SEASONAL CARBOHYDRATE AND TOTAL NITROGEN DISTRIBUTION IN ROSE PLANTS: DEVELOPMENTAL AND GROWTH IMPLICATIONS

Year round production of cut flowers in rose crops, demands a continuous supply of sugars for the growing flower bud, which competes with the need for sugar at the root level, in particular, at roots devoted to absorption processes. This led to the study of carbohydrate distribution in rose plants at the peak of their flower productive period. Rose plants, cv. Dallas, were grown in a greenhouse with a perlite hydroponic system. Growing practices were the usual in all-year-round commercial production, which included the application of the shoot bending technique. Flower production was registered throughout the year and a set of four plants was sampled in April, July, November and February. These were separated into three parts, roots, leaves and stems (photosynthetic compartment), and flowers. Each part was weighed and analysed for soluble sugars and starch and total nitrogen. The results showed that, compared to the other seasons, soluble sugars were highest in the aerial parts in the spring and the starch and the nitrogen contents were lower in the spring than in the winter. All this coincided with high levels of incident solar radiation on the canopy and intense flower production. In the summer the content of soluble sugars and starch decreased. The drop in sugars was most outstanding in the aerial parts in this period. This coincided with the use of a shading net over the canopy, which reduced the incident radiation and coupled this reduced radiation with high temperatures and saturation deficit. The result was the reduction of biomass production in this season. Thermal time in the autumn was similar to the spring and was lower than in the summer. This resulted in an increase in soluble sugars in the photosynthetic compartment with respect to the summer. Nitrogen contents reach their highest value in the aerial parts in this period. The increase of starch and soluble sugars in roots in the winter, suggests a notable change of sink strength, from the photosynthetic compartment to the roots. This seems to agree with a slow down in the plant activity in this season. These results suggest a relationship between the distribution of assimilates and nitrogen among the roots and aerial parts and plant growth. The demand for sugars by the different sinks can result in critical competition. More detailed studies are needed to better understand these processes and their link with nitrate uptake in order to apply them to the improvement of crop management. INTRODUCTION Not long ago productivity was the main aim in horticulture. However, the objectives have changed and together with the improvement of the quality of products the environmental impact of crops and cropping systems have become matter of interest. One of the concerns of environmentalists in relation to horticulture is the excessive use of fertilizers, particularly nitrates. Hydroponic culture offers the possibility of controlling and optimizing mineral supply. In cut rose cultivars grown in glasshouses, the annual nitrogen supply exceeds by far plant requirements. Critical N concentration is defined as the minimum N concentration in the plant tissue which allows maximum growth rate. The use of critical N concentration is proposed as a mean to discern situations of both sub-optimal and supra-optimal N supply and it can be related to biomass accumulation through several physiological processes, such as N uptake, C assimilation and C and N distribution between organs (Gastal and Lemaire, 2002). Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilan Acta Hort. 697 ISHS 2005 214 Nitrate uptake, in particular, is dependent upon energy provided by respiration in root cells, as the N transport systems are associated with H cotransport, ultimately dependent on metabolic energy (Glass et al., 1990). Therefore the availability of assimilates at the root level, might explain some aspects of the kinetics of nitrogen. Although nitrate uptake was proved to be in proportion with readily assimilated carbon in annual plants, most likely through a constant supply to the roots, the competition for current photosynthates is believed to be greater in perennials than in annuals and, hence, an important part of the metabolic needs in the former is covered by the mobilization of the reserves for C and N (Jordan et al., 1998; Gastal and Saugier, 1989). Jeschke and Hilpert (1997) also demonstrated a sink dependent stimulation of nitrate uptake and of net photosynthesis. Attention has been paid to remobilization of reserves in deciduous plants (Zapata et al., 2004; Grossman and Djong, 1995). In roses, however the influence of remobilization on new growth is not so clear (Kool et al., 1996, Kool et al., 1997, Marcelis-van Acker, 1994). There is also a number of studies in photoassimilate partitioning in crops grown for their fruit, in which its interest resides in its role in fruit development (Grossman and Dejong, 1995; Gary et al., 2003). Less work has been devoted to the dynamics of assimilates in flower development (Aloni et al., 1996; Garcia-Luis et al., 1995). The year round production of cut flowers in rose crops, demands a continuous supply of sugars for the growing flower buds which competes with the need for sugar at the root level, in particular, at roots devoted to absorption processes. However, flower production is not even throughout the year. The nitrate uptake was related to irradiance and to the cyclical pattern of production for cut roses by Cabrera et al. (1995a and 1995b) who stated that irradiance did not control the periodicity of the N uptake cycles, but that it affected the average daily plant N demand. Besides, low irradiance has been reported to cause a reduction in rose flowering (Zieslin and Moe, 1985; Zieslin and Mor, 1990). In previous studies Roca et al. (2003) and Martinez et al. (2004) reported on relations for nitrate absorption by rose plants, grown in a close hydroponic system, based on environmental parameters. In the present work, variations on the distribution of nonstructural carbohydrates in the rose plant, throughout the year, are used to explain N content in connection with radiation and temperature. MATERIALS AND METHODS An all-year-round rose crop, cv. Dallas, was grown in a policarbonate greenhouse, equipped with convective heating (minimum 16oC), high pressure fogging and roof ventilation. A closed perlite growing system was used. The nutrient solution was the currently used by rose growers (mmol. l: NO3 13.3; H2PO4 1.5; SO4 1.0; NH4 0.9; K 4.9; Ca 4.5; Mg 1.0; pH 5.5; EC 1.9 dS/m). This basic formula was modified depending on the climatic conditions of the different seasons. The water for the nutrient solution was previously treated with reverse osmosis and ion columns. The climate variables, in and out solar global radiation, temperatures of the air, substrate and nutrient solution and air humidity, were recorded every 15 seconds by means of sensors connected to a data acquisition system. The number of flowers/plant/month was registered and 30 flowering shoots were taken for the following calculations on growth each week: Monthly flower biomass production (dry weight/plant/month); Flower diameter (mm/bud/plant/month); Leaf area (cm/shoot/plant/month) and Shoot length (mean/plant/month). For the study of soluble sugars, starch and nitrogen partitioning, one sample of 4 whole plants was taken at each season of the year: spring (April); summer (July); autumn (November); winter (February). Results are presented considering three plant compartments: roots (fine and woody roots), photosynthetic compartment (leaves and stems) and flowers (peduncles and buds). Plant parts were lyophilised and milled and soluble sugars and starch were analysed using a colorimetric method based on McCready et al. (1950). The results are recorded in a spectrophotometer (Uvikon XS, Bio-Tek, USA) at 630 nm. For total nitrogen determinations we followed the Kjeldahl method described by Bremmer (1965) and the analyses were performed with a semiauthomatic analizer (Tekator).