Sucrose accumulation in sugarcane stalks does not limit photosynthesis and biomass production

Until now raw sugar has been the predominant commodity produced from sugarcane (Saccharum spp. hybrids) with the exception of Brazil where fermentable sugars are used to produce ethanol. Worldwide interest in renewable energy has focussed attention on total biomass production of 'energy canes' rather than sucrose yield alone. Clones selected for biomass tend to have high fibre contents derived from the wild type, S. spontaneum. It is possible that high fibre genotypes can produce higher biomass yields than high sucrose types due to feedback on photosynthesis either by sucrose or sucrose signalling compounds as proposed in several recent publications on feedback responses in sugarcane leaves. Up to 20 sugarcane clones with either high fibre or high sucrose content were grown in one field and three pot experiments to elucidate some of the processes from source to sink that could be responsible for high rates of biomass accumulation expected in high fibre clones. We were particularly interested in the possibility that clones with high sucrose content may have reduced photosynthesis as sucrose levels increased in upper internodes due to feedback mechanisms. Photosynthesis of whole plants and of single leaves decreased with crop development as much as 60% in some cases. Maintenance of photosynthesis was not associated with low content of sugars in leaves or in internodes. Sink strength for sucrose storage in the upper internodes was strong in both high fibre and high sucrose clones despite plants being grown for 12 months in conditions controlled to achieve high sucrose contents. Our data supported previous conclusions about localised feedback on photosynthesis by sugars accumulating in the leaf resulting in reduced photosynthesis of small segments of individual young leaves. However, whole-plant photosynthesis did not decline through the day indicating that older leaves may compensate for reduced photosynthesis in younger leaves in the afternoon. While photosynthesis declined with crop age and sucrose content increased we found no evidence to suggest that photosynthesis declined because sucrose content increased. An increase in biomass yield through breeding and selection may not necessarily result in reduced sucrose content and increased fibre content.