Cultivo bacteriano

3.5.1 Herbicide-Tolerant Species Interactions in Four Growing Media

In Experiments 1 and 2 significant differences were often recorded in responses of the tolerant species to the different herbicides evaluated as well as between the growing media. Alachlor is mostly absorbed through the emerging coleoptile of a plant and its foliar uptake is extremely limited (Jaworski 1969). Thus, when applied post-emergence to onions it should have been well tolerated by the crop (Scherp 1971; Tsiropoulos & Miliadis 1998). The damage to onion plants observed in these experiments indicates that there was significant root uptake. The differing responses between the growing media (Table 3.7) confirm that the damage was from root uptake. If the damage had been due to foliar uptake, the level of damage should have been similar in all the growing media. Generally, the onions were more tolerant of alachlor when grown in either the Mangateretere soil or the paper pulp and were more susceptible when grown in either the Horotiu silt loam soil or the sawdust.

When comparing these results with the physico-chemical properties of soils (Table 3.1) commonly known to influence herbicide behaviour (Bailey & White 1970), it appears that the well subscribed rules do not apply to the novel growing media used in this study. For example, the sawdust has twice the organic carbon content of the paper pulp and ten times that of the Mangateretere soil, yet onion plants grown in sawdust were generally more susceptible to alachlor compared to the other media. Also the pH values of the sawdust and Horotiu silt loam soils are intermediate to those of the paper pulp and the Mangateretere soil while for bulk density the converse is true. Thus it is difficult to correlate any of these properties to the herbicide response of onions.

Alachlor is primarily active on monocotyledonous plants especially Poaceae with less activity towards dicotyledonous species (Scherp 1971; El-Nahhal et al. 1998). This herbicide has also been used for pre-emergence weed control in broccoli (Scherp 1971). However, in these experiments, alachlor was safe only in the Mangateretere soil and the paper pulp when used at the recommended rate. It caused significant damage to broccoli plants in all media when used at twice the recommended rate. Similar to the situation for

and by plant roots (Copping & Hewitt 1998). The differentiation of herbicide phytotoxicity in the different growing media was not as marked for chlorpropham and pendimethalin as observed for alachlor. However, for chlorpropham the onion grown in the sawdust were more susceptible to damage than when grown in the other three media and, although not always significant, there were differences between the media similar to that for alachlor.

3.5.2 Dose-Response Curves for Sensitive Species in Four Growing Media

Many of the dose response curves produced from Experiments 3 and 4 appear to have little symmetry e.g. foxtail millet and alachlor (Figs 3.11 and 3.12) and annual ryegrass and chlorpropham (Figs 3.15 and 3.16). For some combinations the lowest herbicide rate used resulted in some plant damage (e.g. Figs 3.11 and 3.14) while the highest herbicide rate used did not stop the plants from growing (e.g. Figs 3.18 – 3.22). Also, the herbicide pendimethalin frequently caused stimulation of plant growth when applied at subtoxic rates (Figs 3.19 and 3.20). These situations would all lead to difficulty in fitting the traditional logit model. A reasonably accurate ED50 value could be determined using the

equation (Fig. 3.23)

(D + C)/2 Eq.3.1 where D is the weight of the untreated control and C is estimated to be zero. However,

the ability to produce a realistic NOEL value would be severely tested. Similar problems associated with calculating the NOEL have been previously identified and discussed but no solutions offered (Rahman 1989). Using the Bayesian smoothing technique overcame these reported difficulties. For the results reported in this thesis, this technique enabled the determination of accurate NOEL values for most of the experimental combinations of herbicide, plant species and growing media. The only exceptions were where the highest herbicide rate evaluated resulted in less than 50% reduction in plant production and the method unable to produce credible NOEL and ED50 values (Figs 3.24 – 3.28).

The NOEL and ED50 values mostly relate well to each other (Tables 3.8 and 3.9). There

is only one herbicide/species combination where having the lowest ED50 value does not

equate to also having the lowest NOEL value (viz. chlorpropham and cress in Experiment 3). The two experiments also relate well in this regard although there is some variation in the numeric value of the two parameters. This variation is realistic as the experiments were carried out about 6 weeks apart and although in a glasshouse, some climatic conditions such as temperature and solar radiation differed between the two (Table 3.6). Such differences have previously been reported with bioassays and are overcome by using both treated (with known rates) and untreated controls when using this technique to

measure phytotoxic herbicide residues (Rahman 1989; Sandral et al. 1997; Nègre et al. 2006).

In Experiments 3 and 4 the herbicides evaluated were generally more phytotoxic in the soils than in the two high organic media (Tables 3.8 and 3.9). ED50 values were often in

the range of 5 to 10 times higher in the high organic media than in the two soils. Notable exceptions to this were for garden cress and pendimethalin and to a lesser extent garden cress and chlorpropham, where the herbicide was more toxic in sawdust.

3.5.3 Bioavailability

For an herbicide to be absorbed by the emerging shoot or by the roots of a plant it must be in solution (Stalder & Pestemer 1980). The two main factors which determine the quantity of the herbicide in solution are its water solubility and its adsorption onto soil particles (mostly organic matter and clay). However, for the herbicide to be solubilised it must be in the vicinity of the soil water. This usually requires movement of the herbicide from the dry soil surface where it usually is applied into the region of the soil where there is free soil water. Normally this is achieved by incorporating the herbicide mechanically with a light cultivation, or through applying irrigation or by waiting for rainfall (Crafts & Yamaguchi 1960) and is referred to as “activation” of the herbicide. In the present experiments the herbicide was “activated” by applying overhead irrigation which ensured the herbicides were in solution and available for plant uptake. Both alachlor and chlorpropham are considerably more soluble than pendimethalin (Table 2.1) and are thus more likely to have been moved deeper into the soil profile by the applied irrigation than pendimethalin was. This could be the reason why these two herbicides were more damaging to the onions in some growing media. However, it could equally be argued that for various reasons onions have a better tolerance of pendimethalin. The probability of alachlor being leached into the root zone of the crop is acknowledged on this product’s label where it states that in cucurbits, crop injury may result from heavy rainfall immediately after application (O'Connor 2003) or by shallow seeding, incorporation, or high rates on lightly textured soil (O'Connor 2006).

variations in the bioavailability of applied herbicides. With organic carbon contents ranging from 4.3 to 46%, the growing media used here were extremely diverse (Table 3.1). However, organic carbon content alone is an insufficient indicator and the nature of that organic matter is also very important (Baskaran et al. 1996b; Ahmad et al. 2001). For instance in this case the sawdust was in a raw, undecomposed state and comprised mostly of relatively large particles of cellulose with a very low surface area to mass ratio. On the other hand, the paper pulp, while only having half the organic carbon content (22%) of sawdust, was much more degraded as the cellulose had been broken down in the chemical process of paper making. The two soils, with considerably less organic carbon than either the sawdust or paper pulp, could be expected to behave very differently from the other two media because of this.

There is little literature available on the behaviour of herbicides in non-decomposed organic amendments. The literature generally reports on either composted, high organic matter amendments (Barriuso et al. 1992b; Baskaran et al. 1996b; Nelson et al. 1998; Sluszny et al. 1999; Cox et al. 2000; Cox et al. 2001) or on their interactions after they have been in the soil for certain periods of time (Iglesias-Jiménez 1997). Without exception all these authors report on enhanced adsorption of herbicides with the addition of high organic matter amendments. Although soil amendment/soil mixtures were not evaluated in this study, the similar bioactivity of the herbicides in both the soil and the high organic matter growing media indicates that sorption in them was fairly similar and thus soil/organic amendment mixtures would also be similar. As the major difference between the substrates used in literature studies and this study is the composting and decomposition of the amendment, then this must be viewed as a significant and a critical difference.

Results from these experiments therefore illustrate that the nature of the organic matter is also critical in terms of herbicide performance as has been suggested by some researchers in recent years (Iglesias-Jiménez 1997; Cox et al. 2000; Ahmad et al. 2001). When considering the 24 herbicide/plant combinations investigated in the four experiments it is clear that herbicides were more active in the Horotiu soil and least active in the paper pulp, with the Mangateretere soil and sawdust somewhat intermediate and exhibiting varied results. Thus the original hypothesis that high organic matter soil amendments lower plant availability, was supported by the results for the high organic matter paper pulp amendment, but generally not supported by results for the high organic matter sawdust amendment, which exhibited some inconsistencies. The reasons for this were clarified by the sorption studies to be discussed in Chapter 4.