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Citrate-phosphate buffer induced olive (Olea europaea L.) leaf abscission at pH 3, 5, or 7. Of several sources of P-induced leaf abscission, NaH₂PO₄ was the most effective. Sensitivity to P was shown by leaf abscission, leaf desiccation, or both among 32 species representing 22 genera. Applied P accumulates in petioles, but its effect on abscission does not seem to depend on ethylene production. The low P content in the ethephon molecule may have an additive effect in ethephon-induced abscission. Stem-fed NaH₂PO₄ or ethephon resulted in total leaf and fruit abscission. Of the chemicals applied as a foliar treatment, only NaH₂PO₄ caused fruit abscission with minimal leaf loss. Adding Al₂O₃ to adsorb P in treatment solutions delayed the abscission effect of ethephon and NaH₂PO₄. Adding glycerol to NaH₂PO₄ increased fruit abscission from 50% to ≈80% and leaf abscission from ≈9% to 18%. The presumed effect of glycerol is from slowing the drying rate and thereby increasing P penetration into the fruit abscission zone. The pedicel-fruit cavity is a collection basin for spray accumulation that is not present in the leaf petiole attachment to the stem. This morphological difference probably leads to greater absorption of abscission-inducing materials by fruit. Chemical name used: (2-chloroethyl)phosphonic acid (ethephon).

Abstract

The Nitrate Compensation Points (NCP) of 4 plum clones, Marianna 2624 (Prunus cerasifera Ehrh. × P. munsoniana Wight & Hedr.?), M-17 (P. cerasifera × P. munsoniana?), Myrobalan 3-J (P. cerasifera) and Myrobalan B. (P. cerasifera) were determined following ${\text{NO}}_3^{-}$ depletion from aerated nutrient solutions. Differences among NCP's were statistically significant; however, since the NCP's were low (ca. 5 μm ${\text{NO}}_3^{-}$ ) relative to the [ ${\text{NO}}_3^{-}$ ] of typical agricultural soils, the relevance to ${\text{NO}}_3^{-}$ uptake effectiveness under field conditions remains unresolved. Maximum ${\text{NO}}_3^{-}$ influx took place above 20 μm ${\text{NO}}_3^{-}$ , but 15 hour exposures to higher ambient [ ${\text{NO}}_3^{-}$ ] resulted in higher NCP's. Both decreased influx and increased efflux of ${\text{NO}}_3^{-}$ may have contributed to higher NCP.