Conclusions

A. fatua does not display the growth characteristics of an extremely 'shade-tolerant' species (Grime, 1979), with reduced light intensity conditions (applied in the absence of any other kind of competition) reducing shoot biomass (Fig. 40.7), seed production (Fig. 40.2), seed weight (Fig. 40.3) and the degree of seed dormancy (Table 40.1), but greatly extending the time of plant development (Fig. 40.1). However, A. fatua did display some evidence of compensating for the reduced light levels,

2000 1500 1000 500 0

2000 1500 1000 500 0

2000 1500 1000 500

Springsure a

m mm

Toowoomba a

Warwick a

Coonabarabran a

Ia a

0 40 80

0 40 80

2000

1500

1000

All lines aaa ULI

0 40 80

Fig. 40.6. The influence of shade on the leaf area of six near isogenic lines of Avena fatua grown under simulated field conditions at the University of Queensland from April to December 1991. Leaf area values followed by the same letter within each line are not significantly different (P < 0.05).

c 30

S 20

IS E

B 10

0

Springsure

Toowoomba a

III!

III!

II.

Coonabarabran a llll

0 40 80

III.

0 40 80

0 40 80

Fig. 40.7. The influence of shade on the shoot dry weight of six near isogenic lines of Avena fatua grown under simulated field conditions at the University of Queensland from April to December 1991. Shoot weight values followed by the same letter within each line are not significantly different (P < 0.05).

by increasing shoot height (Fig. 40.5), which may allow its upper leaves to avoid shading in the crop canopy in a cropping situation. Reduced light intensity caused similar detrimental effects to all of the lines studied and although some variability in response was observed in these lines, this was small and indicates that few differences would be detectable among lines in a field population.

The findings from this study have confirmed that reduced light intensity can be detrimental to the persistence mechanisms of A. fatua, by reducing seed production and by reducing the degree of dormancy in the seed produced. Fewer seeds with a lower degree of dormancy will probably lead to a more rapid depletion of the soil seed bank through earlier seedling emergence. Canopy shading would effect all A. fatua plants emerging into the crop, but would have its greatest influence on those emerging later in the season when the crop canopy is closing. An interesting effect of reduced light intensity, observed in this study, is the delay it causes in plant development (up to ~70 days). This could result in some plants not reaching maturity before the crop is harvested. While in others, only a few seeds will be produced by this time. Consequently, this would prevent most of the late emerging plants from returning large numbers of seeds to the soil seed bank. In addition, reduced light intensity may dramatically reduce the competitive ability of A. fatua in the crop as it drastically reduces shoot and tiller production. Thus, one aspect of a highly competitive crop might be its ability to shade the weeds within it, reducing their growth, extending their development, reducing their seed set and lowering their seed dormancy.

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