How does density and nutrient stress affect allometry and fruit production in the heterocarpic species Atriplex sagittata (Chenopodiaceae)?

Abstract

Plants of Atriplex sagittata Borkh. were grown under different density and (or) nutrient conditions, and the effect of stress on plant allometry and variation in particular biometrical parameters was studied. Increasing density significantly reduced total weight, basal diameter, stem length, and number of branches. Nutrient deficiency had a negative effect on the total weight and basal diameter only, and the density × nutrients interactions were all nonsignificant. Basal diameter was the best predictor of the total weight, explaining 93.6% of the variation in the whole data set. The relationship between total plant weight and basal diameter was unaffected by growth conditions, indicating that competition determined the position of a plant along the line describing the allometric relationship between both variables but did not seem to alter the relationship. However, the allometric relationships between total weight and stem height and between total weight and number of branches found at low density were significantly different from those at high density, indicating a faster increase in total weight at low densities. Branching, and consequently plant form, were affected by competition. Branch length consistently increased with total plant weight in plants grown under high density, but for low density plants, such an increase was recorded only in branches of the middle section of the stem. In each fruit type, the total weight of fruits produced was more closely predicted by branch weight than by branch length. Allometric relationships between fruit production and branch weight differed among particular fruit types, with a faster increase in the production of type A than in that of types B or C. Competition had less effect on the mean size of type B fruits compared with that of types A or C. Fruits of types C and A had higher variation in mean weight, than type B fruit. Variation in total weight of fruits per plant was of a higher order of magnitude than that in the mean fruit weight. The results show that plants of A. sagittata grown under contrasting conditions change both their allometry and the number of particular fruit types produced. Nevertheless, the question of whether the shift in the fruit ratio is a function of allometric constraints or the result of a basic shift in allometry needs further study.Key words: allometry, Atriplex sagittata, Chenopodiaceae, density stress, nutrient stress, heterocarpy.