14C-Photosynthesis Partitioning in Avocado Trees as Influenced by Shoot Development

Abstract

The influence of shoot age on 14 C partitioning in potted avocado (Persea americana var. americana Mill.) trees was determined. The oldest leaf of actively growing shoots and the youngest leaf of previously matured shoots were exposed to 14 CO 2 18 and 34 days after budbreak (DABB) of new shoots. At these times, treated leaves had a positive net CO 2 assimilation rate and, therefore, were considered to be net C exporters. Sixteen days after 14 C exposure, separate plant tissues were harvested, dried, weighed, and oxidized. The percentage of 14 C in each tissue was determined by liquid scintillation spectrometry. Photoassimilates were translocated acropetally and basipetally from all treated leaves. However, at 18 DABB, developing leaves of actively growing shoots seemed to be the strongest sink for C assimilated by the oldest leaf of these shoots, whereas the roots were the strongest sink for C assimilated by the youngest leaf of the previously matured shoots. By 34 DABB, roots were the strongest sink for C assimilated by leaves of new and previously matured shoots. These data are useful in developing improved management strategies for controlling phytophthora root rot (incited by Phytophthora cinnamomi Rands) in avocados by systemic phosphonate fungicides translocated in the photoassimilate pathway. Thus, phosphonates should be applied after shoots have matured and most of the canopy is in a quiescent state for maximum translocation to the roots.