4.1 ReproductionSugarcane can reproduce both sexually and asexually. Sexual reproduction is via true seed, often called fluff due to the presence of soft hairs. As discussed in Section 2.3.1 the ability of sugarcane to reproduce asexually is exploited for the production of planting material.
4.1.1 Asexual reproductionAsexual reproduction can be via nodal buds which are found on setts, via rhizomes or via stools (Amalraj & Balasundaram 2006). The parent species of Saccharum spp. hybrid differ in their ability to form rhizomes and tillers, with S. spontaneum forming dense mats of rhizomes and many tillers, whereas S. officinarum forms fewer tillers and rhizomes (Moore 1987; Amalraj & Balasundaram 2006).
4.1.2 Sexual reproductionThe ability of sugarcane to reproduce sexually was not recognised until the mid to late 1800’s due to its lack of importance as an economic product (Buzacott 1965). Sugarcane flowering is a complex process consisting of a number of steps which are regulated by different photoperiods (Moore & Nuss 1987). Flowering is dependent on interaction of genotypes and environmental factors such as daylength and temperature. Some cultivars can flower profusely in their natural environment but flower sparingly when introduced to other regions (Bull & Glasziou 1979). Cultivars which evolved at high latitudes usually flower earlier than those which originated at lower latitudes, suggesting that they require longer daylengths for floral initiation (Moore & Nuss 1987). Experiments have also indicated that early flowering cultivars often flower more profusely than later flowering cultivars, possibly due to environmental effects (Moore & Nuss 1987). At Meringa field station, south of Cairns in QLD, an average of 40% of the parental clones used for crossing flowered each year between 1978 – 2003. However, this varied from 13% in 2003 to 75% in 1994 (Berding et al. 2004a). A study of three cultivars in commercial cane fields in the Burdekin region (QLD) in 2006, indicated that one cultivar did not flower, one flowered in some locations and one flowered in all five locations (Bonnett et al. 2007). Observations from the Mulgrave Mill area and the Herbert River region (both in QLD) indicated that commercial sugarcane does flower in these areas (Bonnett et al. 2007).
In Australia, floral development is initiated by shortening day length and occurs from late February to early March (Kingston 2000). Flowering is mostly reliable between latitudes 7º and 12º and non-existent above 30° latitude (Fauconnier 1993). Floral development is induced by photoperiods of approximately 11.5 hours, which often coincides with a natural day length of 12.5 hours. As a result, the period of floral initiation is more defined further from the equator (Bakker 1999). Annual variations in flowering times in a given location are mostly attributable to the differences in night time temperature (Bakker 1999). Cool night temperatures, high day temperatures and lack of moisture interfere with flower initiation. The older and more vigorous stems in a stool are the most likely to initiate flowering (Moore & Nuss 1987). Flower initiation causes the apical meristem to switch from vegetative to floral development. Consequently, flowering of the crop can adversely affect yields (Bakker 1999).
Flowering is not desirable in commercial cane as it uses both energy and sucrose and may lead to pithy islands in the stems (Purseglove 1972). The loss of apical dominance and consequent formation of side shoots leads to reduction in the sucrose content in the stalk. However, if harvesting occurs within 2–3 months of flowering this effect is negligible (Bakker 1999). Measures that have been trialled to prevent flowering include altering planting dates, cutting back the stems, lighting the field for 30 mins at night, applying chemical sprays, applying nitrogen and withholding water (Purseglove 1972; El Manhaly et al. 1984). However, there is some conflicting data on the impact that flowering has on reducing sucrose content in sugarcane stems. As discussed in Moore (1987), some of the conflicting data is due to inappropriate comparisons. Different sugarcane cultivars are affected differently by flowering, and plants that flower may have altered physiology that led to flowering rather than being caused by flowering itself. For example, a series of 35 field trials using epheron, a plant growth regulator, showed reduced flowering and an overall increase in cane weight and sugar yield. However, there was little correlation between reduced flowering and increased yield due to variability between fields (Moore & Osgood 1989). More recent Australian data from experimental plots has shown that cane yield, CCS and sugar yield were all decreased following flowering (Berding & Hurney 2005).
4.2 Pollination and pollen dispersalSugarcane spikelets open from the top of the panicle, with the outermost spikelets opening first. It takes 5–15 days for all the spikelets on the panicle to open. Spikelets open at sunrise, with anther dehiscence occurring about three hours later, although this is delayed by high humidity (Purseglove 1972).
Sugarcane pollen grains are very small, hairy and wind dispersed. The round-ellipsoidal grains vary in size from 38.25 µm x 42.75 µm to 67.5 µm x 72.0 µm (Dutt 1929) and are yellow in colour.
Pollen viability from commercial sugarcane fields is low and varies between cultivars and locations in the Burdekin region, showing a range from 1.2–4.4% viability (Bonnett et al. 2007). Studies showed that sugarcane pollen began to lose viability rapidly in less than 30 min (Venkatraman 1922). S. spontaneum pollen is rapidly desiccated after dehiscence, having a half-life of only 12 minutes, and is no longer viable beyond 35 minutes, under unmodified environmental conditions (26.5º C and 67% relative humidity) (Moore 1976). At higher humidity the pollen longevity was increased (Moore 1976). Tests with another cane cultivar (Saratha Desi, which is thought to be derived from S. barberi) indicated that pollen viability was maintained for two hours in the lab, or one hour when exposed to sunlight (Dutt & Ayyar 1928). Sugarcane pollen stored at 4°C under 90–100% relative humidity retains some viability for up to fourteen days (Moore & Nuss 1987).
Little data is available on sugarcane pollen dispersal. Information from breeding work in which plants were isolated by 20 m in open forest has shown that viable pollen is dispersed over this distance (Skinner 1959). From this work, it was suggested that to prevent contamination of controlled crosses, plants should be isolated by 100 m in open forest, or 300 m in open ground (Skinner 1959).
Sugarcane is a cross-pollinating species although selfing occurs at low levels (Moore & Nuss 1987; McIntyre & Jackson 2001; Tew & Pan 2010). In seven experimental polycrosses the selfing frequencies ranged from from 0–45%. Although the sample size was small, it indicated that the progeny resulting from crosses with a high degree of self-pollination had reduced ability to survive the winter, suggesting reduced vigour (Tew & Pan 2010). The reduction in vigour following self-pollination has been observed previously (Skinner 1959). Sugarcane produces protogynous flowers, where the pistil matures before the anthers. Thus, an individual flower may be cross-pollinated prior to pollen shed from its own anthers (James 2004).
Sugarcane flowers often have reduced male fertility or are male sterile and some are self-sterile (Skinner 1959).
4.3 Fruit/Seed development and dispersalAfter fertilisation it takes approximately three weeks for the fruit to mature and to be shed (Purseglove 1972). The seed at the top of the panicle which was fertilised first is also the first to mature (Breaux & Miller 1987). These seed are shed as the inflorescence starts to disintegrate, before the seeds at the base reach maturity (James 1980). The mature fruit contain whorls of silky hairs at the base and are adapted for wind dispersal (Purseglove 1972) (Figure 6). No further information has been found in the literature on seed dispersal.
Data from crosses has suggested that a low percentage of florets set fertile seed. One estimate of seed germination showed a maximum of 17.2% in a “very heavy” germinator (Price 1961). Another study showed germination rates of between 3.1 and 22.7% (Rao 1980). Seed collected from commercial fields in the Mulgrave Mill area had variable germination, ranging from 0.9 to 23.6 viable seed g-1, depending on the cultivar (Bonnett et al. 2007). Similarly in the Herbert region, even in October, which is four months after the normal flowering time, inflorescences were found containing between 0 and 53.3 viable seed g-1. A very small proportion of seed collected further south in the Burdekin region was viable (three seedlings germinated from 30 arrows) (Bonnett et al. 2007).
The naked seed has been measured as 1.5±0.03 x 0.64 ±0.005 mm and weighing 0.54±0.05 mg, which is approximately 1850 seeds g-1 (Rao 1980). One of the sugarcane parent species S. spontaneum has seed which weighed 0.39 mg with fuzz, or 0.25 defuzzed (Ellis & Hong 2007).
Mature fuzz consists of the mature dry fruit (caryopsis), glumes, callus hairs, anthers and stigma (Breaux & Miller 1987). The additional parts of the inflorescence are generally handled, stored and sown with the seed because it is not practical to separate them. Although many commercial cultivars of sugarcane can produce seed, it is only used in breeding programs, because the proportion of sugarcane seedlings with agronomic qualities near to those of the parental commercial cultivars is extremely low.
Figure 6. S. spontaneum seed. Photo by Kristin Saltonstall, Smithsonian Tropical Research Institute, Panama.
4.4 Seed germinationSome wild species of sugarcane such as S. aegyptiacum (now classified as a subspecies of S. spontaneum) have significant seed dormancy, whereas modern cultivars have little seed dormancy (Ellis, Hong and Roberts 1985 as cited in Simpson 1990).
Sugarcane seed has short viability. Preliminary data from on-going experiments has shown that germination of seeds after shedding varied between sugarcane cultivars, but for more than half of the 13 cultivars tested it stayed high for 10–12 weeks when stored under lab conditions at 22°C. Other samples showed a decline in germination from eight weeks (Powell et al. 2008). If stored in polythene at room temperature, fuzz remained viable for 90 to 120 days (Verma et al. 2002). Artificially dried sugarcane seed lost 90% of its viability in 70 days at 28ºC if not desiccated (Rao 1980). Modelling of seed longevity using data on germination at different temperatures and moisture contents has predicted that under hermetic storage at -20°C, seed from the parent species S. spontaneum will not last as long as ten other crop species, with only potato (Solanum tuberosum) showing shorter viability (Ellis & Hong 2007).
Generally in breeding programs the fuzz is sown. However, the fuzz can encourage growth of microorganisms and a large mass of fuzz can prevent seed contact with the soil (Breaux & Miller 1987). Improved germination has been seen when the non-seed parts of the fuzz are removed (Breaux 1981 as cited in Breaux & Miller 1987).
Germination of sugarcane seed occurs better in the light, requires heat and humidity, and takes 25 days for small seedlings to appear (Buzacott 1965; Purseglove 1972). Preliminary data from on-going experiments has shown that seed germinated at 15–42°C under lab conditions, with an optimum germination at 30–36°C (Powell et al. 2008).
As the seed germinates, the primary root emerges first followed by elongation of the plumule. The leaves of the plumule then emerge rapidly. Tiller branches emerge from a bud which forms in the axil of each leaf. Adventitious roots form near the leaf bases (Moore 1987).
The young seedlings are delicate and require optimum temperature, moisture, nutrients and protection from fungal diseases (Buzacott 1965; Breaux & Miller 1987). Information in Breaux and Miller (1987), in part obtained from a survey of sugarcane breeders, suggests that the conditions required to germinate and grow sugarcane seedlings are exacting. Constant care and attention is needed to give seeds and seedlings the conditions required for survival, especially in the first 3–4 weeks post-germination.
Vivipary, when the seed germinates before it detaches from the parent plant, has been observed under experimental conditions in both the parent species S. spontaneum and in hybrid sugarcane (Ragavan 1960). It is feasible that moist conditions, similar to the experimentally induced ones, could occur naturally.
4.5 Vegetative growthAs discussed previously, sugarcane is propagated from stem cuttings which are referred to as setts, seed, seed-cane or seed-pieces (Purseglove 1972). During the initial stages of germination, root primordia around the nodes of the sett produce a flush of roots, known as sett roots (Bakker 1999). These roots are not connected directly to the primary shoot but are important in maintaining the moisture in the sett. Following formation of the shoot roots, the sett roots blacken and die (Bakker 1999). The primary shoot is made up of a number of closely spaced internodes and nodes below ground. Each node develops new bud and root primordia that are the basis of stool establishment. These root primordia germinate to produce the shoot roots that support further plant growth. The shoot is then independent of the original sett (Bull 2000).
While the shoot roots are developing, some of the new buds below ground also germinate to produce secondary shoots or tillers. These, in turn, develop their own root systems and give rise to shoots (Bull 2000). Shoots usually appear above the soil approximately twelve days after planting, with the first leaf unfurling approximately eight days later (Bakker 1999).
Stem elongation is initially rapid and during this phase the fibre content of the stem is relatively high, whereas the CCS levels are still quite low. Breeding for high above ground biomass in modern sugarcane cultivars means the plant is very top heavy and consequently sugarcane is prone to lodging. Plants recover from lodging by curving of the stem to again grow upright. In Australia, studies have shown that lodging is associated with yield losses in both the wet and dry tropics (Singh et al. 2002).
Growth rate slows and sucrose content increases approximately 120 days after planting (Bull 2000). Maturation and ripening are reversible processes and are associated with the lower rainfall and cooler temperatures of the winter months. During stem growth, each internode operates as an independent unit. While it has a green leaf attached, the internode completes cell elongation and cell wall thickening, and fills with sucrose. Hence internodes generally complete their cycle by the time the attached leaf dies, and the lower internodes are essentially ripe while the upper part of the stem is still growing. The stored sugar is, however, available for translocation to support further tillering and/or growth when conditions are not favourable for photosynthesis (Bull 2000).
As the stem matures, more internodes reach the same condition and sucrose content rises. During this period, the most recently expanded internodes near the top of the stem stop elongating and photosynthates are channelled into storage as sucrose. Factors that affect the maturation of the sugarcane stem include age, nitrogen status and moisture. Environmental factors that can influence sucrose accumulation include water stress, nutrient status and temperature (Bull 2000).