Fig. 5. Relationship between nitrogen nutrition index NNI and RUEaRUEa
max
. Bars represent S.E..
However, since the NNI value of N270 was not ]
1 after the pod formation stage, Eq. 6 was used to calculate an estimate of RUEa
max
at this later stage for the treatment N270. Fig. 6 shows
the relationship between periodic mean air tem- perature T and RUEa
max
for stages B6 6 leaves to G1 beginning of pod formation. A significant
relationship was obtained between temperature and RUEa
max
; the following exponential model gave a satisfactory fit to the data:
RUEa
max
= 3.5 × [1 − 13.1 × e
0.6 × T
] 6
Statistical indicators were acceptable: RMSE = 0.27 g MJ
− 1
PARa; r
2
= 0.907 4 d.f.. This may
be considered as an approximation of the temper- ature effect on RUEa for oilseed rape. However,
this clearly shows that below a given threshold 6 – 7°C the RUEa was dramatically reduced. On
the other hand, the temperature does not explain the low value of RUEa
max
at early stages after emergence B1 – B5 leaves or during the ripening
stage.
4. Discussion
4
.
1
. The ad6antage of using generated TDM and PARa to calculate RUEa
The mean RUEa over the whole crop cycle excluding the end of ripening was 1.76, 2.34 and
2.63 g MJ
− 1
PARa for N0, N135 and N270, respectively. When calculated from TDM exclud-
ing dead DM, then mean RUEa was ca. 20 less: 1.38, 1.88 and 2.17 g MJ
− 1
PARa for N0, N135 and N270, respectively. Thus, including
dead DM ensured that the calculated RUEa is always positive especially after periods of severe
winter frost as occurred here in January, and should avoid misinterpretation of the data.
The maximum theoretical yield of photosynthe- sis RUEa calculated as energy equivalent is in
the order of 5 g MJ
− 1
PARa Russell et al., 1989. The maximum values obtained in this work
were substantially lower: 3.9 g TDM MJ
− 1
PARa for N270 at flowering, despite the inclusion of
DDM. However the values of RUEa
max
obtained in this work are similar to the value of 4.0 g
Fig. 6. Relationship between periodic mean air temperature and RUEa
max
from emergence until flowering Bars represent S.E..
In a second step, the apparent effect of develop- ment stage on RUEa was investigated, by relating
it to mean air temperature over each period see Table 3. In order to alleviate interactions with N,
only the RUEa values corresponding to a NNI greater than unity were used N non-limiting situ-
ations, which entailed RUEa equal RUEa
max
.
MJ
− 1
PARi used by Kiniry et al. 1995 in the EPIC model aerial parts plus roots and that of 4
g MJ
− 1
PARa observed by Leach et al. 1989 during the post-flowering stage of winter rape.
Calculation of RUE using PARi instead of PARa gives lower values of RUEi, on average
about 13 lower than the RUEa values results not shown. This is explained by the large propor-
tion of PAR which is reflected by the crop partic- ularly
during flowering
and the
soil Varlet-Grancher et al., 1989.
4
.
2
. Effect of N on RUEa Andersen et al. 1996 did not show a signifi-
cant effect of N on RUEi for oilseed rape. How- ever, an effect of N on RUEa was observed in the
experiment; this is in good agreement with the results obtained by Be´langer et al. 1992 for tall
fescue swards and Lemaire et al. 1997 for maize. Be´langer et al. 1992 showed that the ratio
RUEaRUEa
max
is very closely correlated with NNI according to a monomolecular equation;
however this kind of function does not improve the relationship between RUEaRUEa
max
and NNI in this study, so a linear regression has been
used. It should be emphasised that with this regres-
sion Eq. 5, RUEa stops increasing as soon as NNI exceeds unity, which is in accordance with
the results of Lemaire et al. 1997. The effect of N on RUEi has already been shown for maize,
rice and soybean by Muchow and Davis 1988 and Sinclair and Horie 1989 for maize, sorghum,
rice and soybean, who related it to the concentra- tion of N per unit area of leaves. Their response
curves were curvilinear, and RUEi changed little for leaf N contents above 2 g m
2
.
4
.
3
. Effect of temperature on RUEa These results suggest a dependency of RUEa
max
on mean air temperature, in agreement with the results of Andrade et al. 1993 and Verheul et al.
1996 for maize. On the other hand, Habekotte´ 1996 did not find any effect of temperature on
RUE, but her measurements were made at later stage after inflorescence formation, probably
when the mean air temperature was high. The optimum temperature for photosynthesis in rape
is about 20 – 25°C Gosse et al., 1983; Paul et al., 1990, with a small linear increase in gross assimi-
lation from 6 to 20°C and no increase from 20 to 30°C Paul et al., 1992. The optimum seems to
vary according to the growing conditions of the rape a plant acclimation phenomenon. Thus,
Gosse et al. 1983 and Rode et al. 1983 found only a small temperature effect on leaf photosyn-
thesis per unit area over the range 14 – 28°C. The results obtained here seem compatible with the
latter, since RUEa
max
is slightly reduced from 12 to 6 – 7°C, and strongly depressed below a
threshold of about 6 – 7°C Fig. 6. Lastly, there remains a clear effect of development stage on
RUEa
max
for the early rosette stages 1 – 6 leaves and late end of pod formation until harvest
stages Table 3.
4
.
4
. Effect of de6elopment stage on RUEa Variations in RUEa with developmental stage
have also been found by Gosse et al. 1983, Rode et al. 1983, Leach et al. 1989, and Habekotte´
1996, 1997b. In these cases, the variation in RUEa occurred mainly in the post flowering
phase when it is attributable to the formation of lipid compounds with a high energy cost, whereby
more radiation is required to achieve the same TDM gain Habekotte´, 1997b. Hence, for late
growth stages, the RUE calculated by taking ac- count of the energetic value of the biochemical
compounds is about 1 g MJ
− 1
PARa, as against a maximum of 3.6 g MJ
− 1
at the beginning of flowering Habekotte´, 1997b. This lower value of
RUEa at the end of the growing period could also partly be explained by a lower photosynthetic
capacity of stems and pods, compared to leaves, which also declines over the course of time. In
fact the net assimilation rate per unit area of flower stems and peduncles would be 25 less,
and that of pods 75 less than that of leaves Rode et al., 1983.
As far as the low value of RUEa at the begin- ning of growth is concerned, this could be ex-
plained by a smaller leaf photosynthetic capacity and perhaps by a saturated leaf photosynthesis
rate which is reached more quickly at a low level of green LAI; this however remains to be verified.
5. Conclusion
