Developing innovative pulse solutions and alternative technologies through understanding physiological processes critical to maintaining postharvest quality in Proteaceae cut flower stems

The project aimed to obtain a better understanding of the postharvest physiology and carbohydrate metabolism of Protea cut flower stems that are susceptible to the postharvest disorder, leaf blackening – in order to improve on current pulsing techniques and/or to develop new innovative postharvest treatments to maintain product quality for the extension of vase life. The efficacy of numerous pulsing reagents were tested: water (control), glucose (industry standard), trehalose, lactulose, ascorbic acid, glycine betaine, amino butyric acid (GABA), sodium nitroprusside (SNP) and nectar biosynthesis inhibitors (cycloheximide and chloramphenicol).

Cut flower products were sourced from producers in the Western Cape, depending on availability during the season.  Products included cultivars or species selections from the genus Protea (P. magnifica, “Sylvia”, “Susara”, “Sharonette”, “Pink Ice”, “Brenda”, “Limelight”). Stems were freshly harvested by producers (untreated) and sent to the SU laboratories directly or collected via Sky Services, Cape Town International airport.  Temperature was monitored throughout the cold chain, and products were kept at a storage temperature of 1-2˚C in standard flower carton boxes. Upon arrival at the lab, stems were graded to 40-50cm, and only 10-20 leaves (closest to the flowerhead) were kept.

Stems were subjected to pulsing with different concentrations of the previously mentioned reagents. Pulsing involved placing single stems in test tubes containing 10ml of the respective pulsing agent and allowing the stems to absorb/ take up the solution. Thereafter, stems were transferred to vases containing tap water and were evaluated for leaf blackening and flower quality over 14 days, unless one treatment reached maximum blackening, in which case the trial was terminated. For certain trials, leaf material was harvested and subjected to a carbohydrate analysis to determine the sugar composition. All scoring data were subjected to a repeated measures ANOVA in Statistica.

Some of the pulsing agents that we used showed potential, whilst others did not have a positive effect at all (e.g. ascorbic acid, cycloheximide, and chloramphenicol). Trehalose and lactulose (both sugars) showed promise; in some experiments, leaf blackening levels were lower compared to stems pulsed with glucose. However, we struggled with replication. Glycine betaine worked for certain cultivars only e.g., ‘Sylvia’ and ‘Brenda’. SNP showed no potential to control leaf blackening but may be used to improve flower quality. GABA has potential to improve flower quality and leaf blackening.

Leaf blackening remains a challenge. It is evident that the disorder is driven by carbohydrate depletion as cultivars respond to sugar pulsing, and leaves of sugar-pulsed stems also contain higher carbohydrate levels. How these sugars are metabolized in the leaves is still unclear, and a better understanding of this is needed. The disorder consists of a complex chain of events, and it is difficult to resolve and produce a “pulsing-mix” that will work for every cultivar. Although we did not find clear solutions to alleviate leaf blackening, we managed to identify two pulsing agents that at least contribute toward better flower quality in Protea cut stems.

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