Introduction

Earlier studies have helped to elucidate how the photoperiod experienced by wild oat (Avena fatua L.) influence some of the important persistence characteristics of weeds. An increase in the day length under which the plants were grown led to a decrease in seed production and a decrease in the depth of seed dormancy possessed by these seeds (Armstrong, 1994). However, little is known about how other components, such as light intensity and quality, modulate the effect of the light environment on the persistence of this serious annual weed of cereal crops worldwide.

In the environment of a growing crop, A. fauta plants are exposed to a range of light intensities and qualities. For example, when the crop is in its early stages of growth, newly emerged A. fatua seedlings are exposed to a low amount of shading. However, as the crop canopy develops, shading will increase, reducing the light level by as much as 90-95% of that experienced in direct sunlight (Puckridge and Ratkowsky, 1971; Fisher and Wilson, 1976; Steven et al., 1986; Cudney et al., 1991). Such shading could be expected to influence plant morphology, phenology and reproductive ability. Shading is known to reduce plant growth because it reduces photosynthesis (Monteith and Elson, 1969), reduces leaf thickness and length, reduces root to shoot ratios, increases apical dominance, reduces tiller production (Holmes and Smith, 1975; Ludlow, 1978; Ballare et al., 1988), increases stem elongation (Carpenter and Hopen, 1985), reduces dry matter production and reduces flower and seed production (Dall'armellina and Zimdahl, 1988; Stoller and Myers, 1989; Stirling et al., 1990). Such developmental changes could be expected to occur in A. fatua plants, especially in those emerging late from the seed bank, into a closing crop canopy.

Although some studies have investigated the effect of shading on A. fatua development (Barnes et al., 1990; Beyschlag et al., 1990) and others have concentrated at the emergence of A. fatua seedlings in a crop (McBeath et al., 1970; Peters and Wilson, 1983; Martin and Field, 1988), the effects of shade alone have not been determined. In order to separate the effects of shade from that of soil water and nutrient status, it might be more appropriate to assess the influence of shade under partially controlled conditions where the plants are grown individually, away from other competition effects. This approach can be problematical, as the quality of light experienced within a crop canopy is different to that produced under controlled conditions using shade cloth. However, assessment of light intensity effects upon plant growth and development, using shade cloth (Yates, 1989), has been undertaken in the past and found to provide significant insight into the effect of light intensity on plant growth and development under plant canopies.

The aim of the present study was to determine whether the light intensity experienced by growing A. fatua plants could have an effect on their reproductive capacity (i.e. seed production, seed weight, seed viability and depth of dormancy) and on features that may determine their long-term persistence in the soil seed bank. The light intensities used represent those that A. fatua would experience if it emerged before or into a closing crop canopy. In order to analyse any changes in reproductive capacity that may occur, a number of phenological and morphological characters were recorded. The A. fatua lines used in this study came from a number of locations within the northern grain growing region of Australia and were used to determine if the genetic variation in natural populations of A. fatua at different locations affected this response.

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