Low irra(liances of 254 nm UV photoreversibly interfere with the capacity of plant cells properly to tutilize auixin for cell division, enlargement and (lifferentiationi (3,8,17). This UV-induced auixinrecalcitrance does not seem to be caused by activation of aulxin oxidase or by alteration of auxin metal)olism, penetration, or translocation. The reqlluirements for establishing synergistic interactions have been eluicidated by Lockhart (10) aind are ftulfilled in these experiments. Repressioln of abscission of explanlts of the puilvinuiis zonie of primary bean-leaf petioles is (lepeii(lnent ulpoll auxinl and growth (5). Puilvinuis explanits (2) were incubated in agar containiing 10 MM IAA at 200; uinder these conditions, 50 % of the explanits absciss in 72 hoturs (9). Irradiation of explanits at zero time with 5 mJ/cm2 UV had no effect on abscission with this concentration of auxin, althouigh higher irradiances increased the percentage abscission (8).Concentration-response experiments with selectedI anitimetabolites and antibiotics presente(d at zero time were conducted to select a concentration which had little or no effect on abscission; higher concentrationis invariably increased abscission. The abscissioni-inicreasing effect of at least 1 antimetabolite from each group was reversible by the appropriate metabolite. All experiments were repeated 3 times with abouit 25 explants per variable: resuilts are presented as the averaged percent abscission.There was no interaction between UV andl any tested anti-thymine (table I) (7), and mytomycini C, which interferes with DNA-dependent RNA and ribosome synthesis (4, 15), all interacted synergistically with UV in increasin-g abscissioni, but chloramphenicol w-as inactive (11).The failure of alnti-thymines to interact withl UV in abscission, anid the synergistic initeractions nioted with anti-uracils, RNA antibiotics, an(ld attimetabolites of proteill syniithesis all ind(licate that UV serves to modify the RNA and(I proteini metabolism niecessary for aulxin actioni in the maintenalnce of the initegrity of the abscission zonie (12). It is possilble that the lack of interaction betweeni antithymines and UV may be duie to a proportional increase in resistance of DNA to UV as inicreasing numbers of thymine sites are occupied by anitithymines, btut the lack of UV-anti-thymine synergism arguies against this possibility.Since UV radiation can cauise dimer fornation in uiracil and uiridine (14) and can directly inlactivate RNA (13, 16) oulr results give presumtnptive evidence (bIlt nlot proof) that UV-induiced aulxinl recalcitrance is caused by a modification i.i that RNA (and hence protein) synthesis involved in atlxin utilization. This conclusion serves as a basis for more (letailed stuldies of the phenomelnon. Acknowledgments