Directory UMM :Data Elmu:jurnal:E:European Journal of Agronomy:Vol11.Issue3-4.Nov1999:

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Productivity and water use e

_ffi

ciency of sweet sorghum as

a

_ff

ected by soil water deficit occurring at di

_ff

erent vegetative

growth stages

Marcello Mastrorilli

_a,

*, Nader Katerji

b

, Gianfranco Rana

_a

aIstituto Sperimentale Agronomico, Via Ulpiani 5, I-70125 Bari, Italy

bINRA-Unite´ de Bioclimatologie, F-78850 Thiverval-Grignon, France

Accepted 10 May 1999

Abstract

The growth and production of sweet sorghum [Sorghum bicolor(L.) Moench] crops under semi-arid conditions in the Mediterranean environment of southern Italy are constrained by water stress. The effects of temporary water stress on growth and productivity of sweet sorghum were studied during three seasons at Rutigliano (Bari, Italy). The aim of this research was to evaluate the sensitivity of phenological stages subjected to the same water deficit. In a preliminary study it was observed that stomata closed when pre-dawn leaf water potential (Y

b) became lower than −0.4 MPa. This criterion was used in monitoring plant water status in three different plots: one never stressed and two stressed at different phenological stages (‘leaf ’ and ‘stem’) when mainly leaves or stems were growing, respectively. An evaluation of the sensitivity of phenological stages subjected to identical water stress was obtained by comparing the above-ground biomass andWUEof drought crops with those of the well-irrigated crop (up to 32.5 t ha−1of dry matter and 5.7 g kg−1). The sensitivity was greatest at the early stage (‘leaf ’), when a temporary soil water stress reduced the biomass production by up to 30%with respect to the control andWUEwas 4.8 g kg−1(average of three seasons). These results help quantify the effects of water constraints on sweet sorghum productivity. An irrigation strategy based on phenological stage sensitivity is suggested. © 1999 Elsevier Science B.V. All rights reserved. Keywords:Biomass; Drought stress; Irrigation; Plant water relationships; Sweet sorghum

1. Introduction _{studies on sweet sorghum have been conducted to} assess its potential productivity and water

require-In addition to being highly productive in terms ment under non-limiting conditions (Mastrorilli

of biomass, sweet sorghum is also known to show et al., 1995a, 1996). However, since water resources

high drought and waterlogging resistance and in the Mediterranean region are limited, the success

salinity tolerance. For these reasons, among the _{of sweet sorghum in this area depends upon the}

biomass energy crops, it is considered as the ‘camel’ _{optimisation of water supplied by irrigation. To}

(Li, 1997). In the Mediterranean regions, previous _{devise a strategy for optimising the amount of}

water provided to sweet sorghum, we had to first answer the question: what is the sensitivity of each * Corresponding author. Tel.:+39-080-5475014;

growth stage to soil water depletion? To this end, fax:+39-080-5475023.

E-mail address:agrobari@interbusiness.it (M. Mastrorilli) we analysed the consequences for final yield of a

1161-0301/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 1 1 6 1 -0 3 0 1 ( 9 9 ) 0 0 03 2 - 5

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 bothYandg

swere 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 Ythreshold value for scheduling irrigation and

managing water stress. When the water status was

higher than theYthreshold, 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 Sperimenexperimen-tale 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

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 whereET(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, Dthe drainage andDW

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

di_fferent 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

Di_fferences 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 m₂ plots from different points

in each of the three treatments. Dry matter was ments onWUEwere 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 m2). 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,Yandg

swere measured

hourly throughout on cloudless days. An example system represent only a small fraction of total

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, twoYtrends 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 minimumYvalue occurred at midday.

For each point standard deviations forg

sare represented by

TheseYdata show the ‘anisohydric’ behaviour _{vertical bars and for}

Y

bby 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 diff

er-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, Yb was plotted against _{values of stomata conductance were intermediate}When Yb was between −0.2 and −0.4 MPa, g

s [Fig. 3(a)]. Generally stomatal conductance

increased sharply asY

bincreased, 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. ThisY}

bthreshold value is the same as

The relationship betweenY

bandgsshows that,

over aY

brange 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

Fig. 3(b) showsY

bvalues 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 1/3 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 well-the drought periods

watered crop was stable over the three years: the at ‘leaf ’ or ‘stem’ stages, irrigation was scheduled

greatest di_fference in relation to the average

con-whenY

bwas 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 inY

bobserved 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 thatY

bvalues 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}

was always lower than−0.6 MPa. Once this value was attained, if water was not promptly supplied, Y

bdecreased 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 cyclesY

bwas checked

daily and it was observed that theY

bin 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 di_ff er-remained open during the crop seasons and sweet

ences appeared betweenY 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 Y

b of the stressed

andY

bof 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,

theY

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 inY

bbetween stressed and control plants; length of the temporary drought period Year and LowestY

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

et al., 1995b). For pepper, the change inY

bunder 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 inY

b( 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 e_fficiency. 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

bvalues were similar to theWUE

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

band

‘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 aNote: values in a column followed by different letters are

significantly different according to DMRT atP<0.05. to those of the ‘C’ treatment in the two available

Table 5

Water use efficiency (g kg₋1) for total biomass (WUE

b) 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 WUEs

‘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

years (1990 and 1992). From comparison between References the two stressed treatments, it was evident that

both in terms of total biomass and stalk, WUE Cosentino, S.L., Riggi, E., Mantineo, M., 1997. Sweet sorghum

[Sorghum bicolor(L.) Moench] performance in relation to was higher when a temporary soil water stress

soil water deficit in south Italy. In: Li, D. ( Ed.), Proc. First occurred at the ‘stem’ stage than at the ‘leaf ’ one.

Int. Sweet Sorghum Conf., Institute of Botany, Chinese These results clearly show that if a water stress

Academy of Sciences, Beijing 100093, China, 430–443.

occurred, the same amount of seasonal consump- _{Dercas, N., Panuntsu, C., Dalianis, C., 1996. Radiation use}

tive water use (see Table 1) did not result in the _{efficiency water and nitrogen effects on sweet sorghum}

sameWUE(Passioura, 1977; French and Schultz, productivity, in: Proc. First European Seminar on Sorghum

for Energy and Industry, Toulouse, France, 1–3 April, 1984; Richards et al., 1993).

218–221.

Feddes, R.A., 1985. Crop water use and dry matter production: state of the art. In: Perrier, A., Riou, C. (Eds.), Crop Water Requirements, Int. Conf., Paris, 11–14 Sept. INRA, Paris, pp. 221–234.

4. Conclusions _{Ferreira, M.I., Katerji, N., 1992. Is stomatal conductance in a} tomato crop controlled by soil or atmosphere? Oecologia 92, 104–107.

The results obtained in the course of this field

French, R.J., Schultz, J.E., 1984. Water use efficiency of wheat study provide the elements for correctly scheduling

in a Mediterranean-type environment. I. The relation irrigation in sweet sorghum.

between yield water use and climate. Aust. J. Agric. Res.

The plant water relationships show that in sweet _{35, 743–764.}

sorghum stomata close when the pre-dawn leaf Gherbin, P., Perniola, M., Tarantino, E., 1996. Sweet and paper

water potential falls to values next to−0.4 MPa. sorghum yield as influenced by water use in southern Italy,

in: Proc. First European Seminar on Sorghum for Energy This threshold is reached if soil water content

and Industry, Toulouse, France, 1–3 April, 222–227. passes the wilting point. These observations led us

Katerji, N., Bethenod, O., 1997. Comparison du comportement

to affirm that irrigation becomes indispensable

hydrique et de la capacite´ photosynthe´tique du mais et du

only when moisture in the soil drops below its _{tournesol en condition de contrainte hydrique. Conclusions}

wilting point. _{sur l’efficience de l’eau. Agronomie 17, 17–24.}

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Coseque´nces d’une contrainte hydrique applique´e a` case of an early stress. Moreover, an early stress _{diffe´rents stades phe´nologiques sur le rendment des plantes}

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with-sorghum in Mediterranean climate: radiation use and

bio-out losing productivity and lowering theWUEwas

mass water use efficiencies. Ind. Crop Prod. 3, 253–260. after the fast growing period. Irrigation should be

Mastrorilli, M., Keterji, N., Rana, G., 1995b. Water e_fficiency

favoured during the early stages of sweet sorghum. _{and stress on grain sorghum at di}

fferent reproductive stages.

This priority should be taken into account in the _{Agric. Water Manag. 28, 23–34.}

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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, twoYtrends 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 minimumYvalue occurred at midday.

For each point standard deviations forg

sare represented by TheseYdata show the ‘anisohydric’ behaviour _{vertical bars and for}

Y

bby 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 diff

er-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, Yb was plotted against _{values of stomata conductance were intermediate}When Yb was between −0.2 and −0.4 MPa,

g

s [Fig. 3(a)]. Generally stomatal conductance increased sharply asY

bincreased, 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. ThisY} bthreshold value is the same as The relationship betweenY

bandgsshows that, over aY

brange 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

Fig. 3(b) showsY

bvalues 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 1/3 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 well-the drought periods

watered crop was stable over the three years: the at ‘leaf ’ or ‘stem’ stages, irrigation was scheduled

greatest di_fference in relation to the average con-whenY

bwas 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 inY

bobserved 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 thatY

bvalues 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

was always lower than−0.6 MPa. Once this value was attained, if water was not promptly supplied,

Y

bdecreased 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 cyclesY

bwas checked daily and it was observed that theY

bin 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 di_ff er-remained open during the crop seasons and sweet

ences appeared betweenY 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 Y

b of the stressed andY

bof 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, theY

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 inY

bbetween stressed and control plants; length of the temporary drought period Year and LowestY

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

et al., 1995b). For pepper, the change inY

bunder 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 inY

b( 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 e_fficiency. 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

bvalues were similar to theWUE 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

band

‘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 aNote: values in a column followed by different letters are

significantly different according to DMRT atP<0.05. to those of the ‘C’ treatment in the two available

Table 5

Water use efficiency (g kg−1) for total biomass (WUE

b) 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 WUEs

‘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

years (1990 and 1992). From comparison between References the two stressed treatments, it was evident that

both in terms of total biomass and stalk, WUE Cosentino, S.L., Riggi, E., Mantineo, M., 1997. Sweet sorghum [Sorghum bicolor(L.) Moench] performance in relation to was higher when a temporary soil water stress

soil water deficit in south Italy. In: Li, D. ( Ed.), Proc. First occurred at the ‘stem’ stage than at the ‘leaf ’ one.

Int. Sweet Sorghum Conf., Institute of Botany, Chinese These results clearly show that if a water stress

Academy of Sciences, Beijing 100093, China, 430–443. occurred, the same amount of seasonal consump- _{Dercas, N., Panuntsu, C., Dalianis, C., 1996. Radiation use} tive water use (see Table 1) did not result in the _{efficiency water and nitrogen effects on sweet sorghum} sameWUE(Passioura, 1977; French and Schultz, productivity, in: Proc. First European Seminar on Sorghum for Energy and Industry, Toulouse, France, 1–3 April, 1984; Richards et al., 1993).

218–221.

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The results obtained in the course of this field

French, R.J., Schultz, J.E., 1984. Water use efficiency of wheat study provide the elements for correctly scheduling

in a Mediterranean-type environment. I. The relation irrigation in sweet sorghum.

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sorghum stomata close when the pre-dawn leaf Gherbin, P., Perniola, M., Tarantino, E., 1996. Sweet and paper water potential falls to values next to−0.4 MPa. sorghum yield as influenced by water use in southern Italy, in: Proc. First European Seminar on Sorghum for Energy This threshold is reached if soil water content

and Industry, Toulouse, France, 1–3 April, 222–227. passes the wilting point. These observations led us

Katerji, N., Bethenod, O., 1997. Comparison du comportement to affirm that irrigation becomes indispensable

hydrique et de la capacite´ photosynthe´tique du mais et du only when moisture in the soil drops below its _{tournesol en condition de contrainte hydrique. Conclusions} wilting point. _{sur l’efficience de l’eau. Agronomie 17, 17–24.}

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well-6–10 July Vol. II., 155–161. watered during the whole cycle, sweet sorghum

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Li, D., 1997. Developing sweet sorghum to accept the challenge the sweet sorghum at the end of the vegetative

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fferent reproductive stages. This priority should be taken into account in the _{Agric. Water Manag. 28, 23–34.}

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