Biomass allocation and leaf chemical defence in defoliated seedlings of Quercus serrata with respect to carbon-nitrogen balance

Abstract

Background and aims: Both nutrient availability and defoliation affect the carbon-nutrient balance in plants, which in turn influences biomass allocation (e.g. shoot-to-root ratio) and leaf chemical composition (concentration of nitrogen and secondary compounds). In this study it is questioned whether defoliation alters biomass allocation and chemical defence in a similar fashion to the response to nutrient deficiency.

Methods: Current-year seedlings of Quercus serrata were grown with or without removal of all leaves at three levels of nutrient availability.

Key results: Plant nitrogen concentration (PNC), a measure of the carbon-nutrient balance in the plant, significantly decreased immediately after defoliation because leaves had higher nitrogen concentrations than stems and roots. However, PNC recovered to levels similar to or higher than that of control plants in 3 or 6 weeks after the defoliation. Nitrogen concentration of leaves produced after defoliation was significantly higher than leaf nitrogen concentration of control leaves. Leaf mass per plant mass (leaf mass ratio, LMR) was positively correlated with PNC but the relationship was significantly different between defoliated and control plants. When compared at the same PNC, defoliated plants had a lower LMR. However, the ratio of the leaf to root tissues that were newly produced after defoliation as a function of PNC did not differ between defoliated and control plants. Defoliated plants had a significantly lower concentration of total phenolics and condensed tannins. Across defoliated and control plants, the leaf tannin concentration was negatively correlated with the leaf nitrogen concentration, suggesting that the amount of carbon-based defensive compounds was controlled by the carbon-nutrient balance at the leaf level.

Conclusions: Defoliation alters biomass allocation and chemical defence through the carbon-nutrient balance at the plant and at the leaf level, respectively.

Fig. 1.

Changes in (A) biomass and (B) the amount of nitrogen in plants after defoliation. Open and closed symbols indicate control and defoliated plants, respectively. Circles, triangles and squares indicate the low, middle and high nutrient treatments, respectively. Mean and s.d. are shown (n = 6). The arrow indicates the day of defoliation. Values for defoliated plants on the day of defoliation were calculated as those of the stems and roots in control plants. Note that plant mass and nitrogen content are expressed on a log scale.

Fig. 2.

Changes in plant nitrogen concentration after defoliation. (A) Low, (B) middle, and (C) high nutrient treatments. Significance levels were assessed for each date using a Student t-test (ns, not significant; ^**P < 0·01; ^***P < 0·001). Symbols are as described in Fig. 1.

Fig. 3.

Changes in leaf nitrogen concentration after defoliation. Symbols are as described in Fig. 1.

Fig. 4.

Allometric relationship between leaf and plant mass in defoliated and control plants grown at (A) low, (B) middle, and (C) high nutrient availability. Open and closed symbols indicate control and defoliated plants, respectively. One point denotes one plant. Plants were harvested at 0, 21, 42 and 63 d after defoliation.

Fig. 5.

(A) Leaf mass ratio (leaf mass per total mass) and (B) leaf nitrogen concentration as a function of plant nitrogen concentration. Open and closed symbols indicate control and defoliated plants, respectively. Circles, triangles and squares indicate low, middle and high nutrient availability, respectively. One point represents one plant. Plants were harvested at 0, 21, 42 and 63 d after defoliation. For leaf mass ratio, the regression lines are y = 0·045 + 25·5x (r² = 0·79, P < 0·0001) for control plants and y = −0·067 + 23·9x (r² = 0·79, P < 0·0001) for defoliated plants. For leaf nitrogen concentration, the regression lines are y = 0·0054 + 1·18x (r² = 0·75, P < 0·0001) for control plants and y = 0·011 + 1·03x (r² = 0·68, P < 0·0001) for defoliated plants.

Fig. 6.

Leaf tannin concentration as a function of (A) plant and (B) leaf nitrogen concentration. Tannin concentration is expressed as a relative value on a dry mass basis.

Fig. 7.

Changes in the concentration of total non-structural carbohydrate in roots after defoliation. Symbols (slightly shifted to avoid overlap of bars) are as described in Fig. 1.

Fig. 8.

The ratio of leaf to root mass newly produced after defoliation as a function of plant nitrogen concentration. Open circles denote control plants (mean of 6 plants), where total mass of harvested roots and leaves is used. Closed circles denote defoliated plants, where the leaf mass 63 d after defoliation is used. The root mass 63 d after defoliation minus the minimum root mass observed though the experiment was used for defoliated plants. Minimum root mass was found at 21 d after defoliation (middle and high nutrient availability) and at 42 d after defoliation (low nutrient availability). The regression line is y = −0·185 + 72·7x (r² = 0·87, P < 0·0001).