Many post-production factors affect vase life of cut flowers including developmental stage at harvest, temperature during the vase period, water loss, and various aspects of the vase solution such as sucrose levels, microbial populations, pH, electrical conductivity (EC), and overall water quality (Sacalis, 1993). To maintain flower quality, studies have focused on storage (Ahmad et al., 2012; Çelikel and Reid, 2002; Jones et al., 2004), floral preservatives (Doi and Reid, 1995; Elhindi, 2012; Gast, 1997), or use of ethylene inhibitors, e.g., silver thiosulfate and 1-methylcyclopropene (Blankenship and Dole, 2003; Chamani et al., 2005), nano-silver (Kim et al., 2005; Liu et al., 2012), and other commercial antiethylene agents (Staby et al., 1993). However, postharvest longevity of cut flowers is also affected by a variety of both preharvest (e.g., harvest season, solar radiation, temperature, relative humidity, water, and mineral nutrient stress) (Halevy and Mayak, 1979; Reid and Jiang, 2012; Slootweg, 2005) and postharvest (e.g., harvest stage, harvest procedures, ethylene, vase water quality, preservatives, storage method and duration, etc.) factors (Halevy and Mayak, 1981; Marissen and Benninga, 2001; van Doorn, 2012). In addition to the aforementioned factors, the time of the day when stems are harvested may be a potentially important consideration (Clarkson et al., 2005; Hasperué et al., 2011; Rapaka et al., 2007).
Generally, stems are harvested by growers in the morning allowing more time for processing and marketing and to avoid field heat, which preserves quality and minimizes transpiration and thereby desiccation. However, for many species, morning harvest may not be the proper time because many plant physiological processes change diurnally and can affect postharvest longevity of cut stems. For instance, diurnal variations in ascorbic acid levels of spinach (Spinacea oleracea L.) (Kiyota et al., 2006) and chlorophyll precursors in bean (Phaseolus vulgaris L.) (Argyroudi-Akoyunoglou and Prombona, 1996) have been reported. However, the most pronounced effect of diurnal fluctuations has been observed on carbohydrate metabolism in plants (Hasperué et al., 2011). Changes in solar radiation greatly affect endogenous carbohydrate reserves, which accumulate during the day and are remobilized by the end of the dark period (Sicher et al., 1984).
Harvesting later in the day compared with morning harvesting increased shelf life considerably for leaves of basil (Ocimum basilicum L.) (Lange and Cameron, 1994), baby salad [Eruca vesicaria (L.) ssp. Sativa (Mill.)] Thell. (Clarkson et al., 2005), and unrooted shoot-tip cuttings of lantana (Lantana comara L.) (Rapaka et al., 2007). These variations in shelf life were attributed to varying carbohydrate levels and diurnal changes in plant metabolic processes.
Genotype and environmental conditions can also impact postharvest longevity. Cultivars of lettuce (Lactuca sativa L.) harvested at different times during the day have varying postharvest quality (Moccia et al., 1998). Butterhead lettuce had the best quality when harvested later in the day, whereas Latin-type lettuce had the best quality when harvested in early morning. Moreover, fluctuations in carbohydrate levels of field-grown crisphead lettuce have been observed between morning and afternoon harvests (Forney and Austin, 1988). Crisphead lettuce harvested in the afternoon had higher sucrose than morning harvests, whereas glucose and fructose were greater in morning harvests. Soluble sugars like sucrose act as signaling molecules for plants under stress and move between cells causing fluctuations in sugar concentrations during senescence (Chuang and Chang, 2013). Moreover, sucrose application reduced ethylene responsiveness in florets of broccoli (Brassica oleracea L.) and carnation (Dianthus caryophyllus L.) flowers (Nishikawa et al., 2005; Verlinden and Garcia, 2004). These diurnal plant carbohydrate changes can greatly affect postharvest longevity of cut flowers. However, limited information is available on the effects of time of harvest on extending postharvest performance (vase life) of particular ornamental crops. Therefore, the following study was conducted to define time of harvest requirements for extending longevity and maintaining quality with or without storage of three popular specialty cut flowers. Storage of cut stems was included in the studies to determine the physiological changes in different sugars taking place during storage and the vase period. Moreover, storage of cut flowers is an essential practice used to extend availability and prolong freshness of cut flowers; therefore, it is necessary to quantify the metabolism of sugars during storage.
The objectives of the present investigation were to 1) study effects of harvesting at different times of the day on water uptake, changes in fresh weight, termination symptoms, LRWC, carbohydrates status, and vase life of cut ‘ABC Purple’ lisianthus (Eustoma grandiflorum), ‘Double Eagle’ African Gold Coin Series marigold (Tagetes erecta), a major flower species in the Indo-Pak sub-continent, and ‘Deep Red’ Benary’s Giant Series zinnia (Zinnia elegans), one of the top three field specialty cut species in the United States; and 2) assess possible mechanisms affecting vase life of fresh or stored cut stems by evaluating changes in water relations and carbohydrate levels. It was hypothesized that harvesting stems in the morning may reduce vase life of lisianthus, marigold, and zinnia.
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