CHANGING SINK STRENGTHS INFLUENCE TRANSLOCATION OF PHOSPHONATE IN AVOCADO (PERSEA-AMERICANA MILL) TREES

Translocation of phosphonic acid (H3PO3) in cv. Hass avocado trees was studied after trunk injection with 20% H3PO3, formulated as potassium phosphonate, at three stages of tree phenology during the growing sea son. Initially, translocation was solely acropetal in the xylem, and H 3PO3 was detected in the leaves 24 h after treatment. Several days aft er injection, H3PO3 concentration in the bark of trunks and in roots i ncreased, indicating basipetal phloem transport of H3PO3 from leaves. The rate of accumulation and the final concentration of H3PO3 in the r oots were directly related to the sink strength of the shoot at the ti me of injection. For example, trunk injection at the beginning of spri ng growth flush, when renewal shoots were strong sinks, resulted in lo w H3PO3 root concentrations (<9 mu g g(fw)(-1)) which peaked about 45 days after treatment. When potassium phosphonate was injected after th e transition of spring-grown shoots from sinks to sources, or at summe r shoot maturity, root concentrations of H3PO3 increased to greater th an or equal to 25 mu g g(fw)(-1) by 30 days after treatment. These res ults suggest that strategic timing of injections according to phenolog ical events may greatly improve fungicide efficacy when targeting spec ific organs for protection.