Dynamics of shoot N, leaf area, and shoot mass for individual plants under controlled conditionsThe changes of N accumulation in shoots in relation to shoot mass for individual plants growing at low density (20 and 40 plants m
–2) under controlled conditions is shown in
Fig. 1. It is possible to fit an allometric relationship between shoot N (
Nsh) and shoot mass (
Wsh) for both densities, according to
equation 1:
| 20 plants m -2 : Nsh = 0.114 (Wsh)0.881R2 =0.997 |
(1.1) |
| 40 plants m -2 : Nsh = 0.112 (Wsh)0.840R2 =0.998 |
(1.2) |
The value of coefficient
a is similar for the two densities, indicating that the level of N supply for the two experiments was roughly the same. The value of coefficient
b is significantly higher at the low plant density (0.881) than at the high density (0.840), indicating that N accumulation in the shoot (
Nsh) increases less rapidly with shoot mass when plant density increases from 20 to 40 plants m
–2.
On the set of plants with a density of 40 plants m–2, it was possible to represent the changes of expansion of leaf area (LA) with the shoot mass. LA increases less than proportionally to shoot mass according to an allometric function:
The corresponding fitted equation is:
| LA = 0.671 (Wsh)0.849 R2 = 0.956 |
(2.1) |
The allometric coefficient
b' is close to the value of
b=0.840 in
equation 1.2 in
Fig. 1. As a result, if it is postulated that
b=
b', it is possible to derive a linear relationship between N accumulation in the shoot and
LA at the individual plant level:
where the coefficient
a/
k1 represents the quantity of N that must accumulate in the shoot in order to elaborate a new
LA unit.
Figure 2 shows the fitted equation for the plants growing at a density of 40 plants m
–2. The intercept of this regression is not different from 0. The slope of the regression is equal to 1.67 g N m
–2, so the quantity of N accumulated in the shoot remains strictly proportional to
LA. Unfortunately, the lack of data on
LA for the experiment at the lower plant density does not make it possible to confirm such a relationship.
Coefficient
k1 of
equation 2 represents the value of
LA for a plant when
W=1. Such a coefficient can be considered as the ‘intrinsic leafiness’ of the plant. Coefficient
a of
equation 1 represents the shoot N content for a plant when
W=1, corresponding to the ‘intrinsic shoot N concentration’. According to
equation 2, as the plant gets bigger, its
LA increases at the same time as its shoot N content at the same fractional rate, leading to proportionality between shoot N accumulation and
LA during plant development.
Dynamics of crop N, leaf area, and biomass at the stand level in the field
The accumulation of N in a lucerne stand in relation to crop biomass was calculated according to equation 1:
| Nsh = 0.161 (Wsh)0.723R2 = 0.981 |
(1.3) |
The value of coefficient
b=0.723 is close to the range of values of 0.64–0.71 obtained by Lemaire
et al. (1985)
for different regrowth periods of lucerne in dense stands in the field. The relationships between leaf area index (
LAI) and shoot mass corresponding to
equation 2 reveal a similar pattern:
| LAI = 0.052 (Wsh)0.809R2 = 0.951 |
(2.2) |
As a consequence, it is possible to derive a linear relationship between the accumulation of N in shoots and LAI, according to equation 3, as shown in Fig. 3. The slope of this regression (1.77 g N m–2) is much closer to the slope obtained under controlled conditions at a much lower plant density (Fig. 2). Nevertheless, this value is obtained by a different combination of coefficients a and k1 where these ratios remain relatively unaffected by the differences in growth conditions: low versus high plant density and controlled versus field conditions. A lower value of a (low intrinsic shoot N concentration) appears to have been entirely compensated for by a lower value of k1 (low intrinsic ‘leafiness’), leading to a remarkably constant shoot N content per unit of LA.
Partitioning of N between individual plants within the canopyFor each group of plants, dominant (D), intermediate (I) or suppressed (S), a relationship was obtained between shoot N accumulation (
Nsh) and shoot plant mass (
Wsh) according to
equation 1:
| D : Nsh = 0.441 (Wsh)0.687R2 = 0.972 |
(1.4) |
| I : Nsh = 0.035 (Wsh)0.674R2 = 0.924 |
(1.5) |
| S : Nsh = 0.026 (Wsh)0.747R2 = 0.862 |
(1.6) |
In Fig. 4A, these regressions were represented on a log–log scale to provide an easier comparison between plants with considerable size differences. For a given class of plants, each data point represents the average value of a set of plants of this class at a given time. The D and I classes of plants show a similar evolution: the values of coefficient b are not different (P >0.05), but the values of the intercept are slightly different. This indicates that the intrinsic shoot N content (i.e. Nsh for Wsh=1 g) of intermediate plants is slightly lower than that of D plants (35 mg N versus 41 mg N). Suppressed plants show a major reduction of shoot N when compared with other plant classes at similar plant mass, but the slope of the regression (coefficient b) is higher (P that the relative difference in shoot N with other plant classes progressively diminishes as the crop develops. Nevertheless, care must be taken when analysing data for S plants. At the successive data points for each category, it was observed that some plants got bigger but maintained the same hierarchical position within the canopy. However, for the S class, the smaller plants progressively died as a result of self-thinning. This inevitably led to a drift in the population because more S plants, i.e. those with a lower intrinsic shoot N concentration, were progressively eliminated from the samples. Such a problem could explain the lower correlation coefficient for this category of plants and the fact that the slope of the regression appears to be different (the S plants remaining at the end of the period being progressively less suppressed than at the beginning). Nevertheless, the data in Fig. 4A confirm the hypothesis that the hierarchical position of plants within the canopy is a reflection of shoot N accumulation, i.e. plants in the dominant position have a higher shoot N content as opposed to plants in a more suppressed position, when compared at similar plant mass. Unfortunately, there was no possibility of measuring LA per plant for each height category. As a result, data on leaf mass were used instead of LA to analyse the developmental change in plant morphology. It was therefore possible to calculate an allometric relationship obtained between leaf mass (WL) and shoot mass (Wsh), according to the general equation:
For the different plant categories, the equations are as follows:
| D : WL = 0.505 (Wsh)0.689R2 = 0.960 |
(4.1) |
| I : WL = 0.448 (Wsh)0.689R2 = 0.873 |
(4.2) |
| S : WL = 0.351 (Wsh)0.768R2 = 0.831 |
(4.3) |
Figure 4B shows the regressions on a log–log scale to provide an easier comparison of plant categories with different sizes. As discussed above, the higher slope obtained for the S plant category could be due to the fact that more S plants progressively died as the canopy developed. The intercept of the regression (value of
k2) makes it possible to discriminate between the three categories of plants, revealing that S plants are less leafy than D plants for a similar plant mass. For a D plant with a shoot mass of W=1 mg, the ratio
WL/
Wsh, that is, the leaf weight ratio (
LWR), is 0.5, corresponding to a leaf:stem ratio (
L:S) of 1, while at the same shoot mass of W=1 mg, an I plant should have a
LWR of 0.45 (
L:S=0.82), and a S plant should have a
LWR of 0.35 (
L:S=0.54). For the three categories, the allometric coefficients are very close to the corresponding values of coefficient
b calculated with
equations 1.4,
1.5, and
1.6. Therefore, by combining equations
1.4,
1.5,
1.6, and
equations 4.1,
4.2, and
4.3, it is possible to derive a linear relationship between shoot N accumulation (
Nsh) and leaf mass (
WL) for each of the three categories of plants:
| D : Nsh = 0.083WL - 0.002 R2 = 0.976 |
| I : Nsh = 0.073WL + 0.002 R2 = 0.938 |
| S : Nsh = 0.073WL + 0.001 R2 = 0.961 |
The value of the intercept is not different from zero for the three plant categories. The slope of the regression appears significantly higher for D plants (0.083 g N g–1, P=0.04) than for I and S plants (0.073 g N g–1). Another way of estimating this ratio between shoot N accumulation and leaf mass is to calculate the ratio a/k2 from equations 1.1, 1.2, 1.3, and 4.1, 4.2, and 4.3, respectively. This implicitly assumes that the two allometric coefficients b and b' between Nsh and Wsh and WL and Wsh, respectively, are the same. The values of 0.082 g N g–1, 0.078 g N g–1, and 0.074 g N g–1 for D, I and S plants, respectively, were obtained. Apparently, D plants accumulated slightly more N for a given leaf mass than the S plants, Furthermore, the difference in shoot N content between plants within a dense canopy appears to be largely determined (i) by their shoot mass, and (ii) given the same shoot mass, by the proportion of leaf mass to shoot mass, i.e. their LWR or their leaf:stem ratio.
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