Simulating the vernalization response of the "Snow Queen

Vernalization response of the "Snow Queen" lily 117 SIMULATING THE VERNALIZATION RESPONSE OF THE “SNOW QUEEN” LILY (Lilium longiflorum Thunb.) Nereu Augusto Streck*; Mariângela Schuh UFSM/CCR - Depto. de Fitotecnia - 97105-900 - Santa Maria, RS - Brasil. *Corresponding author <> ABSTRACT: Vernalization is a process required by certain plant species, including lilies (Lilium spp.), to enter the reproductive phase, through an exposure to low, non-freezing temperatures. The objective of this study was to evaluate a nonlinear vernalization response function for the “Snow Queen” lily. An experiment was carried out in Santa Maria, RS, Brazil, to provide an independent data set to evaluate the performance of the model. Lily bulbs were vernalized at -0.5, 4.0, and 10oC during two, four, six, and eight weeks. The daily vernalization rate (fvn) for each treatment was calculated with a beta function, and the effective vernalization days (VD) were calculated by accumulating fvn. The thermal time from plant emergence to visible buds at different VD treatments was used as the observed response to VD. Lily plants were not vernalized at values less than eight effective vernalization days and were fully vernalized at values greater than 40 days. The generalized nonlinear vernalization function described well the “Snow Queen” lily developmental response to VD, with a root mean square error of 0.178. Key words: low temperature, flowering, bulb, model, plant development SIMULAÇÃO DA RESPOSTA À VERNALIZAÇÃO DO LÍRIO “SNOW QUEEN” (Lilium longiflorum Thunb.) RESUMO: A vernalização é um processo que certas espécies de plantas, incluindo lírios (Lilium spp.), necessitam para entrar no período reprodutivo, por meio da exposição a baixas temperatures. Avaliou-se uma função não linear de resposta a vernalização para o lírio a -0,5; 4,0 e 10,0 oC durante dois, quatro, seis e oito semanas. Dados de duração do subperíodo compreendido do plantio dos bulbos até o aparecimento do botão das plantas com lírio, cultivar “Snow Queen”, provenientes de um experimento realizado em Santa Maria, RS, Brasil, foram usados como dados independentes para testar a função não linear de vernalização. Plantas de lírio não foram vernalizadas com menos de oito dias efetivos de vernalização enquanto que as plantas estavam completamente vernalizadas com 40 ou mais dias efetivos de vernalização. A função geral não linear de vernalização descreveu bem a resposta de desenvolvimento à vernalização com uma raiz quadrada média do erro de 0,178. Palavras-chave: baixa temperatura, florescimento, bulbos, modelo, desenvolvimento vegetal INTRODUCTION Lilies (Lilium spp.) belong to one of the six more important genera of flower bulbs produced worldwide (De Hertogh & Le Nard, 1993). The importance of this genus in the world flower market is in large part due to the diversity of the species and the large number of hybrids and cultivars commercially available (De Hertogh, 1996). Major markets for lily include fresh cut flowers, potted flowering plants, gardens and landscapes (De Hertogh, 1996; Grassotti, 1996; Mynett, 1996). Three phases can be distinguished during the development of lilies (Langens-Gerrits et al., 2003): the juvenile, the vegetative, and the flowering phase. Plants from juvenile bulblets have a “rosette” growing habit, i.e., very short internodes, and will not flower. After one or two growing seasons, bulbs sprout with a stem that elongates and flowers (Langens-Gerrits et al., 2003). Most of lily species require an exposure to low, non-freezing, temperatures to accelerate shoot emergence and flowering (Weiler & Langhans, 1968; Roh & Wilkins, 1977a; Roh, 1985; Choi et al., 1996). The exposure to low temperatures either in natural or in artificial cold treatment is called vernalization (Purvis & Gregory, 1937; 1952; Purvis, 1961). Vernalization is a natural survival mechanism common to certain fall planted species to tolerate low temperatures during the winter (Jedel et al., 1986; Wang et al., 1995; Rawson et al., 1998). Lily plants respond to vernalization by decreasing their time to flowering (i.e., there is an increase in the development rate towards flowering as vernalization progresses). The decrease in the time to flowering is caused by a decrease in the number of primordia that become leaves, i.e., a decrease in final leaf number (Roh & Wilkins, 1977a; 1977b; Roh, 1985). Sci. Agric. (Piracicaba, Braz.), v.62, n.2, p.117-121, Mar./Apr. 2005 118 Streck & Schuh Plant response to vernalization is given by the combination of two factors, the temperature during the vernalization period, and the duration of the period. The duration of the exposure to vernalizing temperatures is measured as effective vernalization days (VD). One VD is attained when the plant is exposed to the optimum temperature for vernalization for a period of one day (24 h). As temperature departs from the optimum, only a fraction of one VD is accumulated by the plant at a given calendar day (Hodges & Ritchie, 1991; Ritchie, 1991). Response to vernalization may also be dependent on plant age and plant physiological stage (Roh, 1985; Slafer & Rawson, 1994; Langens-Gerrits et al., 2003). Because the exposure to low temperatures affects plant development in many species, vernalization models have been developed for several crops such as winter wheat (Triticum aestivum L.) (Ritchie, 1991; Wang & Engel, 1998; Streck et al., 2003), carrot (Daucus carota L.) (Yan & Hunt, 1999), calabrese (Brassica oleracea var. italica) (Wurr et al., 1995), lily (Lilium spp.) (Streck, 2002), and onion (Allium cepa L.) (Streck, 2003). These vernalization models account for the effect of VD on the development rate in many crop simulation models. Vernalization models are composed by a response function for VD [vernalization function, f(VD)], which varies from 0 to 1, as a modifier of the development rate (e.g. Weir et al., 1984; Ritchie, 1991; Wang & Engel, 1998). Streck (2002) proposed a generalized nonlinear f(VD) for lily. The nonlinear vernalization function has coefficients with biological meaning and described well the lily developmental response to VD in 10 lily genotypes with a wide range of vernalization responses. However, the vernalization response function proposed by Streck (2002) was not evaluated for the Lilium longiflorum Thunb, cultivar “Snow Queen”, one of the most important lily cultivars grown in Brazil, which constitutes the rationale for this effort. The objective of this study was to evaluate the nonlinear vernalization response function proposed by Streck (2002) for the “Snow Queen” lily. MATERIAL AND METHODS Results on the effect of VD on time to flowering in other species (e.g. wheat) suggest a sigmoidal shaped curve for describing the plant developmental response to VD (e.g. Chujo, 1966; Wang et al., 1995; Brooking, 1996; Rawson et al., 1998; Mahfoozi et al., 2001). Streck (2002) proposed the following MMF response function (Morgan et al., 1 (...truncated)