Factors that affect Quality in Post-harvest

The respiration of cut flowers, an integral part of growth and senescence, generates heat as a byproduct. Additionally, as the ambient temperature rises the respiration rate increases. For example, a flower at 30ºC is likely to respire (and therefore age) up to 45 times faster than a flower at 2ºC. The rate of aging can be reduced dramatically by cooling the flowers. Rapid cooling accompanied by a stable cold chain are therefore essential to ensure the quality and satisfactory vase life of the majority of cut flowers currently on the market.

The minimum cutting maturity for a given flower is the stage of development at which the buds can fully open and display a satisfactory vase life. Many flowers respond well to being cut at the bud stage, opening after the storage, transportation and distribution process. This technique has many advantages including a reduced growing period for single-harvest crops, higher packing density, simplified temperature management, lower susceptibility to mechanical damage, and reduced desiccation. Many flowers are currently harvested when the buds begin to open (rose, gladiolus), although others are cut when they are fully open or close to being open (chrysanthemum, carnation). Flowers for the local market are generally harvested much more open than those intended for long-distance storage and/or transportation.

It occurs when small air bubbles (emboli) enter the stem at the time of cutting. These bubbles cannot rise within the stem, so their presence obstructs the vertical flow of the solution, which does not reach the flower. Emboli can be removed by cutting the stems back into the water (removing about 2.5 cm), making sure the solution is hot, but not extremely hot, or in an ice-cold solution, dipping. Briefly soak the stems (10 seconds to 10 minutes) in a weak detergent solution (e.g. 0.02% dishwashing liquid), or by submerging the stems in a deep container filled with solution (at least 20 cm).

The starches and sugars stored within the stems, leaves and petals provide most of the food needed for flowers to open and maintain. The levels of these carbohydrates reach their maximum level when the plants have been grown with high light and with appropriate cultural management. The concentration of carbohydrates is in fact generally highest during the afternoon – after a day of full sunlight. However, it is preferable to harvest the flowers early in the morning, when temperatures are low, plant hydration is high and you have the entire day to process the cut flowers.

The cut surface of a flower stem releases the contents of the cells - proteins, amino acids, sugars and minerals - into the water of the container where they are located. This is ideal food for bacteria and these tiny organisms grow rapidly in the anaerobic environment of the vase. The slime produced by bacteria, and the bacteria themselves, can plug the water-conducting system within the stems.

Hard water often contains minerals that make it alkaline (high pH), which drastically reduces water movement within the stems. This problem can be solved by removing the minerals present (with a deionization, distillation or reverse osmosis system) or by acidifying the water. Commercial floral solutions do not contain enough acid to lower the pH of highly alkaline waters, and in this case it is necessary to add acid directly to the water. In some countries, the most obvious solution is to use rainwater to prepare post-harvest solutions.

Some flowers respond to environmental stimuli (tropisms) in ways that result in loss of quality. The most important are geotropism (twisting against gravity) and phototropism (towards light). Geotropism often reduces the quality of spike flowers such as gladiolus, snapdragons and stock, lisianthus, roses, and gerberas, as the flower stems (pedicels) or main stem bend. upwards when the flowers have been stored horizontally. The Lisianthus shoots bend up if stored at warm temperatures horizontally. geotropic response is substantially reduced when flowers are kept at low temperatures; If this is not possible, then it is of utmost importance to keep them upright. Treatment with low concentrations of naphthyl-phthalamic acid (NPA), an inhibitor of auxin transport, is very effective in preventing geotropic folding, but this compound is not commercially available or registered for this use.

Flowers are very susceptible to disease, not only because their petals are fragile, but because the secretions of their nectaries offer an excellent supply of nutrients even for weak pathogens. To make matters worse, moving flowers from a cold storage area to a warmer handling area often causes condensation on the foliage. The most frequently found organism is gray mold (Botrytis cinerea), capable of germinating whenever free water is present. In the moist environment of the flower head, it can grow (although more slowly) even at temperatures close to freezing. Good management of greenhouse hygiene, temperature and other measures to minimize condensation on cut flowers will reduce losses caused by this disease. Some fungicides such as Ronalin, Rovral (Iprodione), and cupric-based Phyton-27 have been approved for use on cut flowers and are very effective against gray mold. Other more modern fungicides such as 'Palladium', a mixture of fludioxinal and cyprodium, have been very effective as post-harvest dips to prevent Botrytis infection and will surely be registered for this use in the near future.

Cut flowers, particularly those with abundant foliage, have a large exposed surface area so they can lose water and wilt quickly. Therefore, they should be stored at relative humidities above 95% to minimize dehydration, particularly during prolonged storage. Water loss is dramatically reduced at low temperatures, all the more reason to ensure prompt and efficient cooling of the flowers. Even after flowers have lost considerable amounts of water (for example during air transport or prolonged storage) they can be completely rehydrated using appropriate techniques. Cut flowers absorb solutions without problems, as long as the flow of water within the stems is not obstructed. Air embolism, bacterial plugging, and poor quality water are all factors that reduce solution absorption.

Bruises and other mistreatment of flowers should be avoided at all costs. Flowers with torn petals, broken stems, or other obvious damage are undesirable for aesthetic reasons. Additionally, pathogenic organisms can infect plants more easily through damaged areas. In fact, some of these organisms can only penetrate plant tissues through wounds. Additionally, respiration and ethylene evolution are generally higher in mistreated plants, further reducing their lifespan.

Some flowers, particularly carnations, gypsophila and some rose cultivars, die quickly if exposed to even very low concentrations of ethylene. Some cut flowers produce ethylene as they age; In carnations and sweet peas, for example, ethylene production is part of the natural process of flower death, while in others, such as calceolaria, snapdragon and delphinium, it induces flower drop.

Premature flower death is a common cause of loss of quality and reduced vase life of many cut flowers. In terms of their senescence process, flowers can be divided into several categories: Some are extremely long-lived, especially those belonging to the daisy and orchid families. Others have a particularly short lifespan, as is the case with many bulb flowers such as tulips, irises and daffodils.

In plants, the death of individual organs and of the plant itself is an integral part of its life cycle. Even in the absence of the senescence process of flowers and leaves, the continuous growth process can lead to a loss of quality, for example in flowers with spikes that bend in response to gravity.

In cut or potted ornamentals, a long life depends almost absolutely on a constant supply of water. If this is interrupted, either due to internal obstruction of the cut stems or because the watering given to the pots is insufficient, rapid wilting of the buds, leaves and petals occurs.

Yellowing of leaves and even other organs (buds, stems) is commonly associated with the end of the useful life of some cut flowers (alstroemerias and lilies being an important example). Leaf yellowing is a complex process that can be caused by a number of environmental factors.

The loss of leaves, buds, petals, flowers or even buds is a process called 'dropping' or 'abscission', and is also a common problem with cut flowers. This problem is often associated with the presence of ethylene in the air, but other environmental factors may also be involved.