ALTERNATE BEARING IN PISTACHIO AS A MASTING PHENOMENON - CONSTRUCTIONCOST OF REPRODUCTION VERSUS VEGETATIVE GROWTH AND STORAGE

Whole-tree effects of alternate bearing on the reproductive growth, ve getative growth, and carbohydrate storage in mature pistachio (scion P istacia vera L., rootstock Pistacia atlantica Desf.) were examined. Al though it is generally accepted that the ''off'' year in pistachio may be used to accumulate carbohydrate reserves, it was found that starch and sugars from the ''off'' year equaled only 8% of the construction cost of the ''on'' year's fruit load. The dry mass increase in perenni al woody tissue of roots, trunk and branches in the ''off'' year (20.2 kg/tree), was 75% of the mass of fruit in ''on'' trees (26.4 kg/tree) . When construction cost of wood and fruit is considered,''off'' year woody tissue growth is 64 % of ''on'' year fruit growth. Since previou s studies have shown that leaf dry mass is generally 30% greater in '' off'' than ''on'' pistachio trees, we conclude that the overall produc tion of biomass in ''off'' and ''on'' year pistachio is similar, There fore, we suggest that the ''off'' year is not a period of substantial storage carbohydrate accumulation, but rather a switch in allocation f rom reproductive to vegetative growth. This is consistent with ecologi cally based theories of the adaptive advantage of masting behavior, an d suggests that withholding water and fertilizer during the ''off'' ye ar may not harm ''on'' year yield.