The effects of chemical pretreatments on the postharvest physiology of lisianthus (eustoma grandiflorum shinn.) cut flowers

Abstract

This study was conducted to investigate the role of exogenously applied 6- benzylaminopurine (BA) and gibberellic acid (GA3) in the postharvest physiology of cut lisianthus flowers. The suitability of biocides that included sodium hypochlorite (Jik "') and aluminium sulphate was tested. Silver thiosulphate (STS) and deionised water were used to compare the efficacies. Combined BA and GA3 treatment of 16 ppm BA + 2 ppm GA3 increased vase life of cut lisianthus inflorescences by 77% compared to the STS positive control and 150% compared to deionised water negative control. Well structured synergistic and additive effect zones were found to characterize the influence of the combined BA and GA3 treatments in delaying cut lisianthus florets senescence and cumulative water balance (marginal fresh weight changes) as these PGRs worked in tandem. High synergistic effect was manifested by the combined BA and GAJ treatment 32 ppm BA + 2 ppm GA3, which emerged overall most effective preservative solution in the prevention of autocatalytic ethylene evolution, producing 2.6. 5.9 and 7.5 times less ethylene than florets treated with STS, 0 ppm BA + 0 ppm GA3 (placebo) and deionised water control respectively. STS prolonged vase life of cut lisianthus flowers by 116% relative to the deionised water control, however, aluminium sulphate and sodium hypochlorite did not significantly influence vz se life. The cumulative water balance or the lisianthus shoots treated with the hormonal combinations was significantly positively correlated to the duration to occurrence of 50% florets wilting and to loss of leaf turgor. This implies that the improvement observed on the above parameters were a direct - 1 - benefit from the improved water balance following the BA and/or GA3 pretreatments. The fact that vase life of cut lisianthus shoots treated with the hormonal combinations was significantly negatively correlated to the ethylene evolution and mean respiration rate; this implies that, vase life termination in lisianthus is ethylene driven. More evidence on ethylene's detrimental effects especially on lisianthus cut flower longevity and chlorophyll content or chlorosis; was unveiled through the comparison of the efficacy of STS relative to that of the deionised water control, the placebo and the hormonal treatment combinations. It is therefore concluded that, lisianthus cut flowers are ethylenesensitive and pre-treatment with BA and/or GA3 suppresses the rate of ethylene biosynthesis as well as sensitivity to ethylene. Possible modes of action are discussed. The use of combined BA and GA3 of the concentration 16 ppm BA + 2 ppm GA3 is recommended in cut lisianthus pre-treatment. These BA/GA3 pretreatments are environmentally friendly unlike STS, and are thus acceptable for all major CLltflower handling protocols such as EUREP-GAP and Kenya-GAP among others. Therefore. the findings of this study are of direct benefit to the entire cut lisianthus handling chain from the grower's grading hall, to the retailer's shop and through to the consumer's vase. -