Environmental stimuli promoting sucker initiation in sugarcane☆
Introduction
Sugarcane is propagated vegetatively from stem pieces containing axillary buds. Like most grass crops, yield (biomass and sugar) comes from mature stalks arising from primary shoots from the axillary buds originally planted and from higher order shoots produced from axillary buds on these primary and other shoots (tillers) during the early stages of growth. Shoot number then declines because competition for light, moisture, and nutrients increases as the crop canopy closes (Borden, 1948, Bell and Garside, 2005, in this issue). Unlike many other grasses, sugarcane produces tillers later in the growth cycle, called suckers. Their morphology differs from tillers produced earlier in the growing cycle (Hes, 1954, Barnes, 1974, Bonnett et al., 2001). Suckers are characterised by thicker stems with shorter, broader, and thicker leaves than found on earlier-produced primary and secondary shoots.
Suckers have negative consequences in mechanically harvested sugarcane production systems as they are harvested together with mature stalks. The overall sucrose concentration of the harvested material is lowered (Berding et al., 2005, in this issue) because of the lower sucrose concentration of suckers (Ivin and Doyle, 1989, Berding et al., 2005). Reducing sucker number would improve sugar concentration and profitability for growers in industries where sugar concentration is part of the payment system.
Environmental factors that affect tiller number in sugarcane have been studied previously. That increasing nitrogen fertiliser increases secondary shoot number has been demonstrated many times (e.g., Borden, 1945, Borden, 1948, Shrivastava and Kumar, 1984). Other factors have been less well studied but have demonstrated some significant effects. Duration, intensity, and spectral composition of light affects grass tillering. Martin and Eckart (1933) demonstrated that reduced light intensity prevented secondary shoot development in sugarcane grown in large pots under three light levels. We can find no reports of manipulation of spectral composition of light and its effect on sugarcane tillering. However, red:far-red ratio has been shown to affect tillering in other grasses (Casal et al., 1987). Ludlow et al. (1990) measured the spectral composition underneath canopies of several sugarcane cultivars but could not determine if observed differences in red:far-red ratio were a cause or effect of measured differences in tillering.
Reports on environmental factors affecting suckering are rare. Borden (1948) and Salter and Bonnett (2000) reported the effect of nitrogen on suckering in single factor experiments. Manipulation of the amount of light reaching the stalk by removing dead leaves increased sucker numbers in the outside rows of sugarcane crops (Bonnett et al., 2001). Long-term observations of suckering resulted in the proposal of a hypothesis that harvest-season soil moisture, soil nitrogen, and increased light affected sucker initiation. The objective of the studies reported here was to determine whether soil moisture, soil nitrogen and light or a combination of these factors stimulate sucker development in a range of cultivars. A companion paper (Berding et al., 2005) reported on the yield and quality components of suckers in the multifactor experiment described here.
Section snippets
Multifactor experiment
An experiment to test the effects of moisture, nitrogen, and light on suckering of two cultivars of sugarcane was established. This experiment has been fully described (Berding et al., 2005 in this issue). Briefly, the experiment was established south of Cairns, north Queensland (17°5′ S, 145°47′ E), and consisted of two moisture regimes, rainfed and a post-monsoonal irrigation treatment. These treatments were used to test the hypothesis that harvest season rainfall (July–October), which has
Multifactor experiment
The number of mature stalks within the core plots in the plant and ratoon crops (Fig. 1) was very similar. There was a slight decline between 180 and 380 days. There were greater differences between crops for number of suckers. Suckers appeared earlier in the development of the ratoon than the plant crop and were more abundant (Fig. 1). At the final count in the plant crop (383 DAP), all main effects, except moisture regimes, showed small but significant differences in mature stalk number (
Discussion
The experiments reported here have shown that nitrogen and moisture have a significant impact upon suckering. The results for experiments where under-canopy light levels were manipulated are however, less clear.
Conclusions
Nitrogen, when applied from May onwards, and increased soil moisture late in the crop's growth period, have been shown unequivocally to influence suckering. The cultivars tested responded similarly to these factors irrespective of their suckering propensity. The role of light remains unresolved though there is evidence that it can promote suckering. The spatially diverse and complex canopy environment makes manipulation and measurement of an altered light environment difficult. Nitrogen and
Acknowledgements
Tom Watters, Alan Zappala, Warren Galligan, and John Dallachy on whose farms this work was performed are thanked for their collaboration and excellent co-operation. This work was funded in part by the Australian Sugar Research and Development Corporation. Barry Salter was additionally supported by the Co-operative Research Centre for Sustainable Sugar Production and Robert Lawn is thanked for his advice and supervision. Fiona Joseph and farm staff at BSES Meringa, Michael Hewitt and Franco
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Present address: CSIRO Plant Industry, Queensland Bioscience Precinct, 306 Carmody Road, St Lucia, Qld. 4067, Australia.