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Résumé :
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Variability among individual plants causes low seed yields in field pea (Pisum sativum L.) crops. To quantify this variability, an empirical principal axis model (PAM) was developed which has two components: (a) a principal axis, representing the relationship between the seed weight (SWT) produced and plant weight (PWT) of individual plants; and (b) an ellipse, which characterizes the scatter of individual values around the axis. To develop the model, plant-to-plant variability was simulated by systematically changing the mean and standard deviation (s.d.) of frequency distributions for SWT and PWT. Changes in the intercept and slope of the principal axis and the location and shape of the associated ellipse were used to describe the plant-to-plant variability. Differences in the mean SWT or PWT altered the location of the ellipse. When changes in the s.d. values were equal for SWT and PWT, the dimensions of the ellipse changed, but the axes ratio was constant. Non-proportionate changes in the s.d. values caused rotation of the principal axis and altered the shape of the ellipse. The effect of differences in the PAM on plant harvest index (PHI) was also examined. When the intercept of the principal axis passed through the origin, PHI was independent of PWT, and equal to the gradient of the axis. However, when the SWT-axis intercept was negative, indicating that a minimum plant weight was required for seed production, the relationship between PHI and PWT was asymptotic. This relationship is of major importance for interpreting differences in PHI distributions, and thus crop harvest index and seed yield among crops. (C) 1997 Annals of Botany Company.
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