SE-CCC
3.5 Discussion
shaded plants showed distribution after adaptation of leaf 6 to changes in sink demand.
The increased allocation of leaf 6-derived 14C to the leaf roll and leaf 3 of shaded plants further indicated prioritisation of young leaf tissues as sinks for carbon from leaf 6. This change in typical leaf 6 partitioning patterns not only indicated a sink-strength-related response to the decreasing levels of sucrose measured in young shaded leaf tissue, but also a change in physiological state from source to sink. Such an event is not uncommon in infected leaves following pathogen attack, where an increase of import to the infected sites occurs (Farrar, 1992; Wright et al., 1995; Ayres et al., 1996), but has not yet been reported in shading experiments. While no overall change in sucrose was observed in leaf 6 after shading for 14 d, a decreased level of 14C-labelled sucrose after 24 h was evident in plants shaded for both 4 d and 10 d. As partial shading did not produce any significant variation in stomatal conductance in leaf 6, reduced labelled sucrose might be indicative of increased sucrose turnover and a higher assimilate transport rate in the phloem of treated plants. This study has thus illustrated the ability of the phloem transport of sugarcane to respond to changes in environment and alter assimilate translocation patterns between various sink and source tissues. These results substantiate the role of sucrose as a signaling molecule in assimilate partitioning (Chiou & Bush, 1998), however, the signaling mechanisms which link phloematic, apoplastic and intracellular sucrose concentrations remain to be fully elucidated (Gibson, 2005).
The changes observed in sugar levels over time in maturing internodes (8, 10 and 12;
Fig. 3.2) are indicative of the many complex factors influencing the overall physiological environment of the plant in shading treatments. Although partial shading produced an acropetal shift in assimilate partitioning from leaf 6 to younger internodes and young leaf tissue, the effect of this on the overall sugar content of mature internodes would be further confounded by the acclimation of leaf 6 to increased sink demand over time and the overall drop in available assimilate for the entire plant. Shading treatments would additionally influence plant water relations. Assuming that water loss from shaded leaves was reduced, this would increase water potential ( P) and possibly reduce the flow of nutrients to culm and leaf tissue. This could influence shaded leaf and root metabolic activities which could in turn reduce the overall demand for CH2O and consequently affect carbon accumulation in mature internodes which typically supply root tissue. As a number of factors may thus affect mature internodal tissue under the
the observed changes and correlations between sugar levels and photosynthesis observed.
Significant increases in photosynthetic rate, carboxylation efficiency and PSII efficiency were measured in leaf 6 over the duration of the shading treatment. A significant linear relationship was further elucidated between maximum photosynthetic assimilation rates (Jmax) of leaf 6 and decreasing levels of sucrose in immature culm tissue (internodes 4 and 6) over the partial shading time treatments. This supports evidence that decreased sucrose at the sink is a likely physiological signal to the source for increased assimilate requirements (van Bel, 2003). A similar effect has previously been observed in pot- grown sugarcane plants, where partial defoliation resulted in only small decreases in culm dry mass (Pammenter & Allison, 2002). However, the dramatic photosynthetic increase in leaf 6 observed here may have been compounded by the sustained presence and required maintenance of other leaves. Furthermore, the depletion or excess of sugars has previously been shown to respectively activate or repress the expression of genes for photosynthetic components and ultimately influence photosynthesis itself (Stitt, 1991; Krapp et al., 1993; Van Oosten & Besford, 1994; 1995;
Basu et al., 1999). The plasticity of leaf assimilation capacity over time observed in sugarcane may thus be linked to regulation of C4 leaf metabolism at the molecular level, such as regulatory phosphorylation of PEPc activity (Vidal & Chollet, 1997) and/or adjustments in several other C4 photosynthetic control mechanisms (Furbank & Taylor, 1995). It is important to note that this study has ‘simulated’ an increase in plant sink strength via an increased demand for carbon from leaf 6. Thus, although it is feasible that the overall sink activity of internodal tissue my have, in fact, declined due to the lack of source supply, this research has provided evidence for the physiological ability of the source to adapt to increased sink requirements. The Saccharum complex is potentially capable of storing more than 25% sucrose on a fresh weight basis (Bull & Glasziou, 1963; Moore et al., 1997). As this estimate is still almost double current commercial yields (Grof & Campbell, 2001), further understanding of source regulation may assist in the eventual utilisation of a greater portion of the potential sink strength of sugarcane.
Interestingly, no relationship was observed between sucrose levels in either unshaded or shaded leaves, and photosynthesis in this study. These results are comparable to studies on maize leaves, where changing sucrose concentrations were shown to have no significant short-term feedback inhibitory effects on the synthesis of sucrose itself in
the leaf (Lunn & Furbank, 1997). Instead, a strong negative correlation was found between hexose and photosynthetic gas exchange variables Jmax and CE in unshaded leaf 6, which implicated hexoses, rather than sucrose, as possible signal factors involved in photosynthetic feedback regulation. In the past, hexoses have been shown to be inhibitors of photosynthesis (Goldschmidt & Huber, 1992). For example, the external supply of glucose (50 mM) to excised Spinacea oleracea leaves over 4 d lead to inhibition of the light harvesting complex (LHC) II-encoding chlorophyll a/b binding protein (cab) genes and a 60% decrease in Rubisco content (Kilb et al., 1995). Thus in sugarcane, a decreased leaf glucose pool could constitute a signal of increased demand from sinks. More recently, hexoses have been shown to play an important role in regulating photosynthesis and leaf development (Ehness et al., 1997; Paul & Pellny, 2003). Hexokinase has been implicated as a putative receptor (Jang et al., 1997), however, the mechanisms involved in hexokinase sensing remain contentious. It has also been demonstrated that glucose itself, and not an analogous phosphorylated metabolite, may be the primary signal that interacts with some putative receptor involved in transduction of the carbohydrate signal (Ehness et al., 1997). Progress has been made (Rolland et al., 2002), but further efforts will be required to fill in the gaps in this complex network, especially for C4 species. Compared to C3 species, relatively little is known about the control of sugar biosynthesis in the leaves of C4 plants, however, sugar induced changes in gene expression are likely to be as important in C4 as in C3 in balancing sink:source interactions (Lunn & Furbank, 1999).
Although it is likely that the hexose concentrations in the leaf tissue are under strict metabolic control, it will ultimately be difficult to elucidate the actual mechanisms of hexose responses, as sugars can act by affecting osmotic potentials as well as by functioning as signal molecules (Gibson, 2005). This may be further complicated by the interactions between carbon and nitrogen levels in leaf developmental processes (Paul
& Pellny, 2003). For instance, the application of moderate (111 mM) concentrations of glucose stimulates the senescence of Arabidopsis, but only under limited nitrogen conditions (Wingler et al., 2004). Furthermore, since most plants can synthesise sucrose when fed with hexoses, it is difficult to attribute the effects of hexoses to their direct sensing, as sucrose sensing could possibly occur. However, our correlations indicate that, although sucrose must play a key role in regulating sink assimilate partitioning, hexoses may be a more proximal component of the signaling mechanism
regulatory effects of hexose sensors such as HXK are well documented in C3 plants (Rolland et al., 2002) and may provide a useful starting point for examining the control of photosynthesis in C4 sugarcane.