EFFECTS OF SHADE ON XYLEM DEVELOPMENT IN SEEDLINGS OF EUCALYPTUS-GRANDIS HILL EX MAIDEN

Seedlings of E. grandis were grown under light regimes of 12.6 and 2.8 E [einstein] m-2 day-1 for 7 wk, during which time secondary xylem development was studied. The rate of cambial cell division was approximately 50% greater in the high light than in the low light treatment. Rates and durations of fiber expansion were similar, but the rate of wall thickening was almost 3 times as great in the high as in the low light treatment. Also, the duration of fiber wall thickening was estimated to be 7 days in high light as compared with 21 days in low light. Secondary vessel lumen area and the cross sectional area of vessels per unit area of leaf surface were greater in high than in low light plants, although the proportion of xylem cross sectional area occupied by vessels did not differ significantly between treatments.