Hot water treatments reduce leaf yellowing and extend vase life of Asiatic hybrid lilies

Abstract

The vase life of Asiatic lilies can be limited by leaf yellowing, which can be caused by exposure to low light or temperature during winter growth or in storage. We examined the use of postharvest hot water treatments (HWTs) as a means of reducing leaf senescence in stored (4 °C for 2 weeks) and non-stored Asiatic hybrid lily ‘Elite’ (Lilium sp.). A range of HWTs (45–55 °C for 2.5 or 5 min) was applied to leaves on cut lily stems (but not flowers). Higher temperatures and the longer duration resulted in heat damage, but treatments of 50 °C for 5 min and 52.5 °C for 2.5 min were found to be optimal for minimising leaf yellowing with trace levels of heat damage for both non-stored and stored stems. The onset of yellowing was delayed by 3–4 d, and the occurrence of an unacceptable level of yellowing eliminated for up to 12 d (compared with <6 d for control stems). The physiological effects of these optimal HWTs were examined in terms of water uptake, chlorophyll fluorescence and chlorophyll degradation. Water uptake for optimal HWTs during shelf life was reduced by more than 50% of the control stems. Chlorophyll fluorescence of leaves on control stems showed a reduction in yield (F_v/F_o) over time, which was more marked in lower than upper leaves (thus correlating with yellowing, which was more severe in the lower leaves). Although both optimal HWTs resulted in an initial reduction in yield, there was a recovery over time resulting in a yield that, by 12 d, was as high or significantly higher than control leaves (particularly for the lower leaves). All treatments showed a reduction in chlorophyll content (total, chlorophyll a and chlorophyll b), but control leaves had significantly lower levels after 7 d. HWTs show potential as a non-chemical, simple means of delaying leaf yellowing of Asiatic lilies and thus increasing vase life.

Introduction

Asiatic and Oriental lilies (Lilium sp.) are a popular cut flower and are grown commercially worldwide. However, leaf yellowing of the cut stems or potted plants is a major issue in some cultivars since its onset can limit vase life of the overall stem, i.e. leaf yellowing occurring before flower senescence. It is triggered by growing-period or postharvest conditions (Ranwala and Miller, 1998, Whitman et al., 2001). Conditions that promote leaf yellowing include reduced light due to high density planting or low levels of incident sunlight, which can lead to a reduction in carbohydrate levels within the plant (Jiao et al., 1986, Sorrentino et al., 1997, Ranwala and Miller, 2000, Han, 2000). Leaf yellowing occurs most commonly following coolstorage of the cut stems associated with the stresses of darkness and low temperature during storage (Ranwala and Miller, 1998), but can also occur directly after harvest. Basal leaves are affected first and yellowing progresses up the stem eventually affecting the entire stem.

A number of different approaches have been taken to ameliorate postharvest leaf yellowing in Asiatic hybrid and Oriental lilies. The use of postharvest hormone-based foliar sprays, particularly combinations of gibberellins and benzyladenine, have been successful in reducing leaf yellowing in both Asiatic hybrid lilies and other Lilium species (Ranwala and Miller, 1998, Ranwala and Miller, 2000, Whitman et al., 2001). Leaf senescence can be induced through ethylene exposure, and in such cases, pretreatment with 1-MCP can delay its onset. However, when leaf senescence can occurs in the absence of ethylene, such as during storage in the dark or storage at high or low temperatures, 1-MCP has no effect (Celikel et al., 2002). Simple, non-chemical postharvest treatments may provide an alternative to these solutions.

Heat treatments have many potential applications in horticulture including disinfestation (killing insects), reducing disease, inducing tolerance to subsequent cold temperatures (reduced chilling injury), maintenance of firmness and/or reduced senescence (Lurie, 1998, Ferguson et al., 2000, Fallik, 2004). Although there has been much work examining the potential for commercial application of heat treatments in fruit and vegetable crops, there has been relatively little such research in flower crops (Lurie, 1998).

Heat treatments have been shown to be effective disinfestation treatments in some flower crops including red ginger (Hansen et al., 1991), bird of paradise (Hara et al., 1993) and Banskia spp. (Seaton and Joyce, 1993), and can reduce disease (Botrytis cinerea) in rose flowers (Elad and Volpin, 1991). Sangwanangkul et al. (2008) have also shown that 40 °C pretreatment in water can improve the tolerance of red ginger flowers to disinfesting irradiation treatments, thus showing that a given stress treatment can induce tolerance to other quite different stressors.

In terms of extending vase life, Hara et al. (1997) showed that pretreatments with hot air, along with HWTs, could lead to significant increases in vase life in red ginger, although vase life was already long (>20 d). HWTs have been examined as a disinfestation treatment for tropical flowers, and small increases in vase life noted (∼1 d; Hansen and Hara, 1994, Hara et al., 1997). Chantrachit and Paull (1998) used HWTs with the direct aim of increasing vase life in red ginger flowers and found a 20–50% increase in vase life if exposed to short HWTs (5–10 min at ≅50 °C). Vase life was increased by reduced senescence, which was expressed as bract-browning and reduced drooping of flowers. In addition, pretreatments with water at 40 °C reduced heat damage due to subsequent high temperature HWTs. However, Seaton and Joyce (1993) found that HWTs reduced vase life of Geraldton wax and Banksia spp. Thus, heat treatments may show potential for improving vase life, something that has commercial significance for lilies.

Our previous work on the reduction in senescence (yellowing) in broccoli florets by HWTs (Tian et al., 1996) led us to hypothesise that there is potential to reduce leaf yellowing in flower crops, specifically leaf yellowing in Asiatic lilies. Other than the examples above — which are large, fleshy flowers adapted to tropical climatic conditions — we could find little published information on reducing leaf or flower senescence in crops of temperate origin. In this paper, we examine the use of postharvest hot water treatments as an alternative non-chemical means to prevent leaf yellowing of Asiatic hybrid lilies (Lilium L.), and study some physiological markers of senescence and response to heat that ameliorate leaf senescence.