Abbreviations: ABA, abscisic acid; ELISA, enzyme-linked immunosorbent assays; RIA, radioimmunoassay 11 Abscisic Acid and Cut Flower Senescence Antonio Ferrante 1*  Paolo Vernieri 2 1 Dipartimento di Produzione Vegetale, University of Milan, Italy 2 Dipartimento Biologia delle Piante Agrarie, University of Pisa, Italy Corresponding author: * antonio.ferrante@unimi.it Keywords: aging, antibody, ethylene, leaf senescence, postharvest ABSTRACT The keeping quality of cut flowers, during the postharvest stages, is very important in the global market, where floricultural items must be extremely competitive. Postharvest physiological analyses of senescence point to plant hormones playing important roles in the promotion or inhibition of flower and leaf senescence. Among them, ethylene and abscisic acid (ABA) are the most important. Ethylene has been extensively studied and much information is available for modulating senescence in ethylene-sensitive flowers. In contrast, the role of ABA during cut flower senescence has been wrongly assigned secondary importance. In this chapter, the effect of ABA on leaf and flower senescence has been briefly reviewed in select species, with a brief discussion of ABA and gene expression. 1. INTRODUCTION The flower longevity of cut flowers is defined by many postharvest disorders that may rapidly lead to visual appearance alteration, which compromise the ornamental value of many floriculture items. The visual appearance is the most important parameter for perishable ornamentals for being appreciated on the local or global market. The ornamental parts that contribute on the visual appearance are leaves and flowers (petals or tepals). Plant hormones may play a crucial role for many cut flowers species in preventing or inducing senescence (Teixeira da Silva 2003). Among plant hormones, ethylene is well known to play an important role during senescence of sensitive species such as carnation, rose and many other cut flowers. However, there is also clear evidence that abscisic acid (ABA) may act as a promoter and accelerate the senescence of flower organs. ABA is involved in several aspects of plant growth and development including embryo maturation, seed dormancy, fruit ripening, water use and responses to environmental stresses (Zeevart and Creelman 1988). The studies carried out on ABA and cut flower senescence have shown that this hormone affects the timing of flower and leaf senescence in a small number of flowers such as carnation, rose and daylily flowers (Mayak and Halevy 1972, Mayak and Dilley 1976, Panavas et al. 1998). 2. ROLE OF ABA DURING FLORAL SENESCENCE Endogenous levels of ABA decrease during flower development from bud to the fully open stage, while it rapidly increases during senescence (Panavas et al. 1998, Ferrante et al. 2006). Treatments with exogenous ABA speed up flower senescence in many cut flowers. Cut carnations treated with ABA showed a reduced vase life (Shibuya et al. 2000). The correlation between ABA and water stress in higher plants is well known and has been extensively studied for understanding the plant response to environmental stresses (Bartels and Sunkar 2005, Roelfsema and Hedrich 2005). Therefore, water stress in detached flowers may be responsible for induction of ABA biosynthesis and it may be considered for some species to be a trigger of flower senescence. In isolated rose petals, the direct relationship between water stress and ABA biosynthesis was demonstrated using labeled ABA. Results showed that progressive tissue water soaking induced an increase in ABA, while no increase in hormone was found in attached flowers (Le Page-Degivry et al. 1991). On the basis of these results, it was suggested that ABA accumulation in cut flowers after harvesting was induced by water stress. This hypothesis was confirmed considering that cut flowers are susceptible to xylem vessel occlusion, which is mainly induced by bacteria or fungi developed in the vase water during postharvest life (Zagory and Reid 1986). Therefore premature ABA biosynthesis may be considered as the trigger of flower senescence. The correlation between ABA content and vase life has been particularly studied in roses. The level of endogenous ABA at harvest time was found to be inversely correlated with the vase life of roses (Mayak and Halevy 1972, Ménard et al. 1996) and this result may be explained considering the alteration of water relations (Eze et al. 1986). In mature tissues, the increase of ABA induces several enzyme activities and affects mRNA biosynthesis, causing the appearance or disappearance of some specific mRNAs. In daylily flowers, treatments with ABA induced ion leakage, lipid peroxidation and proteinase activity (Panavas et al. 1998), leading the flowers to collapse and die. The daylily flowers treated with ABA induced nucleases activity that was detected by the TUNEL test, and showed an induction of DNA degradation