The role of sett roots and shoot roots in the establishment of sugarcane planted into yield decline soils.

Abstract

TO EXAMINE the role of sett roots in the early growth of sugarcane, the production of sett roots was impaired by damaging root primordia and the number and biomass of primary and secondary shoots was measured 70–100 days after planting. Interactions between sett roots and the soil-borne pathogens typically found in association with sugarcane were examined by damaging root primordia and then planting the setts into soil that was fumigated with methyl bromide, treated with a nematicide or left untreated. The results of two of three experiments showed that the size of the sett root system did not affect the number of primary shoots established, presumably because the parent stem cutting was capable of supporting the developing bud until shoot roots became functional. However, the results of a third experiment suggested that the effect of sett roots on primary shoots depended on the quality of the stem cutting and the inherent dormancy of buds. Mortality of primary shoots was relatively high when root primordia were removed from setts taken from the upper part of the stem, while shoot establishment from relatively dormant buds on cuttings taken from the lower stem was reduced when root primordia were removed. Loss of sett roots had a greater impact on the weight of secondary shoots than primary shoots, as primary shoot weight in untreated soil was significantly reduced in only one experiment whereas 38–51% reductions in secondary shoot weight were observed in all experiments. Soil fumigation and nematicide treatments markedly improved plant growth but regardless of soil treatment, the removal of root primordia generally reduced the number and weight of secondary shoots. This result demonstrates that sett roots are important in sustaining the initial growth of sugarcane. The fact that removing root primordia sometimes had a greater impact in untreated soil than in fumigated soil indicates that damage to sett roots from soil-borne pathogens may affect crop establishment. Introduction The sugarcane crop is propagated vegetatively by planting stem cuttings (setts or billets) in rows 1.4–2.0 m apart. The stem cuttings have two or three nodal buds, each with an associated ring of sett root primordia and, under favourable conditions, sett roots grow from these primordia and primary shoots develop from the buds. When the crop is planted, good primary shoot establishment is important because sufficient secondary tillers must be produced to optimise light interception and effectively capture the water and nutrients that are Blair, B.L. & Stirling, G.R. Proc. Aust. Soc. Sugar Cane Technol., Vol. 28: 2006 __________________________________________________________________________ 368 available in the field. The results of experiments done within the Sugar Yield Decline Joint Venture have shown that when soil is fumigated or a sugarcane monoculture is broken by a rotation crop or bare fallow, shoot production increases and the response is apparent within 50–80 days of planting (Garside et al., 2002). Although shoot and stalk numbers later decline, differences between treatments are often maintained until harvest and are partly responsible for the increased yields that are observed following these treatments. Pankhurst et al. (2004) later confirmed the importance of the period immediately after sugarcane is planted by showing that early plant growth responses are a good indicator of the final yield response that is likely to be obtained from biocide treatments in the field. Although there have been few studies of early plant growth in sugarcane, sett roots appear to play an important role in crop establishment. Increases in the weight of primary shoots and the number of secondary shoots following soil fumigation are associated with the improved growth and functionality of sett roots (Pankhurst et al., 2004), and primary shoot growth is inhibited when sett roots are physically removed or are damaged by root-rotting fungi (Ryker and Edgerton, 1931). The objective of the experiments reported in this paper was to manipulate the number of sett roots by damaging root primordia, and measure the effects on crop establishment. A further objective was to examine the impact of soil-borne pathogens, sugarcane variety and the maturity of planted material on the interactions between sett roots and early plant growth. Materials and methods General methods Experiments 1 and 2 were established on a granitic sandy loam at Tully in north Queensland, and were planted in mid-spring 2002 and 2003, respectively. Experiment 3 was located on a fine sandy loam at Bundaberg in south Queensland, and was planted in earlyspring 2001. Each site had been cropped to sugarcane for at least 30 years. NPK fertiliser was applied at 300 kg/treated ha in Experiments 1 and 2 and at 200 kg/treated ha in Experiment 3. In Experiments 1 and 2, stem cuttings were soaked for 5 minutes in 0.15 g/L methoxy ethyl mercuric chloride (Shirtan) to prevent fungal invasion of the stem cutting, whereas Sportak (10 mL/50 mL of water) was used in Experiment 3. In the latter experiment, lorsban (0.5 mL/L of water) was also sprayed onto stem cuttings and the soil surrounding the cuttings to protect against insect pests. In Experiments 1 and 3, sett root primordia were removed from stem cuttings by grinding off part of the circumference of the root band to a depth of 2 mm with a grindstone. In Experiment 2, a scalpel was used to shave the root band. Stem cuttings were planted and covered with about 8 cm of soil to reflect standard industry practice. Experiment 1 In this experiment, 0, 60, 90 and 100% of the root primordia were removed from the stem cuttings planted in untreated soil, while 0, 60 and 90% of the root primordia were removed from the stem cuttings planted in fumigated soil. Fumigated plots were covered with plastic sheeting and gassed with methyl bromide (1200 kg/ha) 25 days prior to planting. A nematicide treatment (2 g fenamiphos/m broadcast onto the soil 50 and 7 days prior to planting and lightly raked into the soil) was also included, and it was planted with unshaved stem cuttings (i.e. no root primordia removed). Stem lengths were selected from a mature crop of first ratoon sugarcane (variety Q117). Portions of the stem with damaged buds were discarded and the lower lengths of stem were divided from the newly grown upper lengths of stem, thereby providing a source of ‘old’ and ‘new’ buds respectively. Ten ‘old’ and 10 ‘new’ stem cuttings, each with three nodes and buds, were planted into four replicate plots. Each plot consisted of two rows, 50 cm apart and 3 m long. Blair, B.L. & Stirling, G.R. Proc. Aust. Soc. Sugar Cane Technol., Vol. 28: 2006 __________________________________________________________________________ 369 Nematode numbers in the soil were assessed at 7 and 50 days after planting (DAP) by collecting 10 sub-samples per plot with a soil auger to a depth of 20 cm. Nematodes were extracted from a 200 mL sample using a Baermann tray for 96 hours (Whitehead and Hemming, 1965). Nematodes were concentrated by sieving twice through a 38 μm sieve, and counted. At harvest, nematode populations were estimated in rhizosphere soil shaken from the root system using the extraction process described above. Endoparasitic nematodes were extracted from sett roots and a sample of shoot roots by misting roots for 96 hours (Hooper, 1990). At harvest (100 DAP), the number of established primary shoots and associated secondary shoots (tillers) were counted. The dry weights of primary shoots, secondary shoots, sett roots and shoot roots were also measured. Primary shoot establishment was reported as a percentage (%) of total buds planted. From selected treatments, the stem cuttings (billets) were split and compared visually for degree of tissue discoloration (i.e. degree of bacterial/fungal invasion).

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