The role of leaf lobation in elongation responses to shade in the rosette-forming forb Serratula tinctoria (Asteraceae)

Abstract

Background and aims: Lobed leaves are considered selectively advantageous in conditions of high irradiance. However, most studies have involved woody species, with only a few considering the role of leaf lobation in herbaceous rosette species. In this study, it is hypothesized that, in addition to its adaptive value in high light, leaf lobation may add to the function of petioles as vertical spacers in herbaceous species in conditions of strong competition for light.

Methods: To test this hypothesis, leaf development was examined under seasonally changing natural light conditions and a field experiment was conducted in which light climate was manipulated in a wooded meadow population of Serratula tinctoria.

Key results: No changes in leaf lobation were observed in response to experimental shading or different natural light conditions. However, in tall herbaceous vegetation, plants with highly lobed leaves achieved significantly greater vertical elongation than plants with less-lobed leaves. In contrast to herbaceous shade, tree shade had no effect on leaf elongation, suggesting differential responsiveness to competition from neighbouring herbs versus overhead shade. In shading treatments, imposed shade could only be responded to by the elongation of leaves that were produced late in development.

Conclusions: The results show that extensive leaf lobation can enable greater leaf elongation in response to shade from surrounding herbaceous vegetation. The different morphological responses displayed by Serratula tinctoria to different types of shade demonstrate the importance of critically assessing experimental designs when investigating phenotypic plasticity in response to shade.

Fig. 1.

Leaf characteristics and examples of leaf shapes of Serratula tinctoria. a, Leaf lobes; b, dissected area of a leaf; c, leaf lamina; d, petiole. Numbers of lobes in leaves from left to right are ten, four, four and zero.

Fig. 2.

Artificial plants used in the experiment. The stems of the artificial plants were placed at equal distances on either side of a study plant (rooting point is shown with a black dot).

Fig. 3.

Dependence of mean height of rosette leaves on mean number of lobes per leaf in repeatedly measured plants of Serratula tinctoria at five consecutive samplings. Least-square regression lines and their 95 % confidence bands are shown. Coefficients of determination (R²) and P-values for relationships at each sampling time are given.

Fig. 4.

The relationship between leaf lamina length and the height of the surrounding herbaceous layer. Data for fully expanded leaves is presented. Leaves were grouped into three categories of leaf lobation: leaves with < 7 lobes, leaves with 7–12 lobes, and leaves with > 12 lobes. See results of the linear model in Table 2.

Fig. 5.

Changes in numbers of leaves in plants of Serratula tinctoria during the course of the experiment. Control plants, constantly shaded plants, plants shaded for the first half of the experiment, and plants shaded for the second half of the experiment, are distinguished. Original differences in mean numbers of leaves between treatments have been set to zero, and absolute changes from the first sampling are shown. For the treatment ‘late shade’, data were only recorded from the third sampling date, when shading was imposed, as plants were assumed to perform as controls prior to shading. The error bars denote 95 % confidence intervals for the mean.

Fig. 6.

(A) Lamina length and (B) density of lobation of fully expanded leaves as a function of leaf ordinal number in plants of Serratula tinctoria grown in control conditions or exposed to three shading treatments. Early and late shade denote shading imposed for the first half and for the second half of the experiment, respectively. The error bars denote 95 % confidence intervals for the mean.