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Productivity and water use efficiency of sweet sorghum (Sorghum bicolor (L.) Moench) cv. “Keller” in relation to water regime
Pergamon
0961-9534(95)000364
Biomass and Eioenergy Vol. 8, No. 6, pp. 401-409, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0961-9534/95 %9.50 + 0.00
PRODUCTIVITY AND WATER USE EFFICIENCY OF SWEET SORGHUM (SORGHUM BICOLOR (L.) MOENCH) CV. “KELLER” IN RELATION TO WATER REGIME M. Dpto
de Producci6n
D. CURT, J. FERNANDEZ Vegetal:
Botanica
y Proteccibn 28040 Madrid,
and M. MARTINEZ Vegetal Spain
Universidad
Polittcnica
de Madrid.
(Receiued 8 June 1994; in revised form 3 March 1995: accepted 22 March 1995) Abstract-Sweet sorghum (Sorghum bicolor (L.) Moench) has been recognized as an alternative crop for energy purposes. In the central area of the Iberian Peninsula, its main growth period coincides with the dry season and irrigation is needed for reasonable sorghum productivity. Knowledge of irrigation-yield relationships is fundamental, since water is a scarce resource there. Our objectives in this work were to study the effect of water regime on the productivity and water use efficiency (WUE) in a sweet sorghum cultivar “Keller” grown in lysimeters in Madrid. Experiments were carried out during three crop cycles. Three irrigation regimes: HI, HZ and Hx, corresponding to a water supply of 5.7, 11.4 and 17. I dm’ m I day - ’were experimented with during the main growth period. Maximum aerial biomass production was 4.0 1O’g DM rn-? in the HJ regime. WUE was quite similar for every irrigation regime but varied between sorghum seasons. 4.6 g aerial biomass DM dm-3 was obtained as the average for a crop cycle length of approximately 130 days. The water regime did not clearly affect the sugar content in stalk sections. The mean value of sugar content in whole stalks was 41.4% w/w on a dry-weight basis. The ratio of ethanol production to evapotranspired crop water was estimated at 0.63 g dm- ’ (mean value). Keyword-Sorghum
bicolor; yield; water
use efficiency:
1. INTRODUCTION
bicolor (L.) Moench,
a member of the Poaceae, is of tropical origin, sensitive to photoperiod and temperature. It is a C4 species of high photosynthetic efficiency under appropriate conditions of light and temperature, indicating a good potential for biomass production. Despite its tropical origin, its cultivation has extended to temperate climatic zones, where the crop cycle goes from spring to autumn; sowing time and harvesting vary according to local temperature conditions. The species shows a great diversity’ and cultivated sorghums include grain sorghums, sweet sorghums and feed sorghums. It is mainly known in the form of grain sorghum, which is one of the most important cereal crops in the world. Sweet sorghum has the capacity to store sugars in the stalk; mainly sucrose, glucose and fructose. Traditionally, it was used to make syrup for confectionery uses. After the 1970s its cultivation was considered for energy purposes, since under non-limiting conditions of water and nutrients it shows a good production of biomass high in sugar contents of good fermentation potential. Cultivation of sorghum Sorghum
irrigation;
sugars.
for sugar production for energy purposes has created great interest in Europe’ in view of the problems of food surpluses. Due to the possibility of damage by cold temperatures, the possible areas for its cultivation in Europe would be the Mediterranean zones. There, the main growth period for sorghum (July, August) coincides with the dry season, therefore irrigation would be necessary for a reasonable productivity. In Spain, studies of sweet sorghum for energy purposes started in the 1980s. Olalla et ~1.‘~~ cultivated this species in Andalucia, in the south of Spain. They estimated water needs in July and August of the order of 9.5 dm - 3 m - ’ day - ‘. They also noted that a 25% reduction (100 dm-’ me2) in irrigation in these months affected the production greatly. Fernandez” studied the productivity of several cultivars in Madrid (central Spain) and pointed to the good potential of sweet sorghum for energy production purposes under favourable irrigation conditions. The shortage of water resources in summer in most parts of Spain makes it necessary to optimize the use of water for crops. It is necessary to study the effect of water regime 401
M. D. CURT et al.
402
on the productivity, as well as the water use efficiency, in the area for cultivation. This paper deals with the effect of water regime on the productivity of the sweet sorghum cultivar “Keller” cultivated in Madrid (Spain) in drainage lysimeters. Experiments were carried out on (i) biomass production, (ii) water use efficiency, and (iii) sugar content in the stalk. 2. MATERIALS
AND METHODS
2.1. Location, plant material and experiment design Experiments were carried out during 1991 and 1992 at the lysimeters station of the Department of Plant Production: Botany and Plant Protection, of the Universidad Politecnica of Madrid (Spain) at latitude 40”26’36’N, longitude 3”44’18’W and altitude 595 m. This station had 12 free-drain lysimeters with fibreglass walls; each lysimeter was 2 x 1 m surface area x 0.5 m deep. The lysimeters were buried 0.80 m apart in an experimental area of 100 m*. Each lysimeter had an individual system of drip irrigation and collection of the drained water. The lysimeters were filled first with a 7 cm layer of coarse stuff and then, on top of this, another 40 cm layer of a mixture of local soil and humus, in the proportion 1O:l (v:v), that had been sifted through a 1 cm mesh. The physical properties of this top layer were: coarse elements ($ 2 2 mm), 22.5%; sandy clay loam
texture-75.8% sand, 7.4% silt, 16.8% clay; field capacity, 14%; wilting point, 6%. Chemical characteristics were determined before sowing and on average were: pH 8.3; electrical conductivity 205 PmS cm - ‘; nitrogen 1.25 o/00; extractable phosphorus (Olsen) 68 ppm P; extractable potassium (extracted in 1N ammonic acetate) 394 ppm K. A fertilizer complex 15:15:15 (W N, PZOs, K,O), at a rate of 70g m - 2, was added immediately before sowing (in spring). Additional N from NH,N03 33.5% was applied as top fertilization at a rate of 30 g m - ‘, in August. The Keller cultivar was selected for this study because in previous trials’,’ it had shown an excellent adaptation to Spanish conditions as well as a higher productivity in relation to other sweet sorghum cultivars. Experiments were carried out in three crop cycles: 1991, 1992a and 19926, the last one corresponding to late sowing. Conditions for these cycles are shown in Table 1. Just before sowing, the lysimeters were set up to field capacity. In each lysimeter, sowing was carried out in two rows, 0.60 m apart, orientated north-south, with 20 cm between seeding points. Five seeds were sown at each point. Seedlings were thinned to 18 plants per lysimeter-equivalent plant density: 9 plants m-‘. The remaining experimental area-including the area between lysimeters-was also sown in the same way, in order to minimize border effects. Weeds were removed by hand throughout the whole cycle.
Table 1. Description of the 1991, 1992~ and 19926 sweet sorghum cv. Keller experiments 1991
1992a
19926
June 27 July 2 October 24 118
May 18 May 22 October 7 141
July 15 July 20 October 21 97
20.6 1269.8 846.4 101.2 628.9
19.6 1360.1 1031.8 173.0 757.5
19.5 932.3 647.4 136.8 481.7
August 5 September 27 _ 22.5 663.3 378.9 17.6 284.4
July 17 September 25 21.9 834.8 531.5 47.4 413.9
August 10 September 25 20.0 460.8 326.3 16.6 242.5
Cron cycle Sowing Emergence Harvest Cycle length (days) Climate data during the crop cycle
Mean temperature (“C) Thermal time* Z PAR (MJ mm*) Rainfall (dm ’m-9 I: ET0 (dm’ m-‘) Period of irrigation treatments
Starting date Finishing date Mean temperature (“C) Thermal time* Z PAR (MJ m-2) Rainfall (dm’ m-r) Z ET0 (dm’ m-*) *Base temperature: 10°C.
Productivity and water use efficiency of sweet sorghum
Each crop cycle was divided into three stages: (1) First stage: from sowing time to the beginning of the differential irrigations. At this stage all lysimeters were kept at non-restricting watering conditions. (2) Stage of irrigation treatments. This stage coincides with the dry period of the Madrid climate, August-September (Table 1). The starting point was established as the thermal time value above 500, except for late sowing which was 400. The stage ended when the daily mean temperature was below 15°C. At the starting point and at the end point the lysimeters were up to field capacity. Three watering regimes were used, H,, Hz and H?, corresponding to different irrigation frequencies: 1, 2 and 3 waterings per week, respectively. The dose was 40 dm3 m - ’ (mm). The actual evapotranspiration in each lysimeter was calculated weekly as the difference between (watering + rainfall) water and supplied drained water. (3) Final stage: from the end of stage 3 until harvesting. Harvesting took place when the daily mean temperature dropped below 10°C. No irrigation was applied during this stage. 2.2. Climatic data and evapotranspiration Climatic data were continuously recorded station every 600 s at the meteorological MeteoData of Geonica* located 30 m from the lysimeters. Air temperature and relative humidity were measured at 1.5 m in a standard shelter (PRS- 103, Geonica), rainfall was measured with a pluviometer of oscillating pans (PCP-210, Geonica), windspeed at 2 m was measured with a cup anemometer (UV-2000, Geonica), solar radiation was measured with a thermoelectric pyranometer (CM6B, class 1) and photosynthetic active radiation (PAR) was measured with a photovoltaic sensor (LI 190s). Potential daily evapotranspiration of the reference crop (ET,) was calculated according to the modified method of Penman’ where the adjusting factor c was calculated according to the regression equation of Frevert et af.:9 c = a0 + a,.RH,,,
+ az.R, + a3.Ud + a.+DNR
+aS.Ud.DNR + a6.RHmrx.Rs.Ud +a,.RH,,,,,.R,.DNR,
*Mention of a trade name or product in this paper does not constitute a recommendation of endorsement for use.
403
where RH,,,,, is the maximum relative humidity (%), R, the solar radiation (mm day - I), U,, the daily mean windspeed (m s - ‘) measured at 2 m, DNR the ratio between the day mean windspeed and the night mean windspeed, and the ai coefficients are: a, = 0.6817006, a, = 0.0027864, az = 0.0181768, a3 = - 0.0682501, a4 = 0.0126514, a5 = 0.0097297, a6 = 0.43025 x 10m4 and a7 = -0.92118 x lo-‘.
2.3. Biomass evaluation and WUE Stalk density, aerial biomass productivity (fresh matter and dry matter), the ratio of the stalks dry weight to aerial biomass dry weight, total biomass productivity and the percentage of plants with visible spikelets were studied. Measurements were carried out by taking each lysimeter as an experimental unit. Production data were referred to m2. Water use efficiency (WUE) was calculated as the ratio of the biomass produced in stages 2 and 3 to the actual water evapotranspired over the same period. The ratios for aerial biomassWUE,-and for total biomass-WUE,-were determined. For WUE calculation, the produced biomass was estimated as the difference between the biomass dry weight at harvest and the biomass dry weight at the beginning of stage 2. This was estimated as follows. At the starting point of stage 2, every lysimeter plant was measured for its stem height. According to this height, representative samples were taken from a nearby Keller field that had been maintained in similar conditions to the lysimeters. Aerial and total biomass dry weight of these samples were determined and the results were extrapolated to lysimeters. 2.4. Sugar content of the stalks The stalks of sample plants were studied for their glucose, fructose and sucrose contents. A random sampling was performed by taking one plant per lysimeter (n = 12). The stalk of every sampled plant was divided into three sections of equal length. The sections were kept in a deep freezer at -20°C until analysis. Every frozen stalk section was weighed and cut into small pieces. Two samples were taken; the first one-about 30 g fresh matter (FM)-to determine the dry matter content, and the second one-about 100 g FM-for sugar analysis. The samples were exactly weighed and triturated in an electrical crusher with 0.200 dm3 of distilled water, obtaining an homogeneous puree. Afterwards, 0.500 and 0.300 dm3 of
M. D. CURT et al.
404
distilled water were added, homogenizing the mixture after each addition. The final puree was then filtered through Whatman No. 40, collecting 0.100 dm3 of colourless filtrate. The glucose, fructose and sucrose contents of this filtrate were determined by enzymatic methods.” Results were expressed in percent w/w of stalk dry weight. 2.5. Data analysis Data were statistically analysed by the MSTAT-C program (Michigan State University). Each lysimeter was considered as an experimental plot. The effect of water regime on productivity was analysed according to Doorenbos and
Kassam.” The relative yield deficit: (1 - (YA/ YM))was related to the relative evapotranspiration deficit: (1 - (ETA/EThl)), where YA is the actual yield of a treatment, YMis the yield of the control treatment, ETA is the actual evapotranspiration and ETM is the control evapotranspiration. Yield was refered to total biomass dry matter. An H, irrigation regime was taken as the control, where evapotranspiration and yield were maximum in these experimental conditions. 3. RESULTS AND DISCUSSION
Figure 1 shows the accumulated data of thermal time, PAR, rainfall and ET,, related to B
A
Thermal
PAR PC. U&J m-2)
time
15007-----
40
0
80
l20
160
DAS
ETo ae. (111-J 800,
0
40
SO DAS
I20
160
0
40
80
I20
1
DAS
Fig. 1. Meteorological data vs days after sowing (DAS) during the sweet sorghum cv. Keller 1991, 1992a and 1992b experiments. (A) Thermal time. (B) Photosynthetic active radiation (PAR). (C) Rainfall. (D) Reference crop evapotranspiration (ETo). Duration of stage 2 (irrigation treatments stage) is shown in (A).
Productivity and water use efficiency of sweet sorghum
the number of days after sowing (DAS), for the three crop cycles. As can be observed, 1992a thermal time went above 1991 only after the 1991 cycle had finished (day 119). This fact shows that 1992a accumulated more thermal time because of the longer duration of the crop cycle. However, referring to PAR, the 1992a cycle accumulated more radiation than the 199 1 cycle from day 100, that is to say several days before the 1991 cycle had finished. Rainfall throughout every cycle was irregular. The dryest cycle was 1991 with a total of 101.2dm3 m-? (mm), 57% v/v of which fell in 3 days at the end of the cycle (DAS 104-106). Rainfall was also badly distributed in the rest of the cycles, as rain fell only on a few days. In 1992a, 49% v/v of the rainfall fell in 6 days, between DAS 11 and 35; the late sowing cycle, 1992b did not benefit from it. 3.1. Biomass productivity and WUE Table 2 shows the results of productivity evaluation and WUE. As can be observed, the stalk density was quite constant, independent of the crop cycle and irrigation regime, showing a mean value of 15.6 stalks m *. This is equivalent to 1.7 stalks plant - ‘. This fact shows the good potential of cv. Keller for energy purposes; because, according to the actual trends, low tillering is desirable for biomass production for energy purposes”. The low number of plants with visible spikelets observed in any experiment indicates an incompleted growth at harvest time. For 1992b, the late sowing experiment, not one plant had visible spikelets. This was expected as Keller
405
is considered a late cultivar. In this cycle, the arrival of low temperatures interrupted the sorghum growing at an early stage. In the two other experiments, the flowering index of the most stressed regime, H,, was lower than the indexes of the other regimes. This suggests that water shortage leads to a retarded plant development in Keller sweet sorghum. Regimes Hz and H3 showed no significant differences on the flowering indexes for the same experiment. Average values in 1991 and 1992a under such regimes were 24.4 and 45.8%, respectively. Belleti et aE.13conducted several trials on sweet sorghum in different localities of the PO Valley (Italy). The Keller trials were carried out at a mean temperature of 19.5”C with an average cycle length of 148 days. In those conditions, the mean number of days from emergence to flowering was 115.6 in Keller. Such a cycle length seems short in our conditions. As for the results for biomass productivity, it should be mentioned that the obtained values are within the range of the ones mentioned for this same variety in several experiments carried out in other Mediterranean countries.5,‘3-‘5 Independently of the irrigation regime, the 1992a experiment was the most encouraging for biomass production and, moreover, the one with the longest cycle length. Within the same experiment, as expected, the higher yield regime was H3, corresponding to the nonstressed regime (control). The lowest productivity regime was given by the H,. However, H, still gave acceptable results of 1.3-2.2 10’g aerial biomass DM m - ?. These figures are considered a good indicator of a favourable
Table 2. Stalks densities, percentages of visible spikelet plants, biomass productions, stalks ratio, evapotranspiration and water use efficiency data for sweet sorghum cv. Keller of HI, HZand HI treatments in the 1991, 1992~ and 1992b experiments. For each experiment, in each row numbers followed by the same letter are not significantly different at P = 5%
H2
H3
HI
H2
H3
Hi
H2
H3
14.5a O.Ob
14.2a 26.0a
15.0a 22.8a
14.5A 8.3B
15.OA 52.8A
18.5A 38.8A
16.5a
16.5a
I5.5a
1.5b
10.6a
11.7a
8.2C
10.5B
14.OA
7.oc
8.5b
10.3a
1.6b 74.1b
2.4a 79.3a
2.7a 79.0a
2.2c 71.3A
3.IB 81.OA
4.OA 77.9A
1.3c 76.4a
1.7b 75.4a
2.la 79.4a
1.9b
2.7a
3.2a
2.7c
3.6B
4.8A
1.8~
2.2b
2.8a
61 1.2AB 8 14.OA 340.1~ 433.2b
518.0a
HI Stalk densitv (stalks m-‘) Spikelet plants (%) Aerial biomass production (IO) g fresh matter m-‘) Aerial biomass production (IO3 g dry matter m-‘) Stalk ratio (% w/w on aerial biomass) Total biomass production (IO) g dry matter m-‘) Evapotranspired water (dn? m-? in stages 2 and 3) Water use efficiency (WUE.) (g aerial biomass dm-‘) Water use efficiency (WUEJ (P total biomass dm-j)
1992h
1992a
1991
339.4~ 534.0b 621 .Oa 403.8B 4.05a
4.01a
4.02a
5.43A
5.07A
4.96A
3.76a
3.74a
4.02a
4.69a
4.51a
4.68a
6.50A
5.86A
5.94A
5.08a
4.96a
5.40a
406
M. D. CURT et al.
perspective for sweet sorghum. In all treatments, the ratio of stalk dry weight to aerial biomass dry weight (stalk ratio, % w/w) was very similar; there were no significant differences either between irrigation regimes or between crop cycles. The mean value observed for the crop was 77% w/w. The WUE values for the main growing period-stages 2 and 3-were quite similar for any irrigation regime with no significant differences within the same crop cycle. So, there is a direct and constant proportionality between productivity and water consumption within each cycle. This was also mentioned by Olalla et a1.5 in their work in 1981 in Andalucia; they did not find differences in the WUE of the various water treatments either. Nevertheless, in our Madrid experiments, significant differences were observed between the various cycles, emphasizing the great importance of the climate cycle conditions. Mean WUE, values ranged from 3.84 to 5.15 g aerial biomass dm- 3, whereas for WUE, they ranged from 4.63 to 6.10 g total biomass dme3. These figures are similar to those mentioned for Keller in west central France,16 about 4.5 and 5 g aerial biomass dm - 3, as well as within the range of WUEs mentioned for diverse genotypes of grain sorghum in Texas,” 3.71-5.65 g aerial biomass dm - 3 transpired water. 3.2. Eflect of irrigation regime on productivity In Figure 2 we have plotted the relative yield deficit (1 - ( YA/YM))against the relative evapotranspiration deficit (1 - (ETA/ET,)) of the three cycles. A linear correlation was established
Fig. 2. Regression line between the relative yield deficit and the relative evapotranspiration deficit of sweet sorghum cv. Keller.
between these two variables; the determination coefficient R2 = 0.844 showed the goodness of the fit. The slope of the regression function KY is the yield response factor to the irrigation regime. The value 0.963 c 1 shows that the total biomass yield falls proportionally faster than the actual evapotranspiration. Similar KY values have been reported for sorghum in other crop conditions. Doorenhos and Kassam” mention, as an average, a value of 0.9 for grain sorghum (grain yield). Perniola et al.” for sweet sorghum cv. Foralco grown in Southern Italy, in their 1990 experiments, estimated a value of 0.90 (yield in aerial biomass). 3.3. Sugar content of the stalks Table 3 shows the mean contents of glucose, fructose and sucrose in the upper, central and lower sections of the stalks of sampled plants, for every treatment. The weighted means for whole stalks are also shown. No clear effect of the irrigation regime on the contents of glucose, fructose, sucrose or total sugars was observed in the culm sections of each experiment. Significant differences between irrigation regimes appeared in only one case in the 1991 experiment: central section, sucrose content. In the 1992a experiment, they were observed in the sucrose of upper and lower sections and in the fructose of upper sections. In this 1992a experiment, the differences may mean that in the most stressed regime the sucrose content is less than in the other two regimes, whereas the fructose and glucose contents are higher-even if there were no significant differences in glucose. However, such effects could also be attributed to a more retarded growth of the Hi plants. As far as the whole stalk data is concerned, no significant differences were observed between irrigation regimes. This agrees with observations by Olalla et al.5 who did not find differences between water treatments in the total sugar contents in the juice and bagasse of the same cultivar in Andalucia. As for the mean values of each culm section, in the 1991 experiment significantly higher contents of sucrose and total sugars were found in the central section than in the other sections. In the 1992a and 1992b experiments the sugar contents in the central and lower sections were slightly higher than in the upper section, but differences were significant only in total sugar contents.
Productivity and water use efficiency of sweet sorghum
407
Table 3. Percentages (w/w) of glucose, fructose, sucrose and total soluble carbohydrates in upper, central and lower culm sections of sweet sorghum cv. Keller in the 1991, 1992~ and 19926 experiments. Results on dry weight basis. Within each item, numbers followed by the same letter are not significantly different at P = 5%. 1991 Culm section
Fr
SC
T
Gl
Fr
SC
HI HZ HI
7.5a 7.6a 7.3a
17.0a 21.9a 21.2a
34.8a 39.3a 38.4a
14.1A 6.1B 5.3B
10.8A 4.IB 4.IB
7.7B 24.3A 18.7A
HI H? H?
lO.la 9.4a 10.3a
8.9a 7.5a 8.2a
29.4a 28.7a 24.lb
49.8a 47.0a 43.8a
13.6A 5.5A 5.9A
9.5A 4.2A 4.OA
Lower
HI HI HJ
11.4a 10.4a 10.2a
7.9a 7.6a 7.la
22.2a 23.2a 18.5a
42.5a 42.4a 36.7a
14.9A 7.8A 6.1A
Upper Central Lower
Mean Mean Mean
9.0b 9.9ab 10.7a
7.5a 8.2a 7.5a
20.0b 27.4a 21.3b
37.5b 46.9a 40.5b
8.2a 7.5a 7.5a 7.7b
24.0a 24.7a 21.0a 23.2a
43.9a 43.2a 39.5a 42.2a
Central
Gl
19921,
9.5a 8.7a 8.7a
Upper
Treatment
1992a
Stalk (Haeighted means) 10.6a HI
HI HI Mean
9.8a lO.Oa lO.lab
Gl
Fr
SC
T
33.OA 35.6A 28.9A
12.la 11.4a 8.3a
9.0a ll.Oa 6.6a
12.la 7.8a
33.8a 34.9a 23.la
15.9A 32.1A 29.6A
39.8A 43.4A 41.OA
13.8a 11.4a 13.5a
11.9a 12.la 10.4a
14.6a 19.7a l5.2a
41.la 44.2a 40.0a
9.7A 4.9A 4.2A
14.6B 29.9A 31.8A
39.9A 44.OA 43.7A
16.2a 11.7a 13.6a
11.4a 12.2a 10.4a
l3.4a 18.7a 21.9a
41.7a 43Sa 47.la
8.5A 8.3A 9.6A
6.3A 5.9A 6.3A
16.9A 25.9A 25.4A
32.5B 41.4A 42.5A
10.6a 12.9a 13.8a
8.9a 11.5a 11.3a
10.6a 16.5a 18.0a
30.6b 41.8a 44.la
14.2A 6.3A 5.9A 8.8b
9.9A 4.4A 4.1A 6.1b
13.5A 29.7A 28.8A 24.0a
38.2A 41.9A 40.2A 40.la
14.6a 11.6a 12.8a 13.0a
l1.2a 12.0a 9.8a ll.Oa
13.9a 18.0a 17.2a 16.4b
40.5a 42.4a 40.8a 41.2a
Figure 3 shows the mean contents of each stalk section, as well as the mean stalk values. It can be observed that sucrose tends to accumulate in the central and lower sections of the stalk, while the fructose and glucose contents tend to be quite uniform throughout the stalk. In the 1991 experiment, higher sucrose and total sugar contents were found in the central section than in the other sections; in the 1992~~and 1992b experiments, differences were significant only in total sugar contents. In sweet sorghum cultivars Wray and MN-l 500, in Italy, Petrini et a1.19found higher sucrose contents in central sections of the stalks; they detected slightly higher glucose and fructose contents in the upper sections. With the Rio sweet sorghum cultivar, in anthesis stage, McBee and Miller”’ found that, in stalks divided into two halves, the upper part contained slightly higher quantities of sucrose and they mentioned that the sucrose distribution in the stalk was generally more uniform than for the other carbohydrates. With regard to whole stalk results, sugars contents were quite similar between the two long cycle experiments, 1991 and 1992a, but somewhat different from those of the short cycle experiment. In this late sowing experiment, the contents of glucose and fructose were higher but the sucrose ones were lower than in the other experiments. A mean content in sucrose of 16.4% w/w was obtained, as opposed to a 23.6% w/w obtained in the two other experiments. Thus, it seems that in the late sowing experiment the sucrose accumulation was
T
I1.9a
somehow stopped by the low temperatures. By relating the growth stage with the sucrose contents, we can assume that, in our conditions, the sucrose accumulation in Keller starts before spikelets are visible. Otherwise, in cv. Wray and Mn-1500, Petrini et aZ.13observed that sucrose accumulation occurred after that stage in the PO Valley (Italy). McBee and Miller” also observed low sucrose contents in the preboot stage on cv. Rio but the total carbohydrates contents were near those found on the anthesis stage. These results suggest that the beginning of the sucrose accumulation depends on the genotype of sweet sorghum. As far as the total sugar content of the whole stalk is concerned, no significant differences were observed between crop cycles; mean content was 41.4% w/w. Almost the same value-41 % w/w-was determined by Belleti et al.13 for the same cultivar in Italy. This fact suggests that in the Mediterranean area this could actually be the potential value of sugar content in Keller stalks. 3.4. Estimated ethanol production Figure 4 shows the mean sugar production for each irrigation treatment, calculated from the productivity results and the mean sugars content in the stalks. According to the most favourable experiment, 1992a, potential sugar yields could be 686,962 and 1240 g m-’ for the H,, Hz and H3 regimes. For the shortest cycle, the yields are about 56% w/w of the previous ones.
M. D. CURTet al.
408
1991
UPPer
UPPer
Middle
Middle
1992b
Weighted means
Middle
0%
2090
do%
Fig. 3. Mean sugar contents in upper, middle and lower culm sections of sweet sorghum cv. Keller and weighted means in the 1991, 1992~ and 19926 experiments. Results on dry weight basis. Numbers followed by the same letter are not significantly different at P = 5%.
The potential for ethanol production from sweet sorghum cv. Keller can be established from the previous data. Assuming a fermentation yield” of 40.8% w/w, the potential ethanol yield would be between 280 and 506 g ethanol m - ’ based on the more favourable 1992~1 experiment, depending on the irrigation regime. As far as the late sowing experiment is concerned, potential yields would be 171-276 g ethanol m - ‘. Relating the potential yields to the evapotranspired water, it can be observed that within each experiment the ratio of ethanol production to evapotranspired water is slightly higher in the most stressed regime (Fig. 4). According to these estimates, we could expect a yield of 59-69 g ethanol 100 dm-3 water,
whereas for a short cycle-a second crop in Spanish conditions-it could be 50-53 g ethanol 100 dm 3 water. 4. CONCLUSIONS
Results indicated that sweet sorghum cv. Keller shows a positive yield response to increasing water supply, and that this response is related to the particular environmental conditions of the crop season. Aerial biomass productions can reach 2.2 and 4.0 lo3 g DM m-* for the water regimes of 5.7 and 17.1 dm3 m* day-‘, respectively. Water use efficiency is, however, uniform for any irrigation regime, but varies between sorghum seasons. As an average,
Productivity
and water use efficiency
6
I
1
-L
8
OH1 OH2 OH3
Fig. 4. Sugar productivities of HI, Hz and H, treatments of the 1991, 1992~ and 1992b sweet sorghum cv. Keller experiments. Ratio of ethanol production to evapotranspired water on each bar.
4.6 g DM aerial biomass dmm3 evapotranspired water is expected for a crop cycle of about 130 days. Water regime does not clearly affect the sugar contents in stalk sections. As a general trend, higher sugar contents are found in the central and lower stalk sections. Mean sugar contents of the whole stalk are quite steady, obtaining an average value of 41.4% w/w (on a dry matter basis). In favourable crop conditions, the potential yield in ethanol could reach 500 g m- ‘. The ratio of ethanol production to evapotranspired water would be estimated at 0.63 g dm3 for a normal crop cycle; this ratio would be lower for short crop cycles. Acknowledgements-This work has been partly financed by the Commission of the European Communities (DG XII) and the Comision Interministerial de Ciencia y Tecnologia (CICYT) of Spain. The authors wish also to thank Mrs M. Fernandez and Mr M. Champion for the English editing of the manuscript.
9
10
11
12
13
14
15
16
17. REFERENCES 1. F. R. Miller and G. G. McBee, Genetics and management of physiologic systems of sorghum for biomass production. Biomass and Bioenergy S(l), 4149 (1993). 2. G. Grassi, P. Moncada and H. Zibetta, Promising industrial energy crop, sweet sorghum. Recent developments in Eurone. EUR-14684-EN. Commission of the European Communities, Brussels (1992). 3. L. Olalla, E. Navarro and A. Mira, Sorgo dulce: aportacion al estudio de su cultivo para la production de azucares y/o alcohol en Andalucia (Espaiia). 1. Introduction general. 2. Tecnicas de siembra. Corn. INIA. Ser. Prod. Veg. 50 (1983). 4. L. Olalla, M. Leon, R. Morillo, E. Navarro and A. Mira, Sorgo dulce: aportacion al estudio de su cultivo para la production de azucares y/o alcohol en Andalucia (Espaiia). 3. Forma de plantacibn y epoca de siembra. An. INIA Ser. Agr. 23,.9-20 (1983). 5. L. Olalla. J. L. Muriel. J. C. Ruiz. E. Navarro and A. Mira, Sorgo duke: aportacion al estudio de su cultivo
18.
19.
20.
21.
of sweet sorghum
409
para la production de azucares y/o alcohol en Andalucia (Espaiia). 4. Estudios sobre riego: agua An. INIA Ser. Agric. 23, aplicada vs producciones. 21-23 (1983). J. Fernandez, Sweet sorghum: an alternative crop for Spain. In Sweet Sorghum (G. Grassi and G. Gosse, eds), pp. 5765. C.E.C., Brussels (1990). M. D. Curt, M. Martinez and J. Fernandez, Potential of sweet sorghum crop for biomass and sugars production in Madrid (Spain). In Biomassfor Energy and Industry, 7th E.C. Conf (D. 0. Hall, G. Grassi and H. Scheer, eds) pp. 632-635. Ponte Press, Bochum (1994). J. Doorenbos and W. 0. Pruitt, Las Necesidades de Agua de 10s Cultivos, Estudio FAO Riego y Drenaje No. 24, Roma (1986). D. K. Frevert, R. Hill and B. C. Braaten, Estimation of FAO evapotranspiration coefficients. J. Irrig. Drain. Eng. 109, 265-270 (1983). Boehringer Mannheim GMBH, UV-Methods for the determination of (i) sucrose and n-glucose and (ii) o-glucose and n-fructose in foodstuffs and other materials. Cat. No. 139041 and 139106 (1989). J. Doorenbos and A. H. Kassam, Efectos del Agua sobre el Rendimiento de 10s Cultivos, Estudio FAO Riego y Drenaje No. 33, Roma (1979). C. Petrini, A. Scozzoli, R. Bazzocchi and A. Belletti, Sweet sorghum breeding for energy production. In Biomass for Energy and Industry 7th, E.C. Conf. (D.O. Hall, G. Grassi and H. Scheer, eds), pp. 447451. Ponte Press, Bochum (1994). A. Belleti, C. Petrini, A. Minguzzi, V. Landini, C. Piazza and F. Salamini, Yield potential and adaptability to italian conditions of sweet sorghum as biomass crop for energy production. Muydica 36, 283-291 (1991). C. Dalianis, M. Christow, S. Sooter, S. Kyritsis, Zafiris and G. Samiotakis, Growth and productivity of sweet sorghum in Greece. In Biomass for Energy and Industry, 7th. E.C. Conf. (D. 0. Hall, G. Grassi and H. Scheer, eds), pp. 636642. Ponte Press, Bochum (1994). M. Mastrorilli, N. Katerji, R. Defilippis and D. Incarnate, Sweet sorghum biomass productivity in the Mediterranean area. In Biomass for Energy and Industry, 7th. E.C. Conf. (D. 0. Hall, G. Grassi and H. Scheer, eds), pp. 700-705. Ponte Press, Bochum (1994). J. L. Durand, Cl. Varlet-Grancher, J. P. Terrason and M. Chartier, Water use efficiency and sensitivity to drought of sweet sorghum productivity under mild water deficit. In Biomass for Energy and Industry 7th. E.C. Conf. (D. 0. Hall, G. Grassi and H. Scheer, eds), pp. 643-647. Ponte Press, Bochum (1994). S. Peng and D. R. Krieg, Gas exchange traits and their relationship to water use efficiency of grain sorghum. Crop Sci. 32, 386391 (1992). M. Perniola, A. R. Rivelli and E. Tarantino, Influenza del regime irriguo sulla produzione de1 sorgo zuccherino (Sorghum uulgare L. var. saccharatum) coltivato in ambiente meridionale. Riv. di Agron. 26(4). 517-523 (1992). C. Petrini, A. Belletti and F. Salamini, Accumulation and distribution of dry matter and soluble carbohydrates in two sweet sorghum cultivars: influence of sowing date and harvesting time. Eur. J. Agron. 2(3), 185-192 (1993). G. G. McBee and F. R. Miller, Carbohydrates in sorghum culms as influenced by cultivars, spacing and maturity over a diurnal period. Crop Sci. 22, 381-385 (1982). V. Lezinou, P. Christakopoulos, L. W. Li, D. Kekos and B. J. Macris, Certain factors affecting direct bio-conversion of sorghum carbohydrates to ethanol. In Biomass for Energy and Environment, Agriculture and Industry 7th. E.C. Conf. Abstracts Book, p. 14.40 (1992).
0961-9534(95)000364
Biomass and Eioenergy Vol. 8, No. 6, pp. 401-409, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0961-9534/95 %9.50 + 0.00
PRODUCTIVITY AND WATER USE EFFICIENCY OF SWEET SORGHUM (SORGHUM BICOLOR (L.) MOENCH) CV. “KELLER” IN RELATION TO WATER REGIME M. Dpto
de Producci6n
D. CURT, J. FERNANDEZ Vegetal:
Botanica
y Proteccibn 28040 Madrid,
and M. MARTINEZ Vegetal Spain
Universidad
Polittcnica
de Madrid.
(Receiued 8 June 1994; in revised form 3 March 1995: accepted 22 March 1995) Abstract-Sweet sorghum (Sorghum bicolor (L.) Moench) has been recognized as an alternative crop for energy purposes. In the central area of the Iberian Peninsula, its main growth period coincides with the dry season and irrigation is needed for reasonable sorghum productivity. Knowledge of irrigation-yield relationships is fundamental, since water is a scarce resource there. Our objectives in this work were to study the effect of water regime on the productivity and water use efficiency (WUE) in a sweet sorghum cultivar “Keller” grown in lysimeters in Madrid. Experiments were carried out during three crop cycles. Three irrigation regimes: HI, HZ and Hx, corresponding to a water supply of 5.7, 11.4 and 17. I dm’ m I day - ’were experimented with during the main growth period. Maximum aerial biomass production was 4.0 1O’g DM rn-? in the HJ regime. WUE was quite similar for every irrigation regime but varied between sorghum seasons. 4.6 g aerial biomass DM dm-3 was obtained as the average for a crop cycle length of approximately 130 days. The water regime did not clearly affect the sugar content in stalk sections. The mean value of sugar content in whole stalks was 41.4% w/w on a dry-weight basis. The ratio of ethanol production to evapotranspired crop water was estimated at 0.63 g dm- ’ (mean value). Keyword-Sorghum
bicolor; yield; water
use efficiency:
1. INTRODUCTION
bicolor (L.) Moench,
a member of the Poaceae, is of tropical origin, sensitive to photoperiod and temperature. It is a C4 species of high photosynthetic efficiency under appropriate conditions of light and temperature, indicating a good potential for biomass production. Despite its tropical origin, its cultivation has extended to temperate climatic zones, where the crop cycle goes from spring to autumn; sowing time and harvesting vary according to local temperature conditions. The species shows a great diversity’ and cultivated sorghums include grain sorghums, sweet sorghums and feed sorghums. It is mainly known in the form of grain sorghum, which is one of the most important cereal crops in the world. Sweet sorghum has the capacity to store sugars in the stalk; mainly sucrose, glucose and fructose. Traditionally, it was used to make syrup for confectionery uses. After the 1970s its cultivation was considered for energy purposes, since under non-limiting conditions of water and nutrients it shows a good production of biomass high in sugar contents of good fermentation potential. Cultivation of sorghum Sorghum
irrigation;
sugars.
for sugar production for energy purposes has created great interest in Europe’ in view of the problems of food surpluses. Due to the possibility of damage by cold temperatures, the possible areas for its cultivation in Europe would be the Mediterranean zones. There, the main growth period for sorghum (July, August) coincides with the dry season, therefore irrigation would be necessary for a reasonable productivity. In Spain, studies of sweet sorghum for energy purposes started in the 1980s. Olalla et ~1.‘~~ cultivated this species in Andalucia, in the south of Spain. They estimated water needs in July and August of the order of 9.5 dm - 3 m - ’ day - ‘. They also noted that a 25% reduction (100 dm-’ me2) in irrigation in these months affected the production greatly. Fernandez” studied the productivity of several cultivars in Madrid (central Spain) and pointed to the good potential of sweet sorghum for energy production purposes under favourable irrigation conditions. The shortage of water resources in summer in most parts of Spain makes it necessary to optimize the use of water for crops. It is necessary to study the effect of water regime 401
M. D. CURT et al.
402
on the productivity, as well as the water use efficiency, in the area for cultivation. This paper deals with the effect of water regime on the productivity of the sweet sorghum cultivar “Keller” cultivated in Madrid (Spain) in drainage lysimeters. Experiments were carried out on (i) biomass production, (ii) water use efficiency, and (iii) sugar content in the stalk. 2. MATERIALS
AND METHODS
2.1. Location, plant material and experiment design Experiments were carried out during 1991 and 1992 at the lysimeters station of the Department of Plant Production: Botany and Plant Protection, of the Universidad Politecnica of Madrid (Spain) at latitude 40”26’36’N, longitude 3”44’18’W and altitude 595 m. This station had 12 free-drain lysimeters with fibreglass walls; each lysimeter was 2 x 1 m surface area x 0.5 m deep. The lysimeters were buried 0.80 m apart in an experimental area of 100 m*. Each lysimeter had an individual system of drip irrigation and collection of the drained water. The lysimeters were filled first with a 7 cm layer of coarse stuff and then, on top of this, another 40 cm layer of a mixture of local soil and humus, in the proportion 1O:l (v:v), that had been sifted through a 1 cm mesh. The physical properties of this top layer were: coarse elements ($ 2 2 mm), 22.5%; sandy clay loam
texture-75.8% sand, 7.4% silt, 16.8% clay; field capacity, 14%; wilting point, 6%. Chemical characteristics were determined before sowing and on average were: pH 8.3; electrical conductivity 205 PmS cm - ‘; nitrogen 1.25 o/00; extractable phosphorus (Olsen) 68 ppm P; extractable potassium (extracted in 1N ammonic acetate) 394 ppm K. A fertilizer complex 15:15:15 (W N, PZOs, K,O), at a rate of 70g m - 2, was added immediately before sowing (in spring). Additional N from NH,N03 33.5% was applied as top fertilization at a rate of 30 g m - ‘, in August. The Keller cultivar was selected for this study because in previous trials’,’ it had shown an excellent adaptation to Spanish conditions as well as a higher productivity in relation to other sweet sorghum cultivars. Experiments were carried out in three crop cycles: 1991, 1992a and 19926, the last one corresponding to late sowing. Conditions for these cycles are shown in Table 1. Just before sowing, the lysimeters were set up to field capacity. In each lysimeter, sowing was carried out in two rows, 0.60 m apart, orientated north-south, with 20 cm between seeding points. Five seeds were sown at each point. Seedlings were thinned to 18 plants per lysimeter-equivalent plant density: 9 plants m-‘. The remaining experimental area-including the area between lysimeters-was also sown in the same way, in order to minimize border effects. Weeds were removed by hand throughout the whole cycle.
Table 1. Description of the 1991, 1992~ and 19926 sweet sorghum cv. Keller experiments 1991
1992a
19926
June 27 July 2 October 24 118
May 18 May 22 October 7 141
July 15 July 20 October 21 97
20.6 1269.8 846.4 101.2 628.9
19.6 1360.1 1031.8 173.0 757.5
19.5 932.3 647.4 136.8 481.7
August 5 September 27 _ 22.5 663.3 378.9 17.6 284.4
July 17 September 25 21.9 834.8 531.5 47.4 413.9
August 10 September 25 20.0 460.8 326.3 16.6 242.5
Cron cycle Sowing Emergence Harvest Cycle length (days) Climate data during the crop cycle
Mean temperature (“C) Thermal time* Z PAR (MJ mm*) Rainfall (dm ’m-9 I: ET0 (dm’ m-‘) Period of irrigation treatments
Starting date Finishing date Mean temperature (“C) Thermal time* Z PAR (MJ m-2) Rainfall (dm’ m-r) Z ET0 (dm’ m-*) *Base temperature: 10°C.
Productivity and water use efficiency of sweet sorghum
Each crop cycle was divided into three stages: (1) First stage: from sowing time to the beginning of the differential irrigations. At this stage all lysimeters were kept at non-restricting watering conditions. (2) Stage of irrigation treatments. This stage coincides with the dry period of the Madrid climate, August-September (Table 1). The starting point was established as the thermal time value above 500, except for late sowing which was 400. The stage ended when the daily mean temperature was below 15°C. At the starting point and at the end point the lysimeters were up to field capacity. Three watering regimes were used, H,, Hz and H?, corresponding to different irrigation frequencies: 1, 2 and 3 waterings per week, respectively. The dose was 40 dm3 m - ’ (mm). The actual evapotranspiration in each lysimeter was calculated weekly as the difference between (watering + rainfall) water and supplied drained water. (3) Final stage: from the end of stage 3 until harvesting. Harvesting took place when the daily mean temperature dropped below 10°C. No irrigation was applied during this stage. 2.2. Climatic data and evapotranspiration Climatic data were continuously recorded station every 600 s at the meteorological MeteoData of Geonica* located 30 m from the lysimeters. Air temperature and relative humidity were measured at 1.5 m in a standard shelter (PRS- 103, Geonica), rainfall was measured with a pluviometer of oscillating pans (PCP-210, Geonica), windspeed at 2 m was measured with a cup anemometer (UV-2000, Geonica), solar radiation was measured with a thermoelectric pyranometer (CM6B, class 1) and photosynthetic active radiation (PAR) was measured with a photovoltaic sensor (LI 190s). Potential daily evapotranspiration of the reference crop (ET,) was calculated according to the modified method of Penman’ where the adjusting factor c was calculated according to the regression equation of Frevert et af.:9 c = a0 + a,.RH,,,
+ az.R, + a3.Ud + a.+DNR
+aS.Ud.DNR + a6.RHmrx.Rs.Ud +a,.RH,,,,,.R,.DNR,
*Mention of a trade name or product in this paper does not constitute a recommendation of endorsement for use.
403
where RH,,,,, is the maximum relative humidity (%), R, the solar radiation (mm day - I), U,, the daily mean windspeed (m s - ‘) measured at 2 m, DNR the ratio between the day mean windspeed and the night mean windspeed, and the ai coefficients are: a, = 0.6817006, a, = 0.0027864, az = 0.0181768, a3 = - 0.0682501, a4 = 0.0126514, a5 = 0.0097297, a6 = 0.43025 x 10m4 and a7 = -0.92118 x lo-‘.
2.3. Biomass evaluation and WUE Stalk density, aerial biomass productivity (fresh matter and dry matter), the ratio of the stalks dry weight to aerial biomass dry weight, total biomass productivity and the percentage of plants with visible spikelets were studied. Measurements were carried out by taking each lysimeter as an experimental unit. Production data were referred to m2. Water use efficiency (WUE) was calculated as the ratio of the biomass produced in stages 2 and 3 to the actual water evapotranspired over the same period. The ratios for aerial biomassWUE,-and for total biomass-WUE,-were determined. For WUE calculation, the produced biomass was estimated as the difference between the biomass dry weight at harvest and the biomass dry weight at the beginning of stage 2. This was estimated as follows. At the starting point of stage 2, every lysimeter plant was measured for its stem height. According to this height, representative samples were taken from a nearby Keller field that had been maintained in similar conditions to the lysimeters. Aerial and total biomass dry weight of these samples were determined and the results were extrapolated to lysimeters. 2.4. Sugar content of the stalks The stalks of sample plants were studied for their glucose, fructose and sucrose contents. A random sampling was performed by taking one plant per lysimeter (n = 12). The stalk of every sampled plant was divided into three sections of equal length. The sections were kept in a deep freezer at -20°C until analysis. Every frozen stalk section was weighed and cut into small pieces. Two samples were taken; the first one-about 30 g fresh matter (FM)-to determine the dry matter content, and the second one-about 100 g FM-for sugar analysis. The samples were exactly weighed and triturated in an electrical crusher with 0.200 dm3 of distilled water, obtaining an homogeneous puree. Afterwards, 0.500 and 0.300 dm3 of
M. D. CURT et al.
404
distilled water were added, homogenizing the mixture after each addition. The final puree was then filtered through Whatman No. 40, collecting 0.100 dm3 of colourless filtrate. The glucose, fructose and sucrose contents of this filtrate were determined by enzymatic methods.” Results were expressed in percent w/w of stalk dry weight. 2.5. Data analysis Data were statistically analysed by the MSTAT-C program (Michigan State University). Each lysimeter was considered as an experimental plot. The effect of water regime on productivity was analysed according to Doorenbos and
Kassam.” The relative yield deficit: (1 - (YA/ YM))was related to the relative evapotranspiration deficit: (1 - (ETA/EThl)), where YA is the actual yield of a treatment, YMis the yield of the control treatment, ETA is the actual evapotranspiration and ETM is the control evapotranspiration. Yield was refered to total biomass dry matter. An H, irrigation regime was taken as the control, where evapotranspiration and yield were maximum in these experimental conditions. 3. RESULTS AND DISCUSSION
Figure 1 shows the accumulated data of thermal time, PAR, rainfall and ET,, related to B
A
Thermal
PAR PC. U&J m-2)
time
15007-----
40
0
80
l20
160
DAS
ETo ae. (111-J 800,
0
40
SO DAS
I20
160
0
40
80
I20
1
DAS
Fig. 1. Meteorological data vs days after sowing (DAS) during the sweet sorghum cv. Keller 1991, 1992a and 1992b experiments. (A) Thermal time. (B) Photosynthetic active radiation (PAR). (C) Rainfall. (D) Reference crop evapotranspiration (ETo). Duration of stage 2 (irrigation treatments stage) is shown in (A).
Productivity and water use efficiency of sweet sorghum
the number of days after sowing (DAS), for the three crop cycles. As can be observed, 1992a thermal time went above 1991 only after the 1991 cycle had finished (day 119). This fact shows that 1992a accumulated more thermal time because of the longer duration of the crop cycle. However, referring to PAR, the 1992a cycle accumulated more radiation than the 199 1 cycle from day 100, that is to say several days before the 1991 cycle had finished. Rainfall throughout every cycle was irregular. The dryest cycle was 1991 with a total of 101.2dm3 m-? (mm), 57% v/v of which fell in 3 days at the end of the cycle (DAS 104-106). Rainfall was also badly distributed in the rest of the cycles, as rain fell only on a few days. In 1992a, 49% v/v of the rainfall fell in 6 days, between DAS 11 and 35; the late sowing cycle, 1992b did not benefit from it. 3.1. Biomass productivity and WUE Table 2 shows the results of productivity evaluation and WUE. As can be observed, the stalk density was quite constant, independent of the crop cycle and irrigation regime, showing a mean value of 15.6 stalks m *. This is equivalent to 1.7 stalks plant - ‘. This fact shows the good potential of cv. Keller for energy purposes; because, according to the actual trends, low tillering is desirable for biomass production for energy purposes”. The low number of plants with visible spikelets observed in any experiment indicates an incompleted growth at harvest time. For 1992b, the late sowing experiment, not one plant had visible spikelets. This was expected as Keller
405
is considered a late cultivar. In this cycle, the arrival of low temperatures interrupted the sorghum growing at an early stage. In the two other experiments, the flowering index of the most stressed regime, H,, was lower than the indexes of the other regimes. This suggests that water shortage leads to a retarded plant development in Keller sweet sorghum. Regimes Hz and H3 showed no significant differences on the flowering indexes for the same experiment. Average values in 1991 and 1992a under such regimes were 24.4 and 45.8%, respectively. Belleti et aE.13conducted several trials on sweet sorghum in different localities of the PO Valley (Italy). The Keller trials were carried out at a mean temperature of 19.5”C with an average cycle length of 148 days. In those conditions, the mean number of days from emergence to flowering was 115.6 in Keller. Such a cycle length seems short in our conditions. As for the results for biomass productivity, it should be mentioned that the obtained values are within the range of the ones mentioned for this same variety in several experiments carried out in other Mediterranean countries.5,‘3-‘5 Independently of the irrigation regime, the 1992a experiment was the most encouraging for biomass production and, moreover, the one with the longest cycle length. Within the same experiment, as expected, the higher yield regime was H3, corresponding to the nonstressed regime (control). The lowest productivity regime was given by the H,. However, H, still gave acceptable results of 1.3-2.2 10’g aerial biomass DM m - ?. These figures are considered a good indicator of a favourable
Table 2. Stalks densities, percentages of visible spikelet plants, biomass productions, stalks ratio, evapotranspiration and water use efficiency data for sweet sorghum cv. Keller of HI, HZand HI treatments in the 1991, 1992~ and 1992b experiments. For each experiment, in each row numbers followed by the same letter are not significantly different at P = 5%
H2
H3
HI
H2
H3
Hi
H2
H3
14.5a O.Ob
14.2a 26.0a
15.0a 22.8a
14.5A 8.3B
15.OA 52.8A
18.5A 38.8A
16.5a
16.5a
I5.5a
1.5b
10.6a
11.7a
8.2C
10.5B
14.OA
7.oc
8.5b
10.3a
1.6b 74.1b
2.4a 79.3a
2.7a 79.0a
2.2c 71.3A
3.IB 81.OA
4.OA 77.9A
1.3c 76.4a
1.7b 75.4a
2.la 79.4a
1.9b
2.7a
3.2a
2.7c
3.6B
4.8A
1.8~
2.2b
2.8a
61 1.2AB 8 14.OA 340.1~ 433.2b
518.0a
HI Stalk densitv (stalks m-‘) Spikelet plants (%) Aerial biomass production (IO) g fresh matter m-‘) Aerial biomass production (IO3 g dry matter m-‘) Stalk ratio (% w/w on aerial biomass) Total biomass production (IO) g dry matter m-‘) Evapotranspired water (dn? m-? in stages 2 and 3) Water use efficiency (WUE.) (g aerial biomass dm-‘) Water use efficiency (WUEJ (P total biomass dm-j)
1992h
1992a
1991
339.4~ 534.0b 621 .Oa 403.8B 4.05a
4.01a
4.02a
5.43A
5.07A
4.96A
3.76a
3.74a
4.02a
4.69a
4.51a
4.68a
6.50A
5.86A
5.94A
5.08a
4.96a
5.40a
406
M. D. CURT et al.
perspective for sweet sorghum. In all treatments, the ratio of stalk dry weight to aerial biomass dry weight (stalk ratio, % w/w) was very similar; there were no significant differences either between irrigation regimes or between crop cycles. The mean value observed for the crop was 77% w/w. The WUE values for the main growing period-stages 2 and 3-were quite similar for any irrigation regime with no significant differences within the same crop cycle. So, there is a direct and constant proportionality between productivity and water consumption within each cycle. This was also mentioned by Olalla et a1.5 in their work in 1981 in Andalucia; they did not find differences in the WUE of the various water treatments either. Nevertheless, in our Madrid experiments, significant differences were observed between the various cycles, emphasizing the great importance of the climate cycle conditions. Mean WUE, values ranged from 3.84 to 5.15 g aerial biomass dm- 3, whereas for WUE, they ranged from 4.63 to 6.10 g total biomass dme3. These figures are similar to those mentioned for Keller in west central France,16 about 4.5 and 5 g aerial biomass dm - 3, as well as within the range of WUEs mentioned for diverse genotypes of grain sorghum in Texas,” 3.71-5.65 g aerial biomass dm - 3 transpired water. 3.2. Eflect of irrigation regime on productivity In Figure 2 we have plotted the relative yield deficit (1 - ( YA/YM))against the relative evapotranspiration deficit (1 - (ETA/ET,)) of the three cycles. A linear correlation was established
Fig. 2. Regression line between the relative yield deficit and the relative evapotranspiration deficit of sweet sorghum cv. Keller.
between these two variables; the determination coefficient R2 = 0.844 showed the goodness of the fit. The slope of the regression function KY is the yield response factor to the irrigation regime. The value 0.963 c 1 shows that the total biomass yield falls proportionally faster than the actual evapotranspiration. Similar KY values have been reported for sorghum in other crop conditions. Doorenhos and Kassam” mention, as an average, a value of 0.9 for grain sorghum (grain yield). Perniola et al.” for sweet sorghum cv. Foralco grown in Southern Italy, in their 1990 experiments, estimated a value of 0.90 (yield in aerial biomass). 3.3. Sugar content of the stalks Table 3 shows the mean contents of glucose, fructose and sucrose in the upper, central and lower sections of the stalks of sampled plants, for every treatment. The weighted means for whole stalks are also shown. No clear effect of the irrigation regime on the contents of glucose, fructose, sucrose or total sugars was observed in the culm sections of each experiment. Significant differences between irrigation regimes appeared in only one case in the 1991 experiment: central section, sucrose content. In the 1992a experiment, they were observed in the sucrose of upper and lower sections and in the fructose of upper sections. In this 1992a experiment, the differences may mean that in the most stressed regime the sucrose content is less than in the other two regimes, whereas the fructose and glucose contents are higher-even if there were no significant differences in glucose. However, such effects could also be attributed to a more retarded growth of the Hi plants. As far as the whole stalk data is concerned, no significant differences were observed between irrigation regimes. This agrees with observations by Olalla et al.5 who did not find differences between water treatments in the total sugar contents in the juice and bagasse of the same cultivar in Andalucia. As for the mean values of each culm section, in the 1991 experiment significantly higher contents of sucrose and total sugars were found in the central section than in the other sections. In the 1992a and 1992b experiments the sugar contents in the central and lower sections were slightly higher than in the upper section, but differences were significant only in total sugar contents.
Productivity and water use efficiency of sweet sorghum
407
Table 3. Percentages (w/w) of glucose, fructose, sucrose and total soluble carbohydrates in upper, central and lower culm sections of sweet sorghum cv. Keller in the 1991, 1992~ and 19926 experiments. Results on dry weight basis. Within each item, numbers followed by the same letter are not significantly different at P = 5%. 1991 Culm section
Fr
SC
T
Gl
Fr
SC
HI HZ HI
7.5a 7.6a 7.3a
17.0a 21.9a 21.2a
34.8a 39.3a 38.4a
14.1A 6.1B 5.3B
10.8A 4.IB 4.IB
7.7B 24.3A 18.7A
HI H? H?
lO.la 9.4a 10.3a
8.9a 7.5a 8.2a
29.4a 28.7a 24.lb
49.8a 47.0a 43.8a
13.6A 5.5A 5.9A
9.5A 4.2A 4.OA
Lower
HI HI HJ
11.4a 10.4a 10.2a
7.9a 7.6a 7.la
22.2a 23.2a 18.5a
42.5a 42.4a 36.7a
14.9A 7.8A 6.1A
Upper Central Lower
Mean Mean Mean
9.0b 9.9ab 10.7a
7.5a 8.2a 7.5a
20.0b 27.4a 21.3b
37.5b 46.9a 40.5b
8.2a 7.5a 7.5a 7.7b
24.0a 24.7a 21.0a 23.2a
43.9a 43.2a 39.5a 42.2a
Central
Gl
19921,
9.5a 8.7a 8.7a
Upper
Treatment
1992a
Stalk (Haeighted means) 10.6a HI
HI HI Mean
9.8a lO.Oa lO.lab
Gl
Fr
SC
T
33.OA 35.6A 28.9A
12.la 11.4a 8.3a
9.0a ll.Oa 6.6a
12.la 7.8a
33.8a 34.9a 23.la
15.9A 32.1A 29.6A
39.8A 43.4A 41.OA
13.8a 11.4a 13.5a
11.9a 12.la 10.4a
14.6a 19.7a l5.2a
41.la 44.2a 40.0a
9.7A 4.9A 4.2A
14.6B 29.9A 31.8A
39.9A 44.OA 43.7A
16.2a 11.7a 13.6a
11.4a 12.2a 10.4a
l3.4a 18.7a 21.9a
41.7a 43Sa 47.la
8.5A 8.3A 9.6A
6.3A 5.9A 6.3A
16.9A 25.9A 25.4A
32.5B 41.4A 42.5A
10.6a 12.9a 13.8a
8.9a 11.5a 11.3a
10.6a 16.5a 18.0a
30.6b 41.8a 44.la
14.2A 6.3A 5.9A 8.8b
9.9A 4.4A 4.1A 6.1b
13.5A 29.7A 28.8A 24.0a
38.2A 41.9A 40.2A 40.la
14.6a 11.6a 12.8a 13.0a
l1.2a 12.0a 9.8a ll.Oa
13.9a 18.0a 17.2a 16.4b
40.5a 42.4a 40.8a 41.2a
Figure 3 shows the mean contents of each stalk section, as well as the mean stalk values. It can be observed that sucrose tends to accumulate in the central and lower sections of the stalk, while the fructose and glucose contents tend to be quite uniform throughout the stalk. In the 1991 experiment, higher sucrose and total sugar contents were found in the central section than in the other sections; in the 1992~~and 1992b experiments, differences were significant only in total sugar contents. In sweet sorghum cultivars Wray and MN-l 500, in Italy, Petrini et a1.19found higher sucrose contents in central sections of the stalks; they detected slightly higher glucose and fructose contents in the upper sections. With the Rio sweet sorghum cultivar, in anthesis stage, McBee and Miller”’ found that, in stalks divided into two halves, the upper part contained slightly higher quantities of sucrose and they mentioned that the sucrose distribution in the stalk was generally more uniform than for the other carbohydrates. With regard to whole stalk results, sugars contents were quite similar between the two long cycle experiments, 1991 and 1992a, but somewhat different from those of the short cycle experiment. In this late sowing experiment, the contents of glucose and fructose were higher but the sucrose ones were lower than in the other experiments. A mean content in sucrose of 16.4% w/w was obtained, as opposed to a 23.6% w/w obtained in the two other experiments. Thus, it seems that in the late sowing experiment the sucrose accumulation was
T
I1.9a
somehow stopped by the low temperatures. By relating the growth stage with the sucrose contents, we can assume that, in our conditions, the sucrose accumulation in Keller starts before spikelets are visible. Otherwise, in cv. Wray and Mn-1500, Petrini et aZ.13observed that sucrose accumulation occurred after that stage in the PO Valley (Italy). McBee and Miller” also observed low sucrose contents in the preboot stage on cv. Rio but the total carbohydrates contents were near those found on the anthesis stage. These results suggest that the beginning of the sucrose accumulation depends on the genotype of sweet sorghum. As far as the total sugar content of the whole stalk is concerned, no significant differences were observed between crop cycles; mean content was 41.4% w/w. Almost the same value-41 % w/w-was determined by Belleti et al.13 for the same cultivar in Italy. This fact suggests that in the Mediterranean area this could actually be the potential value of sugar content in Keller stalks. 3.4. Estimated ethanol production Figure 4 shows the mean sugar production for each irrigation treatment, calculated from the productivity results and the mean sugars content in the stalks. According to the most favourable experiment, 1992a, potential sugar yields could be 686,962 and 1240 g m-’ for the H,, Hz and H3 regimes. For the shortest cycle, the yields are about 56% w/w of the previous ones.
M. D. CURTet al.
408
1991
UPPer
UPPer
Middle
Middle
1992b
Weighted means
Middle
0%
2090
do%
Fig. 3. Mean sugar contents in upper, middle and lower culm sections of sweet sorghum cv. Keller and weighted means in the 1991, 1992~ and 19926 experiments. Results on dry weight basis. Numbers followed by the same letter are not significantly different at P = 5%.
The potential for ethanol production from sweet sorghum cv. Keller can be established from the previous data. Assuming a fermentation yield” of 40.8% w/w, the potential ethanol yield would be between 280 and 506 g ethanol m - ’ based on the more favourable 1992~1 experiment, depending on the irrigation regime. As far as the late sowing experiment is concerned, potential yields would be 171-276 g ethanol m - ‘. Relating the potential yields to the evapotranspired water, it can be observed that within each experiment the ratio of ethanol production to evapotranspired water is slightly higher in the most stressed regime (Fig. 4). According to these estimates, we could expect a yield of 59-69 g ethanol 100 dm-3 water,
whereas for a short cycle-a second crop in Spanish conditions-it could be 50-53 g ethanol 100 dm 3 water. 4. CONCLUSIONS
Results indicated that sweet sorghum cv. Keller shows a positive yield response to increasing water supply, and that this response is related to the particular environmental conditions of the crop season. Aerial biomass productions can reach 2.2 and 4.0 lo3 g DM m-* for the water regimes of 5.7 and 17.1 dm3 m* day-‘, respectively. Water use efficiency is, however, uniform for any irrigation regime, but varies between sorghum seasons. As an average,
Productivity
and water use efficiency
6
I
1
-L
8
OH1 OH2 OH3
Fig. 4. Sugar productivities of HI, Hz and H, treatments of the 1991, 1992~ and 1992b sweet sorghum cv. Keller experiments. Ratio of ethanol production to evapotranspired water on each bar.
4.6 g DM aerial biomass dmm3 evapotranspired water is expected for a crop cycle of about 130 days. Water regime does not clearly affect the sugar contents in stalk sections. As a general trend, higher sugar contents are found in the central and lower stalk sections. Mean sugar contents of the whole stalk are quite steady, obtaining an average value of 41.4% w/w (on a dry matter basis). In favourable crop conditions, the potential yield in ethanol could reach 500 g m- ‘. The ratio of ethanol production to evapotranspired water would be estimated at 0.63 g dm3 for a normal crop cycle; this ratio would be lower for short crop cycles. Acknowledgements-This work has been partly financed by the Commission of the European Communities (DG XII) and the Comision Interministerial de Ciencia y Tecnologia (CICYT) of Spain. The authors wish also to thank Mrs M. Fernandez and Mr M. Champion for the English editing of the manuscript.
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10
11
12
13
14
15
16
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