In southern Europe, the intensive use of 2,4-D (2,4-dichlorophenoxyacetic acid) and tribenuron-methyl in cereal crop systems has resulted in the evolution of resistant (R) corn poppy (Papaver rhoeas L.) biotypes. Experiments were conducted to elucidate (1) the resistance response to these two herbicides, (2) the cross-resistant pattern to other synthetic auxins and (3) the physiological basis of the auxin resistance in two R (F-R213 and D-R703) populations. R plants were resistant to both 2,4-D and tribenuron-methyl (F-R213) or just to 2,4-D (D-R703) and both R populations were also resistant to dicamba and aminopyralid. Results from absorption and translocation experiment revealed that R plants translocated less [14C]-2,4-D than S plants at all evaluation times. There was between four and eight-fold greater ethylene production in S plants treated with 2,4-D, than in R plants. Overall, these results suggest that reduced 2,4-D translocation is the resistance mechanism in synthetic auxins R corn poppy populations and this likely leads to less ethylene production and greater survival in R plants.

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To better understand the key processes involved in the ripening of attached fruit, we have investigated physico-chemical and biochemical changes occurring in 'Blanquilla' pear during on-tree (attached fruit) and off-tree ripening (harvested fruit). Flesh firmness, sugars, acids and the volatile profiles as well as ethylene metabolism, PG and PME enzyme activities and oxidative damage were measured. Firmness loss in detached 'Blanquilla' pear (off-tree), was initially mediated by oxidative stress (higher accumulation of malondialdehyde) and then by ethylene in a process in which 1aminocyclopropene 1-carboxylic acid (ACC) synthase was the limiting factor. In contrast the progressive but slower softening observed during on-tree fruit ripening was not associated to oxidative damage but rather to a delayed production of ethylene limited, in turn, by the activity of ACC oxidase. An interesting association was found between the initiation of the ethylene production and a concomitant increase of sucrose levels during on-tree ripening also accompanied by a decline in hexanal. The putative role of these compounds as a tree-associated factor modulating on-tree pear ripening is discussed.

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