It is a brave move on the part of the SRDC and the researchers to sort of restart strategic research aimed at improving sugar accumulation in canes. The interests and capacity of the various institutions for research into sugar accumulation will also be more clearly defined, and the potential for synergistic collaboration strengthened.
R. WILSON
Australia is generally regarded as one of the most efficient producers of raw sugar in the world. I will therefore begin by describing the nature of the marketing environment facing the Queensland sugar industry.
L. CREES
dr. Henzell (SRDC President) outlined SRDC's interest in the workshop topic and interest in the outcome of the workshop to guide research and research. A holistic approach to addressing the balance between CCS level and season length was advocated.
TECHNICAL SESSIONS
C. COX BSES
Various crop management strategies have been developed based on empirical relationships established through field trials (Gosnell, 1970; Leverington et al, 1970; Kingston, 1972). While low temperature, water stress, and nitrogen stress are known to promote sugar accumulation (see review by Liu and Kingston, 1992), there are no quantitative relationships between these stresses and sugar accumulation (Bull and Glasziou, 1975). In worst cases, sucrose can be removed from storage to cover respiratory costs (Figure 1).
The rate and extent of sugar accumulation will depend on the relative shape of the relationship between stress and consumption and stress and production of fixed carbon. Again, the sugar accumulation is probably due to the greater sensitivity of consumption than the production of fixed carbon. There is much evidence (Alexander, 1973; Clements, 1980; Bowen and Anderson, 1992) to suggest that: (i) if too little nitrogen is applied, cane becomes stressed prematurely and cane yield and sugar accumulation suffer; (ii) if too much is applied, vegetative growth continues with little sugar accumulation in the stems, and in addition, many stems in solid canes will break and rot, and violent suction will suffocate the primary and secondary stems that have become attached; and (iii) sufficient nitrogen should be supplied for maximum growth early in the season, but the supply should run out before the end of the growing season to ensure sufficient sugar accumulation.
To use this new technology, we need to develop quantitative relationships between voltages and sugar accumulation so that they can be incorporated into models. Increase in daily canopy photosynthesis in relation to canopy nitrogen distribution pattern.
KINGSTON BSES
When applied to sugarcane at a non-herbicidal rate, glyphosate promotes sucrose storage by inhibiting meristem growth (Maretzki et al., 1976; Maretzki and Thorn, 1978). Ethrel's ripening activity is restricted to the upper, less mature parts of the stem (Clowes, 1978), while glyphosate has the ability to ripen immature internodes and "load" sucrose into the lower stem (Maretzki and Thorn, 1978; Clowes, 1980; Tianco and Gonzales, 1980). In areas such as Swaziland and parts of South Africa, where Ethrel is used every year, purity testing and measuring commercial responses have been omitted due to the perceived reliability of the response.
There is large seasonal and inter-district variation in the prevalence of good early-season ripening conditions in the Australian sugar belt; purity testing prior to spraying is likely to be an important criterion until there is sufficient understanding of environmental control of the ripening processes to issue chemical ripening forecasts. Extending the milling season in Australia would increase opportunities for more widespread use of chemical ripeners. An improved understanding and quantification of the sucrose accumulation process in relation to environmental variables will improve the ability to target crops suitable for chemical ripening.
It was noted that every 0.1 point increase in CCS increases crop value by $2.70 per ton of cane. Paul Moore noted that in other areas of the world temperate varieties tend to be less responsive to rotting than tropical varieties.
S. HAWKER
In a general discussion of sugar metabolism in plants, one must remember that we are looking for control steps in the accumulation of sucrose in sugar cane, from C02 and the trapping of light through the leaves throughout the plant to the maintenance of sucrose concentrations in the sugar cane stems . It is important to resolve the whole question of the route and mechanism of transport of sucrose from the leaves to the vacuoles of the stem to determine the limiting steps. Other speakers will discuss these processes in more detail, but it is relevant to note here that there is likely to be controversy over the role of sucrose synthase in wells as an indicator of well strength, as supported by Claussen et al. ( 1986) and the Black (Sun et al, 1992) and Davies (Ross and Davies, 1992) schools and recently denied by Geigenberger and Stitt (1992).
Since xylem concentrations of sucrose are not high, there cannot be high concentrations of sucrose throughout the stem-free space. It is important to note that the equilibrium constant for sucrose phosphate synthase is close to that of sucrose synthase (Barber, 1985) which may change future approaches to research on sucrose storage. Phloem transport of sucrose, sucrose-degrading enzymes, sucrose synthase, sucrose phosphate synthase, the phosphorylation of sucrose phosphate synthase, turgor regulation of sucrose synthesis and sucrose as a new plant growth regulator are some of the topics.
Further recent work on sugar metabolism illustrating the importance of sucrose phosphate synthase, invertases and hexose transporters in sugar synthesis and accumulation in plants has appeared recently (Rausch, 1991; Influence of sucrose and hormones on activity of sucrose synthase and invertase in detached leaves and leaf parts of eggplants (Solanum melongena), J.
T. FURBANK
The first step in solving the problem of improving sugar accumulation in sugarcane by metabolic engineering must be to identify the rate-limiting levels in the process as a whole. A program to express maize SPS in C4 Flaveria bidentes is currently underway at CSIRO, Plant Industry Division. Although modern sugarcane cultivars have low stem starch content, leaf starch can form up to 4% of leaf dry matter at the end of the photoperiod (Alexander, 1973).
As discussed above, overexpression of enzymes in sucrose synthesis can shift carbon allocation, although evidence from C3 mutants deficient in the starch synthesis enzyme phosphoglucose isomerase (PGI) suggests that limiting starch synthesis may inhibit photosynthesis rather than shift carbon toward sucrose (Kruckeberg et al. al., 1989). Finally, if the rate of sucrose synthesis in the leaves can limit sugar accumulation in the stem, and if ultimately all photosynthate is efficiently converted to stem sucrose, then we should strive to improve the rate of photosynthesis. At the time, there was debate that physiological experiments were needed to determine whether sugar accumulation in leaves during the day caused feedback inhibition of photosynthesis in sugarcane.
Robert Furbank (under pressure from Robert Henry) agreed that the overexpression of sucrose phosphate synthase in sugarcane leaves was likely to be successful in achieving higher levels of sugar accumulation in the leaves. The issue of transport to the stem and the resulting accumulation in the stem was not so easy to predict.
W. PATRICK
For zinc-limited conditions, sucrose accumulation from the phloem pathway should ultimately be determined by the capacity of the stem sink tissues to metabolically interconvert or compartmentalize sucrose. This claim is examined beginning with a consideration of the possible cellular pathways for radial transmission. Cellular structures, plasma membrane surfaces and plasmodesmal frequencies of the stem of Phaseolus vulgaris L.
Developmental changes in sugarcane stem anatomy in relation to phloem unloading and sucrose storage. Gradients of turgor, osmotic potential and water potential in the cortex of the hypocotyl of growing Ricinus seedlings. Effect of supply of water from the xylem and of dissolved substances from the phloem.
Further evidence of apoplastic discharge in the bean stem: Identification of the phloem buffer pool. Symplastic isolation of the sieve element - companion cell complex in the phloem of Ricinus communis and Salix alba stems.
MARETZKI
How this new evidence affects membrane transport in the stem sink of the sugarcane plant is still controversial. One approach that then becomes feasible is to determine the distribution of expression of the relevant protein in the tissues. The problem is likely the low abundance of messenger RNA in the tissue.
How phloem is read into the apoplastic space of the storage parenchyma is still unanswered. Current research integrating studies at the structural, physiological, biochemical and molecular levels should provide us with data to develop a better understanding of the controls of sucrose accumulation in the sugarcane stem. 1992) Developmental changes in sugarcane stem anatomy in relation to phloem unloading and sucrose storage.
The discussion then moved to consider (i) the gaps in our knowledge that need to be filled in order to select the best candidate genes for genetic manipulation, and (ii) the best experimental approaches to fill them. This paper covers some other metabolic approaches to increase the capacity of the sugarcane plant to accumulate more sucrose. Information on the specific roles of the two isoforms of sucrose synthase is lacking.
Molecular modification of any process requires the identification and isolation of the gene to be modified.
![Figure 1 Whole plant respiration as a percentage of gross photosynthesis in sugarcane [adapted from Glover 1973].](https://thumb-ap.123doks.com/thumbv2/5docco/10423799.0/91.892.82.614.267.610/figure-plant-respiration-percentage-photosynthesis-sugarcane-adapted-glover.webp)
OVERVIEW OF THE WORKSHOP
The aim of the workshop was to assess the potential for increasing sugar yield in sugarcane by increasing sucrose accumulation. Below is a brief summary of some of the key points raised during the workshop. 20-30% increase in sugar yield over the years, largely due to higher biomass than higher CCS.
