• *

    Present address: Botany Building, School of Biological Sciences, Sydney University, Sydney, N.S.W. 2006, Australia.

  • †Present address: Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80521, U.S.A. while on leave from CSIRO, Rangelands Rest-arch Unit, Deniliquin, N.S.W. 2710, Australia.


Populations of Arabidopsis thaliana were established in acid-washed sand culture from sowing densities of 1, 10, 100 and 1000 seeds/pot at three levels of mineral nutrition; undiluted, 1/10 diluted and 1/100 diluted Hoagland's solution.

Mortality increased with sowing density. At the highest sowing density, mortality was greatest at the lowest and least at the intermediate level of mineral nutrition.

Proportionally fewer of the surviving plants reached fruiting as sowing density increased and as the nutrient supply became more dilute.

Seed output/fruiting plant was highest in low sowing density populations supplied with undiluted nutrient solution and least in high sowing density populations supplied with diluted nutrient solutions.

The range of variation of mean seed output/fruiting plant was 100-fold over the different types of population, and that of its components, capsule number/fruiting plant and number of seeds/capsule 20-fold and 5-fold, respectively. The range of the components of number of seeds/capsule, that is, capsule length and seeds/unit length of capsule was 21/2-fold and 2-fold, respectively.

Number of seeds/capsule varied with developmental stage, largely because later-produced capsules were shorter. The rate of capsule development was affected by density and nutritional stress more readily than the number of seeds/capsule at a particular developmental stage.

Most capsules were borne on main stems of the plants, and an allometric relationship was demonstrated between mean seed output/fruiting plant and mean length of the main stem. Departures from this relationship are discussed and the statistical properties of the frequency distributions of main stem lengths of fruiting plants examined.

Attributes of fruiting plants in nine of the twelve types of population were linearly related to the density of all surviving plants on a log/log plot, a separate line being characteristic of each nutritional level. The possibility of allometric relationships between attributes and mean plant weight and of linear log/log relationships between mean plant weight and density is discussed.

Seed output was constrained by plant density in nine of the twelve types of populations and free from this constraint in two of the remaining three types. The specific area for seed outputs of individual plants to be unconstrained by available space decreases as mineral nutrition level increases.

Maximum seed output/population occurred over a wider range of sowing density the higher the level of mineral nutrition. At the lowest level of mineral nutrition, seed output displayed a well-defined optimum with sowing density, but at higher levels it rose with sowing density to reach a maximum which was subsequently maintained.

Mean weight of seed harvested was similar over all sowing densities at the highest level of nutrition, but it decreased with sowing density as nutrient level decreased. This decrease in seed weight was correlated with decreasing capacity to germinate 33/4 years after harvest.