Materials and methods Directory UMM :Data Elmu:jurnal:E:Environmental and Experimental Botany:Vol44.Issue2.Oct2000:
physiological responses have received relatively little attention Mcdonald and Davies, 1996. In
beans Shimshi, 1970, coffee Tesha and Kumar, 1978, and winter wheat Shangguan, 1997, the
stomatal conductance g
s
increased with nitrogen nutrition under well-watered conditions and be-
came more sensitive to leaf water potential. It decreased as soil water became less available.
Other works on tea Nagarajah, 1981 and cotton Radin and Ackerson, 1989 have indicated an
opposite response, i.e. the stomatal sensitivity to leaf water potential was decreased by high nitro-
gen nutrition. A strong correlation was found between leaf conductance and leaf nitrogen con-
tent; however, this relationship was weaker than that between stomatal conductance and water po-
tential Radin and Parker, 1979; Bolton and Brown, 1980; Morgan, 1986; Shangguan and
Chen, 1990; Ciompi et al., 1996. Sugiharto et al. 1990 found a significiant positive correlation
between the photosynthetic capacity of leaves and their leaf nitrogen concentration suggesting that
most of the nitrogen used for synthesis of compo- nents of the photosynthetic apparatus. In particu-
lar, Rubisco, the leaf protein playing the major role in carbon assimilation, was strongly affected
by nitrogen deficiency Seemann et al., 1987. Although CO
2
and NO
3 −
assimilation are linked, it is not completely clear to what extent they are
coupled Lawlor, 1995. Therefore, further eluci- dation of the relation of leaf nitrogen content to
the gas exchange and water use is needed.
Water use efficiency indicates the performance of a crop growing under any environmental con-
straint. At the leaf level, intrinsic water use effi- ciency
WUE
i
is defined
as the
ratio of
photosynthetic rate P
n
to transpiration rate. To achieve the same P
n
at a lower g
s
, a higher Rubisco activity and capacity for electron trans-
port is required and thus a higher concentration of nitrogen in the leaf does not necessarily mean a
proportional increase in the rate of photosynthesis Shangguan et al., 1999. In crop plants with C
3
photosynthetic pathway, carbon isotope discrimi- nation D has been used to provide time-inte-
grated estimates of plant intrinsic water use efficiency Farquhar and Richards, 1984; Far-
quhar et al., 1989. Foliar D values of C
3
plants have also been used as an integrated measure of
the response of photosynthetic gas exchange to environmental variables such as humidity Winter
et al., 1982, salinity Guy et al., 1986, light intensity Zimmerman and Ehleringer, 1990, soil
water availability Meinzer et al., 1992, and ele- vated CO
2
Picon et al., 1996. A high but nega- tive correlation was found between carbon isotope
discrimination D and WUE
t
Ehdaie and Hall, 1991; Ismail and Hall, 1992 and D with WUE
i
Wright et al., 1988, 1994. Meinzer et al. 1990 reported simultaneous reductions in stomatal con-
ductance and D with increased WUE for water stressed cowpea. Rao and Wright 1994 showed
significant effects of location and water regime on D
for cowpea. Ehleringer et al. 1991 reported a high correlation between D and C
i
C
a
. To our knowledge, there is little information on the ef-
fects of the two major environmental constraints on photosynthetic gas exchange and D in winter
wheat.
Since different stress histories could signifi- cantly have effects on a number of physiological
mechanisms in wheat Morgan, 1984, the present study was designed to eliminate the uncertain
effect of nitrogen nutrition stress with growing plants in solution culture and imposing water
stress with polyethylene glycol PEG 6000. In this study, it was hypothesized that: 1 there would be
interaction between drought and nitrogen nutri- tion on photosynthetic gas exchange and water
use efficiency, and the effects of nitrogen nutrition on plants depend on solution water status; 2
WUE
t
and WUE
i
would be increased by the nitrogen nutrition supply. For verifying the hy-
potheses winter wheat was grown under different combinations of nitrogen nutrition and water sup-
ply levels and the study was focused on the effects of nitrogen deficiency and water stress on leaf
water status, photosynthesis, nitrogen content, and water use.