SYMPLASTIC SOLUTE TRANSPORT AND AVOCADO FRUIT-DEVELOPMENT - A DECLINEIN CYTOKININ ABA RATIO IS RELATED TO APPEARANCE OF THE HASS SMALL FRUIT VARIANT/

Studies on the effect of fruit size on endogenous ABA and isopentenyla denine (iP) in developing avocado (Persea americana Mill. cv. Hass) fr uit revealed that ABA content was negatively correlated with fruit siz e whilst the iP/ABA ratio showed a linear relationship with increasing size of fruit harvested 226 d after full bloom. The effect of this ch ange in hormone balance on the relationship between symplastic solute transport and appearance of the small fruit variant was examined follo wing manipulation of the endogenous cytokinin (CK)/ABA ratio. Applicat ion of ABA caused seed coat senescence and retarded fruit growth but t hese effects were absent in fruit treated with equal amounts of ABA pl us iP. Thus, the underlying physiological mechanisms associated with A BA-induced retardation of Hass avocado fruit growth appeared to be ine xtricable linked to a decline in CK content and included: diminution o f mesocarp and seed coat plasmodesmatal branching, gating of mesocarp and seed coat plasmodesmata by deposition of electron dense material i n the neck region, abolishment of the electrochemical gradient between mesocarp and seed coat parenchyma, and arrest of cell-to-cell chemica l communication.