1. Introduction
The efficiency of conversion of solar energy into biomass by a crop is usually represented by
a synthetic value which is the conversion effi- ciency of intercepted radiation or radiation use
efficiency RUE Monteith, 1972. The RUE is defined as the quantity of dry biomass DM
produced per unit of radiation intercepted or absorbed by the crop. The radiation used may be
either: i the total short wave radiation R; Ri for intercepted R, Ra for absorbed R; or ii the
photosynthetically active radiation PAR; PARi for intercepted PAR, PARa for absorbed PAR
Varlet-Grancher et al., 1989. The value of the RUE is expressed in grams of aerial dry matter
ADM or total dry matter TDM shoots + roots per megajoule of radiation g MJ
− 1
; it varies depending on whether it is calculated as
PAR absorbed RUEa or as PAR intercepted RUEi.
The RUE of oilseed rape has already been estimated by several authors, whose values show
a great deal of variability. For example, accord- ing to Mendham et al. 1981, the RUEi calcu-
lated on the basis of the aerial parts of oilseed rape until flowering, without any stress, was on
average 2.4 g MJ
− 1
PARi whereas Rao and Mendham 1991 calculated RUEi values from
2.71 to 3.5 g MJ
− 1
PARi and, before flowering, Mendham and Salisbury 1995 used the value of
3.5 g MJ
− 1
PARi in the EPIC model for the aerial parts and 4.0 g MJ
− 1
PARi for the total biomass Kiniry et al., 1995. Concerning RUEa,
large variations were also observed: Gosse et al. 1983 and Rode et al. 1983, for spring-sown
oilseed rape, calculated a constant RUEa for the aerial parts at 2.19 g MJ
− 1
PARa until 21 days after flowering, and then only 1.06 g MJ
− 1
PARa, whereas Leach et al. 1989 measured RUEa varying from 1.8 to 4 g MJ
− 1
PARa during the vegetative phase of winter rape, and
about 4 g MJ
− 1
PARa during the post-flowering stage. Likewise Habekotte´ 1996, 1997b found
important variations in RUEa for ADM be- tween the beginning of spring or near maturation
1.1 g MJ
− 1
PARa and during the onset of flowering 2.62 g MJ
− 1
PARa; she then imple- mented RUEa according to the development
stage in
the LINTUL-BRASNAP
model Habekotte´, 1997a. This literature review illus-
trates significant variations in RUEa, which make it questionable to select a single value, for in-
stance for use in a crop simulation model for oilseed rape.
Several causes of variation in RUE due to physical factors of the crop environment or in-
trinsic characteristics of oilseed rape have been suggested, such as the significant effect: i of
extreme temperatures, either very low or very high Mendham and Salisbury, 1995; ii of de-
velopmental stage Gosse et al., 1983; Rode et al., 1983; Leach et al., 1989, particularly in the post
flowering phase due to the high energy cost of lipid compounds Habekotte´, 1997a; iii of win-
ter and spring sowing period Gosse et al., 1986; iv of sowing density or number of plants per m
2
Morrison and Stewart, 1995; or v of water stress Mendham and Salisbury, 1995; Andersen
et al., 1996. However, other authors have not found
any significant
effect of
temperature Gosse et al., 1983; Rode et al., 1983; Habekotte´,
1996, or of the plant density per m
2
Habekotte´, 1996.
Moreover, causes of variation in RUE may be related to the method of calculation, notably due
to failure to take account of: i large losses of leaf biomass during flowering which can even
lead to negative RUE Leach et al., 1989; Yates and Steven, 1987; or ii the ground cover ratio
at the beginning of growth; this can explain the row width or year to year effects observed by
Morrison and Stewart 1995.
If the effect of N on LAI expansion is well known and integrated in oilseed rape crop simula-
tion models DAISY model: Petersen et al., 1995; CERES model: Gabrielle et al., 1998b, the effect
of N on RUE remains to be assessed, as far as oilseed rape is concerned. In fact, although Be´-
langer et al. 1992 showed a large effect of N on RUE for tall fescue Festica arundinacea Schreber
pastures, Gosse et al. 1983 and Rode et al. 1983 only found a very small N effect on RUEa
for spring rape, and Leach et al. 1989 and Andersen et al. 1996 did not find any relation
between N fertilisation and RUEa or RUEi for
winter rape, respectively. However, none of these authors quantified the actual N nutritional state
of the crop. The N status of a crop may be assessed by using the N nutrition index NNI
proposed by Lemaire and Gastal 1997. Be´langer et al. 1992 used the NNI successfully to take
account of the effect of N on the RUEa of forage grasses. In the same way, Lemaire et al. 1997
shown that for maize RUEa was strongly reduced by N deficiency: the relationship between RUEa
and NNI was linear for values of NNI between 0.5 and 1. This high sensitivity of RUEi to N
deficiency was also observed by Muchow and Davis 1988 and Sinclair and Horie 1989 for
maize, sorghum, rice and soybean. Until now there have been no studies on the variation of
oilseed rape RUEa as a function of its N nutri- tion. Recently, Gabrielle et al. 1998a proposed a
CERES-Rape model including N stress on LAI and obtained significant over-estimation of DM
for unfertilised crops; they hypothesised that RUEa decreased in response of N deficiency.
Now, RUEa is a widely used parameter in crop simulation models; it seems then necessary to
assess the magnitude of the N effect on it.
The objective of this work was to investigate the effect of N on the RUEa of winter oilseed
rape canopies which had received different doses of N fertiliser, using the NNI. After having ac-
counted for the influence of N status on RUEa, the effect of temperature for all development
stages of the crop was also investigated, which enabled one to avoid confounding interactions
between N and temperature. Moreover, to limit the influence of experimental artefacts, the RUEa
was calculated on the basis of the radiation ab- sorbed by the crop which yielded RUEa, and by
taking account the total biomass of the crop, including the measured dry matter of the leaves
which fell onto the soil during growth generated total dry matter.
2. Materials and methods
Kategori