213 Response of Tomato Transplant to Nitrate Phosphate Conditioning: Effects on Development, Growth and Early Yield M. Guzmán, F. Arcos and A. Sánchez Dept. Plant Production Almería University, Spain Keywords: fertigation, clormecuat, post-transplant, etiolation Abstract The nursery industry in Almería constitutes the first step of intensive horticulture in the area. Annual tomato transplant production amounts to almost 300 million plants. Transplant quality and early yield response in tomato plants are currently dependent on the use of chemical growth regulators. However, nutrient conditioning may be a useful alternative technique to obtain adequate transplant quality, faster resumption of growth after transplanting and a higher early yield. It should, therefore, be possible to regulate the vegetative and early reproductive growth rate of tomato by controlling the balance between N and P during transplant production in the nursery. As may be expected, the maximum effect on growth regulation was obtained with the application of the chemical growth regulator clormecuat. Nevertheless, our results show that similar growth reduction can be achieved by maintaining low N and high P levels in nutritive solution during the first stages of transplant production, and increasing P more than N shortly before transplanting. When chemical growth regulator was used, resumption of vegetative growth was delayed for more than two weeks after transplanting. In contrast, nutritively conditioned transplants resumed growth almost immediately. In addition, fruit weight and early yield are more than double in the nutrient conditioned tomato transplant. Nutritive conditioning induces changes in transplant growth that are helpful in preparing the plant for the resumption of post-transplant growth and for increasing early yield production. INTRODUCTION The transplant of the selected cultivars from the nursery is the starting point for almost all European protected crops. The percentage of open field production using this method is also increasing rapidly. In Spain, especially in the South-East region, nurseries have becomes an indispensable link in the intensive horticulture production chain. Their function is two-fold: they specialize in seed germination and in the production of high quality transplants. Nutrient conditioning consists of preparing plants nutritionally during the nursery stages in order to increase quality and reduce post-transplant stress as much as possible. According to Masson et al (1991) the best post-transplant response was obtained with medium nitrogen concentration values of 150 to 300 mg.L -1 . At nitrogen concentration below 150 mg.L -1 the growth rate was too slow, as was post-transplant response. Phosphorous produces a less notable effect than nitrogen. At higher P concentration (60 mg.L -1 ) both growth rate and post-transplant responses were better. Melton and Dufault (1991) showed that K concentrations between 10 to 250 mg.L -1 did not produce significant effects on growth of tomato transplant. Transplant growth rate can be regulated in nursery by adapting the concentration of nitrogen, and probably its balance with other nutrients. Transplant quality can be defined according to seedling response (Hoyos, 1996). Dry weight seems the best parameter to evaluate the sensitivity to transplant stress. A second useful parameter is the Specific Leaf Area (SLA). SLA usually increases with plant age, but not indefinitely. Leskovak et al (1994) also states that transplant leaf area only increases yield up to a certain value, beyond which this parameter becomes negative. Lescak (2001) showed a linear increase in the ratio of yield to stem diameter and Proc. IS on Greenhouse Salinity Eds: A. Pardossi et al. Acta Hort 609, ISHS 2003