SE-CCC
3.2 Introduction
The accumulation of phenomenal levels of sucrose by sugarcane has been the focus of intense study (Moore, 1995; Lakshmanan et al., 2005; Moore, 2005). Sugarcane is a C4
species that accumulates high sucrose concentrations in the mature internodes with less accumulation in younger internodes. The differences in sucrose accumulation between young and mature culm tissues are the consequence of varying rates of cycling of
1997; Casu et al., 2003; Walsh et al., 2005), but the integration of source (photosynthetic) and storage (culm) processes in plants is still not fully understood (Koch et al., 2000; Pego et al., 2000).
For many plant species, the activities of source photosynthetic production and sink growth appear to be closely co-ordinated, such that a balance is maintained between source supply and sink demand (Wardlaw, 1990; Ho, 1992; Foyer et al., 1995).
Evidence increasingly supports a sink-dependent relationship (Paul & Foyer, 2001), whereby sink-strength influences the net photosynthetic activity and carbon status of source organs (Paul et al., 2001). Apart from possible feedback through product accumulation, there is increasing evidence that the activity of photosynthesis-related enzymes and expression of associated gene transcripts is modified by sink demand (Sheen, 1990; Sheen, 1994; Black et al., 1995; Koch, 1996; Pego et al., 2000; Paul &
Foyer, 2001; Rolland et al., 2002).
Although there have been limited studies on sugarcane focusing on the relationship between source and sink tissue (Marcelis, 1996; Pammenter & Allison, 2002), in various other plant species the dominant influence of sink activity on source photosynthesis and carbon partitioning has been demonstrated. In Solanum tuberosum, a high sink demand in the form of rapidly growing tubers caused increased rates of photosynthesis (Dwelle et al., 1981) and enhanced translocation of photosynthate (Moorby, 1978). Removal of the tubers led to a marked decrease in net photosynthesis due to the imbalance between source and sink activity (Nosberger & Humphries, 1965). Irrespective of the presence or absence of water stress conditions, plants with artificially lowered sink- strength (tuber excised) accumulated carbohydrate in the leaves and displayed a considerably reduced maximum photosynthetic rate (Amax), electron transport rate (ETR) and quantum yield (Fv/Fm) (Basu et al., 1999). Cold girdling of the leaves of Citrus unshiu to reduce carbon export and defruiting have also reduced rates of photosynthesis (A), and this reduction coincided with an accumulation of carbohydrate in the source leaf (Iglesias et al., 2002). Sugar accumulation in leaves also represses the expression of photosynthetic genes (Sheen, 1990). In transgenic Nicotiana tabacum leaves, the expression of a yeast invertase in the cell wall resulted in increased carbohydrate content, especially soluble sugars, which gradually inhibited photosynthetic levels as sugars accumulated (Von Schaewen et al., 1990; Stitt et al., 1991). Similarly, mature
leaves of Spinacia oleracea supplied with glucose through the transpiration stream lost Rubisco activity over a 7 d period (Krapp et al., 1991).
In sugarcane, the sucrose accumulating processes within the maturing stem are likely to be strong sinks for photoassimilate (Marcelis, 1996). Sucrose accumulation in the sugarcane culm has recently been shown to receive high priority in the allocation of assimilates (Pammenter & Allison, 2002). Coincidently, large differences in photosynthetic rates have, in the past, been reported for individual sugarcane leaves related to the age of the plant, with young plants typically assimilating at significantly higher rates than older plants (Hartt & Burr, 1967; Bull & Tovey, 1974). Gross photosynthesis has been found to be lower in eight-month-old sugarcane plants compared to four-month-old plants, regardless of the light intensity (Allison et al., 1997).
Another study reported that three-month-old sugarcane exhibited photosynthetic rates of 45 mol m-2 s-1 under intense illumination, while young leaves on ten-month-old plants only photosynthesised at 25 mol m-2 s-1 (Amaya et al., 1995).
In plants, sugar status modulates and coordinates growth and development (Smeekens, 2000) and, although the regulatory role of sugar on photosynthesis and metabolism is well known, progress has only recently been made in determining the molecular mechanisms of sugar sensing and signaling (Rolland et al., 2002; Gibson, 2005).
Components of sugar sensing systems that have been identified include glucose, sucrose and trehalose sensing systems. For example, hexokinase (HXK) functions as a glucose sensor that modulates gene expression and sucrose non-fermenting 1 (Snf1)- related protein kinases (SnRKs), which are known to have diverse functions in carbon metabolism and sugar signaling (Rolland et al., 2002). Since the details of sink regulation of photosynthetic source relations in C3 plants are only now emerging, the picture is even less clear in the more complex C4 species, such as sugarcane. In part, regulation of C4 photosynthesis is achieved through compartmentation of the process between mesophyll and bundle sheath cells and control of metabolite transfers through a set of cell-specific organelle metabolite translocators (e.g. dicarboxylic acid transporters) together with symplastic connections (Edwards et al., 2001). Various specialisations have been demonstrated for C4 leaf carbon metabolism, including bundle sheath cell- specific storage of starch for a range of species (Downton & Tregunna, 1968; Laetsch, 1971; Lunn & Furbank 1997) and preferential localisation of genes involved in sucrose
uncover new aspects of the control mechanisms involved in C4 photosynthesis (Kubien et al., 2003; von Caemmerer et al., 2005), however little is known about the unique regulatory interactions that determine assimilatory flux in C4 plants, such as sugarcane.
However, many of the controls elucidated for C3 systems also operate in C4 plants (Sheen, 2001); for example carbamylation of Rubisco by Rubisco activase has been shown to be essential for photosynthesis in the C4 dicot, Flaveria bidentis (von Caemmerer et al., 2005).
The existence of a sugar-dependent relationship between source and sink tissues in sugarcane could represent a potentially fundamental limiting factor for sucrose accumulation in the stalk and consequently play a major role in overall sucrose accumulation and crop yield. In the current study, the relationship between photosynthetic source tissue and sink material was examined through manipulation of sink demand and total sink-strength in field-grown sugarcane. To artificially increase sink-strength by manipulating the sink:source ratio, all leaves, except for the third fully expanded leaf, were enclosed in 90% shade cloth. In this way leaves that served as source were converted to sinks, producing an overall increase in plant sink-size. The effects on gas exchange characteristics and PSII efficiency were investigated and changes in photosynthesis were explained on the basis of leaf sugar levels and variations in sugar partitioning based on the uptake of a 14CO2 label.