THE INFLUENCE OF NEW METHODS OF CORM COATING ON FREESIA GROWTH, DEVELOPMENT AND HEALTH

Abstract

Recently in agriculture, there has been a growing interest in new proecological technologies whose main aim is the protection of biological material against negative effects of direct contact with the environment. We examined the effect of coating freesia corms with natural polysaccharides such as chitosan and its derivatives on growth and plant health. Two experiments were conducted under covers, to test how particular coating methods affect the length of the vegetative and generative stages, the quality and number of flowers, plant health and corm quality. Vegetative and generative parts of the plants were measured, leaf green index was determined and the leaves and corms were analyzed to detect viruses and fungal diseases e.g. Fusarium sp. It was found that, irrespective of the coating method, encapsulated plants emerge earlier and their vegetative stage was 10-14 days shorter in comparison with control plants. Although the effects on the flower quality and plant health varied and depended on the kind of coating chemistry, the results for either of the coating variants were inferior to the results in control plants. The studies show that coating of corms clearly improves their health, reduces selected fungal infection, inhibits the development of fungal pathogens, reduces and eliminates, depending on the coating chemistry, some virus and fungal occurrence. The results of DAS-ELISA test for freesia corms for some coating chemistries were the same or slightly differed from the control, untreated plants, whereas corms produced from corms without coating showed virus infection. Coating turned out to have no influence on virus presence in freesia leaves. INTRODUCTION Corms used for freesia reproduction must be regenerated every few years due to their ageing process, decreasing vigour and increasing problems associated with diseases and pests (Bach, 1992). The rate of senescence and quality deterioration depends both on cultivar characteristics and growing conditions as well as corm reproduction methods. The production of healthy corms, their preparation in order to obtain even, abundantly flowering, good quality plants as soon as possible are of utmost importance in freesia cultivation (Imanishi, 2002; Startek et al., 2002). In recent years there has been a growing interest in using various types of coatings as an effective method of protecting seeds, bulbs and corms against negative effects of direct contact with exterior environment. Despite widely-described improvements in immobilisation technologies, the problem of finding appropriate substrates and reaction conditions compatible with natural materials remains unsolved. The majority of recommended agrotechnical coating methods concern the application of coat polymers for Proc. IX Intl. Symp. on Flower Bulbs Eds.: H. Okubo, W.B. Miller and G.A. Chastagner Acta Hort. 673, ISHS 2005 612 covering granulated mineral fertilizers. However, both the synthetic polymers and coating conditions are not suitable for coating plant corms and seeds. In horticulture, growth stimulants and substances enhancing the natural plant resistance to pathogens, including ionic polysaccharides such as chitosan (Pospieszny et al., 1991), have aroused great interest. The literature on the application of chitosan in the cultivation of some ornamental plants indicate that it can strongly inhibit disease development, stimulate plant growth, shorten developmental cycles and affect plant quality (Ohta et al., 1999; Saniewska, 2001). Chitosan’s molecular weight is one of the most important parameters affecting its biological properties. At the moment there is no available information concerning the relationship between its molecular weight and its effects on plant properties. The aim of the studies commenced in 2002 was to examine the effects of freesia corm coating on their health, the course of developmental stages, basic morphological traits, flowering quality and corm yield. Natural anion polysaccharides together with chitosan varying in molecular weight were used for corm coating. MATERIALS AND METHODS Two experiments were conducted on “Easy Pot” freesia ‘Gompey’ in Experiment 1 and ‘Popey’ in Experiment 2. The plants were grown in a foil tunnel from June to mid December without any summer cooling. For both the experiments, the corms (>5 cm) had been stored for 12 weeks at 18-28°C. Prior to planting they were coated according to the technology described in Polish Patent Application P 359797 (Bartkowiak et al., 2003). Four coating variants were made using polysaccharides: 1% gellan and 1% iota-carrageenan in the solution of which corms were dipped for 30 seconds and 2 kinds of chitosan solution containing 0.2% of molecular weight 2,000 and 100,000 g mol in which the corms were soaked for 10 minutes. Sodium salts were used for coating in Experiment 1 and iron salts (III) in Experiment 2. Control corms were soaked in distilled water. Freesia corms were planted into peat substrate supplemented with minerals with a spacing of 12.5 cm x 10 cm, in a randomised block design. Every experimental object consisted of 40 corms, 10 per each replication. Multicomponent fertiliser was used to maintain the nutrient content at the level recommended for freesia. During each experiment, observations were made concerning the course of developmental stages, i.e. emergence, heading, flowering and the end of vegetation. Morphological characteristics of the plants were measured, e.g. in generative stage – the length and number of inflorescences, diameter and number of flowers per inflorescence. At the beginning of flowering the leaf greening index, which is closely correlated to chlorophyll content, was measured using a chlorophylmeter Minolta SPAD-502. After vegetation had died and corms were lifted, their weight and number were determined and the coefficient of increase in weight and number were calculated. Twice during the experiments, at full blooming and after death of the vegetation, the health of the corms were assessed. DAS-ELISA tests were conducted to identify the presence of freesia mosaic virus (FMV), using method described before (Kaminska, 1991). Samples of corms and leaves were homogenised with extracting buffer in 1:10 ratio. The results were based on spectroscopic measurements (405 nm) using Multiscan apparatus. Disinfected corm surface sterile fragments of parenchyma were also taken and inoculated on potato dextrose agar (PDA) on Petri dishes on PDA with streptomecyne. Petri dishes were incubated for 5-10 days at 24-25oC and 90-95% humidity. Fungal colonies appearing successively around parenchyma were transferred on PDA and after 20-days incubation, identified according to Domsch et al. (1980). The results of morphological characteristics and coefficient of corm weight and number increase were subjected to analysis of variance and Tukey test at 5% level of confidence (Table 1-2). The course of developmental stages depending on experimental object is presented in Fig. 1-2 and the evaluation of corm health in Table 3.