Introduction

Sugarcane (a complex hybrid of Saccharum spp.) is an important industrial crop and has been exploited globally in sugar production and bioenergy feedstock as one alternative energy sources (McMillan 1997). It is well known that sugarcane accounts for approximately 80% of global sugar production (https://www.isosugar.org/sugarsector/sugar). Starting from many years ago, Brazil took the lead in a project of biomass production of ethanol as fuel by fermenting sugarcane sucrose (Matsuoka et al. 2009; Neves et al. 2011; Grassi and Pereira 2019). The sustainability of ethanol production from sugarcane in Brazil was the most successful program of renewable energy from biomass (Leal and Da Silva Walter 2010). It has been approved that sugarcane is an important biofuel feedstock crop and accounts for approximately 40% of the biofuel production worldwide because of its favorable energy input/output ratio (Lam et al. 2009). In tropical and subtropical regions, sugarcane is the most efficient crop for both commercial sugar and bioenergy (Waclawovsky et al. 2010). When conventional sugarcane is crossed with S. spontaneum, the progeny shows a wide range of sugar-to-fiber ratios which could be useful for molecular, biochemical, physiological, and/or agronomic studies regarding to source–sink system, partitioning, and composition of all carbohydrates (Wang et al. 2013; Yang et al. 2019).

It is well known that sugarcane is a complex interspecific hybrid of several species, including S. officinarum with high sucrose and low fiber, and S. spontaneum with low sucrose and high fiber as well as stress tolerance, and then backcrossing to improve sucrose concentration, disease resistance, and abiotic stress tolerance. Energy cane is a hybrid of commercial and wild sugarcanes, and it is bred for high tonnage, high fiber content, and relatively low sucrose (Kim and Day 2011). Essentially, two canes (i.e., sugarcane and energy cane) come from the same genus, Saccharum. Both are high-yielding biomass crops suitable for producing renewable fuels such as ethanol and second-generation biofuels (Woodard and Prine 1993; Woodard et al. 1993; Tew and Cobill 2008). Saccharum is a C4 plant with high efficiency in use of nutrients, water, and solar radiation (Somerville et al. 2010), especially under unfavorable conditions for other field crops.

In addition to sugar production, therefore, sugarcane and/or energy cane may be the ideal candidates of bioenergy feedstock to produce biomass for biofuel production in locations with marginal soils. The lignocellulosic ethanol conversion technologies have led scientists to develop stress tolerant, high-biomass, and low-sucrose varieties of Saccharum for the purpose of converting the fiber into bio-ethanol energy. These varieties are referred to as energy cane in contrast to sugarcane with high sucrose and low fiber mainly for sugar production. The growing interests in bioenergy in recent decades thrust scientists to develop improved energy cane varieties via conducting related studies in the USA (Tew et al. 2011; Sandhu et al. 2015; Sandhu and Gilbert 2017) and the worldwide (Matsuoka et al. 2009; Carvalho-Netto et al. 2014; Santchurn et al. 2014, 2019) and to better understand plant physiology of source-to-sink process as an obvious basic step to get efficiency either in the process of capturing solar radiation by the plant or its subsequent accumulation of sugar and ultimately use of the resultant total plant by mankind (Waclawovsky et al. 2010; Santchurn et al. 2014; Lingle 1999; de Souza et al. 2013; Matsuoka et al. 2014). As a C4 plant, sugarcane/energy cane is very efficient in the photosynthetic process and biomass production in a wide arrange of growth environment, so that its dry matter accumulation is one of the highest among the known plants (Jakob et al. 2009; Byrt et al. 2011).

Sucrose in sugarcane plants is the dominate form for both transportation from source (leaves) to sink (stalks) and storage in stalks as final product (Wang et al. 2013). Recently, van Heerden et al. (2010) suggested that high concentration of sucrose in sugarcane stalks probably resulted in the feedback inhibition of leaf photosynthesis capacity, which in turn could have contributed toward the decline in biomass accumulation. Inman-Bamber et al. (2011) used 20 sugarcane clones with either high fiber or high sucrose concentration to elucidate some of the processes from source to sink in order to validate the feedback mechanisms as reported by van Heerden et al. (2010). They revealed that sucrose accumulation in sugarcane stalks did not limit leaf photosynthetic rate and biomass production (Inman-Bamber et al. 2011). The similar results were found in sugarcane and energy cane with no consistent difference in leaf net photosynthetic rate between sugarcane and energy cane (Zhao et al. 2017). Carvalho-Netto et al. (2014) reviewed biomass crops for the cellulosic industry and pointed out that energy cane was one of the greatest potentials for biomass production among the fibrous plants.

In a recent study, Zhao et al. (2017) compared sugarcane with energy cane in growth and physiology in plant cane, and first- and second-ratoon crops. They found that although there are no consistent differences between the two canes in leaf relative chlorophyll level (SPAD reading), net photosynthetic rate, or canopy normalized difference vegetation index (NDVI) of plant cane, energy cane has significantly greater NDVI value than sugarcane in the two ratoon crops during tillering and fast growing phase (i.e., grand growth) because of more tillers, taller plants, and faster growth in energy cane than in sugarcane. Energy cane has higher shoot dry biomass yield than sugarcane based on results from six harvests (two locations and three crops) (Zhao et al. 2017). The differences in biomass yields between sugarcane and energy cane are mainly detected in ratoon crops. Increased biomass yield of energy cane is primarily associated with higher stalk population, longer stalks, and greater NDVI values rather than leaf net photosynthetic rate or stalk diameter as compared with sugarcane (Zhao et al. 2017). Sugarcane is one of major crops on sand soils in south Florida, but yields and profits are low compared to sugarcane grown on organic soils in the region (VanWeelden et al. 2018). Energy cane may be an alternative crop on sand soils in the future to improve profits if the market and facility are available because of the increasing interest of high biomass for energy, especially cellulosic ethanol production and sustainability.

In most sugarcane production systems, fresh mature cane tonnage (i.e., cane yield) and sucrose content in fresh cane are two major yield components of sugar yield. In energy cane production systems, however, dry biomass and its chemical composition are more important compared with fresh biomass. A better understanding of plant material structural and nonstructural carbohydrate concentrations and composition on a dry biomass basis for both sugarcane and energy cane can improve knowledge of management practices, crop production, postharvest process, and sustainability. It is especially important when both sugarcane and energy cane grow on mineral (sand) soils with much high proportion of sand and relative high water table in south Florida, a major sugarcane production region in the USA. The specific objectives of this study were as follows: (1) to identify sugar composition and concentration of stalk juice in sugarcane and energy cane by growing on sand soils during ripening for three crops of plant cane, first ratoon, and second ratoon, and (2) to determine differences between the two canes in composition and concentrations of nonstructural and structural carbohydrates on the basis of dry biomass for the three crops at maturation.

Materials and Methods

Experimental Background

The experiment was carried out in commercial sugarcane production fields at two grower’s farms of Pahokee Produce Incorporated (PPI) and Townsite in 2011–2013 in three crops of plant cane, first ratoon, and second ratoon. The PPI farm (80° 23′ 42.0″ W, 26° 45′ 43.2″ N) is located approximately 30 km east side of Canal Point, and the Townsite farm (80° 58′ 32.0″ W, 26° 44′ 21.9″ N) is west side of the Lake Okeechobee near Clewiston, FL. Details of soil types and properties and field management practices at the two experimental sites have been described in a recent report (Zhao et al. 2017).

Two commercial sugarcane cultivars ‘CP 78-1628’ (Tai et al. 1991) and ‘CP 80-1743’ (Deren et al. 1991) and two genotypes (US 78-1013 and US 84-1066) of energy cane were planted on Oct. 7, 2010, in the Townsite farm and on Dec. 9, 2010, in the PPI farm. Mechanical harvest dates of the plant-cane, first-ratoon, and second-ratoon crops completely relied on the dates when the industry harvest of adjacent sugarcane fields and were as follows: Feb. 12, 2012, Feb. 13, 2013, and Jan. 10, 2014, respectively, at PPI; and on April 9, 2012, March 20, 2013, and Feb. 21, 2014, respectively, at Townsite. The experiment was planted in a random complete block design with four replications. The plot size was 20.7 m2 (4.6 m long × 4.5 m wide) with 3 rows per plot and 1.5 m row spacing at both experimental sites.

Stalk Juice Brix and Sugar Concentrations

During cane ripening or harvest season from September through February, stalk juice in an internode of the halfway of stalk was collected using a handhold punching device from three stalks in each plot on the PPI farm monthly for six, five, and four times, respectively, on plant cane, first ratoon, and second ratoon. The specific sampling dates for each crop are given in Table 1. A subsample of 0.05 ml juice was placed into a 1.5-ml microvial with 0.95 ml of 80% ethanol after determination of juice Brix (% of juice total soluble solids) values using an Atago PAL-1 Digital Brix Refractometer (Spectrum Technologies, Inc., Plainifield, IL). The ethanol-diluted juice subsamples were stored in a freezer for further analyzing sucrose, glucose, and fructose concentrations in juice at the end of harvest season according to Zhao et al. (2010) with some modifications because of much lower concentrations of reducing sugars (glucose and fructose) than sucrose in stalk juice. Briefly, the ethanol-diluted juice subsamples were centrifuged (3000 ×g) for 10 min before further analysis or dilution. The supernatants were directly used to quantify juice glucose and fructose concentrations using the microplate enzymatic assays (Zhao et al. 2010). The supernatants were further diluted to 400 to 500 times with 80% of ethanol to determine sucrose and total sugar (i.e., glucose, fructose, and sucrose) concentrations using the same method. Additionally, Brix and sugar components of juice extracted from 10-stalk crushed samples harvested in late December from each plot at PPI and Townsite and milled at Canal Point were measured using the same methods as described above and by Zhao et al. (2010).

Table 1 Specific dates (Month/Day/Year) of four to six times of collecting Juice for quality analysis during sugarcane and energy cane ripening of plant cane (2011), first ratoon (2012), and second ratoon (2013) crops
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Aboveground Biomass Yields

Stalk populations of plant cane, first ratoon, and second ratoon were estimated by counting millable stalks in the middle row of all plots in August at the both the PPI and Townsite experimental sites. To estimate fresh shoot biomass yield of mature cane in each harvest season, 10-stalk samples were collected randomly from the middle row of each plot in mid-to-late December for the plant-cane, first-ratoon, and second-ratoon crops to ensure the samples could represent the whole plot. Stalk samples were transported to a laboratory at Canal Point where the aboveground fresh and dry biomass yields were assessed (Zhao et al. 2017).

Carbohydrate Concentrations and Composition of Mature Canes

The oven dried mature cane samples were ground to pass a 2-mm screen in a Wiley mill (Arthur H. Thomas Co., Philadelphia, PA), and a subsample was ground afterward through a 1-mm screen in a cyclone mill (Udy Corporation, Fort Collins, CO) for measurements of concentrations of structural (cellulose, hemi-cellulose, and lignin) and nonstructural (glucose, fructose, sucrose) carbohydrates in the dry ground materials. Concentrations of cellulose, hemi-cellulose, and lignin in the ground biomass samples of mature sugarcane and energy cane were determined according to standard laboratory procedures of forage quality analysis outlined by Ankom Technology (Fairport, NY) (http://www.ankom.com/09_procedures/procedures.shtml). Extraction and concentrations of nonstructural carbohydrates, including glucose, fructose, and sucrose in the ground biomass samples, were carried out and analyzed according to Zhao et al. (2010) with proper dilution of extracts as described above for juice.

Data Analyses

Predata analyses of variance indicated that there were no statistical differences in most parameters measured between the two cultivars (lines) within sugarcane or energy cane (Data not shown). Therefore, the combined energy cane data of US 78-1013 and US 84-1066 were used to compare with the combined sugarcane data of CP 78-1628 and CP 80-1743 at each location in order to determine differences between sugarcane and energy cane.

Statistical analyses of variance for biomass carbohydrate composition and yield data were performed using PROC MIXED of SAS (SAS Inc. 2010). To determine main effects of cane type (i.e., sugarcane and energy cane), crop cycle (i.e., plant cane, first ratoon, and second ratoon), and their interactions on biomass yield and chemical composition, the data of juice sugar concentration on each sampling date, dry biomass yields, carbohydrate concentrations were statistically analyzed separately by the experimental sites. Both crop cycle and cane type were considered as fixed effects in this study. Significance of each fixed effect and their interactions were analyzed using the MIXED procedures of SAS (SAS Inc. 2010). If the hypothesis of equal means for measured traits between two canes or among three crops were rejected and/or any interactive effect was significant by the F test, trait means were separated with the least significant difference (LSD) at P = 0.05. The LSD values were calculated with the standard errors (SE) generated by the Diff option in the MIXED procedures of SAS.

Results and Discussion

Juice Brix and Sugar Composition Responses to Sampling Date

Commercial sugarcane harvest season in Florida lasts more than 6 months, from the mid-October through late April in order to fully and efficiently use mill’s capacity. To determine dynamics of stalk juice Brix and sugar composition during the harvest season, juice samples were collected monthly for five or six times from mid-September through February using a handhold puncher device. The results indicated that juice Brix value and the concentrations of sucrose and hexose (glucose + fructose) closely depended on cane type (i.e., sugarcane, energy cane), crop cycle (i.e., plant cane, first ratoon, second ratoon), and sampling date during ripening or harvest season (Fig. 1).

Fig. 1
figure 1

Dynamics of juice a Brix value, b sucrose concentration, and c hexose (glucose + fructose) concentration for sugarcane and energy cane as responses to sampling date for plant-cane, first-ratoon, and second-ratoon crops. Note: juice samples were collected from the middle Sect. (1.2 m from ground level) of three stalks in each plot using a handhold punching device. The specific sampling dates can be found in Table 1. Each data point is mean ± 1SE of 8 replicates; the *, **, and *** indicate the significant levels reach P ≤ 0.05, 0.01, and 0.001, respectively; and NS = not significant

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Brix values and sucrose concentration increased rapidly with sampling date from mid-September through mid-November (for energy cane) or December (for sugarcane) and fluctuated then after. Sugarcane had significantly greater juice Brix values (Fig. 1a) and sucrose concentration (Fig. 1b) than energy cane at all sampling dates (P ≤ 0.01 to 0.001). In contrast to sucrose concentration, juice hexose (glucose + fructose) concentration sharply declined with the sampling date (Fig. 1c). Sugarcane had significantly higher juice hexose in mid-September and lower juice hexose for most other sampling dates than energy cane. Averaged across sampling dates and three crops, energy cane had 43% lower juice Brix value and 53% lower sucrose concentration than sugarcane. Significant interactions were also detected in juice hexose concentration among sampling date, crop cycle, and cane type (Fig. 1). Recently, Zhao et al. (2019) reported that sugar composition and concentration varied greatly among sugarcane genotypes especially in early harvest season, and some genotypes had significantly higher sucrose than others. Therefore, better understanding sugarcane cultivar sugar dynamics and genotype differences can optimize harvest time and improve juice quality and production profits.

Juice Quality Traits of Crushed Canes

As described above, the timely juice samples were collected from the halfway of stalks with a handhold puncher device to investigate juice sugar dynamics. However, juice in the crushed mature cane from a mill house actually came from whole stalks. There might be some differences between the punched and crushed juices in sugar composition and concentrations, because these quality traits changes with not only sampling date in harvest season, but also the node or internode position on a stalk (Zhao et al. 2017; Glasziou 1961; Tai and Miller 2002; Watt and Cramer 2009). Further determination of Brix and sugar components of mill-house juice from crushed cane in the present study indicated that crop cycle, cane type, and the interactions of the crop cycle and cane type significantly affected juice sucrose and hexose concentrations (Fig. 2). The main effect of crop cycle on Brix did not differ, but cane type and the crop cycle × cane type interaction on Brix was highly significant (P ≤ 0.05 to 0.001).

Fig. 2
figure 2

Comparison of sugarcane and energy cane on a Brix value, b sucrose concentration, and c hexose concentration of Juice collected from milling 10-stalk samples in late December for plant cane (PC), first ratoon (FR), and second ratoon (SR). Data are mean values + SE (n = 8)

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Averaged across the two experimental sites and three crops, energy cane had 28% lower juice Brix value and 35% lower sucrose concentration than sugarcane. The differences in degree of low Brix and sucrose between sugarcane and energy cane were mainly caused by high hexose concentration in energy cane juice, because Brix value reflected juice total soluble solid concentration (Lingle et al. 2010). Compared to sugarcane, energy cane had 3- to 4-fold higher hexose concentration in mature cane juice collected from the crushed cane samples in the mill house (Fig. 2).

Aboveground Dry Biomass Yield

Aboveground dry biomass yield at Townsite (43.3 Mg ha−1) was much higher than that at PPI (39.4 Mg ha−1) when combining data of three crops and two canes (Fig. 3). Lower aboveground dry biomass yield (especially in plant cane, 15.3% lower) at PPI than that at Townsite was probably due to differences in planting date and in N fertilizer application between the two sites (Zhao et al. 2017). The planting dates at Townsite (7 Oct.) was 2 months earlier than that at PPI (Dec. 9, 2010). Amount of N application (90 to 100 kg ha−1) at the PPI farm seemed to be not adequate for sugarcane and energy cane on sand soils in Florida (Zhao et al. 2017) according to the University of Florida fertilizer recommendations for sugarcane on sand soils (Ezenwa et al. 2005; McCray et al. 2016). Based on sugar yield responses to N rate on the Florida sand soils reported by McCray et al. (2014), N fertilizer recommendations for sugarcane grown on sand soils in Florida (McCray et al. 2016) currently are 224 to 246 kg ha−1 with multiple times of split applications.

Fig. 3
figure 3

Aboveground dry biomass yields of mature sugarcane and energy cane for plant cane, first ratoon, and second ratoon at the PPI and Townsite farms. Data are mean values + SE of eight replications

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The aboveground dry biomass yields of sugarcane and energy cane were 37.4 and 45.2 Mg ha−1, respectively, averaged across the two sites and three crops. The increased dry biomass yield for energy cane relative to sugarcane was mainly associated with stalk population (Zhao et al. 2017). The differences in dry biomass yield between the two canes increased with ratooning crops (Fig. 3). The interaction of crop cycle and cane type on the dry biomass yield was also detected. Zhao et al. (2017) reported that the difference between sugarcane and energy cane in dry biomass yield was greater than that in fresh biomass yield, because energy cane had greater ratio of dry to fresh biomass than sugarcane.

Studies have indicated the differences between sugarcane and energy cane in biomass yields (Santchurn et al. 2014; Kim and Day 2011; Fedenko et al. 2013; Leon et al. 2015), but results varied greatly in these previous studies. Our results indicated that the overall dry biomass yield of energy cane was 21% higher than that of sugarcane on sand soils in south Florida and the differences in dry biomass yield between the two canes were mainly detected in the ratoon crops rather than plant cane. The results from our study suggested that energy cane was more tolerant to unfavorable growth conditions (i.e., water, temperature, and nutrients) than sugarcane, because these issues frequently occur on sand soils in Florida (Ezenwa et al. 2005; Gilbert et al. 2008; Zhao et al. 2013). The differences in energy cane biomass yield and its responses to crop cycles among previous studies as well as in the present study are probably associated with varieties used, soil type, planting date, and harvest time (Zhao et al. 2017).

Carbohydrate Concentrations and Composition in Dry Biomass

Carbohydrate concentrations and composition in shoot dry biomass differed significantly among three crops and between sugarcane and energy cane (Table 2). The significant interactions of the crop cycle and cane type on all carbohydrate components including structural (cellulose, hemi-cellulose, and lignin) and nonstructural (sucrose, glucose, and fructose) carbohydrates were also detected, although the total (structural + nonstructural) carbohydrate concentrations of two canes were similar (Table 2). Compared to sugarcane, energy cane had much lower nonstructural carbohydrates (especially sucrose), but higher structural carbohydrate concentrations. Averaged across the two experimental sites and three crops, concentrations of cellulose, hemi-cellulose, lignin, sucrose, glucose, fructose, and total carbohydrates in dry shoot biomass of mature sugarcane were 214.1, 142.6, 41.4, 285.0, 86.5, 71.7, and 841.2 g kg−1, respectively. These carbohydrate concentrations in energy cane were 384.9, 232.0, 73.0, 88.7, 30.9, 31.5, and 841.1 g kg−1, respectively (Table 2).

Table 2 Concentrations of structural (cellulose, hemi-cellulose, and lignin) and nonstructural (sucrose, glucose, and fructose) carbohydrates in mature shoot dry biomass of sugarcane and energy cane for three crops of plant cane (PC), first ratoon (FR), and second ratoon (SR) at the two experimental sites and their analyses of variance.†
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The complexities in using fresh and dry weight to estimate energy cane quality traits have been highlighted in a recent study in Mauritius (Santchurn et al. 2019). Carbon partitioning in sugarcane has been reviewed by Wang et al. (2013). It is well known that energy cane has much lower sugar and higher fiber concentrations than sugarcane (Kim and Day 2011; Santchurn et al. 2014; Matsuoka et al. 2014). In the present study, we found that there were much great differences in results of sugar concentration and composition between sugarcane and energy cane when measuring sucrose and hexose concentrations on the basis of the dry biomass level (Table 2) and mature stalk juice level (Fig. 2). For instance, sugarcane had 221% higher sucrose and 153% higher hexose concentrations than energy cane in dry biomass, when averaged across the experimental sites and crop cycles. In contrast, sugarcane had only 35% higher juice sucrose concentration than energy cane. Former had much lower juice hexose concentration compared to the later (Fig. 2).

Further analyzing the fractions of each carbohydrate component in the total structural and nonstructural carbohydrate indicated that the great differences existed between sugarcane and energy cane (Fig. 4). Averaged across two experimental sites and three crops, cellulose, hemi-cellulose, lignin, sucrose, glucose, and fructose accounted for 25, 17, 5, 34, 10, and 9% of total carbohydrates, respectively, in sugarcane (Fig. 4a); and accounted for 46, 27, 9, 10, 4, and 4% of total carbohydrates, respectively, in energy cane (Fig. 4b). The differences in carbohydrate composition between the two canes may influence ethanol production and processing cost when using them as biofuel or ethanol production as reported by Leal (2007), Kim and Day (2011), Shields and Boopathy (2011), and Thammasittirong et al. (2017).

Fig. 4
figure 4

Carbohydrate partitioning in aboveground dry biomass of mature a sugarcane and b energy cane. Data are the percentage of total carbohydrate for each component and are mean values of plant-cane, first-ratoon, and second-ratoon crops at the PPI and Townsite farms

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Conclusions

This study revealed sugar concentrations and composition in juice of sugarcane and energy cane as affected by sampling date during ripening and harvest season. Results indicated that juice Brix value and sucrose and hexose (glucose + fructose) concentrations highly depended on cane type, crop cycle, and sampling date. Averaged across sampling dates and three crops, energy cane had 43% lower juice Brix value and 53% lower sucrose concentration than sugarcane. The differences in percentages of juice Brix and sucrose between two canes were mainly associated with higher hexose concentration in energy cane than sugarcane. Energy cane had significantly higher shoot dry biomass than sugarcane at the harvest time of mature canes. The increased dry biomass for energy cane was mainly associated with high stalk population (Zhao et al., 2017). Carbohydrate concentrations and composition of shoot dry biomass or carbon partitioning in hexose, sucrose, cellulose, hemi-cellulose, and lignin differed significantly between sugarcane and energy cane, although their total (structural + nonstructural) carbohydrate concentrations were similar. Fast growth and high shoot dry biomass yield of energy cane versus sugarcane could be associated with carbon partitioning. These results can help better understand physiological and biochemical basis of two canes, and be useful for sucrose and bio-ethanol production from sugarcane to energy cane on unfavorable sand soils in the future.