Biosynthesis of zeatin from N-(Delta-isopentenyl)adenine in Actinidia: Sites and seasonal changes in activity.

In Actinidia arguta (hardy kiwifruit) plants, the potential to accumulate the cytokinin zeatin (io(6)Ade) during feeding with a precursor, N(6)-(Delta(2)-isopentenyl)adenine (i(6)Ade), varies depending on the tissue. This can be demonstrated by incubating explants for 24 hr on a basal nutrient medium supplemented with 30 muM i(6)Ade and then extracting the tissues and analyzing cytokinin contents using HPLC methodology. Under these conditions, the potential for io(6)Ade accumulation in tissue slices from growing roots was 93 nmol/g at the root tip, >200 nmol/g immediately behind the tip (1.0-mm diameter), approximately 100 nmol/g in 2-mm diameter root, and progressively lower in older tissues. A similar gradient of io(6)Ade accumulation was detected in growing stems, with relatively low activity (40 nmol/g) in the terminal 0.5 cm, approximately 170 nmol/g in the 5- to 15-cm interval, and about 25 nmol/g in stem tissues taken 15-100 cm from the tip. Growing leaves accumulated little io(6)Ade (7-26 nmol/g) during feeding, as did fruits (0-8 nmol/g) at various stages of development and maturation. Seasonally, root activity was detected as early as March 1, whereas stem activity did not appear until March 15. Thus, the activation of i(6)Ade metabolism in both of these organs preceded sap flow (March 29) and bud break (April 16) by several weeks. The results suggest that root and stem tissues may be sites of cytokinin biosynthesis in growing Actinidia plants and that cytokinin production may not be the critical factor controlling the beginning of shoot growth in the spring.