Nasholm T
Removal of Nitrogen During Needle Senescence in Scots Pine (Pinus-Sylvestris L)
Oecologia: 1994 99:290-296
The concentrations of arginine, protein and total nitrogen (N) and the abundance of N-15 were measured in 3- and 4-year-old needles of Scots pine trees fertilized with either 0 (C), 36 (N1) or 73 (N2) kg N ha(-1) year(-1) annually for 22 years (average doses of N). Remaining green needles and needles that were shed were compared and removal of N from total, protein and arginine pools was calculated. Earlier investigations had shown that high arginine concentrations are found in needles of trees that have an excessive N supply (Nasholm and Ericsson 1990). This study aimed to elucidate the fate of the accumulated arginine during needle senescence. It was speculated that a low removal of arginine during senescence would implicate that the primary function of arginine is in N detoxification and not in N storage. Moreover, litter quality would be altered if needles are shed with high concentrations of arginine and this might affect the turnover of N in forest ecosystems. In remaining green needles, the concentration of total N increased with increasing N supply. Protein N concentrations were higher in fertilized trees, but did not differ between the two N treatments. Arginine N was low in C and N1 trees but high in N2 trees. Senescent needles from C and N1 trees had about equal total N concentrations while in N2 trees this concentration was significantly higher. Protein N in senescent needles did not differ between treatments. Arginine N, however, was less than 0.1 mg g(-1) dw in C and N1 trees but was higher than 1.5 mg g(-1) dw in N2 trees. Removal of N was highest in N1 trees followed by C trees while N2 trees removed least N from senescing needles. The high concentration of total N in senescent needles from N2 trees was to a great extent explained by a high arginine concentration. The delta(15)N value of remaining, green needles was higher (less negative) in N2 trees than in C and N1 trees. The same pattern was found for senescent needles. Comparisons of delta(15)N values between remaining, green and senescent needles within each treatment showed a significant increase in delta(15)N for all treatments during senescence possibly indicating losses of N as NH3 (g) from needles during senescence. It is concluded that arginine, accumulated in response to high N supply, is retranslocated only to a small extent during needle senescence. The ecological and physiological implications of this finding are discussed.
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