Photosynthesis, Storage, and Allocation
137
by limiting carbon availability, but also through other mechanisms (BastowWilson 1988).
The leaf area maintained per leaf dry weight (specific leaf area, SLA, the
reciprocal of the specific leaf mass) increased when photosynthesis was
inhibited genetically (Fig. 7.1C), i.e., the investment of biomass per leaf
area produced declined when photosynthates were limiting. This resembles
changes in leaf investment patterns when light (Impatiens, Evans and Hughes
1961; sun and shade plants, Corre 1983; birch, Margolis and Vezina 1988) or
CO 2 (soybean, Allen et al. 1988; Cure et al. 1988; five annuals, Garbutt et
al. 1990) was altered. Lower carbon gains due to lower rates of photosynthesis are in part compensated for by an increased leaf area per leaf biomass
invested. In tobacco, limiting N nutrition decreased SLA at comparable
rates of photosynthesis (Fig. 7.1C). This was mainly due to increasing
amounts of starch present in the leaves at N deficiency (cf. Fig. 7.2A). If
SLA was corrected for starch the difference between N treatments largely
disappeared (see Fichtner et al. 1993), although SLA still increased with
decreasing photosynthesis.
The leaf area maintained per total plant dry weight (leaf area ratio, LAR)
can be viewed as the integration of the partitioning of whole-plant biomass
to root and shoot, (the latter primarily leaves in the case of the young
tobacco plants), and of leaf biomass to leaf area. LAR, similarly to SLA
(compare Fig. 7.1D and C), decreased with photosynthesis and increased
with N supply. The range of LAR within the HN treatment was higher than
the range of SLA because of the concomitant increase in shoot partitioning
when photosynthates were limiting (Fig. 7.1B). The difference between the
N treatments was larger for LAR than SLA due to the lower shoot/root
patitioning at decreasing N supply. Thus, the leaf area maintained per total
plant dry weight is enhanced when the carbon gain of the photosynthetically
active leaf area is low, but is reduced by low N supply, which favors biomass
partitioning to roots rather than leaves (e.g., Corre 1983).
7.2.3 Carbon and Nitrogen Storage in Relation to Photosynthesis
Carbon stored as starch in the leaves increased nonlinearly with photosynthesis and decreased with N supply at comparable rates of photosynthesis
(Fig. 7.2A). Starch content increased with photosynthesis most markedly
when growth did not respond, as in the LN treatment and the MN treatment
at high photosynthetic rates (Fig. 7.1A). Similar observations were made in
a more qualitative way when light (Waring et al. 1985; McDonald et al.
1986) or CO2 (Raper et al. 1973; Allen et al. 1988) was increased. In
general, the starch content increases with higher availability of photosynthates or lower N availability, i.e., when carbohydrate availability exceeds
the demand for growth.
137
by limiting carbon availability, but also through other mechanisms (BastowWilson 1988).
The leaf area maintained per leaf dry weight (specific leaf area, SLA, the
reciprocal of the specific leaf mass) increased when photosynthesis was
inhibited genetically (Fig. 7.1C), i.e., the investment of biomass per leaf
area produced declined when photosynthates were limiting. This resembles
changes in leaf investment patterns when light (Impatiens, Evans and Hughes
1961; sun and shade plants, Corre 1983; birch, Margolis and Vezina 1988) or
CO 2 (soybean, Allen et al. 1988; Cure et al. 1988; five annuals, Garbutt et
al. 1990) was altered. Lower carbon gains due to lower rates of photosynthesis are in part compensated for by an increased leaf area per leaf biomass
invested. In tobacco, limiting N nutrition decreased SLA at comparable
rates of photosynthesis (Fig. 7.1C). This was mainly due to increasing
amounts of starch present in the leaves at N deficiency (cf. Fig. 7.2A). If
SLA was corrected for starch the difference between N treatments largely
disappeared (see Fichtner et al. 1993), although SLA still increased with
decreasing photosynthesis.
The leaf area maintained per total plant dry weight (leaf area ratio, LAR)
can be viewed as the integration of the partitioning of whole-plant biomass
to root and shoot, (the latter primarily leaves in the case of the young
tobacco plants), and of leaf biomass to leaf area. LAR, similarly to SLA
(compare Fig. 7.1D and C), decreased with photosynthesis and increased
with N supply. The range of LAR within the HN treatment was higher than
the range of SLA because of the concomitant increase in shoot partitioning
when photosynthates were limiting (Fig. 7.1B). The difference between the
N treatments was larger for LAR than SLA due to the lower shoot/root
patitioning at decreasing N supply. Thus, the leaf area maintained per total
plant dry weight is enhanced when the carbon gain of the photosynthetically
active leaf area is low, but is reduced by low N supply, which favors biomass
partitioning to roots rather than leaves (e.g., Corre 1983).
7.2.3 Carbon and Nitrogen Storage in Relation to Photosynthesis
Carbon stored as starch in the leaves increased nonlinearly with photosynthesis and decreased with N supply at comparable rates of photosynthesis
(Fig. 7.2A). Starch content increased with photosynthesis most markedly
when growth did not respond, as in the LN treatment and the MN treatment
at high photosynthetic rates (Fig. 7.1A). Similar observations were made in
a more qualitative way when light (Waring et al. 1985; McDonald et al.
1986) or CO2 (Raper et al. 1973; Allen et al. 1988) was increased. In
general, the starch content increases with higher availability of photosynthates or lower N availability, i.e., when carbohydrate availability exceeds
the demand for growth.