Time-course expression of soluble acid invertase (SAI) gene mirroring post-harvest cane quality deterioration: effective treatments cause reduction of SAI gene expression

Sugarcane is a crop valued mainly for the high sucrose content in its cane stalks. Since sucrose is the most essential part of commercial cane sugar (CCS), it automatically becomes the top priority for farmers, and consequently, for breeders and agronomists as well. In the mature internodes, sucrose content can reach around 25% of the fresh weight of the culm. However, the rapid inversion and deterioration of sucrose in harvested cane, makes it a perishable commodity. This deterioration is associated with a variety of factors such as ambient temperature, humidity, cane variety, period of storage, activities of invertases, maturity status, etc.. However, the sucrose loss mainly occurs due to endogenous invertase as well as invertase of bacterial origin (resulting from bacterial invasion through the cut ends or damaged sites of stalk) and consequent production of secondary metabolites such as organic acid, dextran, gum and alcohol. This biochemical process becomes pronounced with the passage of time, making the cut-tocrush time lag of crucial importance for maximum sugar recovery. The problem has been alarming, especially in India, as the delay between harvesting and milling of cane sometimes even exceeds the usual 3 days, incurring huge loss due to cane staling (Figure 1). Studies have reported up to 2.0 units decline in CCS% in cane, within a period of 72 h after harvest, depending upon the sugarcane genotype and milling season. This ultimately leads to a loss of nearly Rs 30 lakhs every day, to a mill having crushing capacity of 5000 tonnes of cane per day (TCD). In the recent past, various physical/chemical methods have been tried to cut down the post-harvest sucrose losses, but their practical use has mostly been restricted by availability, high cost and environmental problems. In the past, many authors have discussed the effect of trash cover, water spray and shade effects in minimizing the sucrose losses in sugarcane. Spraying of water is perhaps considered beneficial because invertase activity is said to go up, with the loss of moisture. Electrolysed water (EW) prepared by electrolysing saline to create a disinfectant, has been reported to cause lesser reduction in post-harvest storage quality of sugarcane and lesser formation of invert sugars compared to untreated cane. Various bactericides such as formaldehyde, DBAC, IFOPOL, DNDT, potassium permanganate and sodium metasilicate, Tsunami-100, Sucroguard, etc. have been suggested to curb deterioration of cane and milled juice. Solomon et al. have reported the efficacy of a few chemical formulations containing both antibacterial (quaternary ammonium compounds/thiocarbamates) and anti-inversion chemicals (sodium metasilicate/sodium lauryl sulphate), thus equipped to check post-harvest sucrose losses (both microbiological and biochemical). These aqueous formulation(s) are sprayed over freshly harvested cane (whole stalk and billets) and the treated cane is covered with a thick layer of dried cane leaves (trash). Formulation containing benzalkonium chloride (BKC) + sodium meta silicate (SMS) has been found to be most effective – the synergistic effect of antibacterial and anti-inversion chemicals reduces the sucrose loss from harvested cane, up to a period of one week, irrespective of storage temperature and variety, improving sugar recovery by about 0.5 units. Solomon et al. observed increase in the activity of both acid and alkaline invertase after 72 h of post-harvest storage of cane, with a corresponding rise in the level of invert sugars. Studies have reported increase in invertase activity in post-harvest storage of cane. In sugarcane, soluble acid invertase (SAI) plays a major role in controlling sucrose content in cane stalk vacuoles through sucrose import and sugar signalling, particularly during the initiation of sink growth and cell-wall expansion, when