209 M. Mastrorilli et al. European Journal of Agronomy 11 1999 207–215
2.4. Water use efficiency The evaluation of water use efficiency WUE
was based on the relation between evapotranspira- tion ET and dry biomass Stanhill, 1986. ET
was determined by applying the soil water balance approach where ET mm is obtained over a seven
day period as:
ET=R+I−D±DW where R is the amount of precipitation, I the
irrigation water applied, D the drainage and DW
Fig. 1. Leaf and stem development as percentage of total
the variation in water content of the soil profile.
above-ground dry matter from emergence until harvest in 1990
Capillary rise was neglected as it was considered
experiment for the well-watered treatment ‘C’. The two tem-
to be negligible from the cracked rocky layer below
porary stress periods are also indicated.
70 cm. Soil water content was determined gravi- metrically every seven days by sampling at three
different sites and two depths 10–30 and 40– 60 cm for each water treatment.
was in favour of leaf or stem development. All The ratio between total above-ground dry
plots were well watered during the whole cycle matter yield g m−2 and cumulative ET from
with the exception of the stage to be stressed. To emergence until the harvest kg m−2 represents
ensure good water supply the plots were irrigated the WUE
b Feddes, 1985 or the ‘biomass water
frequently, each
time Y
, measured
daily, ratio’ g kg−1 according to Monteith 1993.
approached the threshold. The temporary water Using the same approach specifically for sweet
stresses were applied by withholding water to sorghum, we propose the ‘stalk water use effi-
individual plots and allowing the soil medium to ciency’ WUE
s .
dry until the target leaf water potential was achieved. For the cv. Keller, the flag leaf is usually
2.5. Statistical analysis leaf 16, but ears do not appear. Thus, most of the
life cycle is vegetative. For each year of the trial an analysis of variance
was applied General Linear Model, GLM; SAS, 1989 to yield data collected at the final harvest.
2.3. Growth and production analysis Differences in dry matter total biomass and stalk
among treatments were evaluated using Duncan Throughout each crop cycle, above-ground bio-
mass and the leaf area were measured weekly from Multiple Range Test, DMRT SAS, 1989.
To assess whether the effects of the water treat- harvests of four 1 m
2 plots from different points in each of the three treatments. Dry matter was
ments on WUE were statistically significant, ‘years’ were taken as replicates in the analysis of variance.
determined after drying the plant samples in a ventilated oven at 90°C for 48 h.
For each treatment yield was estimated from three sample areas each 4×10 m
2. Since sweet
3. Results and discussion
sorghum is considered a biomass crop, the criteria for analysing yield were the final dry matter of
3.1. Calibration aerial part of plants total biomass and stalk
stem including leaf sheath. Panicle and root While soil was drying, Y and g
s were measured
hourly throughout on cloudless days. An example system represent only a small fraction of total
biomass Mastrorilli et al., 1995a. of the hourly variations of Y is given in Fig. 2.
210 M. Mastrorilli et al. European Journal of Agronomy 11 1999 207–215
Fig. 2. Hourly variations of leaf water potential measured in ‘C’ and ‘leaf ’ treatments August 6 1990. Vertical bars repre-
sent standard deviations.
On the same day, two Y trends are compared: the first one was monitored in plot ‘C’; the second in
plot ‘leaf ’, before irrigation. Both curves are char- acterised by Y values which changed during the
day. In particular we observed that the maximum values corresponding to the minimum evaporative
Fig. 3. a Stomatal conductance g s
measured at noon vs. pre-
load occurred twice a day at sunrise and sunset
dawn leaf water potential Y b
during different stress cycles.
and that the minimum Y value occurred at midday.
For each point standard deviations for g s
are represented by
These Y data show the ‘anisohydric’ behaviour
vertical bars and for Y b
by horizontal bars. b Actual soil
Katerji et al., 1987 of sweet sorghum. Thus,
water content mm as a function of pre-dawn leaf water
either the minimum or the maximum Y value
potential.
could be used. Maximum values, measured at pre- dawn Y
b were preferred because: i the differ-
ence between ‘C’ and stressed treatments was When Y
b became lower than −0.4 MPa,
stomata tended
to close
completely. At
greater; ii they were less affected by meteorologi- cal conditions and thus were more stable low
Y b
− 0.6 MPa, wilting was evident and the plants
could no longer attain full leaf turgor by the end standard deviation; and iii pre-dawn Y is
directly dependent on soil water content. of the night. Moreover, fluctuations around mean
values were minimal. To assess sensitivity of stomatal conductance
g s
to crop water status, Y b
was plotted against When Y
b was between −0.2 and −0.4 MPa,
values of stomata conductance were intermediate g
s [Fig. 3a]. Generally stomatal conductance
increased sharply as Y b
increased, but two patterns between those previously described.
We concluded that −0.4 MPa represented a were evident, in the ranges Y
b −
0.2 MPa and Y
b −
0.4 MPa. threshold for separating non-stress from stress
conditions. This Y b
threshold value is the same as The relationship between Y
b and g
s shows that,
over a Y b
range between 0 and −0.2 MPa, values observed for corn Katerji and Bethenod, 1997
and is between the threshold for sunflower of stomatal conductance were high and accompa-
nied by high variability large standard deviation. −0.6 MPa,
Rana et
al., 1997a,
soybean −0.5 MPa, Rana et al., 1997b and grain sorghum
In previous work Ferreira and Katerji, 1992 this high variability was interpreted as a consequence
−0.2 MPa, Mastrorilli et al., 1995b. In addition to plant water status, soil water
of the effect of air saturation deficit on stomatal conductance.
content was measured throughout the crop cycle.
211 M. Mastrorilli et al. European Journal of Agronomy 11 1999 207–215
Fig. 3b shows Y b
values as a function of avail- able soil water while the soil was drying. It is
evident that soil water was below wilting point measured at −1.5 MPa, which corresponds to
120 mm of water actually stored in the soil profile when Y
b was lower than −0.4 MPa. In other
words, at −0.4 MPa the crop completely depleted the available soil water, whereas corn, according
Katerji and Bethenod 1997, at the same Y b
threshold value, consumes about 13 of available soil water. The criterion used, Y
b , could then also
be retained as a tool for diagnosing soil water
Fig. 4. Daily values of minimum and maximum temperatures
availability.
lines and rainfall vertical bars in the 1990 sweet sorghum season May–September. Phasic development S=sowing, E=
Data shown here suggest that pre-dawn leaf
emergence, FG=fast growth phase, H=harvest.
water potential was directly linked to soil water content and it was a good indicator of daytime
gas exchange through the stomata Steduto et al., storms, highly unreliable and frequently of a low
1997. From these results one can conclude that effectiveness.
for sweet sorghum Y b
could be retained as a Soil water content and rainfall were below the
criterion of plant water status and that −0.4 MPa water requirement for sweet sorghum and this was
could be taken as a threshold for irrigation sched- reflected in the frequent irrigation and high volume
uling. In the second part of this experiment irriga- of irrigation water applied during growth cycle.
tion was applied when Y b
, daily monitored, was Crop water use during the three seasons is given
approaching the threshold value. As well as for in Table 1. It is evident that water lost by the well-
the stressed treatments, during the drought periods watered crop was stable over the three years: the
at ‘leaf ’ or ‘stem’ stages, irrigation was scheduled greatest difference in relation to the average con-
when Y b
was lower than −0.6 MPa. sumptive use 555 mm was ±5
. Also the amount of water lost by the stressed treatments
was stable: it represented about 80 of that of the
3.2. Evaluation well-watered crop.
Since for the three years the meteorological 3.3. Water status in plant and crop growth
conditions during the growing seasons were similar to the long-term average and cultural techniques
An example of the trend in Y b
observed in well- were the same, the results obtained for these years
watered and stressed treatments is presented in could be generalised to others. Climatic conditions
Fig. 5. It should be noted that Y b
values observed during the 1990 trial are illustrated in Fig. 4 along
on the well-watered plot were remarkably constant. with the dates of phenological events from sowing
to harvest. Maximum temperatures approached 40°C on several occasions during the three years
Table 1 Consumptive water use mm in the different treatments: C,
while minimum daily temperatures did not repre-
control well-watered, and stressed during the ‘leaf ’ and ‘stem’
sent a limit for this crop. During the three years
stages
the crop was harvested at an average of 111 days after seedling emergence. Total rainfall from
Year Leaf
Stem C
sowing May to maturity September was 69,
1990 470
470 580
127 and 84 mm, respectively in 1990, 1992 and
1992 496
440 560
1993 the 17 year average for the same period is
1993 363
– 526
150 mm, usually in the form of high intensity
212 M. Mastrorilli et al. European Journal of Agronomy 11 1999 207–215
was always lower than −0.6 MPa. Once this value was attained, if water was not promptly supplied,
Y b
decreased from −0.6 to −1.5 MPa in two to three days.
After the temporary stress period, irrigation assured good soil and plant water status were
restored. During the wetting cycles Y b
was checked daily and it was observed that the Y
b in the plots
submitted to stress, took two days to come back to the same values observed on well-watered plots
and no differences appeared until the end of the crop cycle. The water status of sweet sorghum
Fig. 5. Pre-dawn leaf water potential in ‘C’ and stressed ‘leaf ’
recovered well from the effect of stress.
and ‘stem’ plots in the 1990 experiment.
On the same experimental farm the same meth- odology was used to follow the development of
water stress for other species. In the case of grain The
three year
average value
of Y
b was
sorghum, after stopping irrigation, about five days −
0.18±0.07 MPa. This means that stomata were necessary before the first significant differ-
remained open during the crop seasons and sweet ences appeared between Y of the stressed and Y
sorghum in the control plots did not experience of the well-watered plants. After that, the mini-
soil water stress. After stopping irrigation, about mum values were attained after a week Mastrorilli
15 days were necessary before the first significant differences appeared between Yb of the stressed
and Y b
of the control plants. The minimum values were attained at almost the same number of days
after the last irrigation: at least 4 weeks, if weather conditions did not delay the stress development.
In particular, the lower threshold value for ‘leaf ’ in 1993 was attained after 6 weeks because of rain,
which coincided with the period of stress treat- ment. During the other two years rainfall occurred
outside the stress periods. As shown in Table 2, the Y
b measured at the end of the stress periods
Table 2 Lowest values of predawn leaf water potential Y
b measured
in the two water stress treatments at the end of the temporary drought period; number of days after stopping irrigation to
attain the first significant difference in Yb between stressed and control plants; length of the temporary drought period
Year and Lowest Y
b Number of days
water treatment MPa
First Drought
difference length
1990 ‘leaf ’
− 1.60±0.09
12 20
‘stem’ −
1.62±0.11 15
24 1992
‘leaf ’ −
1.16±0.20 15
27 Fig. 6. Change from emergence until harvest of a LAI and
‘stem’ −
0.60±0.24 17
26 b above-ground biomass for the well-watered ‘C’ and
1993 ‘leaf ’
− 0.90±0.08
13 43
stressed treatments ‘leaf ’ and ‘stem’ in the 1990 season.
213 M. Mastrorilli et al. European Journal of Agronomy 11 1999 207–215
et al., 1995b. For pepper, the change in Y b
under trol plots at the end of their growth cycle, is
reported in Table 3. drought soil conditions was more rapid: 48 h since
last irrigation was enough to obtain significant The total biomass yield obtained at our experi-
mental site
and that
obtained in
similar differences in Yb Katerji et al., 1991.
Mediterranean environments under non-limiting conditions about 30 t ha−1, Cosentino et al., 1997
3.4. Sensitivity of different vegetative stages to water stress
show that the yields we obtained were representa- tive of the sweet sorghum productivity of the
region. The change with time in leaf area index and
above-ground biomass is shown in Fig. 6 for the The data of the other treatments are presented
in Table 4 and compared with the well-watered three water treatments as measured in the 1990
experiment. The above-ground dry matter in the crop. Temporary soil water stress reduced the final
yield but its reduction was dependent on develop- three seasons considered, for the well-watered con-
ment stage when stress occurred. These results show that sweet sorghum is highly sensitive to
Table 3
water stress during the early vegetative stage. A
Total and stalk dry biomass t ha−1 obtained in three years under well-watered conditions C plots
stress at ‘leaf ’ stage significantly reduced both final biomass and stalk production.
1990 1992
1993 Total
Stalk Total
Stalk Total
Stalk
3.5. Water use efficiency
32.5 21.4
30.8 22.8
31.7 22.1
The relationship between water used in evapo- transpiration and dry matter production gives an
estimation of the water use efficiency. These values
Table 4
for the total biomass WUE b
and for stalk
Total and stalk biomass dry biomass in t ha−1 as the average
WUE s
are given in Table 5. Under well-watered
of the three experiment seasons. The three water treatments are indicated as ‘C’ control, never stressed, ‘leaf ’ and ‘stem’ tem-
conditions, WUE b
values were similar to the WUE
porary water stress at leaf or stem stage, respectively a
reported in
the recent
literature for
other Mediterranean sites Cosentino et al., 1997; Dercas
Total Stalk
et al., 1996; Gherbin et al., 1996. For ‘leaf ’
‘C’ 31.67 a
22.1 a
treatments, in all the years of the trial, WUE b
and
‘leaf ’ 20.84 b
14.99 c
WUE s
were from 12 to 17
lower than ‘C’
‘stem’ 28.32 a
18.62 b
treatments. For ‘stem’ treatments, water use effi- ciency for both total and stalk biomass was similar
a Note: values in a column followed by different letters are significantly different according to DMRT at P0.05.
to those of the ‘C’ treatment in the two available
Table 5 Water use efficiency g kg−1 for total biomass WUEb and stalk WUEs. The three water treatments are indicated as ‘C’ control,
well-watered, ‘leaf ’ and ‘stem’ temporary water stress at leaf or stem stage, respectively a
Year WUE
b WUE
s ‘C’
‘leaf ’ ‘stem’
‘C’ ‘leaf ’
‘stem’ 1990
5.62 4.72
5.99 3.69
3.07 3.66
1992 5.49
4.61 6.10
4.06 3.42
4.52 1993
6.02 4.95
– 4.20
3.70 –
Average DMRT 5.71 a
4.76 b 6.04 a
3.98 b 3.4 a
4.09 b a Note: values in the average line followed by different letters are significantly different according to DMRT at P0.05.
214 M. Mastrorilli et al. European Journal of Agronomy 11 1999 207–215
years 1990 and 1992. From comparison between
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4. Conclusions
