Barley [photo] 


The fundamentals of sucrose accumulation in sugarcane

By Stephanie Roberts (Regional Manager: Agronomic Research and Development)

Often in crop production, fertilizer practices are aimed at achieving maximum yields, with little consideration given to the effect of nutrition (positive or negative) on crop quality.

In sugarcane, the sucrose content of cane is the primary driver of on-farm profitability. This is partly because the logistical cost of harvesting and transporting large volumes of cane with a low sucrose content is prohibitive, and also because sucrose is the primary component of the RV payment system. The RV payment system works as follows:

RV % cane = S - dN - cF
Where S = sucrose % cane
N = non-sucrose % cane 
F = fibre % cane 
and d = the relative value of sucrose which each unit of non-sucrose diverts from sugar production to molasses  
c = the loss of sucrose from sugar production per unit of fibre 

(South African Sugarcane Association – Cane Testing Service)

Sucrose and fibre content generally have an inverse relationship, thus, achieving an increase in sucrose content will generally result in a higher RV%.

The sucrose cycle in sugarcane

Sugarcane leaves produce sucrose through photosynthesis. From there, the sucrose is transported to the stem via the phloem. It can then either be stored in the stem, or be converted to glucose and fructose which are used to provide the energy required for new growth (Fig. 1). Although new growth reduces the sucrose content in the stem, it also allows the ''factory'' of the plant to increase both its sucrose ''production sites'' (leaves) and sucrose ''storage sites'' (stem internodes). Thus, the grand growth stage is the most critical growth stage for sucrose accumulation as this is where the balance is found between sucrose used for growth, sucrose production and sucrose storage (Bull, 2000). During the maturation phase, any remaining glucose, fructose and other soluble carbons are then converted to sucrose again for storage (Whittaker and Botha, 1997). Although the maturation phase plays an important role in the final sucrose content of cane, optimal maturation conditions and/or the use of chemical ripeners cannot overcome a low RV yield resulting from either poor growth (low cane yield) or excess growth (high fibre/low sucrose) during grand growth.

Scientists are hard at work to understand the enzymatic process involved in sucrose accumulation in sugarcane to aid in the breeding of high-sucrose cultivars, but the genetic complexity of cane and the complex nature of the process means that there has been minimal success (Wang et al., 2013). Until new high-sucrose cultivars are available, we need to focus on what role crop production factors play in enhancing sucrose yields. Many factors that affect RV yield are out of farmers' hands e.g. climate and water availability, but crop nutrition is something that farmers can manage in order to improve RV yield.

Figure 1: The sucrose cycle in sugarcane (Wang et al., 2013)

The role of crop nutrition in sucrose content

Any element that is required for photosynthesis and is in short supply will naturally lead to a reduction in the crop’s ability to produce sucrose. As an example, in high pH soils positive sucrose responses are often observed when iron (Fe) and zinc (Zn) are applied to sugarcane, as deficiencies of these elements are likely under these conditions and will result in a reduction of photosynthesis. Only elements that are expected to have a direct impact on sucrose are covered further in this article.

Excessive nitrogen (N) fertilization is one of the biggest causes of low RV levels in cane. During grand growth, when a significant amount of N is available and temperature and soil moisture allow, growth is then stimulated at the cost of sucrose storage (Bull, 2000). The presence of nitrate within the plant also has a direct negative effect on sucrose synthesis by inhibiting the action of sucrose phosphate synthase (Champigny and Foyer, 1992). Cane grown on an autumn/winter cycle is more likely to show a low RV% in response to higher N application rates (Meyer and Wood, 2001).

Phosphorus (P) is one of the most important elements for sucrose production. The enzyme responsible for the final step of sucrose synthesis is sucrose phosphate synthase. The name says it all – without adequate P, sucrose cannot be synthesised. The synthesis of sucrose is also heavily dependent on other P-containing enzymes as well as ATP (adenosine triphosphate).

Potassium (K) has long been associated with transport of sugars in crops, as it plays a critical role in maintaining osmotic potential so that sucrose can be stored against a concentration gradient (Marschner, 1995). Hartt (1969) clearly demonstrated the effect that a K deficiency has on reducing sucrose transport from the leaf to the stem in sugarcane. Crucially, this reduction in sucrose transport often occurs in the absence of visible deficiency symptoms in the leaves.

Magnesium (Mg) deficiency also typically results in the accumulation of sugars in the leaves of deficient crops. This is because Mg is essential for the activity of the enzymes responsible for the loading of sucrose into the phloem (Marschner, 1995). Although trials to demonstrate the role of Mg in sugarcane have not been conducted as it was for K by Hartt (1969), Mg has been clearly shown to affect sucrose accumulation in sugar beet (Hermans et al., 2005).

Boron (B) has also long been associated with sugar transport in crops, but its exact role is yet undetermined and may be of a secondary nature (Marschner, 1995). However, the numerous trials reporting benefits in sugarcane quality to B applications are clear (Ferraz di Siqueira et al., 2013).

Iron (Fe) is essential for the conversion of ACC (an ethylene precursor) into ethylene (Marschner, 1995). Where ripeners like ethephon are not applied, natural endogenous ethylene plays a critical role in sugarcane maturation and signalling the plant to convert available soluble carbon to sucrose. If Fe is deficient, endogenous ethylene production will be lower and the final phase of sucrose accumulation will be negatively affected.

Zinc (Zn) plays a critical role in sucrose content as it is required for the activity of the enzyme fructose-1,6-piphosphatase. Without the functioning of this enzyme, sucrose synthesis cannot proceed. Importantly, this shortage is not associated with visible Zn deficiency symptoms and may easily go undiagnosed.

Silicon (Si) has been shown directly to increase sucrose content of cane (Meyer and Keeping, 2000), over and above benefits associated with liming and Eldana infestation. The cause of this benefit is unknown but may be explained by Si’s ability to enhance Zn bioavailability (Marschner, 1995).

In summary, it is critical to maintain the balance between growth and storage during the grand growth phase and ensure that nutrient supply is adequate. This is important, considering that most fertilizer programmes come to an end just when sucrose accumulation begins in earnest. Sucrose content of cane is not purely dependent on conditions and management practices during drying-off – careful management during the grand growth stage is as important.

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