Postharvest browning of cut Backhousia myrtifolia stems

Backhousia myrtifolia Hook. & Harv. is a species native to Australia that shows potential as a cut flower crop. During spring and summer, it bears numerous small florets with prominent white sepals and glossy deep-green foliage. B. myrtifolia is harvested either when tight white buds are present in the centre of the star-shaped sepals or following bud burst, after the petals and stamens have abscised to leave only the sepals. However, cut stems can suffer from unsightly browning that was suggested by cut flower growers to be the result of water deficit after harvest. Experiments revealed that no freshly harvested non-stored B. myrtifolia stems developed the postharvest browning syndrome during vase life experiments, irrespective of the wilt level. Vase solution chlorination and physical anti-transpirant films did not initiate symptoms of the postharvest browning syndrome. Instead, stems stored at 2 or 5 °C for 1 week often developed brown lesions, regardless of the severity of water deficit stress. Vase life was reduced for stored stems subjected to transient postharvest water stress due to accelerated wilting, and all stems developed brown leaf lesions post storage, even when they were fully rehydrated soon after harvest. Pre-storage sucrose, anti-oxidant, and fungicide treatments had no effect on the post-storage browning syndrome. Stems stored at 2 °C had a shorter vase life and more extensive browning symptoms than those stored at 5 °C. Further research showed that brown lesions developed on B. myrtifolia leaves stored at 0 to 10 °C, but symptom appearance was highly variable. The critical storage temperature below, and the period beyond which symptoms develop, were also highly variable. Brown lesions frequently developed on stems stored at ≤5 °C for 1 week and less frequently developed on stems stored at >5 to ≤10 °C for 1 week. Symptoms on stems stored at >5 to ≤10 °C were only very slight, rarely caused vase life termination and required up to 2 weeks storage to develop. If browning symptoms developed, then both the depth of colour and distribution of leaf symptoms was highly variable. The largest and most intense brown lesions developed on stems stored at 2 °C. Attempts to objectively measure and predict low temperature injury with chlorophyll fluorescence and respiration measurements were unsuccessful, possibly because of difficulties reproducing symptoms, non-uniformly affected tissues (i.e. isolated lesions), and/or very rapid browning following injury. Postharvest options to manage low temperature injury, such as low temperature conditioning and diurnal harvests, were largely unsuccessful. Hot water treatments inhibited the postharvest browning disorder and arrested flower opening, but caused flower browning. However, there may be potential to further optimise the hot water treatment protocols. Several forms of discolouration characterised over the 2004 - 2006 flowering seasons were collectively termed the ‘post-harvest browning syndrome’. Browning appeared to be a common symptom of stressed or injured B. myrtifolia, but several leaf and flower symptoms were associated with low temperature storage. Localised or extensive black or reddish-black leaf lesions that often formed a V-shape from the tip towards the base were found on low temperature injured young mature leaves. These leaves were fully expanded glossy deep-green coloured leaves near the stem tip. Tan-brown to dark-brown sporadic, and sometimes sunken lesions, developed in low temperature injured older mature leaves. These leaves were lighter green and less glossy and located near the cut stem bases. Sepals, hypanthiums, or buds of B. myrtifolia flowers turned brown or black when injured by low temperatures. Pre-harvest conditions during tissue growth and development influenced B. myrtifolia susceptibility to low temperature injury and therefore contributed to the observed symptom variability. Leaves that developed during mild pre-harvest temperatures (20 to 30 °C) with non-limiting water and nutrients had proportionally larger brown lesions than those that developed at more extreme temperatures. However, there was no relationship between pre-harvest temperatures and postharvest low temperature injury for commercially grown stems. Instead, it is hypothesised that unknown pre-harvest factor(s) that are influenced by temperature, such as growth rate, determined susceptibility to postharvest low temperature injury. Leaf age also influenced the colour of low temperature injured leaves. Freeze injured young mature leaves became glossy black upon thawing whereas freeze injured older mature leaves turned a lighter greyish-black. The number of young mature leaves per stem was highly variable within and between plants (coefficient of variation 60%). A model system was developed to further investigate the influence of leaf age and wilting on the tissues potential for discolouration caused by phenolic compound oxidation. The model system involved a freeze-thaw cycle to more reproducibly disrupt leaf cells than low temperature storage. The system also modified atmospheres to induce multiple levels of wilt in young and old mature leaves. The model system successfully modified wilt levels, disrupted cells and induced leaf discolourations similar to those observed on stems with the postharvest browning disorder. That is, freeze injured older mature leaves became light-tan-brown to brown, whereas young mature leaves became dark-brown to black. Furthermore, freeze injured and wilted leaves appeared lighter coloured than did freeze injured and non-wilted leaves. Biochemical analysis suggests that dark pigments derived from the oxidation of phenolic compounds potentially caused B. myrtifolia leaf browning. Total phenolic compounds were reduced by ca. 33%, and browning indexes 357 and 418 increased ca.1.36 and 4.25 fold, respectively, for freeze injured young mature leaves. Young mature leaves had an ellagic acid content of 45.6 - 45.8 mg/g DW which declined to 25.2 – 28.5 mg/g following freeze injury, accounting for up to 73% of the observed decline in total phenolic compounds. Reduced phenolic compound oxidation in freeze injured old mature leaves appeared to account for the effect of leaf age on symptom colour variation. However, the effects of leaf water content on symptom colour appeared to be independent of changes in phenolic compounds and chlorophylls. The effects of wilt may alternatively be due to physical properties such as air ingress or altered cell structure. The postharvest browning syndrome is a low temperature storage disorder that manifests as a highly variable group of flower and leaf discolourations. These symptoms represent a continuum of susceptibility to low temperature injury, biochemical browning potential and leaf reflectance. Pre-harvest factor(s), influenced by temperature, determine cut B. myrtifolia stems susceptibility to low temperature injury. Susceptible stems stored at ≤5 °C became discoloured during, or upon removal, from storage. The appearance of this discolouration was influenced by leaf age and water content