Effects of pre-harvest factors and postharvest treatments on vase life of cut Acacia holosericea foliage stems

Abstract

Cut Acacia holosericea foliage stem have ornamental value in their silvery phyllodes. The cut stems fulfil most requirements for cut foliage. However, their inherent weakness is a relatively short vase life. Currently, no comprehensive research on the relationship between pre-harvest practices and postharvest life of this cut foliage line is available. Better understanding of pre-postharvest is required towards addressing vase life limiting factors will give a view to full commercialisation of A. holosericea as a cut foliage line. A retrospective desktop study of existing primary vase life data was undertaken. This approach based on previously collected vase life data was taken to acquire general information on vase life determinants. Cut A. holosericea in deionised water (DIW) had an averaged vase life of 6.4 ± 1.2 days. Vase life varied with season. Harvesting the cut stems in Summer and Autumn provided longer vase life of ≥7 days as compared to harvesting in Winter and Spring with a vase life of ≤5 days. Based on a mixed model analysis, season showed greater influenced on the cut stems vase life than maturity stage. In addition to pre-harvest conditions, the relative efficacy of postharvest treatments applied to the cut stem vase life was also assessed. Based on meta-analysis, investment in postharvest treatment to prolong vase life is worthwhile since 68.3% of the treatments have a high effect size (d). The analysis also showed that cut stem vase life was more affected by chemical postharvest treatments than physical postharvest treatments. The study was extended into elucidating senescence events in the cut stems. The findings confirmed that the short vase life problem was rooted in stem water deficit. Senescence in cut A. holosericea stems was associated with an increase in electrolyte leakage (EL) after 72 h for cut stems were held in DIW. Senescence was only detected visually when relative water content (RWC) of the cut foliage stems decreased to <92%. Among attributes studied, the senescence of cut A. holosericea showed high correlation with RWC (r = 0.90), hue value (0.85), EL (-0.78), and malondialdehyde (MDA) concentration (r = -0.76). Seasonal relationship with vase life of cut A. holosericea stems were investigated in detail from March 2011 to February 2012. Vase life proved to be inversely associated with pre-harvest relative humidity; vase life = -0.37RH + 34.65; R2 = 0.64. Temperature as reflected in season was correlated with the relative maturity of stems at harvest as indicated by significant relationship of acid detergent fibre (ADF) and acid detergent lignin (ADL) with temperature; ADF = 1.02 temperature + 30.71, R2 = 0.74 and ADL = 0.31temperature + 6.04, R2 = 0.73, respectively. Lignification of xylem helped support cut stems in the face of water deficit. Stem lignification increased with advancing maturity such that the stems became physically tougher. To characterise cut A. holosericea stems in term of relative lignification and vase life, a novel cutting device was devised and developed to measure their toughness. In this investigation, cut stems were sorted by visual apex maturity as very soft, soft, tough and very tough. However, vase lives recorded for these cut stems did not show any correlation with toughness of the cut stems ends. Visually, the physical characteristics of cut stems with long vase lives were <5 cm green apex tissue, narrow phyllodes (<6.60 cm width), and small stem diameters (<0.6 cm). Citric acid has been reported as a promising postharvest treatment to prolong the vase life of several cut Acacia species. Experimentation established that citric acid can also prolong vase life of cut A. holosericea foliage stems. A concentration of 10 mM citric acid was best for both vase solution and pulsing solution treatments. Ten mM citric acid treatments increased vase lives between 2.0 – 2.2 fold as a vase solution and 1.1 – 1. 4 fold as a pulsing solution. Stems held in citric acid vase solution had greater hydraulic conductance (Kh) as compared to stems held in either DIW or in 10 mM citric acid pulsing solution. Ten mM citric acid as a perfusion solution also improved Kh as compared to deaerated DIW perfusion. Sucrose alone ranging from 0.5 to 4%, or combination of 10 mM citric acid and 1% sucrose had negative effects on cut stems longevity. The sequential combination of 2.2 mM Cu2+ pulsing with 10 mM citric acid vase solution showed increased efficacy over citric acid alone. Overall, this study determined that short lived cut A. holosericea foliage stems suffer internal tissue water deficit early during senescence and that pre-harvest variables which include harvest time, maturity stage, and stem physical characteristics are indices of relative longevity. Postharvest experiments with citric acid alone and in combination with sucrose and Cu2+ established that citric acid in particular has potential as a postharvest treatment for extending cut A. holosericea foliage stem vase life.