Prevention of bud and floret abscission in Cestrum cut flowers is related to the mode of transport and metabolism of synthetic auxins

Red Cestrum (Cestrum elegans Schlecht.) cut flowers are considered as a new export crop from Israel. However, the cut flwers exhibit both bud and floret abscission during shipment. Pulsing cut flowers for 4 h at 20°C and 16 h at 4°C with 2,4-dichlorophenoxyacetic acid (2,4-D) combined with sliver thiosulfate (STS) significantly reduced abscission, while a similar treatment with 1-naphthaleneacetic acid (NAA) failed to do so. Contrary to its inhibitory effect on floret abscission, 2,4-D induced a higher level of ethylene evolution in the inflorescences compared to NAA. The data suggest that 2,4-D moved acropetally in a significant amount, sufficient to reduce floret abscission, while NAA did not. These unexpected results led us to study the mode of transport of 2,4-D and NAA in stem sections and cut flowers. In stem sections, transport of NAA was polar with a velocity of 5.9 mm per h. Although 2,4-D exhibited a polar transport, it was also transported non-polarly showing a fast transport during the first 15 minutes both in the acropetal and basipetal directions, followed by progressively increased basipetal polar transport and decreased acropetal transport. During the transport period of 3 to 11 h, 2,4-D moved at a rate of 1.9 mm per h in the basipetal direction. Studies with cut flowers also showed a significant acropetal transport and accumulation of 2,4-D in the upper parts of the stem, leaves and florets, while NAA mainly accumulated in the lower parts of the stem. In addition, NAA was metabolized faster in florets and leaves compared with 2,4-D. This suggests that like in other abscission systems, the level of free auxin in the tissues controls abscission of buds and florets in cut flowers.