Sucrose Metabolism and Growth in Transplanted Loblolly Pine Seedlings

Loblolly pine (Pinus taedu L.) seedling height, root collar diameter (RCD) and the specific activities of three sucrose metabolizing enzymes, namely, sucrose spthase (SS), acid invertase, and neutral invertase, were measured to assess seedling responses to transplant stress. Bare-root nursery-grown loblolly pine seedlings were lifted and transplanted immediately into nearby nursery beds in February 199 1. It was found that transplanted seedlings grew less in height and RCD and had lower SS activity than nontransplanted controls during the first 7 months after transplanting. However, the seasonal patterns for seedling growth and SS activity were not changed by transplanting. Stems, with SS as the dominant sucrose metabolizing activity, were a stronger growth sink than taproots between spring aridearly fall. Taproot SS activity decreased from spring to summer and increased again in late summer. Toward late fall when terminal buds were set and RCD growth stopped, taproot became a dominant growth sink. It was concluded that i) SS was the dominant enzyme for sucrose metabolism in actively growing pine stems and. taproots; ii) there were decreases in seedling growth and SS activity in transplanted seedlings as compared with nontransplanted controls; and iii) SS can be used as a biochemical indicator for growth sink strength and for stress caused by transplanting. ,