It is widely accepted that sucrose is the sugar that is translocated in sugarcane and in many other plants (Hatch and Glasziou, 1964; Rae et al., 2005). Hartt et al. (1963) followed the pathway of assimilates when translocated from the leaves to various parts of the sugarcane plant. They reported that the major pathway followed by labelled photosynthates from the leaf is down the parallel veins to the midrib and sheath. However a significant amount of the labelled photosynthate was observed to pass directly down the lamina and into the sheath, bypassing the midrib. The appearance of labelled sucrose in the midrib was explained by Alexander (1973) as due to the gradual union of the midrib with the veins along the flow course from the leaf tip to the leaf collar (dewlap).
Movement of photoassimilates in the leaves begins immediately after their synthesis.
Radioactivity was detected some 55cm away from a leaf exposed to CO2 within 5 minutes. Hatch and Glasziou (1963) observed a much faster rate of assimilate movement.
The outer green part of the central part of the sheath conducts more photoassimilates than the inner white part. The presence of bundles closer to the outer epidermis than to the
inner epidennis accounts for this observation (Hartt et al., 1963). Assimilates enter the stem from the sheath nearly in a horizontal direction (direction of the dewlap) to the centre of the stem, and then take a perpendicularly downward direction for as many as eight internodes, before some of the assimilates reverse direction upwards (Artschwager, 1925). Some of the ascending assimilates may reach spindle leaves, immature storage tissue and meristematic area before the descending assimilates reach the roots (Hartt et al., 1963). It was also shown in the results of the study by Hartt et al., (1963) that produced assimilates firstly get accumulated in the nodal section (a region with high concentration of conducting tissue) rather than in the internodes. Some assimilates were also observed to move out of the roots into the nutrient solution. A rather striking finding by Harttet al. (1963) was the translocation of assimilates from a leaf of one stalk to other stalks within a stool. In one experiment it was found that radioactive 14C incorporated into leaf blade number 3 of one stalk was detected as far as in leaf blades number 2 of 15 other stalks within the same stool.
1.7 Enzyme activity and sucrose metabolism
A number of enzymes operate in the sugarcane plant during its development (Glasziou, 1961; Sacher et al., 1963a). Complexity arises through the activity of these different enzymes in the same overall reaction but in opposite directions (Ebrahim et al., 1998a).
The concentration of various sugars in the sugarcane stalk is influenced by the balance of the activity of these enzymes (Zhu et al., 1997). The primary enzymes of sucrose metabolism are the invertase (EC 3.2.1.26), which cleaves sucrose into glucose and fructose; sucrose synthase (EC 2.4.1.13), which can either cleave sucrose into UDP- glucose and fructose or catalyse sucrose synthesis from glucose and fructose; and sucrose phosphate synthase (EC 2.4.1.14), which synthesises sucrose-6-phosphate, which in turn, is dephosphorylated into sucrose by sucrose-phosphate phosphatase (Lingle and Smith, 1991). The sucrose pool is thus under continuous rapid cycle of synthesis and cleavage, depending on conditions affecting the enzymes involved in the cycle (Whittacker and Botha, 1997). The activity of the enzymes varies with season during sugarcane's
Three invertase enzymes operate in the metabolism of sucrose in sugarcane, namely soluble acid invertase (SAl), neutral invertase (NI) and cell wall acid invertase (CWAI) enzymes (Albertson et al., 2001). The CWAI in sugarcane internodes has not had much attention from researchers (Lingle, 2004). The activity of this enzyme seems to increase with internode age in the same way as sucrose content. It is thought that CWAI contributes to sucrose storage (Lingle, 2004). Immature internodes of sugarcane contain high levels of SAl (optimum activity between pH 5 and 5.5); whereas mature internodes contain high levels of NI (optimum activity at pH 7) (Hatch and Glasziou, 1963). In their studies (Sacher et al., 1963b; Hatch and Glasziou, 1963; Gayler and Glasziou, 1972) reported that these two invertases (SAl and NI) have a key role in regulating sucrose translocation in conducting tissues, and hence its subsequent utilization for growth or storage. High positive correlation between internode extension and SAl was found, whereas a high positive correlation between movement of sugar into storage bodies and NI was found. It was concluded that the NI regulates the translocation of sugars from vascular to storage tissue of mature internodes. However, the results of the study by Zhu et al. (1997) and Veith and Komor (1993) did not support this idea.
Low sucrose-storing cultivars of sugarcane retain relatively high levels of SAl in their mature internodes, whereas high sucrose-storing cultivars contain relatively high levels of NI in their mature tissue and no SAl (Hatch and Glasziou, 1963). The early idea that NI and SAl played the major role in controlling sugar flux in mature storage tissue of sugarcane stalks was counteracted by Zhu et al. (1997) and by Veith and Komor (1993).
The activity of SAl should be low before sucrose can accumulate, but there are other factors that affect accumulation (Zhu et al., 1997). Zhu et al. (1997) showed that SAl enzyme activity varied with internode age, and genotype; and that the SAl activity was highest in the most immature internodes of low sucrose-storing cultivars and lowest in the high sucrose-storing cultivars. SAl activity values of 350 and <50 Jl.mol min-1 g-l were found for low sucrose-storing cultivar Mol5829 and for high sucrose storing cultivar LA Purple, respectively. SAl activity was reported to prevent most, but not all sucrose accumulation. Low SAl in high sucrose-storing cultivars was not sufficient to account for
sucrose accumulation. In the same study by Zhuet al. (1997), it was shown that there was no correlation between NI, SS, SPS and sucrose concentration of individual intemodes of the high sucrose-storing cultivar LA Purple; whereas a nonlinear correlation (?=0.70;
P<0.002) between sucrose concentration and the activity of SAl and between sucrose concentration and the difference of SAl and SPS activities (r2=0.71; P<0.002) was found, indicating that both the SAl activity and the difference between the SAl and SPS activities account for sucrose flux in sugarcane intemodes. However, analysis of mean values of enzyme activities of individual intemodes per stalk showed that the nonlinear correlation between sucrose concentration and SAl activity was low (r2=0.52; P<0.003);
whereas the correlation between sucrose concentration and the difference between SPS and SAl activity was high and positive (r2=0.86; P<0.001). Hence sucrose accumulation and storage on the whole stalk basis was said to be accounted for by the balance between SAl activity and SPS activity more than it is accounted for by the SAl activity alone (Glasziou and Gayler, 1972). Similar results were found by Lingle (1999).
A study by Lingle (2004), to investigate the effect of transient temperature change on sucrose metabolism in sugarcane intemodes, showed that there was no significant temperature effect on sucrose concentration for any intemode. A slight increase in sucrose concentration in the most immature intemodes as a result of a chilling effect was observed, but this slight increase was not associated with sucrose metabolism enzymes (SAl, NI, SS, SPS and CWAD, since there was no significant change in the activity of these enzymes due to low temperature. Hence she rejected the hypothesis that short time exposure of plants to low temperatures affects sucrose metabolism in sugarcane. She associated the slight increase in sucrose concentration in response to transient chilling effect with low-temperature suppression of sucrose cleavage enzymes.