Delayed ripening of 'Bartlett' pears treated with nitric oxide

Preclimacteric European pears (Pyrus communis L. 'Bartlett') were either untreated or treated with 10 mul l(-1) nitric oxide (NO) at 20degreesC for 2 h and then kept at 20degreesC. Other lots were treated with 0, 10 or 50 mul l(-1) NO for 12 h. In pears treated with 10 mul l(-1) NO for 2 h the ethylene climacteric was delayed 4 d, maximum ethylene production decreased by 28% and firmness and colour changes had a 2 d delay. An additional treatment with 10 mul l(-1) NO after 4 d decreased ethylene production even further (48%). In contrast, pears treated with 50 mul l(-1) NO for 12 h displayed a 719-fold increase in ethylene biosynthesis 1 d following treatment. This sharp increase in ethylene emission, characteristic of a stress response, did not accelerate the normal progress of ripening-related events. In fruit treated with 10 and 50 mul l(-1) NO for 12 h, yellowing was delayed 2 d in comparison with control fruit while the softening rate was almost unaffected. Thus, different ripening processes are not influenced in the same fashion by NO treatments. Results suggest a time X concentration effect when NO is applied.