ABSTRACT
Optimization of crop nutrition is essential to avoid plant stress and to obtain high yields and qualities of horticultural products. In this respect, soilless systems are of great interest because they allow the management of the different factors affecting plant nutrition, such as nutrient solution composition and concentration or nutrient solution temperature. In this thesis, efforts have been done in this direction in order to optimize nutrition of rose plants cultivated for cut flower production. This general aim has been addressed in three chapters.
   Chapter 3 has the aim of understanding the factors affecting daily water and nutrient uptake by rose plants and developing empirical models that could explain it. Five models for nutrient uptake (nitrate, phosphate, potassium, calcium and magnesium) and one for water uptake were developed with the interest of being practical for their application in real conditions. This is due to the fact that the models were developed with data of more than a year of cultivation and include the effects of some common practices in cut flower production of rose plants such as renewal of old bent shoots, the use of shade screen or the synchronization of flower shoot development for scheduling purposes. In addition, other independent variables were nutrient solution concentration, vapour pressure deficit, radiation integral inside the greenhouse, air temperature, nutrient solution temperature, flower shoot production or unknown internal factors. Nutrient uptake models also integrated the effect of water absorption.
   Chapter 4 has the objective of testing the tolerance of rose plants to low nutrient solution temperatures by studying their effect on several physiological parameters. Rose plants were tolerant to a nutrient solution temperature of 10 °C during winter by means increasing the production of thin roots, nitrate uptake, nitrate reductase acivity, photochemical activity and carbohydrates content, and by enhancing the partitioning of N and carbohydrates towards the roots. Nevertheless, this response decreased in the beginning of spring, maybe because of the interaction between the effect of nutrient solution temperature and the improved air climatic conditions.
   Chapter 5 aims at studying the effect of using a lower nutrient solution concentration compared to the standard on the subsequent vase life of cut roses. Although interesting from an environmental standpoint, a 40% dilution of the nutrient solution shortened vase life of rose flowers by one day. This resulted from a higher water loss to water uptake ratio during the first day after harvest, which was the main factor affecting vase life duration, and from a faster decrease of flower shoot fresh weight during postharvest life. The second objective was to apply chlorophyll fluorescence imaging to analyze the functioning of the photosynthetic apparatus throughout postharvest life in order to understand the response mechanisms of the flower shoot to harvest. One day after harvest, an activation of the photoprotective mechanisms in the leaves was observed. These mechanisms began to be less operational with the progression of water loss throughout postharvest life and this eventually led to a decrease in the fraction of PSII centres that are capable of photochemistry. The best chlorophyll fluorescence parameter to describe the changes during rose vase life was phi(NPQ)/phi(NO) and the less informative was Fv/Fm.