NITROGEN USE WITHIN THE GROWING LEAF BLADE OF TALL FESCUE

Leaf elongation rate (LER) of grasses depends on N supply and is expre ssed mostly through cell production, whereas most N in mature leaf tis sues is chloroplastic. Our objective was to evaluate a possible compet ition for N between cell production and chloroplast development proces ses, utilizing the gradient of cell development along the leaf growth zone of tall fescue (Festuca arundinacea Schreb.). Under the two contr asting N regimes, total N content was highest in the cell production z one, declined sharply as cells elongated, and remained relatively cons tant in more distal positions, at values close to those measured in ma ture tissues. A similar pattern was found for N in proteins and nuclei c acids that were not soluble in 80% ethanol. Content of N compounds s oluble in 80% ethanol was higher in the cell production and elongation zones than in mature parts of the leaf. NO3- N content was low in the cell production zone and increased in the cell elongation zone for hi gh-N plants. The deposition rate of total N in the growth zone was muc h higher with plants in high N than in those shifted to no N. For both N regimes, most N was deposited during cell production and early cell elongation. Little N was deposited during cell maturation where ribul ose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was being activel y synthesized. This suggests that synthesis of Rubisco, and probably o ther chloroplastic proteins, occurs largely from recycling of N that w as previously incorporated into proteins during cell production. Thus, Rubisco content in mature tissues is more closely associated with N d eposited during cell production than with N deposited during its biosy nthesis.