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Comparison of two Sorghum genotypes for sugar and fiber production
Industrial Crops and Products 7 (1998) 265 – 272
Comparison of two Sorghum genotypes for sugar and fiber production Ivano Dolciotti, Stefania Mambelli *, Silvia Grandi, Gianpietro Venturi Department of Agronomy, Uni6ersity of Bologna, Via Filippo Re 6, 40126 Bologna, Italy Received 6 August 1996; accepted 14 November 1996
Abstract A comparative analysis of the growth and yield performances of two late maturing and productive sweet and fiber sorghums has been conducted, with the perspective of their introduction in temperate Italian climate areas as competitive multi-product crops. Sweet type sorghum ‘Wray’ (Sorghum bicolor (L.) Moench ssp. bicolor), and the nonsweet type, ‘H173’ (hybrid Sorghum bicolor (L.) Moench × Sorghum docna var. technicum), were grown in a field trial under well-watered conditions in northern Italy (latitude 44°3%N, longitude 11°2%E). During the crop cycle, growth analysis were performed by collecting data from both non-destructive and destructive samplings. Fundamental growth indexes were calculated as a function of accumulated growing degree days (GDD) from sowing. Yield traits were evaluated at soft dough maturity. Sweet and fiber crops reached soft dough maturity after 1250 GDD and did not statistically differ for total and main stem yields. Mean values of 27 and 20 t ha − 1 dw, respectively, were detected. The sucrose content was more than three times higher and the cellulose and lignin contents 40 – 50% lower in ‘Wray’ as compared to ‘H173’, whereas the level of reducing sugars was similar. Both sorghum types can be considered as interesting new crops which might provide an energy production higher than 10 000 kcal m − 2, a potential production of around 6000 l ha − 1 of ethanol (sweet), and up to 15 t ha − 1 of structural polysaccharides (fiber). The rate of leaf formation on the main stem and their final number were similar between the two genotypes. Until the growing differentiation point, one new leaf was visible every 40.5 GDD, thereafter the same growth process required around 123 GDD. During the period of early leaf formation, the fiber type showed a greater tillering ability which positively affected early canopy area and growth parameters. On the other end, the sweet sorghum crop presented enhanced dry matter accumulation capacity after the growing differentiation point as compared to the fiber crop (42.7 and 27.7 g m − 2 d − 1, respectively). This could be the result of higher leaf thickness and leaf area duration. © 1998 Elsevier Science B.V. Keywords: Sorghum; Growth analysis; Biomass; Fermentable sugars; Fiber
* Corresponding author. Fax: +39 51 351545. 0926-6690/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 6 6 9 0 ( 9 7 ) 0 0 0 5 7 - 5
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1. Introduction The genus Sorghum is characterized by a vastly diverse germ plasm in terms of phenotypic and morphological traits. Many of these have been exploited to give genotypes suitable for grain and forage production, as well as, alternative uses, such as energy, pulp for paper, food products, high grade chemicals and building products (Duncan et al., 1991). Much of the work related to non-food agricultural production of sorghum has been conducted on sweet sorghum because of the increased interest in sugar crops as potential renewable resources that can be converted into ethanol. It also has a shorter growing season than sugarcane, and is therefore suitable to be grown in geographical areas with a temperate climate. It also has rapid rates of growth. In several studies, sweet types have been evaluated for fermentable sugar production and theoretical ethanol yields (Smith et al., 1987; Copani et al., 1989; Belletti et al., 1991), for the relationships between agronomic practices and yield (Broadhead and Freeman, 1980; Massantini and Masoni, 1983) and among growth parameters (Shih et al., 1981; Ferraris and Charles-Edwards, 1986a; Tarantino et al., 1992), for the pattern of soluble carbohydrates accumulation (McBee and Miller, 1982; Bosetto et al., 1986; Ferraris and Charles-Edwards, 1986b; Petrini et al., 1993), and for relevant physiological aspects of this metabolic process (Lingle, 1987; Vietor and Miller, 1990; Tarpley et al., 1994). Unlike sweet genotypes, nonsweet, and especially those characterized by stalk storage organ with high fiber content, have had little attention, so far. A comparative analysis of the growth and yield performances of two late maturing and productive sweet and fiber sorghums has been conducted, with the perspective of their introduction in temperate Italian climate areas as competitive multiproduct crops.
2. Materials and methods The sweet type, ‘Wray’, belonging to Sorghum bicolor (L.) Moench ssp. bicolor and the nonsweet
type, ‘H173’, hybrid Sorghum bicolor (L.) Moench× Sorghum docna var. technicum (Snowden, 1936), were compared in a field study conducted in 1993 at Bologna University Experimental Station (latitude 44°3%N, longitude 11°2%E). The crops were sown on 4 May in rows spaced 70 cm apart on an aquic aplustalf soil, characterized by a particle-size distribution of 40% sand, 38% silt and 22% clay. A randomized block design consisting of 10 rows × 13 m plots for each entry, with four replications was used. Plant population was thinned to 14 plants m − 2 after emergence, which took place on 11 May. Irrigation water (70 mm) was applied by the sprinkler method as needed to obtain uniform plant emergence. During the crop cycle, growth analysis were performed by collecting data from both non-destructive and destructive samplings. The non-destructive measurements were collected from the ‘collar of 5th leaf visible’ to the ‘final leaf visible in whorl’ (Vanderlip and Reeves, 1972) stages on
Fig. 1. Patterns of climatic parameters during the crop growing season. ——, growing degree days; - - -, evaporation; i, precipitation; , irrigation. S, sowing; E, emergence; 5thL, collar of the 5th leaf visible; 7thL, collar of the 7th leaf visible; GPD, growing differentiation point; FLV, collar of the final leaf visible; F, flowering; SDM, soft dough maturity.
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Fig. 2. Patterns of tiller formation, total leaves, leaves on the main stem and leaves on the tillers per plant as a function of the accumulated GDD from sowing. , ‘WRAY’ (sweet); , H173 (fiber). Vertical bars represent S.E. of the means (n =4).
a weekly interval. For each genotype the number of tillers, leaves per main and secondary stalks per plant were counted on a 10-plant sample. Destructive samplings were performed at the ‘collar of 7th leaf visible’, ‘growing point differentiation’ and ‘collar of the 14th leaf visible’ (Vanderlip and Reeves, 1972) stages, and at soft-dough maturity. Whole plants, from an area of 1.4 m2 selected randomly within each plot, were harvested at ground level and separated into main and secondary stalks, leaves and panicles. Leaf area was determined with a leaf area meter (LI-COR 3100). Plant parts were dried to constant weight at 105°C and then weighed to obtain dry matter values. At soft dough maturity, yield traits were evalu-
ated on an area of 7 m2 per plot. In addition, some qualitative traits were measured. A main stalk sub-sample was frozen at − 20°C and used for the determination of fermentable carbohydrates (glucose, fructose and sucrose) and a second stalk sample was dried to constant weight at 60°C and ground (size 1 mm) for chemical analysis of fibers. Sugars were extracted from 10 g of frozen material macerated for 30 min at 60°C in 100 ml of distilled water. After filtration, the level of soluble carbohydrates (glucose, fructose and sucrose) was determined enzymatically (Anon., 1984). Main stalk fiber composition was analyzed according to the detergent system procedure (Goering and Van Soest, 1970) as previously described (Mambelli and Grandi, 1995).
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Accumulated GDD from sowing to each sampling date were calculated considering 13°C as the base temperature, as suggested by Ferraris and Charles-Edwards (1986a), and 30°C as the upperlimit. From plant biomass and leaf area development data, and following an interval approach (Hunt, 1978), these fundamental growth parameters were calculated: leaf area index (LAI); leaf area ratio (LAR); specific leaf area (SLA); leaf weight ratio (LWR) and the integrated leaf area duration (LAD); crop growth rate (CGR); relative growth rate (RGR); and net assimilation rate (NAR). Differences between genotypes for each sampling date were analyzed by standard analysis of variance procedures.
3. Results and discussion
which fixes leaf number, was therefore similar. Two distinct phases of leaf formation were observed: from the ‘collar of the 5th leaf visible’ stage until the growing differentiation point, the linear rate of formation was rapid and equal to one new visible leaf every 40.5 accumulated thermal units (r 2 = 0.92**), and thereafter the same growth process slowed down, requiring around 123 GDD (r 2 = 0.83**) (Fig. 2). Therefore, ‘H173’ produced more total leaves per plant mainly as a consequence of the contribution of the leaves on the tillers. Canopy leaf area index was higher for the fiber crop (68%) at 250 GDD, corresponding to the ‘collar of the 7th leaf visible’ stage. Rates of leaf formation similar for both genotypes resulted in a common sharp increase up to 450 GDD, when mean value around 10 was reached (Fig. 3). This represented the maximum index for H173 and
The patterns of climate parameters registered during the sorghum growing season were generally average for the area. Rainfall measured 232 mm and a short period of drought was detected in mid-August, when both the environmental evapotranspiration and mean temperature reached the maximum values (Fig. 1). During vegetative growth, the fiber genotype showed a higher production of basal tillers as a function of thermal units accumulated from sowing than the sweet genotype (Fig. 2). The tiller density of the fiber type increased linearly until the ‘growing point differentiation’ stage was reached. At this point, which represents the switch from vegetative to reproductive growth, around 3.5 tillers per plant on average, were present. Then their number markedly decreased and at the ‘final leaf visible’ stage, both the sweet and fiber crops presented on average, less than one secondary stalk per plant. Both the number of the leaves on the main stem and their rates of appearance were similar between the two genotypes. The display of leaves was dependent on temperature and increased almost linearly during the leaf formation period. At 800 GDD both genotypes were characterized by the presence of a final number of 16 leaves per main stem. The process of floral initial formation,
Fig. 3. Patterns of the development of total leaf area index (top) and main stem leaf area index (bottom) as a function of the accumulated GDD from sowing. , ‘WRAY’ (sweet); , H173 (fiber). Vertical bars represent S.E. of the means (n =4).
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Fig. 4. Leaf growth indices as a function of the accumulated GDD from sowing. l, ‘WRAY’ (sweet); i, H173 (fiber). Vertical bars represent S.E. of the means (n=4).
after that its foliage surface decreased linearly because of the senescence of the leaves on the tillers. The sweet sorghum showed a greater canopy area persistence. No statistical differences were detected between the two genotypes for the ratio of leaf area to leaf dry weight (LAR) but the sweet type was characterized by a lower ratio of leaf area to leaf mass (SLA). This is an indication of higher leaf thickness and, in fact, ‘WRAY’ plants allocated a higher fraction of total biomass in leaves (LWR) (Fig. 4). ‘H173’ showed higher initial biomass accumulation rates with respect to ‘Wray’ both in terms of absolutely dry matter increase (CGR) and relative to the productive mass of the plant (RGR) (Fig. 5). This could be the result of initial higher LAI, considering that the leaf area is the primary determinant of the proportion of the incident light
energy intercepted by the canopy. From mid-cycle to soft dough maturity, on the contrary, the sweet sorghum crop showed a mean growth rate 50% higher as compared to the fiber crop (42.7 and 27.7 g m − 2 d − 1, respectively), in spite of significant lodging seen after the formation of the final leaf. The larger amount of leaf tissue could have positively influenced its photosynthetic assimilation capacity (Gutschick, 1988) as can be derived considering the values of the growth parameter, NAR, which estimates the efficiency of assimilatory organs in producing new growth. Fewer competing sinks to utilise assimilates could have also enhanced the main stem internode elongation, as previously seen by Ferraris and CharlesEdwards (1986a) who did a comparison between ‘Wray’ and a forage sorghum genotype. When reproductive growth of the sweet type starts, also the accumulation of sucrose in the
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Fig. 5. Crop growth rate, relative crop rate and net assimilation rate as a function of the accumulated GDD from sowing. l, ‘WRAY’ (sweet); i, ‘H173’ (fiber). Vertical bars represent S.E. of the means (n = 4).
main stem takes place. The developing panicle represents a less competitive sink than do the elongating internodes, then the sugar concentration increases linearly from boot to soft dough stage (Lingle, 1987). Almost all assimilate partitioned to the stem during this period of time are allocated to sucrose (Vietor and Miller, 1990) and its construction cost has been estimated in 1.13 g of glucose g − 1, taking glucose as the standard substrate for biosynthesis (Chiariello et al., 1991). On the contrary, the energy costs for the constructions of structural carbohydrate compounds, such as cellulose and hemicellulose, and especially for the synthesis of more complex compounds, such as lignin, are higher (1.21, 1.24 and 2.58, respectively). Therefore, the detected lower net increase of dry matter of the fiber type could have been ascribed to the incorporation of assimilates before in continual tillering, and then in the accu-
mulation of higher energy requiring structural components. A plant growing point requires a certain minimum flux of assimilate to remain viable (Charles-Edwards, 1984) so the senescence of secondary tillers, which started as soon as the plant entered reproductive growth, could be related to the beginning of the fiber accumulation phase in the main stem, but the timing of this metabolic process is not well known. After 1250 GDD, when the soft dough maturity was reached, the total plant biomass accumulated and its partitioning among the plant organs were not statistically different (Table 1). At this stage the main stem, which accounted for 80% of the fresh weight on average, was characterized by a chemical composition in accordance with the genotypic attitude previously described (Table 2). The sucrose content was more than three times higher and the cellulose and lignin contents 40–
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Table 1 Yield traits determinated at soft dough maturity Genotype
Total biomass (t ha−1)
Main stem biomass (t ha−1)
Biomass components (% dw)
Fresh
Dry
Fresh
Dry
Stalks
Leaves
Panicle+seeds
27.59 9 3.44 27.57 9 2.15
100.69 920.09 82.35 95.43
20.73 93.43 20.98 91.99
83.71 9 0.64 79.61 9 0.67
14.82 9 0.49 17.54 90.71
1.47 9 0.16 12.85 9 0.20
WRAY (sweet) 127.36 9 21.40 H173 (fiber) 100.21 94.43
Table 2 Main stem quality traits determined at soft dough maturity Genotype
Fermentable sugars
Fibers
Content (% dw)
WRAY (sweet) H173 (fiber)
Yield (t ha−1 dw)
Content (% dw)
Yield (t ha−1 dw)
Sucrose
Glucose
Fructose
Total
Total
Cellulose
Hemicellulose Lignin
Total
Cellulose+ Hemicellulose
27.61a 7.90b
8.74 6.62
7.23 5.07
43.58a 19.59b
10.03a 4.32b
25.41b 41.85a
22.34b 27.21a
51.59b 76.88a
11.11b 15.15a
3.84b 7.82a
Means followed by different letters are significantly different at P= 0.05 (NK test).
50% lower in ‘Wray’ with respect to ‘H173’, whereas the level of reducing sugars was similar, confirming an intermediate role for these in transport processes (Lingle, 1987).
4. Conclusion Duration of growth in terms of thermal sum to reach soft dough maturity was similar for both sorghums (1250 GDD). Furthermore, the biomass accumulated by the two crops was not statistically different. Sweet and fiber genotypes showed a different growth pattern. The fiber type was characterized by a greater tillering ability during the period of early leaf formation. This positively affected early canopy area and growth parameters. On the other end, the sweet sorghum crop presented enhanced rates of growth after the growing differentiation point as a consequence of higher leaf thickness, area duration and dry matter accumulation capacity, as well as the attitude to accumulate less energy requiring compounds, such as soluble carbohydrates instead of structural ones and lignin.
Considering a biomass conversion factor of 4000 kcal kg − 1 of dry matter, both sweet and fiber crops might provide an energy production superior to 10 000 kcal m − 2. The measured fermentable sugar content of the sweet type ‘Wray’ can result in a potential ethanol production of around 6000 l ha − 1, whereas the main stem of the fiber type ‘H173’ is suitable for the production of up to 15 t ha − 1 of structural polysaccharides. Both sorghum types can be considered as interesting new multi-product crops for temperate Italian environments. To fully exploit the potentiality of these renewable sources of energy and pulp for paper, future work should be directed in solving some physiological bottlenecks, for example chilling and lodging susceptibility, as well as in the implementation of a complete mechanization and transformation products chain. References Anon., 1984. Methods of Enzymatic Food Analysis. Boehringer Mannheim GmbH, Mannheim, pp. 27 – 73. Belletti, A., Petrini, C., Minguzzi, A., Landini, V., Piazza, C., Salamini, F., 1991. Yield potential and adaptability to
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Italian conditions of sweet sorghum as biomass crop for energy production. Maydica 36, 283–291. Bosetto, M., Fusi, P., Arfaioli, P., 1986. Dinamica degli zuccheri liberi in due varieta` di sorgo, ‘Soave’ (italiana) e ‘Beledweyin’ (somala) nelle varie parti della pianta e a vari stadi di maturazione. Riv. Agri. Subtrop. Trop. 53, 339– 356. Broadhead, D.M., Freeman, K.C., 1980. Stalk and sugar yield of sweet sorghum as affected by spacing. J. Agron. 72, 523 – 524. Charles-Edwards, D.A., 1984. On the ordered development of plants. I. An hypothesis. Ann. Bot. 53, 699–702. Chiariello, R.N., Mooney, H.A., Williams, K., 1991. Growth, carbon allocation and cost of plant tissues. In: Plant Physiological Ecology. Chapman and Hall, London, pp. 327 – 365. Copani, V., Patane`, C., Tuttobene, R., 1989. Potenzialita` produttiva del sorgo zuccherino (Sorghum bicolor (L.) Moench) quale fonte di biomassa a fini energetici. Riv. Agron. 23 (6) 428 – 434. Duncan, R.R., Bramel-Cox, P.J., Miller, F.R., 1991. Contributions of introduced Sorghum germoplasm to hybrid development in the USA. Use of Plant Introductions in Cultivar Development. Part I, Crop Science Society of America, pp. 69– 102. Ferraris, R., Charles-Edwards, D.A., 1986a. A comparative analysis of the growth of sweet and forage sorghum crops. I. Dry matter production, phenology and morphology. Aust. J. Agri. Res. 37, 495–512. Ferraris, R., Charles-Edwards, D.A., 1986b. A comparative analysis of the growth of sweet and forage sorghum crops. II. Accumulation of soluble carbohydrates and nitrogen. Aust. J. Agri. Res. 37, 513–522. Goering, H.K., Van Soest, P.J., 1970. Forage Fiber Analysis (Apparatus, Reagents, Procedures and some Applications). USDA Agriculture Handbook. Gutschick, V.P., 1988. Optimization of specific leaf mass, internal CO2 concentration and chlorophyll content in crop canopies. Plant Physiol. Biochem. 26, 525–537. Hunt, R., 1978. Plant Growth Analysis. Edward Arnold,
London. Lingle, S.E., 1987. Sucrose metabolism in the primary culm of sweet sorghum during development. Crop Sci. 27, 1214 – 1219. Mambelli, S., Grandi, S., 1995. Yield and quality of kenaf (Hibiscus cannabinus L.) stem as affected by harvest date and irrigation. Ind. Crops Products 4, 97 – 104. Massantini, F., Masoni, A., 1983. Problemi agronomici inerenti la produzione di alcool da sorgo zuccherino. Riv. Agron. 17 (1), 232 – 237. McBee, G.G., Miller, F.R., 1982. Carbohydrates in sorghum culms as influenced by cultivars, spacing and maturity over a diurnal period. Crop Sci. 22, 381 – 385. Petrini, C., Belletti, A., Salamini, F., 1993. Accumulation and distribution on dry matter and soluble carbohydrates in two sweet sorghum cultivars: influence of sowing date and harvesting time. Eur. J. Agron. 2 (3), 185 – 192. Shih, S.F., Gascho, G.J., Rahi, G.S., 1981. Modeling biomass production of sweet sorghum. Agron. J. 73, 1027 – 1032. Smith, G.A., Bagby, M.O., Lewellan, R.T., Doney, D.L., Moore, P.H., Hills, F.J., Campbell, L.G., Hogaboam, G.J., Coe, G.E., Freeman, K., 1987. Evaluation of sweet sorghum for fermentable sugar production potential. Crop Sci. 27, 788 – 793. Snowden, J.D., 1936. The Cultivated Races of Sorghum. Adlard and Son, London. Tarantino, E., Perniola, M., Quaglietta, F., Rivelli, A.R., 1992. Growth analysis and sugar yield of sweet sorghum under different irrigation times. Proc. 2nd ESA Congress, pp. 142 – 143. Tarpley, L., Lingle, S.E., Vietor, D.M., Andrews, D.L., Miller, F.R., 1994. Enzymatic control of nonstructural carbohydrate concentrations in stems and panicles of sorghum. Crop Sci. 34, 446 – 452. Vanderlip, R.L., Reeves, H.E., 1972. Growth stages of sweet sorghum (Sorghum bicolor (L.) Moench). Agron. J. 64, 13 – 16. Vietor, D.M., Miller, F.R., 1990. Assimilation, partitioning and nonstructural carbohydrates in sweet compared with grain sorghum. Crop Sci. 30, 1109 – 1115.
Comparison of two Sorghum genotypes for sugar and fiber production Ivano Dolciotti, Stefania Mambelli *, Silvia Grandi, Gianpietro Venturi Department of Agronomy, Uni6ersity of Bologna, Via Filippo Re 6, 40126 Bologna, Italy Received 6 August 1996; accepted 14 November 1996
Abstract A comparative analysis of the growth and yield performances of two late maturing and productive sweet and fiber sorghums has been conducted, with the perspective of their introduction in temperate Italian climate areas as competitive multi-product crops. Sweet type sorghum ‘Wray’ (Sorghum bicolor (L.) Moench ssp. bicolor), and the nonsweet type, ‘H173’ (hybrid Sorghum bicolor (L.) Moench × Sorghum docna var. technicum), were grown in a field trial under well-watered conditions in northern Italy (latitude 44°3%N, longitude 11°2%E). During the crop cycle, growth analysis were performed by collecting data from both non-destructive and destructive samplings. Fundamental growth indexes were calculated as a function of accumulated growing degree days (GDD) from sowing. Yield traits were evaluated at soft dough maturity. Sweet and fiber crops reached soft dough maturity after 1250 GDD and did not statistically differ for total and main stem yields. Mean values of 27 and 20 t ha − 1 dw, respectively, were detected. The sucrose content was more than three times higher and the cellulose and lignin contents 40 – 50% lower in ‘Wray’ as compared to ‘H173’, whereas the level of reducing sugars was similar. Both sorghum types can be considered as interesting new crops which might provide an energy production higher than 10 000 kcal m − 2, a potential production of around 6000 l ha − 1 of ethanol (sweet), and up to 15 t ha − 1 of structural polysaccharides (fiber). The rate of leaf formation on the main stem and their final number were similar between the two genotypes. Until the growing differentiation point, one new leaf was visible every 40.5 GDD, thereafter the same growth process required around 123 GDD. During the period of early leaf formation, the fiber type showed a greater tillering ability which positively affected early canopy area and growth parameters. On the other end, the sweet sorghum crop presented enhanced dry matter accumulation capacity after the growing differentiation point as compared to the fiber crop (42.7 and 27.7 g m − 2 d − 1, respectively). This could be the result of higher leaf thickness and leaf area duration. © 1998 Elsevier Science B.V. Keywords: Sorghum; Growth analysis; Biomass; Fermentable sugars; Fiber
* Corresponding author. Fax: +39 51 351545. 0926-6690/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 6 6 9 0 ( 9 7 ) 0 0 0 5 7 - 5
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I. Dolciotti et al. / Industrial Crops and Products 7 (1998) 265–272
1. Introduction The genus Sorghum is characterized by a vastly diverse germ plasm in terms of phenotypic and morphological traits. Many of these have been exploited to give genotypes suitable for grain and forage production, as well as, alternative uses, such as energy, pulp for paper, food products, high grade chemicals and building products (Duncan et al., 1991). Much of the work related to non-food agricultural production of sorghum has been conducted on sweet sorghum because of the increased interest in sugar crops as potential renewable resources that can be converted into ethanol. It also has a shorter growing season than sugarcane, and is therefore suitable to be grown in geographical areas with a temperate climate. It also has rapid rates of growth. In several studies, sweet types have been evaluated for fermentable sugar production and theoretical ethanol yields (Smith et al., 1987; Copani et al., 1989; Belletti et al., 1991), for the relationships between agronomic practices and yield (Broadhead and Freeman, 1980; Massantini and Masoni, 1983) and among growth parameters (Shih et al., 1981; Ferraris and Charles-Edwards, 1986a; Tarantino et al., 1992), for the pattern of soluble carbohydrates accumulation (McBee and Miller, 1982; Bosetto et al., 1986; Ferraris and Charles-Edwards, 1986b; Petrini et al., 1993), and for relevant physiological aspects of this metabolic process (Lingle, 1987; Vietor and Miller, 1990; Tarpley et al., 1994). Unlike sweet genotypes, nonsweet, and especially those characterized by stalk storage organ with high fiber content, have had little attention, so far. A comparative analysis of the growth and yield performances of two late maturing and productive sweet and fiber sorghums has been conducted, with the perspective of their introduction in temperate Italian climate areas as competitive multiproduct crops.
2. Materials and methods The sweet type, ‘Wray’, belonging to Sorghum bicolor (L.) Moench ssp. bicolor and the nonsweet
type, ‘H173’, hybrid Sorghum bicolor (L.) Moench× Sorghum docna var. technicum (Snowden, 1936), were compared in a field study conducted in 1993 at Bologna University Experimental Station (latitude 44°3%N, longitude 11°2%E). The crops were sown on 4 May in rows spaced 70 cm apart on an aquic aplustalf soil, characterized by a particle-size distribution of 40% sand, 38% silt and 22% clay. A randomized block design consisting of 10 rows × 13 m plots for each entry, with four replications was used. Plant population was thinned to 14 plants m − 2 after emergence, which took place on 11 May. Irrigation water (70 mm) was applied by the sprinkler method as needed to obtain uniform plant emergence. During the crop cycle, growth analysis were performed by collecting data from both non-destructive and destructive samplings. The non-destructive measurements were collected from the ‘collar of 5th leaf visible’ to the ‘final leaf visible in whorl’ (Vanderlip and Reeves, 1972) stages on
Fig. 1. Patterns of climatic parameters during the crop growing season. ——, growing degree days; - - -, evaporation; i, precipitation; , irrigation. S, sowing; E, emergence; 5thL, collar of the 5th leaf visible; 7thL, collar of the 7th leaf visible; GPD, growing differentiation point; FLV, collar of the final leaf visible; F, flowering; SDM, soft dough maturity.
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Fig. 2. Patterns of tiller formation, total leaves, leaves on the main stem and leaves on the tillers per plant as a function of the accumulated GDD from sowing. , ‘WRAY’ (sweet); , H173 (fiber). Vertical bars represent S.E. of the means (n =4).
a weekly interval. For each genotype the number of tillers, leaves per main and secondary stalks per plant were counted on a 10-plant sample. Destructive samplings were performed at the ‘collar of 7th leaf visible’, ‘growing point differentiation’ and ‘collar of the 14th leaf visible’ (Vanderlip and Reeves, 1972) stages, and at soft-dough maturity. Whole plants, from an area of 1.4 m2 selected randomly within each plot, were harvested at ground level and separated into main and secondary stalks, leaves and panicles. Leaf area was determined with a leaf area meter (LI-COR 3100). Plant parts were dried to constant weight at 105°C and then weighed to obtain dry matter values. At soft dough maturity, yield traits were evalu-
ated on an area of 7 m2 per plot. In addition, some qualitative traits were measured. A main stalk sub-sample was frozen at − 20°C and used for the determination of fermentable carbohydrates (glucose, fructose and sucrose) and a second stalk sample was dried to constant weight at 60°C and ground (size 1 mm) for chemical analysis of fibers. Sugars were extracted from 10 g of frozen material macerated for 30 min at 60°C in 100 ml of distilled water. After filtration, the level of soluble carbohydrates (glucose, fructose and sucrose) was determined enzymatically (Anon., 1984). Main stalk fiber composition was analyzed according to the detergent system procedure (Goering and Van Soest, 1970) as previously described (Mambelli and Grandi, 1995).
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Accumulated GDD from sowing to each sampling date were calculated considering 13°C as the base temperature, as suggested by Ferraris and Charles-Edwards (1986a), and 30°C as the upperlimit. From plant biomass and leaf area development data, and following an interval approach (Hunt, 1978), these fundamental growth parameters were calculated: leaf area index (LAI); leaf area ratio (LAR); specific leaf area (SLA); leaf weight ratio (LWR) and the integrated leaf area duration (LAD); crop growth rate (CGR); relative growth rate (RGR); and net assimilation rate (NAR). Differences between genotypes for each sampling date were analyzed by standard analysis of variance procedures.
3. Results and discussion
which fixes leaf number, was therefore similar. Two distinct phases of leaf formation were observed: from the ‘collar of the 5th leaf visible’ stage until the growing differentiation point, the linear rate of formation was rapid and equal to one new visible leaf every 40.5 accumulated thermal units (r 2 = 0.92**), and thereafter the same growth process slowed down, requiring around 123 GDD (r 2 = 0.83**) (Fig. 2). Therefore, ‘H173’ produced more total leaves per plant mainly as a consequence of the contribution of the leaves on the tillers. Canopy leaf area index was higher for the fiber crop (68%) at 250 GDD, corresponding to the ‘collar of the 7th leaf visible’ stage. Rates of leaf formation similar for both genotypes resulted in a common sharp increase up to 450 GDD, when mean value around 10 was reached (Fig. 3). This represented the maximum index for H173 and
The patterns of climate parameters registered during the sorghum growing season were generally average for the area. Rainfall measured 232 mm and a short period of drought was detected in mid-August, when both the environmental evapotranspiration and mean temperature reached the maximum values (Fig. 1). During vegetative growth, the fiber genotype showed a higher production of basal tillers as a function of thermal units accumulated from sowing than the sweet genotype (Fig. 2). The tiller density of the fiber type increased linearly until the ‘growing point differentiation’ stage was reached. At this point, which represents the switch from vegetative to reproductive growth, around 3.5 tillers per plant on average, were present. Then their number markedly decreased and at the ‘final leaf visible’ stage, both the sweet and fiber crops presented on average, less than one secondary stalk per plant. Both the number of the leaves on the main stem and their rates of appearance were similar between the two genotypes. The display of leaves was dependent on temperature and increased almost linearly during the leaf formation period. At 800 GDD both genotypes were characterized by the presence of a final number of 16 leaves per main stem. The process of floral initial formation,
Fig. 3. Patterns of the development of total leaf area index (top) and main stem leaf area index (bottom) as a function of the accumulated GDD from sowing. , ‘WRAY’ (sweet); , H173 (fiber). Vertical bars represent S.E. of the means (n =4).
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Fig. 4. Leaf growth indices as a function of the accumulated GDD from sowing. l, ‘WRAY’ (sweet); i, H173 (fiber). Vertical bars represent S.E. of the means (n=4).
after that its foliage surface decreased linearly because of the senescence of the leaves on the tillers. The sweet sorghum showed a greater canopy area persistence. No statistical differences were detected between the two genotypes for the ratio of leaf area to leaf dry weight (LAR) but the sweet type was characterized by a lower ratio of leaf area to leaf mass (SLA). This is an indication of higher leaf thickness and, in fact, ‘WRAY’ plants allocated a higher fraction of total biomass in leaves (LWR) (Fig. 4). ‘H173’ showed higher initial biomass accumulation rates with respect to ‘Wray’ both in terms of absolutely dry matter increase (CGR) and relative to the productive mass of the plant (RGR) (Fig. 5). This could be the result of initial higher LAI, considering that the leaf area is the primary determinant of the proportion of the incident light
energy intercepted by the canopy. From mid-cycle to soft dough maturity, on the contrary, the sweet sorghum crop showed a mean growth rate 50% higher as compared to the fiber crop (42.7 and 27.7 g m − 2 d − 1, respectively), in spite of significant lodging seen after the formation of the final leaf. The larger amount of leaf tissue could have positively influenced its photosynthetic assimilation capacity (Gutschick, 1988) as can be derived considering the values of the growth parameter, NAR, which estimates the efficiency of assimilatory organs in producing new growth. Fewer competing sinks to utilise assimilates could have also enhanced the main stem internode elongation, as previously seen by Ferraris and CharlesEdwards (1986a) who did a comparison between ‘Wray’ and a forage sorghum genotype. When reproductive growth of the sweet type starts, also the accumulation of sucrose in the
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Fig. 5. Crop growth rate, relative crop rate and net assimilation rate as a function of the accumulated GDD from sowing. l, ‘WRAY’ (sweet); i, ‘H173’ (fiber). Vertical bars represent S.E. of the means (n = 4).
main stem takes place. The developing panicle represents a less competitive sink than do the elongating internodes, then the sugar concentration increases linearly from boot to soft dough stage (Lingle, 1987). Almost all assimilate partitioned to the stem during this period of time are allocated to sucrose (Vietor and Miller, 1990) and its construction cost has been estimated in 1.13 g of glucose g − 1, taking glucose as the standard substrate for biosynthesis (Chiariello et al., 1991). On the contrary, the energy costs for the constructions of structural carbohydrate compounds, such as cellulose and hemicellulose, and especially for the synthesis of more complex compounds, such as lignin, are higher (1.21, 1.24 and 2.58, respectively). Therefore, the detected lower net increase of dry matter of the fiber type could have been ascribed to the incorporation of assimilates before in continual tillering, and then in the accu-
mulation of higher energy requiring structural components. A plant growing point requires a certain minimum flux of assimilate to remain viable (Charles-Edwards, 1984) so the senescence of secondary tillers, which started as soon as the plant entered reproductive growth, could be related to the beginning of the fiber accumulation phase in the main stem, but the timing of this metabolic process is not well known. After 1250 GDD, when the soft dough maturity was reached, the total plant biomass accumulated and its partitioning among the plant organs were not statistically different (Table 1). At this stage the main stem, which accounted for 80% of the fresh weight on average, was characterized by a chemical composition in accordance with the genotypic attitude previously described (Table 2). The sucrose content was more than three times higher and the cellulose and lignin contents 40–
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Table 1 Yield traits determinated at soft dough maturity Genotype
Total biomass (t ha−1)
Main stem biomass (t ha−1)
Biomass components (% dw)
Fresh
Dry
Fresh
Dry
Stalks
Leaves
Panicle+seeds
27.59 9 3.44 27.57 9 2.15
100.69 920.09 82.35 95.43
20.73 93.43 20.98 91.99
83.71 9 0.64 79.61 9 0.67
14.82 9 0.49 17.54 90.71
1.47 9 0.16 12.85 9 0.20
WRAY (sweet) 127.36 9 21.40 H173 (fiber) 100.21 94.43
Table 2 Main stem quality traits determined at soft dough maturity Genotype
Fermentable sugars
Fibers
Content (% dw)
WRAY (sweet) H173 (fiber)
Yield (t ha−1 dw)
Content (% dw)
Yield (t ha−1 dw)
Sucrose
Glucose
Fructose
Total
Total
Cellulose
Hemicellulose Lignin
Total
Cellulose+ Hemicellulose
27.61a 7.90b
8.74 6.62
7.23 5.07
43.58a 19.59b
10.03a 4.32b
25.41b 41.85a
22.34b 27.21a
51.59b 76.88a
11.11b 15.15a
3.84b 7.82a
Means followed by different letters are significantly different at P= 0.05 (NK test).
50% lower in ‘Wray’ with respect to ‘H173’, whereas the level of reducing sugars was similar, confirming an intermediate role for these in transport processes (Lingle, 1987).
4. Conclusion Duration of growth in terms of thermal sum to reach soft dough maturity was similar for both sorghums (1250 GDD). Furthermore, the biomass accumulated by the two crops was not statistically different. Sweet and fiber genotypes showed a different growth pattern. The fiber type was characterized by a greater tillering ability during the period of early leaf formation. This positively affected early canopy area and growth parameters. On the other end, the sweet sorghum crop presented enhanced rates of growth after the growing differentiation point as a consequence of higher leaf thickness, area duration and dry matter accumulation capacity, as well as the attitude to accumulate less energy requiring compounds, such as soluble carbohydrates instead of structural ones and lignin.
Considering a biomass conversion factor of 4000 kcal kg − 1 of dry matter, both sweet and fiber crops might provide an energy production superior to 10 000 kcal m − 2. The measured fermentable sugar content of the sweet type ‘Wray’ can result in a potential ethanol production of around 6000 l ha − 1, whereas the main stem of the fiber type ‘H173’ is suitable for the production of up to 15 t ha − 1 of structural polysaccharides. Both sorghum types can be considered as interesting new multi-product crops for temperate Italian environments. To fully exploit the potentiality of these renewable sources of energy and pulp for paper, future work should be directed in solving some physiological bottlenecks, for example chilling and lodging susceptibility, as well as in the implementation of a complete mechanization and transformation products chain. References Anon., 1984. Methods of Enzymatic Food Analysis. Boehringer Mannheim GmbH, Mannheim, pp. 27 – 73. Belletti, A., Petrini, C., Minguzzi, A., Landini, V., Piazza, C., Salamini, F., 1991. Yield potential and adaptability to
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