Reproductive development

When considering all the lines together, the average number of primary seeds produced per plant was reduced from 187 to 109, as the light intensity was reduced from full sunlight to 20% of that value (Fig. 40.2). The reduction in the numbers of seeds produced under shade may have been due to the observed reduction in growth rate (Fig. 40.1), which in turn reduced flower production and/or promoted flower abortion. This result would suggest that plants growing under reduced light intensities in the field may produce less seeds than plants growing outside the crop canopy, and such effects have been previously reported (McBeath et al., 1970; Peters and Wilson, 1983; Martin and Field, 1988). This response was only observed in the Springsure line, where seed production in full sunlight was considerably higher than in all other lines.

80 40 0

80 40 0

Toowoomba

Warwick

b

Time (days)

Fig. 40.1. The influence of shade on the duration of the vegetative (□), reproductive (0) and seed maturation (H) growth phases of six near isogenic lines of Avena fatua grown under simulated field conditions at the University of Queensland from April to December 1991. Total development time values within each line followed by the same letter are not significantly different (P < 0.05).

The mean seed weight of all the lines declined (from 253 to 204 mg) as the light intensity was reduced (Fig. 40.3). This would suggest that later-emerging seedlings, exposed to a closing crop canopy, would produce smaller seeds. However, this trend was not statistically significant in every one of the individual lines (e.g. Narrabri and Moree). The overall reduction in seed weight, under reduced light

500

400

300

200

100

0

500

400

c

ra

Ü

300

<d

Ü

m -o

200

ffi

ffi

m

100

co

fc

□l

0

500

400

300

200

100

0

40 80

500 400

40 80

40 80

500 400

40 80

300 200 100 0

All lines a

0 40

Fig. 40.2. The influence of shade on the seed production of six near isogenic lines of Avena fatua grown under simulated field conditions at the University of Queensland from April to December 1991. Seed numbers followed by the same letter within each line are not significantly different (P < 0.05).

H 200

M 100

Fig. 40.3. The influence of shade on the mean seed weight of six near isogenic lines of Avena fatua grown under simulated field conditions at the University of Queensland from April to December 1991. Seed weight values followed by the same letter within each line are not significantly different (P < 0.05).

H 200

M 100

All lines

All lines

Fig. 40.3. The influence of shade on the mean seed weight of six near isogenic lines of Avena fatua grown under simulated field conditions at the University of Queensland from April to December 1991. Seed weight values followed by the same letter within each line are not significantly different (P < 0.05).

0 40 80

0 40 80

intensity, may be because of a reduced growth rate of these plants (Fig. 40.1). The mean seed weight produced under full sunlight (253 mg) is comparable with that produced in field studies (Armstrong, 1994) on the same lines.

All primary seeds produced in these studies were >95% viable; therefore, no differences were observed between the different light intensity treatments or different lines. All lines produced seeds that had dormancy, as observed in earlier studies (Armstrong, 1994). However, under the lowest light intensity used (i.e. 80% light reduction), the degree of dormancy was lowest (Table 40.1; i.e. 37% germination as compared with 11% under full sunlight). The reason for this decrease in the degree of dormancy, under the reduced light conditions, is unknown and is under investigation. In a cropping situation, the decrease in seed dormancy, brought about by plants being exposed to shading inside a crop canopy, may not occur in all seeds produced on a plant since panicles are able to elongate and emerge above the crop canopy. Seeds produced on these panicles may not be affected in the same way as those produced within the canopy (Beyschlag et al., 1990; Cousens et al., 1991). However, seeds that are produced on the non-elongating secondary tillers are likely to have a decreased level of dormancy. Thus, A. fatua plants establishing into a developing crop canopy will have an increased proportion of non-dormant seeds entering the soil seed bank.

Was this article helpful?

0 0

Post a comment