EFFECT OF CORM STORAGE TEMPERATURE ON THE GROWTH AND FLOWERING OF GLADIOLUS L. IN THE GLASSHOUSE

Małgorzata Zalewska¹, Małgorzata Antkowiak^2
1 Department of Ornamental Plants and Vegetable Crops, University of Technology and Life Sciences in Bydgoszcz, Poland
² Department of Ornamental and Vegetable Plants, University of Technology and Life Sciences, Bydgoszcz, Poland

ABSTRACT

The present research covered the effect of gladiolus corm storage temperature on the growth and flowering of three cultivars: 'Amsterdam', 'Energy' and 'White Friendship', grown in the glasshouse, starting from February 14. Corms of each cultivar were divided into four groups: I – 12-week storage at 17°C, II – 12-week storage at 5°C, III – 6-week storage at 17°C and then 6-week storage at 5°C as well as IV – exposed to preparation treatment at 22-23°C, having been stored for 4 weeks at 17°C. It was demonstrated that maintaining only the temperature of 17°C during corm storage was most favorable, since the plants of all the cultivars flowered at similar dates as those from the corms additionally exposed to preparation. At the same time plant flowering occurred three weeks earlier than the flowering from the corms stored at 5°C. In all the cultivars researched, in each combination, there was recorded a high share of flowering plants, ranging from 84 to 100%. Irrespective of the cultivar, in the group of corms stored throughout the storage period at 5°C or at 17°C, there were reported shorter stems than those from the corms stored at 17°C and then prepared. The corm storage temperature did not affect the spike length, the number of florets and foliage leaves or the stem diameter.

Key words: Gladiolus, corm storage temperature, corm preparation.

INTRODUCTION

Gladioli can constitute a perfect ornamental piece of our gardens late in summer, depending on the cultivar earliness, mid July through mid September, as well as give a great look as cut flowers in vases. Flowering plants can be produced much earlier or later than at their natural flowering date outdoors. Such a possibility is offered by forced or delayed cultivation. Forced cultivation makes it possible to produce flowering plants as early as May or in June [6,14], while white gladiolus cultivars can well replace lilies, gillyflowers, freesias or roses of the same color in wedding and first-communion decorations. The cost-effectiveness of the forced cultivation is determined mostly by the flowering date, flower yield per area unit and flower quality. To produce the plants which would flower earlier than the second half of summer, flowers are grown under shields. The flowering date can be forced by applying different methods. The most frequent method is preparing corms prior to planting [6,14]. The treatment involves the corm storage at 30-33°C for 4 weeks, 25-30°C for 6 weeks or 22-23°C for 8 weeks, at the relative air humidity of 50-60% [7]. The treatment aims at accelerating the bud growth and root radicles. It can be also well-combined with retardant application. When grown under shields, gladioli usually produce stems which are too long, while corm soaking in these growth regulators prior to planting is an effective method limiting their growth [1,16]. An excessive height makes cultivation, nursing and harvest more difficult. Excessively long stems can result in plant breaking, which can lead to their total disqualification. Applying retardants, it is not necessary to provide beds with supporting nets which support plants and thus prevent them from breaking-off. Additionally, gladioli with shorter stems are much easier to pack and to transport further. They are then less likely to get damaged. Besides, the preparation treatment performed during corm soaking in the retardant solution enhances the share of flowering plants and delays the flowering date less considerably [12,13]. Accelerating flowering in gladiolus is also possible thanks to cultivation under warmed soil conditions. Boulard and Baille [2] demonstrated that increasing the temperature of the substrate at the beginning of cultivation, during underground growing period, forced rooting and stimulated the overground plant growth. As a result, flowering is 2-3 weeks earlier. Grabowska [7], however, suggests placing gladiolus corms in boxes filled with peat or in earthen pots, paper pots or pots with pierced bottom in the room at 16-18°C, and then planting rooted plants on permanent spot before producing contractile roots. Yet another possibility of shortening the cultivation period is offered by applying higher temperature during corm storage. Escher [4] recommends initially maintaining the temperature of 17°C, and then 20°C, as it affects the growth of gladioli after planting and then indirectly flowering [3]. According to Zalewska and Woźny [17], the application of gibberellic acid by soaking the corms prior to planting can also accelerate flowering in gladioli by an average of one week. It is possible, since gibberellins initiate the flower budlets formation and stimulate flower development [11]. Most probably, however, it is a quality which is cultivar-specific since the gladiolus cultivars reported reacted to the treatment differently. The longer the treatment, the bigger the number of flowering plants.

Delayed gladiolus cultivation is much less common, which is due to a much lower demand for these plants autumn through winter. To delay the flowering date, it is best to plant smaller corms, about two weeks later than in the case of field cultivation [9] and to increase the planting density [8].

During storage, from harvest to planting, gladiolus corms are in dormancy, initially absolute and then relative dormancy. Its depth is, e.g. cultivar-specific; early cultivars end it earlier than the late ones, and it depends on the corm size; bigger corms before the smaller ones. The absolute dormancy disappears at the temperature ranging from 6°C to 27°C, while to keep corms in relative dormancy, it is necessary to maintain low temperature [7].

An adequate corm preparation, cultivar selection and meeting the growing requirements make it possible for gladioli to flower as early as the beginning or in mid-May.

The present experiment aimed at evaluating the effect of corm storage temperature on the growth and flowering of gladioli grown in the glasshouse under forced growing conditions.

MATERIAL AND METHODS

The research was carried out over 2006-2007. The first stage, which covered corm storage, took place from November 21, 2006 to February 13, 2007 at the Department of Ornamental Plants and Vegetables Crops of the University of Technology and Life Sciences in Bydgoszcz, while the second one, concerning heated glasshouse cultivation lasted from February 14 to July 20, 2007 on 'Florita' Horticultural Farm at Grzybno, in the vicinity of Unisław. The research involved three gladiolus (Gladiolus L.) cultivars: 'Amsterdam', 'Energy' and 'White Friendship'; all of them with white spikes due to a potential increase in the supply of white flowers May through June. Corms treated by soaking for 0.5 hours in the mixture of Merpan 50 WP (1.5%), Sarfun 500 SC (0.5%), Topsin M 500 SC (0.7%), Sumilex 500 SC (0.5%), and Actellic 500 EC (0.1%) were stored for 12 weeks before being planted in different temperature conditions. Each combination was represented by 25 corms representing the first-size group, with the circumference over 14 cm. Corms of each cultivar were divided into four groups. The first one included corms to be planted after 12-week storage at 17°C, the second one – the corms stored for 12 weeks in the cool-house at 5°C, while the third group was stored at the combination of two successive temperatures: 6 weeks at 17°C and the successive 6 weeks at 5°C, and the fourth one – exposed to preparation after 4-week storage at 17°C. Preparing involved 8-week corm storage, starting from December 19, 2006, in the room at 22-23°C and mean relative air humidity of 60%. Covering corm scales were removed to make the warm air reach the buds better and to make them grow freely up to the length of 1-3 cm. The same was performed for non-prepared corms. Prior to planting out, all the corms were soaked for 0.5 hours using the preparations applied before. Additionally, soaking eliminated the effects of an excessive drying of corms treated with high temperature. They were planted out into the growing beds on February 14, 2006 in 8-9 cm deepth. The distance between rows was 15 cm, between corms – 8 cm, which ensured the plant density of 64 pcs·m^-2. The growing medium was made up of a composition of peat, sand and compost in the volume ratio of 5:2:1 of pH 6.0. The temperature of the medium over planting was 12°C. The plants were fertilized following the shielded gladiolus guidelines. Over the cultivation the average temperature of medium was 15.4°C, the air temperature was 16.9°C, while the average relative air humidity – 79.3%. To produce a single spike of high quality, the plants were grown for a single stem only by breaking-off the others at the corm base at an early growth stage. The beginning of plant flowering was considered to be the moment when 3-4 florets per spike were colored. The other growing and nursing treatments were performed following the technology recommended for glasshouse gladiolus growing. The stems were cut off right over the growing medium surface.

The average flowering date was determined based on the weighted mean. With that, the number of days was calculated from corm planting to flowering. There was calculated a percentage share of flowering plants. The following measurements were made: stem length, spike length, stem diameter at the cut plant base, with the use of the slide caliper, the length of foliage leaf, namely, the highest reaching one. There was also determined the number of florets per spike and the number of foliage leaves. The numerical data recorded expressed as a percentage were converted following Freeman-Tukey to be statistically verified. The results were statistically verified with the analysis of variance, while the means were compared with the Tukey's test at the significance level α = 0.05.

RESULTS

The cultivars researched started flowering at the end of May ('Energy') and at the beginning of June ('Amsterdam' and 'White Friendship'). The plants grown from the corms stored all the time at 5°C flowered last, while the corms stored at 17°C produced plants flowering at similar date as those additionally prepared – on average more than 3 weeks earlier in 'Amsterdam' and 'Energy' and 19 days earlier in 'White Friendship'. Treated for 6 weeks with the temperature of 17°C and the successive 6 weeks – with 5°C delayed the flowering by 18 days in 'Amsterdam', 11 days in 'Energy' and 12 days in 'White Friendship', as compared with the plants whose corms were stored only at 17°C or prepared after an earlier storage at 17°C (Figs. 1-3).

In all the three plant cultivars, the share of flowering plants from the corms stored at 17°C and additionally exposed to preparation accounted for 100%. A slightly lower share of flowering plants was recorded in the plants whose corms were stored only at 5°C or 17°C (on average 93%). Irrespective of the cultivar, the corm treatment with the temperature combination (first high temperature and then low temperature) decreased the share of flowering plants to 87% (Table 1).

The spike quality is determined e.g. by the plant height connected with the spike length and the number of florets per spike. The longest stems were produced in 'Amsterdam', while the shortest ones – in 'Energy'. Irrespective of the cultivar, the gladioli whose corms were stored for 12 weeks at 5°C and for 12 weeks at 17°C reached a similar length. On average the stems were 7.5 cm shorter than the ones grown from the corms exposed to 17°C and additionally prepared. There was found, however, no difference between these groups and the corms stored for 6 weeks at 17°C and for successive 6 weeks at 5°C. The statistical analysis of the results did not demonstrate such reaction in any of the cultivars individually (Table 2).

The corm storage temperature did not affect the diameter of the flowering stem. The smallest diameter was recorded in 'Amsterdam'; the diameter in 'Energy' and 'White Friendship' was similar (Table 2).

Similarly as in the case of stems, the longest spikes were produced by 'Amsterdam'. 'Energy' and 'White Friendship' produced spikes of a similar length. The storage temperature did not affect their length; neither did it affect the number of florets per spike. Different temperature conditions did not cause any changes in the number of florets per spike (Table 3).

The corm storage temperature affected the length of the highest reaching foliage leaf in two cultivars: 'Amsterdam' and 'Energy'. In 'White Friendship' no such relationship was noted. The shortest leaves in 'Amsterdam' were produced from corms exposed to 5°C throughout the storage period; on average they were 8 cm shorter than those grown from corms treated with the temperature of 17°C and additionally prepared and 7.5 cm – shorter than those grown from the corms initially stored at the temperature of 17°C, and then at 5°C, unlike in 'Energy'; the plants grown from the corms stored at 5°C produced the longest leaves. They differed significantly from those grown from the corms stored exclusively at 17°C and from the additionally prepared corms (Table 4). There was recorded no significant effect of the corm storage on the number of foliage leaves (Table 4).

DISCUSSION

Accelerating gladiolus flowering under shields has been present for a long time. The period of their availability is prolonged and thus the assortment of plants on the florist market in spring  has been supplemented. Differentiating gladiolus flowers takes place only after sprouting and it starts when 3 to 7 leaves are visible [3]. One cannot, therefore, force gladiolus the way tulips can be forced; in gladiolus one can only speed up flowering. It is possible, e.g. by applying the treatment of preparing corms prior to planting [7]. The reports by Grabowska [6] demonstrated that gladioli grown in the heated glasshouse the corms of which, prior to planting, were treated for 6 weeks with the temperature of 20-24°C flower earlier than those exposed to 5-8°C throughout the storage period. Other experiments showed that this acceleration could be even up to three weeks [14,15], which has been also confirmed by the present research. Additional corm preparation prior to planting, after corm storage at 17°C, forced the plant flowering of all the cultivars considerably, but the flowering date did not differ significantly from those exposed to 17°C throughout the storage period. Serocka and Zalewska [14] did not identify any such relationship. Gladioli exposed to the preparation treatment flowered from 11 to 20 days earlier than those stored all the time at 16°C. In the present research shortening the cultivation time was similar in both cases. The greatest flowering acceleration was recorded in 'Amsterdam' – by 25 days, in 'White Friendship' by 24 days, and in 'Energy' – by 19 days, as compared with gladioli the corms of which were all the time exposed to 5°C. Decreasing the corm storage temperature from 17°C (6 weeks) to 5°C (6 weeks) delayed the flowering, as compared with those exposed throughout the storage period to 17°C or additionally prepared; on average: by 18 days in 'Amsterdam', 11 days in 'Energy' and 12 days in 'White Friendship'. According to Zalewska et al. [15], the highest share of flowering plants is recorded from the corms exposed to preparing, which coincides with the present experiment results. In all the cultivars researched, 100% of the corms additionally treated in such way flowered. As reported by Grabowska [6], gladiolus corm storage at the temperature of 5-8°C resulted in the elongation of stems and spikes, as compared with those stored at 20-24°C, which does not coincide with the results of the present research. Irrespective of the cultivar, the corms stored at 5°C produced significantly shorter stems than those additionally exposed to preparation. According to Cohat [3], the differences in the plant growth must have been due to the fact that corms over the planting period must have been in different physiological states. Shilo and Simchon [12], on the other hand, showed that decreasing the temperature to 5-10°C, following the storage at the higher temperature (30°C), decreases the spike quality. In the present research there was observed no effect of the corm storage on the spike length. A similar experiment was reported by Spanish research which involved Gladiolus tristis [5] who found that corm treatment for 9 and 6 weeks with 18-25°C, and then for 3 and 6 weeks at the temperature of 5°C, accelerated the plant flowering by 20 or by 11 days, as compared with the corms stored only at 18-25°C and inhibited the leaf length increase. In the present experiment the results have been opposite; the exposure to the temperature of 5°C following an earlier storage at 17°C delayed the flowering and the leaf length increase. Gladiolus tristis, however, is a species flowering under natural conditions very early, already in February, unlike garden gladioli grown in Poland which, when planted outdoors, flower as late as in the second half of summer.

Cohat [3] demonstrates that gladiolus growth and development is forced by the following sequence of temperatures: first low (over 10°C) which activates growth, disturbing the dormancy, and then high (over 20°C), which has a directly differentiating effect and affects the bud growth. According to Grabowska [7], absolute dormancy disappears at 6-27°C, while for the corms to remain in the relative dormancy low temperature is required, which can account for the present results. Gladiolus corm dormancy is broken by both low and high temperature. Most probably higher temperature breaks it earlier, while low temperature – later, which coincides with the reports by González et al. [5] claiming that storage at the temperature of 18-25°C can be sufficient to break the gladiolus corm dormancy. Similar results were reported in the present experiment. The plants the corms of which were stored at the temperature higher than 5°C flowered much earlier, and so the temperature below 10°C does not seem to be necessary to break dormancy. According to Hosoki [10], it is connected with the production of endogenous growth regulators responsible for its dormancy. In his experiment the content of growth inhibitors decreased more rapidly during storage at the temperature of 30°C than of 5°C, while the content of auxins increased.

The results of the present research demonstrate that gladiolus corms can be well stored for 12 weeks at the temperature of 17°C loose in boxes (with no medium) and with no need of planting in pots. Neither is the suggestion made by Escher [4] confirmed; what Escher proposes is gladiolus corm storage from November for 6 weeks in the room at the temperature of 17°C and for further 6 weeks at the temperature increased up to 20°C. Corm storage throughout the storage period at 17°C makes it possible to obtain a high share of flowering plants, high-quality flowers and to shorten the growing period considerably, which is very important for forced growing of this species. One can produce plants with stems similar in length as those from corms stored at the temperature of 5°C and, at the same time, significantly shorter than those grown from the corms stored at 17°C, and then exposed to preparation. From the economic point of view, to ensure that the corms are stored for 12 weeks only at 17°C seems to be more justifiable than their storage for 4 weeks at 17°C, and then exposure to preparation for 8 weeks. Besides, this corm storage method is feasible, e.g. under window-sills in the glasshouse while growing other plants in winter. The temperature of 17°C conditions a better corm drying, as compared with the storage at 5°C, and thus poses less risk of fungal diseases infection.

In all the cultivars researched, the corm storage temperature affected the flowering date the same way and thus one can assume that it is a trait which is not cultivar-specific, while the typically cultivar-specific trait can be the effect of corm storage temperature on the biometrics, which suggests that it is worth verifying upon introducing new cultivars to forced growing.

CONCLUSIONS

1. The temperature of 17°C, throughout the corm storage period, accelerates the flowering of 'Amsterdam', 'Energy' and 'White Friendship' plants similarly as additional preparation. The plants flower by an average of 3 weeks earlier than those grown from the corms stored at the temperature of 5°C. The highest share of flowering plants is recorded from the corms stored at 17°C and then exposed to preparation.

2. Irrespective of the cultivar, corm storage at 17°C results in the stem growth inhibition as compared with the plants grown from the corms additionally exposed to preparation. From the corms stored at 17°C one obtains plants with shorter foliage leaves as compared with those stored at 5°C.

3. The corm storage temperature affects neither the length of spikes nor the number of florets per spike. Nor does it influence the number of foliage leaves or the stem diameter.

REFERENCES

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SPECIAL THANKS

We hereby address Mr Piotr Pukszto, MSc, the owner of 'Florita' Horticultural Farm, for providing gladiolus corms used in the present research and for making place in the glasshouse available for growing free-of charge.

Accepted for print: 23.10.2008

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.