1CSIRO Plant Industry, GPO Box 1600, Canberra, ACT, 2601.
2 Farrer Centre, Charles Sturt University, Wagga Wagga, NSW, 2678.
Aqueous extracts of undecomposed residues of 2 wheat varieties (cv. Janz and Swift) were tested for their effect on the germination and radicle growth of lupins, oats and canola. Overall, the wheat residues were more toxic to canola than the other species although varietal differences were also evident. Both wheat cultivars caused greater inhibition of radicle growth of the canola cultivars than the other species. Extracts of Janz were more toxic to canola cv. Oscar germination than the other species while Swift was more toxic to oats cv. Echidna germination. These observations may explain the poor emergence and vigour of canola sown into wheat straw. Selection of canola varieties tolerant to wheat stubble phytotoxins may be possible for increasing the yield potential of canola crops in a wheat-canola rotation.
KEYWORDS Allelopathy, stubble, germination, radicle growth
The beneficial effects of retained crop residues for erosion control has been well documented (Fettell and Gill 1995; Malinda 1995) however the yields of crops can be reduced. Retained wheat residues have been shown to reduce yield in several studies (McCalla and Army 1961; Guenzi and McCalla 1962; Putnam and Weston 1986; Purvis 1990). More recently, there has been mounting circumstantial evidence for reduced yield of canola crops sown into retained wheat residues.
Several factors may contribute to the poor growth of canola sown into wheat residues including (1) poor soil-seed contact, (2) ineffective pre-sowing herbicide incorporation resulting in increased weed competition, (3) nitrogen immobilisation and (4) increased incidence of root disease. Anecdotal evidence suggests that these cannot always explain the poor growth of canola in the southern wheat belt of NSW.
Recent investigations into poor growth under crop residues have demonstrated that phytotoxins may be contributing to the decline (Bhowmik and Doll 1982; Purvis 1990). These toxins may be a result of leaching, through processes of decomposition or synthesised by microbes (Rice 1974; Purvis 1990). The toxins identified include more than 300 secondary plant and microbial compounds (Purvis, 1990). Many factors may contribute to the production and detrimental action of these chemicals including the state of decomposition of crop residues, the quantity of stubble, the species and variety of the stubble and the environmental conditions during the growth of the next crop (Einhellig, 1996).
With the move toward stubble retention in cropping systems, identifying stubble and crop combinations in which the risk of phytotoxicity is high is necessary to avoid yield reduction. For example Purvis (1990) found that wheat yield was significantly higher under canola stubble than under peas and attribituted this to the greater phytotoxicty of pea stubble. As the area of canola and the desire to retain stubble in wheat-canola rotations increases, it is important to assess the risks of allelopathy.
This paper reports preliminary studies designed to investigate the potential allelopathic effects of undecomposed wheat residue on the germination and radicle growth of two canola varieties, and to compare the response of canola with that of lupins and oats.
Wheat (cv. Janz and Swift) residues were collected from the NSW Agricultural Research Institute field site in Wagga Wagga NSW following harvest in 1997 and stored in a growth house prior to evaluation. Stubble was air-dried and chopped into 3 cm lengths. Stubble extracts were prepared by agitating 20g of stubble in 500 mL of distilled water for 2 hours at room temperature (the concentration tested was determined in a preliminary trial). The leachate was decanted, centrifuged to remove particulate matter, passed through a millipore filter (Millipore 0.45 μm) to remove micro-organisms and stored at 4 0C.
Seeds of lupins (cv. Merritt), oats (cv. Echidna), and two canola cultivars (cv. Oscar and Rainbow) were examined for their response. The species were chosen because they are typically sown into wheat residues.
A germination and radicle length bioassay was used to determine the phytotoxic effect of stubble residues. Twenty seeds of each species were placed in petri dishes on Whatman filter paper no. 42. The leachates were added to separate petri dishes, different volumes depending on seed size. They were 4mL, 6mL, and 15 mL for canola, oats, and lupins respectively. Distilled water was used for control treatments. The treatments were replicated 4 times and placed in a randomised design. The seeds were germinated in a growth cabinet in the dark at an average temperature of 150C. Germination was measured when radicle length was > 1mm at 48 hours, 72 hours, 96 hours and 120 hours, when all controls had germinated. Radicle length was measured twice to estimate radicle elongation rate and to avoid confounding radicle growth with germination time. For canola (cv. Rainbow) and oats, radicle length was measured at 72 hours and 120 hours. All other species were measured at 48 hours and 96 hours. Preliminary trials had been used to determine the average germination rate for all the species and hence the most appropriate time for measuring radicle length.
Germination counts and radicle lengths were averaged within each petri dish. Germination for each replicate was presented as a percent of the control. Radicle growth rate was measured over a 48 hour interval and was expressed as a percentage of the mean of the control radicle elongation rate for the same period. All data were analysed using ANOVA followed by least significant differences test (LSD, p=0.05).
Figure 1: Effect of aqueous extracts of a) wheat cv. Janz and b) wheat cv. Swift on cumulative germination of 4 plant species/cultivars. Vertical bars are lsds ( p = 0.05).
The final germination % of most crops was unaffected by wheat leachates with the exception of Oscar which was reduced by Janz (Fig 1a), and oats which was reduced by Swift (Fig1b). In contrast, the rate of germination was affected by the leachates and two types of responses were observed. Lupins, oats and Rainbow had germination delayed by Janz at 48 hours but this effect diminished over time. The same response occurred for lupins, Oscar and Rainbow to Swift leachate (Fig 1b). In contrast, the early inhibition of Oscar by Janz leachate persisted over time (Fig 1a) and a similar response occurred for oats to Swift leachate (Fig 1b).
Figure 2: Rate of radicle growth of 4 plant species/cultivars to aqueous extracts of wheat cv. Janz and wheat cv. Swift. Vertical bar showing lsd (p = 0.05)
Radicle elongation was reduced by both leachates however there were significant varietal interactions. Generally the canola was more sensitive to the leachates than oats and lupins. Oscar was particularly sensitive to the leachate of Janz with radicle growth reduced to 3.4% of the control.
The results reported here indicate that undecomposed wheat (cv Janz and Swift) residues were more toxic to canola than the other species tested. Varietal differences in both straw phytotoxicity and crop sensitivity were also observed. For example, Oscar was inhibited by Janz more than the other species and cultivars for both germination and radicle length. Interestingly, Oscar sown into Janz straw would be the most common stubble/crop combination in southern NSW where poor growth has been observed. Similar effects have been noted by An et al. (1997) who found different species and cultivars to have different phytotoxic responses to Vulpia residues. Kimber (1967, 1973) and later Wu et al. (1998) found that different wheat cultivars produced different phytotoxic effects in the same test species. Wu et al. (1998) also found that phytotoxicity of wheat straw correlated with total phenolic content of the straw. Further screening of wheat and canola cultivars will be necessary to examine the extent of these varietal differences. To comprehensively assess the above effect and to determine the relative importance of allelopathy in relation to disease incidence and nutrient immobilisation, pot-trials and field trials will be undertaken using different environmental conditions and different quantities of stubble and states of decomposition. A major aim of such trials would be to determine when allelopathy is a risk and to create guidelines for farmers for achieving high yields within a stubble retention system.
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