6.1 Abiotic stresses

6.1.1 Nutrient stress

The cultivation of sugarcane relies on the extensive use of fertilizers. It has been estimated that a crop of 74 tons of cane ha-1 removes 107 kg nitrogen, 60 kg phosphorus oxide and 300 kg potassium oxide ha-1 (Purseglove 1972). The sugarcane plant requires nitrogen for optimum development for yield and sugar content of the canes. Symptoms of nitrogen deficiency are thin, stunted stalks, yellowing leaves with necrosis at the edge and tips and reduced root mass (Calcino et al. 2000). However, excess nitrogen can prolong the crop maturation, resulting in a plant with an excessive leafy canopy, which in turn can make the plant more susceptible to leaf diseases and attack by pests (Bakker 1999). It can also cause excess growth with little storage of sucrose (Irvine 2004).

Phosphorus is required for optimum growth. Deficiencies may manifest in plants with short, thin stalks and stools with a low number of primary stalks, a poorly developed root system and sometimes leaves that are green-blue in colour. Conversely, an excess of phosphorus can lead to a deficiency of other trace elements such as zinc and iron, thus reducing sugar yields (Bakker 1999).

Potassium is required for many physiological processes. It helps to promote the formation and translocation of sugars, and thus may improve the extraction and purity of the cane juice. Supplementing sugarcane plants that are exposed to excessive nitrogen with potassium can alleviate the symptoms of over-supply of nitrogen. Potassium deficiency results in depressed growth, thin stalks and yellowing of the older leaves with chlorotic spots and ultimately death of the leaf (Bakker 1999). Potassium may also play a role in the ability of sugarcane to withstand dry conditions (Wood & Schroeder 2004). An excess of potassium increases the ash content of sugarcane juice and reduces the recovery of sugar, and, as with phosphorus, it may also lead to a deficiency of other trace elements (Calcino 1994).

Calcium is an important element for plant growth and also a regulator of soil acidity. A deficiency in calcium results in leaf chlorosis and reduced stem diameter. Increasing soil acidity, often due to lime application, can result in an increased fixation of phosphorus, aluminium, iron, manganese and nickel, which may lead to toxicity (Bakker 1999).

Magnesium is important for photosynthesis, being required for chlorophyll function, and is responsible for the green colour in the leaves (it absorbs the blue and red light spectrum). Deficiencies result in leaf chlorosis and stalks of reduced diameter with internal browning (Bakker 1999). Magnesium is usually abundant in Australian soils, although it may become depleted in old canegrowing soils (Calcino 1994).

Other micro element requirements include sulphur, iron, aluminium, zinc, copper, boron, silicon, molybdenum and manganese. Both deficiencies and toxicity to these elements can occur, resulting in symptoms such as reduced growth, reduced root development and a reduction in photosynthesis (Bakker 1999).

6.1.2 Temperature stress

Low temperatures
Sugarcane cultivars differ in their degree of temperature sensitivity but in general sett germination is slow at soil temperatures below 18C and the setts may succumb to attack by fungal pathogens before they germinate. Sett germination is increasingly rapid up to about 35C (Bull 2000).

Experiments have shown that sugarcane plants grow more slowly and have fewer, shorter internodes and fewer leaves at 15C than when grown at 27C. The low temperatures also inhibited sucrose export from the leaves to the stalk so the leaves accumulated sugar and starch (Ebrahim et al. 1998).

Flowering is also affected by low temperatures. Cool night temperatures, high day temperatures and lack of moisture interfere with both flower initiation and sucrose accumulation. Temperatures below 18.3C are non-inductive for flower development (Coleman 1963). In temperate South Africa, pollen fertility has been shown to be limited at temperatures below 21C (Brett (1952) as cited in Berding 1981). In Meringa, in QLD, artificially increasing the night-time temperature of sugarcane plants to 22–23C led to increased and earlier flowering (Berding 1981). Experiments have also shown that heated pollen lanterns, used for crossing, can increase seed setting, due to improved fertilisation and embryo development (Berding & Skinner 1980).

Preliminary data from on-going experiments has shown that seed germination is reduced by 60% at temperatures below 30C (Powell et al. 2008).

Sugarcane is susceptible to frost damage (Griffee 2000). Freezing reduces yields by delaying crop development in spring and by terminating sugar accumulation in autumn (Moore 1987). In northern NSW about a third of the cane is affected by frost, leading to yield losses of 10–30% annually. Frosts may also affect production in southern QLD and limit the growth of sugarcane in southern regions of Australia (Weaich et al. 1993). The degree of damage varies with the severity of the frost. Leaf browning occurs at temperatures from 0 to -2C, with temperatures down to - 4C causing damage to terminal and lateral buds and death of some young internodes. Temperatures as low as -11C can cause freezing and subsequent cracking of entire stalks. The cracks or damaged buds can allow entry of anaerobic bacteria such as Leuconostoc mesenteroides which can replicate in the damaged tissues and produce dextran. Dextran reduces the sucrose yield at the mill by preventing the crystallisation of sucrose (Irvine 2004). Frost damage varies between sugarcane cultivars, and this is thought to be due to differences in tolerance, rather than differences in morphology which might protect against frosts (avoidance) (Weaich et al. 1993). Management practices, such as retention of a trash blanket increases the susceptibility to frost by preventing radiation of warm air from the soil (Kingston 2000).

Hot temperatures
Sugarcane can survive temperatures as high as 45C, or higher for short periods of time but growth slows at temperatures above 40C (Moore 1987). However, in Iran sugarcane is grown in the Hapft Tappeh region where the average temperature over the summer months is 45.8 C (Sund & Clements 1974). Sugarcane grown in the Ord River region of WA, which has a mean November temperatures of 39.4C Bureau of Meterology accessed 17 Sept 2010, has a lower sucrose content than that grown in cooler regions (Bonnett et al. 2006). Sugarcane exposed to temperatures between 25–38C had a larger number of shorter internodes which contained lower sucrose levels than similar sugarcane plants grown at 23–33C (Bonnett et al. 2006). High daytime temperatures (above 31C) may also inhibit flowering, and very high temperatures at anthesis may reduce seed set. However, it has been suggested that these responses to high temperatures may be due to a water stress effect (as discussed in Moore & Nuss 1987).

6.1.3 Water stress

Sugarcane is relatively drought resistant but water stress results in a reduction of sugar production (FAO 2004). It is estimated that irrigation can add 3 t sugar ha-1, a figure modelled on an average irrigation of 500 mm (Meyer 1997). Sett germination does not occur in dry soil (Smit 2011). Sugarcane flowering is also reduced by water stress (Moore & Nuss 1987) with watered crops showing a greater number of panicles and a higher percentage of plants flowering (Berding 1995).

6.1.4 Other abiotic stresses

Fires do not generally kill sugarcane plants, in fact fire may be used to facilitate easier harvesting. It does not destroy the suckers and the plant will subsequently shoot from the nodes or regrow from the stools (FAO 2004).

Sugarcane plants can withstand short periods of flooding (FAO 2004). After four days the growing point of the sugarcane plant will die, but it may continue to grow from side shoots once the water has receded (BSES Ltd 2008a). Generally yield loss will be 15–20% after five days submergence, 30–60% yield loss after ten days and 37–100% after fifteen days, but this depends on the height of the stalks, with younger cane being more affected than those at 2.5 m tall (BSES Ltd 2008a). Prolonged periods of waterlogging will result in a decline in sugar content (FAO 2004). Waterlogging also results in cooler soil temperatures so germination of setts will be slower and losses from disease may be higher (Ridge & Reghenzani 2000).

Sugarcane can be grown in a range of altitudes from just above sea level to as high as 3000 m above sea level (FAO 2004).

High winds, especially when combined with heavy rain, can lead to lodging of cane stalks in the field. This leads to problems with harvesting, reduced cane yield and reduced CCS. In northern QLD (an area of high rainfall) a 15–35% decrease in sugar yields have been recorded in a lodged crop compared to an unaffected crop (Singh et al. 2000; Singh et al. 2002). This may be due to rat damage, suckering, and stalk and stool death following lodging (Inman-Bamber et al. 2008).

Soil pH
Sugarcane prefers a soil pH of 5.0–5.8, although it will tolerate between pH 4–10 (Fauconnier 1993).

Salt tolerance
Sugarcane is sensitive to soil salinity. It has been estimated that it will show no reduction of growth in soil with salinity up to 1.1 decisiemens per metre (dS m-1) and a 10% growth reduction at 2.2 dS m-1 (Evans 2006). Sugarcane production is not economic in areas with soil salinity above 4.0 dS m-1 (Rozeff 1995). It has been further estimated that 10% of the area under sugarcane cultivation in Australia is affected by salinity (Christiansen 2000). Field studies in the Burdekin region showed a negative correlation between cane yield and soil salinity, and showed yield reductions even at salinity levels usually considered too low to be detrimental (Nelson & Ham 1998). It was estimated that for the Burdekin area, there is a 14% decrease in yield for every 1 unit increase in ECe (saturation extract electrical conductivity) of the 0–0.5 m depth soil layer (Nelson & Ham 2000). Salinity affects both growth rate and yield of sugarcane, but also the sucrose content of the stalk (Rozeff 1995). Shoot growth has been shown to reduce, although the severity varies between cultivars (Akhtar et al. 2001b), and root growth may be stimulated by increased salinity (Gerard 1978). High salinity has been shown to reduce stalk height and weight, due to reduction in both the number of internodes and the internode length, but not the number of stalks, and may be related to reduced water content (Lingle et al. 2000; Akhtar et al. 2001a). Different life stages may have different sensitivities to salinity, with seed germination showing the least sensitivity (Wahid et al. 1997). In experiments under saline conditions, ratoon crops have shown 2.2–3.7 times greater yield loss compared to plant crops (Bernstein et al. 1966). The addition of potassium and silicon have been shown to help ameliorate the decreases in plant growth and juice quality caused by salinity, and actually have more effect on salt sensitive genotypes compared to salt tolerant genotypes (Ashraf et al. 2009).

Aluminium tolerance
High aluminium levels are associated with acid soils, and aluminium toxicity can cause a major reduction in yield in many crops (Delhaize & Ryan 1995). Sugarcane is relatively tolerant of high aluminium levels, although differences in tolerance have been seen between cultivars (Hetherington et al. 1986). Cultivars of the S. officinarum parent species generally have higher levels of tolerance than the S. spontaneum parent species (Landell (1989) as cited in Drummond et al. 2001). In an experiment comparing the aluminium tolerances of sugarcane, navybeans, soybeans and maize, which may be grown in rotation with sugarcane, the sugarcane cultivars showed the greatest tolerance. The concentration of aluminium which led to a 10% reduction in root growth were up to ten-fold higher for sugarcane than the other crops tested (Hetherington et al. 1988). Symptoms of toxicity include root stubbing, which leads to susceptibility to water stress and yield loss (Calcino 1994).

Other metals
Sugarcane has been shown to tolerate up to 100 μM copper in laboratory experiments (Sereno et al. 2007). Tolerance to cadmium is higher, with laboratory experiments showing no toxicity at 500 μM cadmium (the highest concentration tested). Plant damage was seen in other experiments at 2 mM cadmium (Fornazier et al. 2002). The high tolerance to cadmium, and the observation that the sugarcane plants can accumulate cadmium have suggested its use in phytoremediation (Sereno et al. 2007).