Role of Nitrous Oxide in Promoting Abscission in Citrus by the Abscission Agent Cmnp

The following study was conducted to determine if the abscission agent CMNP (5-chloro-3-methyl-4-nitro-1H-pyrazole) can produce nitrous oxide, if nitrous oxide (NO) promotes fruit abscission, and if alcohol dehydrogenase (ADH) can produce NO.  One-half of a ‘Valencia’ tree was sprayed to drip with 300 ppm CMNP in April, 2011.  Fruit detachment force (FDF) of treated fruit decreased from 80 N to 10 N.  NO increased in treated fruit from undetectable to about 1.2 nM/g fresh wt by 48 hours after application and declined to below 0.2 nM/g fresh wt by 120 h.  Additional untreated fruit were clipped from a ‘Valencia’ tree and dipped in a 2.0 mM solution of sodium nitroprusside, which releases NO upon exposure to water.  FDF of fruit dipped for 45 min in the solution and held at 25 °C declined similarly as application of CMNP to the field grown tree.  FDF of treated fruit held at 10 °C did not decline.  Fruit dipped in solutions had ADH activity ranging from 35 nM/min to 0 nM/min for the control and 1.5 mM CMNP, respectively, a result that has been reported previously.  Purified ADH in solution with CMNP added produced NO.  The results indicate that CMNP applied to field grown trees may be converted to NO by ADH and that the NO promotes abscission of sweet orange.