Light and the growth of prairie grass (Bromus willdenovii) and short-rotation ryegrass (Lolium perenne x L. multiflorum) swards

Abstract

For many reasons, a great deal of recent agricultural research has been directed towards the maximization of pasture and crop yield per unit area of land. One facit of such research has been the evaluation of light as a limiting factor in plant yield.

Because it is very difficult to control the level of incident radiation, the problem is one of maximum utilization of the incident radiation which is variably available to a plant surface. As will be reviewed subsequently, the plant leaf area, which is an indirect measure of the size of the photosynthetic system, has been found to be correlated with rate of growth and the vegetative and reproductive yield of a plant population. In addition, the leaf area required to maintain a plant population at a maximum rate of growth, varies depending upon the level of incident radiation and growth habit of that particular plant population. Growth habit is implicated because of the effect of orientation and distribution of plant parts on light penetration into a pasture or crop.

The practical implication of these concepts is that a pasture sward or forage crop might be maintained at its maximum growth rate, even with seasonal variation in the level of incident radiation, by controlling through cutting or grazing the leaf area at optimum values.

In view of the small experimental evidence to support these concepts and disagreement by some workers, two grasses (short-rotation ryegrass and prairie grass), of very different growth habit (rate of tillering and leaf production, leaf size and plant stature) were sown in pure swards to determine:- (a) whether each species had an optimum leaf area at which growth rate was maximal, (b) whether these optimum leaf areas changed with the level of incident radiation, (c) whether there was a species interaction with the level of incident radiation, (d) whether the chlorophyll content was a more direct measure of the size of the photosynthetic system than leaf area and hence more closely correlated with growth rate and vegetative yield.

In addition, reduction of the level of incident radiation by 50% and 70%, enables summer, spring – autumn and winter light conditions to be simulated. Thus, species responses to seasonal changes in the level of incident radiation can be evaluated.