This paper quantifies the effects of N-stress on development and growth of
sorghum by identifying critical values for stover N% (SNC) and specific lea
f nitrogen (SLN) for a range of physiological processes (rates of leaf appe
arance, leaf expansion, leaf area increase, and biomass accumulation). It a
lso compares the merits of the SNC and SLN approach for implementation in t
he N-routines of crop growth simulation models. Data from experiments cover
ing a range of nitrogen treatments, grown at three contrasting locations in
Australia, were used in the analyses. For the rate of biomass accumulation
, the critical and minimum SLN were constant for most of the growing season
, except for early stages prior to approximately panicle initiation. For SN
C, however, the critical and minimum values continued to decline throughout
the growing season, although the decline was limited at later development
stages. During the stage of leaf growth, the critical N content was similar
for all processes considered. Although critical SLN was less sensitive to
development stage than SNC, identification of a critical SLN was hampered i
f environmental effects on specific leaf area were present. The merit of ea
ch approach in simulation models will depend upon their ability to capture
genotypic differences in N-dynamics. (C) 2001 Elsevier Science B.V. All rig
hts reserved.