Salicylic Acid Inhibits Gravitropic Response
Researchers in Israel have investigated the effect of salicylic acid (SA) on the gravitropic bending response in flowering shoots. Gravitropic bending, which may occur during shipping, decreases the flower quality of some species. Shoots of snapdragon, lupinus, anemone and ranunculus were tested. After harvest, the shoots were held vertically overnight to allow for standard straightening. A 24-hour pulse treatment was given in the following solutions: SA at concentrations ranging between 0.1 and 5 mM, or citric acid, acetic acid, or formic acid at concentrations ranging between 1 and 50 mM. After the pulse, stems were placed in distilled water and placed at a horizontal orientation. The curvature angles were measured with a protractor at hourly intervals for 24 hours.
None of the SA concentrations tested inhibited the bending of the ranunculus stems. However, SA did inhibit gravitropic bending in the other species in a dose-dependent manner. In snapdragons, a 5 mM SA pulse completely prevented the gravitropic response during the 24 hours of observation while a 1 mM pulse inhibited bending only during the first 10 hours of gravistimulation. Anemone shoots displayed a significant inhibition of the bending response at SA concentrations of 0.5 mM. SA concentrations above 0.6 mM prohibited the bending of lupinus during the first 6 hours of horizontal placement. Up to 1 mM, no deleterious effects were observed on the flower shoots, however at 5mM, an increased number of wilted flowers were observed in lupinus and snapdragon.
The tests of citric, acetic and formic acid had no effect on snapdragon shoot bending. This disputed the hypothesis that the bending response is exerted via acidification of the cytoplasm.
By measuring three carefully defined zones of the snapdragon spikes for postharvest growth, the researchers observed that 5 mM SA inhibited both the gravity-induced process and general processes such as vertical growth. They concluded that SA specifically interferes with downstream processes of the gravitropism signaling pathway resulting in the inhibition of gravitropic bending of various cut flower shoots.
Friedman, H., S. Meir, A.H. Halevy, and S. Philosoph-Hadas. 2003. Inhibition of the gravitropic bending response of flowering shoots by salicylic acid. Plant Science 165 pp. 905-911.
Inhibition of Leaf Senescence in Alstroemeria
The first sign of senescence in Alstroemeria is often not flower drop, but yellowing of the leaves. Previous research has demonstrates that the yellowing is associated with chlorophyll breakdown that may be delayed with a treatment of gibberellins and cytokinins. Collaborating researchers from Italy and the University of California, Davis, explored the role of cultivar, environment and vase solution components on leaf and flower senescence in Alstroemeria.
Cut Alstroemeria were placed in solutions containing 100mM gibberellic acid (GA3), 100mM 6-benzylaminopurine (BA), a standard floral preservative containing 1.0% sugar, or 1 mM thidiazuron (TDZ), then exposed to ethylene gas for 24 hours. They also conducted pulse treatments of various concentrations of TDZ, other substituted phenyl-ureas (4-CPPU and 1,3-DPU), BA, or GA3 for 24 hours. Data collected included chlorophyll content, days to initiation of visible leaf yellowing and days to abscission of the first petal. Among the cultivars tested, wide variations were observed for the rate of leaf yellowing and petal drop. For example, within 5 days, leaf yellowing was visible in ‘Cuba’, ‘Saba’, ‘Petra’, and ‘Tamara’; however, leaf yellowing of ‘Rio’ did not occur until day 18. Leaf yellowing was substantially delayed in the flowers placed in GA3 and after the duration of the experiment (70 days), leaves on the TDZ-treated stems (1 mM) were still green. Pulse treatments of 10 mM TDZ were very effective, preventing leaf yellowing for more than 60 days, though TDZ pulse treatments at concentrations below 1mM were ineffective.
The variation in senescence rates among Alstroemeria cultivars should be considered in future breeding efforts to increase flower longevity. Currently, a GA4 pretreatment is part of the standard practice to prevent leaf yellowing. The TDZ results may provide a less expensive treatment alternative. TDZ is already registered in the US as an agricultural chemical for the defoliation of cotton (Dropp). Additional preliminary experiments have shown that TDZ also retards leaf yellowing in a range of other cut flowers and potted flowering plants including lilies, stock, tulip, iris, poinsettia and miniature roses.
Ferrante, A., D.A. Hunter, W.P. Hackett, and M.S. Reid. 2002. Thidiazuron—a potent inhibitor of leaf senescence in Alstroemeria. Postharvest Biology and Technology. 25(3), pp. 333-338.
Prolonging Flower Longevity in Alstroemeria
While many treatment options have been identified to inhibit leaf yellowing in Alstroemeria, little research has been focused on the flower longevity. Researchers in the United Kingdom considered the effects of exogenous ethylene and a sucrose vase solution on the flower longevity of cut Alstroemeria stems. Flowers were harvested from Alstroemeria ‘Rebecca’ just after the outer sepals separated. Longevity was terminated when at least one tepal had abscised.
This study also compared the longevity of flowers on cut stems to those blooming on an intact plant. The flowers remaining on the plant lasted approximately 4 days longer (12 days) than those on cut stems. They were also more deeply pigmented and were larger than flowers blooming on the cut stems.
Previous studies have reported that Alstroemeria produce very little ethylene. This study exposed the stems to various concentrations of CEPA and ACC, concluding this cultivar is very sensitive to exogenous ethylene.
A pre-treatment with 2mM silver thiosulfate (STS) for 1 or 3 hours prolonged flower longevity; however, a 6 hour pre-treatment resulted in blackened bracts indicating a toxic effect at this duration. Sucrose concentrations from 0.5 to 5.0% steadily increased longevity. After 5 days, those stems treated with 2.0 and 5.0% sucrose exhibited bract yellowing. Sucrose at 1% did not result in yellowing and prolonged flower longevity to 11.2 days. Combining the STS and sucrose treatments did not significantly alter vase-life compared to the individual treatments.
Additional experiments considered the effect of removing unopened buds from the axil of each flower. This practice extended the vase-life from 4.8 to 16.6 days and more than doubled the flower fresh weight of the remaining flowers. This result may suggest an internal competition for a limited supply of nutrients. It is important to note that this study only considered the first flower and by removing the unopened buds, vase-life may be decreased by eliminating the potential for further flowering.
Chanasut, U., H.J. Rogers, M.K. Leverentz, G. Griffiths, B. Thomas, C. Wagstaff, A.D. Stead. 2003. Increasing flower longevity in Alstroemeria. Postharvest Biology and Technology. 29, pp. 324-332.
Salicylic Acid Inhibits Gravitropic Response