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Ice Nucleation Active (INA) Agents in Freezing of Young Citrus Trees


Aqueous suspensions of ice nucleation active (INA) bacteria [Pseudomonas syringae van Hall and Erwinia herbicola (Löhnis) Dye] and suspensions of nonbacterial agents, silver iodide, phenazine, and flurophlogopite, were used to induce freezing in young citrus trees, ‘Valencia’ orange [Citrus sinensis (L.) Osbeck] on sour orange (C. aurantium L.) rootstock. Trees sprayed with INA agents froze at higher temperatures than unsprayed trees. INA bacteria-induced freezing was significant only when leaf surfaces were allowed to dry prior to freeze tests. Leaves with dry surfaces supercooled 1° to 3°C more than wet leaves, which started to freeze, and about 1° sooner with than without INA agents. Differences between INA bacteria and nonbacterial agents were not significant in the moment of freeze of wet leaves. INA agents induced freezing in citrus leaves usually before −5°, and in water drops, before −4°. Freezing was easier to induce on the underside (abaxial) than top (adaxial) surfaces of leaves. Sucrose, proline, and expressed sap were nonINA on citrus leaves and in drops of water.