Fisiologia postraccolta dei fiori recisi

The study of the postharvest physiology of cut flowers has the aim of understanding the behaviour and disorders of harvested flowers. The dramatic differences between the conditions in the growing environment and in the storage facility affect the physiology of cut flowers. Understanding the biological mechanisms that are activated or inhibited during storage is essential to developing technical strategies for avoiding quality losses. External quality (appearance) is extremely important to the consumer. Although consumer choice is driven by external quality, the postharvest life, or vase life, is key to convincing the consumer to re-purchase cut flowers. Postharvest physiology therefore has a double goal: to preserve external quality during the distribution chain and to extend the vase life of the flowers. The key to extending flower life after harvest is understanding the biological processes governing postharvest physiology: respiration, ethylene production, water loss (transpiration), hormonal imbalance and the activation of enzymes associated with flower discoloration and leaf yellowing (chlorophyll loss). Respiration is the fundamental metabolic process responsible for providing energy in all living cells. During postharvest life, cut flowers depend on stored reserves (starch and sugars) for respiration. Because flowers are poikilotherms, the rate of respiration is dependent on temperature, increasing in an exponential fashion with increasing temperature. For cut flowers, the Q1 0 (the ratio of respiration at temperature T to that at temperature T- 10) is usually at least 3, and may be as high as 9. This means that respiration (and deterioration) at 20 C is at least 9 times more rapid than at 0 C; rapid cooling and maintenance of temperature is therefore key to maintaining the vase life of harvested flowers. After harvest, the hormone equilibrium in cut flowers is frequently altered. Stress conditions stimulate ethylene biosynthesis, which leads to rapid senescence or abscission in ethylene-sensitive cut flowers. The physiological effects of ethylene are expressed when tissues are sensitive to this plant hormone, which requires the presence in the cells of ethylene receptors. Plant ethylene receptors have been studied in model systems (principally Arabidopsis thaliana) and, although much is now known of their structure and function, new details are continually being discovered. Cut flowers that are sensitive to ethylene respond fully to concentrations of 1 to 3 μL L- 1, and last longer when treated after harvest with inhibitors of ethylene biosynthesis or action. Abscisic acid (ABA) is another plant hormone that can play an important role during flower senescence. ABA accumulation is induced in leaves and petals by water stress, and may result from carotenoid degradation in leaves. Increased ABA concentrations may trigger flower senescence in some species. Once harvested, flowers no longer receive cytokinins from the roots of the mother plant. The lack of these hormones results in the induction of leaf yellowing. Exogenous treatment with natural cytokinins or substituted phenyl-ureas with cytokinin-like activity can inhibit chlorophyll degradation and leaf yellowing. Excessive water loss frequently compromises the storage and vase life of harvested cut flowers. Continued transpiration in detached flowers leads to water imbalance, resulting in flower wilting. To avoid excessive water losses, cut flowers should be cooled immediately after harvest. Storage and transportation conditions strongly affect cut flower physiology and subsequent vase life. The environmental factors that directly affect cut flower physiology are temperature, relative humidity, exogenous ethylene and light. Temperature is the most important factor affecting the life of all perishables. Low temperatures reduce respiration, transpiration, ethylene biosynthesis and the response to ethylene, microbial growth and enzyme activities. Relative humidity (RH) in the storage chambers or packages strongly affects cut flower transpiration. High RH conditions reduce water loss, which is driven by the vapour pressure gradient across the stomata. This effect is very temperature dependent: at high temperatures the vapour pressure gradient is much steeper. The RH in storage rooms should range between 90 and 95%; at these high humidities, accurate temperature control is essential in order to avoid condensation and consequent mould growth. Environmental ethylene contamination is usually the result of incomplete combustion or senescent plant parts. Its effect can be dramatic in sensitive cut flowers. Light can have a major effect on postharvest performance of some flowers, but since cut flowers are stored and transported in dark conditions it is of minor importance in the commercial distribution chain. The main postharvest disorders that affect the ornamental value of cut flowers occur in the floral organs (petals, sepals and pistils) or leaves. Floral senescence symptoms vary among species, and even among cultivars. Leaf senescence may be considered as the last stage of leaf development. It is a highly regulated process that ends with leaf death. Leaf senescence symptoms include leaf yellowing, brown spots, necrotic zones and desiccation