208 M. Mastrorilli et al. European Journal of Agronomy 11 1999 207–215
temporary stress occurring during the growth 1990 to 1993 at a density of 11.5 plants m−2. In
1991 violent storms reduced cycle length, and only cycle. From the results, we suggest how to optimise
the use of a limited water supply in the manage- the results from the other three years are given
here. Sowing dates were: 14 May 1990 Julian day ment of the crop.
We adopted a method previously used success- 135, 29 April 1992 120, and 25 May 1993 145.
Harvest dates were: 9 October 1990 Julian day fully both in the glasshouse Katerji et al., 1993
and in the open field Mastrorilli et al., 1995b. 282, 5 October 1992 279, and 18 October 1993
291. The method consisted of stopping watering during
a given stage and monitoring directly the plant Irrigation water was uniformly distributed all
over the field by means of a drip irrigation system. water status by measurements of leaf water poten-
tial Y . Watering was resumed when Y attained Each year the same experimental design covering
a field area of 2 ha was repeated in different a certain threshold value, which was the same for
all stages. As the water deficit provoked the same positions within the farm.
degree of stress Katerji et al., 1991 it was possible to compare the consequences for yield of a given
2.1. Calibration stress occurring during a particular growth stage.
This study was carried out in two stages: first, To characterise the reaction of crop water status
to soil water depletion, leaf water potential Y calibration of the method for detecting the occur-
rence of water stress; and second, evaluation and stomatal conductance g
s were measured
hourly, under conditions of different evaporative during the vegetative phase of the sensitivity of
two stages to a water stress of the same intensity. demand. In practice both Y and g
s were measured
on a sample of 10 well-developed mature leaves Sensitivity was expressed in terms of yield and
water use efficiency. Our analysis is limited to the from the top of the canopy. Measurements were
performed by means of a pressure chamber vegetative phase because it represents the relevant
part of the life cycle, as the stalks, harvested before Scholander et al., 1966 and a steady state poro-
meter Li-Cor 1600 respectively. The objective of grain maturity, constitute the commercial product
of this crop. this eco-physiological characterisation was to find
a Y threshold value for scheduling irrigation and managing water stress. When the water status was
higher than the Y threshold, the crop was consid-
2. Materials and methods
ered to be growing without water supply limitation, while when leaf Y was lower than the threshold
The field trial was carried out at the experimen- tal farm of Istituto Sperimentale Agronomico
the crop was considered to be under stress. Bari located at Rutigliano 41° lat. N, 17° long.
E, 122 a.s.l., in Southern Italy, 7 km from the 2.2. Evaluation
Adriatic coast. The soil contained 44 clay and
26 silt. Soil depth was up to 0.70 m because of
During the vegetative growth, two phases can be defined: in the first, leaf growth is predominant;
a cracked rocky layer, which limited root develop- ment but, at the same time, ensures optimal drain-
in the second, stem growth is predominant. We evaluated the sensitivity of these two growth phases
age of excess water. Total water content of the profile at field capacity is 213 mm, and available
‘leaf ’ and ‘stem’ to a temporary soil water stress using as comparison a well-watered crop ‘C’.
water, calculated as that between field capacity 26.5
measured in field; as weight of water on Each year the field was divided into three plots of
equal size corresponding to three water treatments: dried soil and wilting point at −1.5 MPa, 15
was 93 mm. The climate of the region is typical of C, control never stressed , while ‘leaf ’ and ‘stem’
treatments were temporarily stressed. As shown in maritime Mediterranean conditions.
Sweet sorghum cv. ‘Keller’ [Sorghum bicolor Fig. 1, the stress was applied early or late during
the vegetative period, respectively, when growth L. Moench] was grown during four seasons from
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
