. Proc. V th IS on New Flor. Crops Eds.: A.F.C. Tombolato and G.M. Dias-Tagliacozzo Acta Hort. 683, ISHS 5 357

2 Experiment 1 In January, plants of H. obconicum were transplanted to plastic pots, 15.5cm in diameter and 11cm high, containing a layer of expanded clay at the bottom, and filled with a mixture of peat and perlite in the proportion of 1:1. These plants were rooted in the previous year and they were pruned in order to promote branching. On 26 January, the average canopy height was -3cm and all the plants were in the vegetative state. Plants were grown under two different conditions: in open field under natural conditions (T) or in a growth chamber with day and night temperatures of 23ºC and 17ºC, respectively, long day (LD) conditions (16hours light/8hours dark), and a relative humidity of 7% (T1). Seventy plants were used for each treatment. Plants were monthly observed and the number of plants with flower was recorded until October. The effect of the treatments in the number of weeks until flowering was determined by Kruskal-Wallis test and to compare means, the Dunn test was used, with a significance level of p=.5. Experiment 2 Adult plants - Rooted cuttings of H. obconicum were transplanted in 1999 to plastic pots, with 13.5cm in diameter and 11cm high, containing a layer of expanded clay at the bottom, and filled with a mixture of peat and perlite in the proportion of 1:1. In December all the plants were pruned in order to promote branching. At the end of March 1, the plants were transplanted to bigger pots, 21.5cm in diameter and cm high and they were grown in the open field, some under natural day (ND) conditions and others under continuous long day (LD) conditions. The long days were achieved by cycles of 7.5min of light supplied by incandescent lamps of 15W followed by 7.5min of darkness, from 19.3 to 7.3. For each condition, natural day (ND) and long day (LD), 96 plants were used. Young plants Plants obtained from seeds germinated between December and February 1 were transplanted to plastic pots, 18cm in diameter and 16.5cm high, containing a layer of expanded clay at the bottom, and filled with a mixture of peat and perlite in the proportion of 1:1. Plants were grown under the same natural day (ND) and long day (LD) conditions of the adult plants (26 plants per treatment). All the plants were monthly observed until March 3 and the number of flowering plants was recorded. RESULTS Experiment 1 Plants subjected to the same temperatures occurring in nature during the reproductive period and to LD conditions (T1), started to flower after three months (April ) (Fig. 1). Plants under natural conditions (T) started to flower in June ; by that time, the average maximum and minimum temperatures of the air were 24.8 and 17.6ºC, respectively, and 14.3 hours of daily light (Table 1). There was a significant difference in the number of weeks to flower between both treatments (23.5 and 17.4wks, for T and T1, respectively) (Table 2). The LD and higher temperatures led to an advance of almost 2 months in the flowering of H. obconicum plants, when compared to those under natural conditions. More than 5% of flowering was obtained after 4 and 6 months for T1 and T plants, respectively (Fig. 1). Experiment 2 The adult plants grown under natural conditions (ND) started to flower in July 1, and the reproductive period ended in December 1; afterwards, the plants were in vegetative state (Fig. 2). In the following year, the reproductive period occurred from June to December 2. Fig.3 shows the average maximum and minimum air temperature occurring during the experiment. 358

3 Adult plants of H. obconicum grown under LD conditions started to flower after two months (Fig. 2). In December 1, % of flowering was achieved, and afterwards plants were continuously in flower until the end of the experiment (March 3); each plant was showing simultaneously buds, florets and seeds. The young plants grown under continuous LD conditions developed well vegetatively, and only started to flower after 5 months (Fig. 2). In October 1, more than 5% of the plants were in the flowering state, and in March 2 all the plants were continuously presenting flowers, buds and seeds; they kept themselves in reproductive state until the end of the experiment (March 3). The young plants grown under ND conditions developed well, but only started to flower in the following year (June 2); the reproductive period ended in December 2 (Fig. 2). DISCUSSION AND CONCLUSIONS Helichrysum obconicum is a long day plant. Helichrysum obconicum can be forced to flower by subjecting the plants to LD conditions. To get more than 5% of flowering in adult plants, they must be submitted to LD conditions for at least 3-4 months; continuous LD conditions will lead to plants in continuous reproductive state (continuous cycles of buds, florets and seeds). When applying LD conditions to young plants (2 months old), 7 months are needed to obtain more than 5% of flowering plants; there is a delay of 3 months comparing with the adult plants. To be ready for flowering, plants had to be at least 7 months old. Under natural conditions, seedlings of H. obconicum grown from January to June did not flower in June/July of that year, maybe because plants were in an early phase of growth during which flowering cannot be induced. Plants were ready to receive the flowering stimulus only in the following year, when they flowered from June to December. The control and manipulation of Helichrysum obconicum flowering broadens its use as foliage and flowering plant. ACKNOWLEDGEMENTS To CITMA (Centro de Ciência e Tecnologia da Madeira) for financing the presentation of this work. Literature Cited Dragovic, M.J. Oliveira, Ferreira, S.G. and Neves, M.C.. Utilisation of Helichrysum obconicum DC. as ornamental plant: preliminary studies on pot adaptation. Acta Hort. 541: Press, J.R. and Short, M.J Flora of Madeira. HSMO, London. Vieira, R Flora da Madeira. O interesse das Plantas Endémicas Macaronésicas. Serviço Nacional de Parques, Lisboa. 359

4 Tables Table 1. Average maximum and minimum air temperatures and daylight hours in Ponta do Sol (source: Lugar de Baixo Meteorological Station, from January to October ). Maximum temperature of the air(ºc) Minimum temperature of the air (ºC) Daylight (hours) January February March April May June July August September October Table 2. Number of weeks until flowering of Helichrysum obconicum plants subjected to different temperature and daylength treatment (mean + standard error). Treatment T natural conditions T1 23/17ºC (day/night) + 16hrs light/day Weeks to flower a b 1 Means with the same letter are not significantly different at p=.5 according to Dunn Test. Figures % of flowering plants Jan 25-Feb 26-Mar 25-Apr 25-May 24-Jun 24-Jul 23-Aug 22Sep 22-Oct Time (months) Fig.1. Flowering of Helichrysum obconicum plants grown under natural conditions (-) and under continuous day/night temperature of 23/17ºC and 16hours of light per day ( ). 36

5 % of flowering plants Mar-1 Apr-1 May-1 Jun-1 Jul-1 Aug-1 Sep-1 Oct-1 Nov-1 Dec-1 Jan-2 Feb-2 Mar-2 Apr-2 May-2 Jun-2 Jul-2 Aug-2 Sep-2 Oct-2 Nov-2 Dec-2 Jan-3 Feb-3 Mar-3 ND-adult plants ND-young plants LD-adult plants LD-young plants Fig. 2. Flowering of adult and young plants of Helichrysum obconicum grown under natural conditions (ND) and under continuous long day conditions (LD). Air temperature (%) Mar-1 Apr-1 May-1 Jun-1 Jul-1 Aug-1 Sep-1 Oct-1 Nov-1 Dec-1 Jan-2 Feb-2 Mar-2 Apr-2 May-2 Time (months) Jun-2 Jul-2 Aug-2 Sep-2 Oct-2 Nov-2 Dec-2 Jan-3 Feb-3 Mar Daylight hours Maximum temperature Minimum temperature Daylight hours Fig. 3. Average maximum and minimum temperature of the air in Lugar de Baixo, Ponta do Sol (source: Meteorological Station of Lugar de Baixo, from March 1 to March 3). 361