Role of nitrogen remobilization from old leaves for new leaf growth of Eucalyptus globulus seedlings

Six-month-old Eucalyptus globulus Labill. seedlings were grown in sand culture irrigated with a nutrient solution containing 6.0 mol N m(-3) for 3 months (November-January). Before rapid growth began in February, seedlings were repotted and irrigated with either 6.0 mol N m(-3) (High-N treatment) or 1.0 mol N m(-3) (Low-N treatment). Seedlings were analyzed during the subsequent flush of growth to determine the role of old leaves, and in particular the leaf protein Rubsico, as a source of N for new leaf growth. During spring growth, the N content of old leaves of High-N seedlings decreased with decreasing leaf dry weight, although there was no change in leaf number. In High-N seedlings, the net loss of N from old leaves provided less than 10% of the N used for new leaf growth, and the new leaves quickly became the dominant sink for N. In contrast, in Low-N seedlings, the net loss of N from old leaves provided 44% of the N used for new leaf growth. During the period of spring growth, the amount of soluble proteins recovered from old leaves of Low-N seedlings dropped, but there was no change in the content of either Rubisco or chlorophyll. The photosynthetic capacity of old leaves remained constant throughout the study period, and there was no evidence that N was remobilized from Rubisco.