Dry matter partitioning of sugarcane in Australia and South Africa

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Abstract

Partitioning of dry matter (DM) in sugarcane is of interest for two fundamental reasons. Firstly, sugar production depends directly on partitioning of crop biomass to the stalk and then to sucrose stored largely in stalk parenchyma. Secondly, various DM components of the stalk and particularly sucrose concentration are used to calculate the value of cane consignments delivered to the mill. In this study we review data from serial harvesting experiments in South Africa and Australia to discover similarities and differences in DM partitioning of sugarcane grown in a wide range of conditions and to gain a better understanding of the factors influencing DM partitioning as a basis for improving functional responses in sugarcane simulation models. Firstly, climatic and genetic variations in DM partitioning to various above ground plant components of sugarcane and to sucrose within the stalk component, are examined at a broad level. We then assess the robustness of sucrose partitioning rules used in the two international sugarcane models (APSIM and CANEGRO) and provide enhancements on predictions of sucrose content (SC) on a DM basis in sugarcane stalks.

Well-adapted cultivars in widely different climatic conditions were similar with respect to partitioning of biomass to various above ground organs. The trash component (dead leaves and dead stalks) was one that varied most between cultivars and growing conditions. The stalk fraction of green biomass (biomass less trash) reached a maximum of about 0.85 when green biomass yield exceeded 60 t ha−1 regardless of cultivar or extremes of water regime. Opportunities for further improvements of this trait through breeding were not obvious but harvesting could be delayed to ensure that stalk fraction is as high as possible.

Variations in SC with respect to age, biomass and seasonal variation in climate, were explained by a simple conceptual model of cane stalk segments which are rapidly filled with sucrose when young and may remobilise sucrose when carbohydrate is required for rapid expansive growth. Season and age effects on whole stalk SC are due to varying proportions of young segments with low SC and older segments with high SC. A mechanistic model capturing these concepts is required but as an interim measure an empirical model based on stalk mass, leaf number per stalk, leaf area index and temperature accounted for a large proportion of the variation in SC across Australian and South African cultivars and conditions. Uses for this model together with CANEGRO or APSIM include improved SC through manipulating irrigation and the harvest schedule and more realistic targets for cane quality.

Introduction

Partitioning of dry matter (DM) in sugarcane is of interest for two fundamental reasons. Firstly sugar production depends directly on partitioning of biomass to the stalk and then to sucrose stored largely in stalk parenchyma. Secondly, various DM components of the stalk and particularly sucrose concentration are used to calculate the value of cane consignments delivered to the mill (Berding, 1997). In South Africa and Australia (and in many other countries) the value of cane consignments is reduced by leaf and trash (dead leaf and stalk) and by non-sucrose constituents of the stalk, even if the total mass of sucrose in the consignment is not reduced (Culverwell, 1996). In the case of the Australian sugar industry the payment formula is designed to accentuate the value of sucrose content (SC) of cane consignments and this has led to a heightened awareness of variations in SC due to cultivar, harvest season, crop age and many other factors. The moisture, fibre and juice purity components of the payment formulas are also important but will not be considered in this paper.

Opportunities for increasing sugar production of sugarcane in Australia and South Africa are limited largely by radiation, temperature and water (Inman-Bamber, 1995, Muchow et al., 1997). Sugarcane has a relatively high radiation use efficiency (RUE) which appears to be consistent across cultivars and climatic zones in different countries (Sinclair and Muchow, 1999). If increases in biomass are not likely to arise from improvements in RUE then more favourable partitioning to sucrose needs to be considered. This can be achieved possibly by increasing the fraction of stalk in total biomass and by increasing the fraction of sucrose in the stalk.

Fraction of stalk in crops with high dry biomass (>6 kg m−2) was about 0.8 for some Australian cultivars (Robertson et al., 1996a) and was 0.66 for two Hawaiian cultivars once biomass exceeded 5 kg m−2 (Evensen et al., 1997). For one South African cultivar, stalk fraction reached a maximum of 0.7 (Inman-Bamber and Thompson, 1989). Robertson et al. (1996a) suggested that the reported range in maximum stalk fraction (0.66–0.8) could be due to variable amounts of trash recovered during sampling.

Muchow et al. (1996) reported remarkable similarity in SC of stalks on a dry weight basis in the published data on some Australian, South African, Mauritian and Hawaiian cultivars. They suggested that maximum SCs have been stable across cultivars and across locations for several decades. Robertson et al. (1996b) concluded that SC tends towards a common maximum of 0.48 for Australian cultivars Q96, Q117, for South African cultivars NCo376, N12 and N14 and for a wide range of water regimes and N supply in the respective countries. Rostron (1972) showed that SC in NCo376 changed little after 56 weeks of growth under irrigated conditions. Julien and Delaveau (1978) regarded their reported range in SC of about 0.44–0.48 g g−1 with crop age and harvest as ‘negligible’.

Berding (1997) challenged the finding of Muchow et al. (1996) that ‘maximum stalk sucrose concentration on a dry weight basis was stable across cultivars and crop classes at a value of 0.48 g g−1. He showed that SC in unselected clones of sugarcane varied substantially in the range 0.44–0.60 g g−1. However, most of these clones had SCs in the 0.50–0.56 g g−1 range. Whilst Berding (1997) challenged the 0.48 g g−1 maximum SC presented by Muchow et al. (1996), his data did not directly challenge the overriding effect of stalk biomass on SC evident in their data (Muchow et al., 1996).

Attempts to provide a functional basis of SC to explain or forecast this important component go back at least to 1972 when Glover (1972) showed an inverse correlation between SC and rainfall, lagged by 112 months. Glover (1972) noted that low temperature and low soil water content led to increased SC. These factors as well as nutrient deficiency, restricted internode elongation more than photosynthesis and resulted in increased SC of the stalk because of reduced demand for photoassimilate in meristematic regions (Bull and Glasziou, 1975, Glasziou et al., 1965, Hatch and Glasziou, 1964, Inman-Bamber and De Jager, 1988). It is thus reasonable to expect that SC depends on various expressions of radiation, temperature and plant water status in addition to stalk biomass which was the dominant factor in the work of Robertson et al. (1996b).

A large body of published and unpublished data on biomass accumulation and partitioning of sugarcane has accumulated from serial harvesting experiments in South Africa and Australia over the past 10 years. While there was no thought of coordinating this research at the time, an agreement was subsequently reached between the South African Sugar Association and CSIRO, Australia to permit the collation of these data in order to discover similarities and differences in DM partitioning of sugarcane grown in a wide range of conditions.

The objective of this paper is to gain a better understanding of the factors influencing DM partitioning in sugarcane as a basis for improving functional responses in sugarcane simulation models. Firstly, broad climatic and genetic variations in DM partitioning to various above ground plant components of sugarcane and to sucrose within the stalk component, are examined. We then assess the capability of partitioning to sucrose in the two international sugarcane models (APSIM and CANEGRO) and provide enhancements on predictions of SC on a DM basis in sugarcane stalks.

Section snippets

Methods

Data from two growth analysis experiments in South Africa and 12 similar experiments conducted in Australia were re-analysed for this paper.

Results and discussion

It is important to note that variation in biomass in these experiments arose largely from sampling crops during their development over time, however variation in growing conditions also influenced biomass yields regardless of crop age.

Conclusions

The data derived from experiments on well-adapted cultivars in widely different climatic conditions, showed similarity between cultivars with respect to partitioning of biomass to various above ground organs. The trash component was one that varied most between cultivars and growing conditions. The stalk fraction of green biomass (biomass less trash) reached a maximum of about 0.85 when green biomass yield exceeded 60 t ha−1 regardless of cultivar or extremes of water regime. This indicates that

Acknowledgements

Permission to use data generated while senior author was employed by the South African Sugar Association Experiment Station, is gratefully acknowledged. Funds provided in Australia by the Sugar Research and Development Corporation and the CRC for Sustainable Sugar Production are also acknowledged.

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