The average total biomass of plants immediately before they were treated with one of four nutrient solutions was 512 ± 31 g d. wt. Thereafter, mean relative growth rates (ranked in order of increasing phosphorus availability) were 8·0, 9·7, 28·0 and 21·1 % d−1. By the end of the experiment, the plants in the four solutions differed considerably in size (Fig. ), with average heights of 23·0, 25·6, 38·3 and 34·7 cm, respectively (ANOVA: P < 0·001), and average dry weights of 2·15, 2·50, 6·24 and 5·05 g (ANOVA: P < 0·001). There were no significant differences in height or biomass between unscathed and wounded plants, and no differences among the various elicitor treatments (Table ). Thus, growth depended only on the phosphorus concentration in the nutrient solution, with the third highest level being the most favourable.
Fig. 2. (A) Total biomass (log-transformed) and (B) mean plant height of unscathed (white), wounded (black), and elicitor-treated plants (hatched columns, from left to right: bacteria, fungi and insects) at the end of the experiment (n = 6). For statistical details (more ...)
Phosphorus and nitrogen contents of leaflets
The phosphorus content of leaf tissue increased with increasing phosphorus concentration of the nutrient solution (ANOVA: P < 0·001; Fig. ). Plants grown at the lowest level of phosphorus availability and, to a lesser extent, those at the second lowest showed typical symptoms of phosphorus deficiency such as purple margins to leaflets and necrotic areas. The phosphorus content of leaflets increased strongly and significantly from the second to the third level of phosphorus availability, and also from the third to the highest level of phosphorus availability. The phosphorus content at the highest level of phosphorus availability (4·66 ± 0·30 mg P g−1 d. wt) was more than twice that at the lowest (2·19 ± 0·18 mg P g−1 d. wt). The nitrogen content of leaflets increased only slightly with increasing phosphorus availability from 43·31 ± 1·05 mg N g−1 d. wt at the lowest level to 49·27 ± 2·61 mg N g−1 at the highest (Fig. ; ANOVA: P < 0·05). As a result, the nitrogen:phosphorus (N/P) ratio of leaflets ranged considerably, from 19·8 at the lowest phosphorus availability to 10·6 at the highest.
(A) Foliar phosphorus and (B) nitrogen content at the end of the experiment. Columns represent block-wise pooled samples (n = 6). Columns that share a letter are not statistically different according to Tukey's honest significant difference.
Except for a small necrotic area around each wound, wounded leaflets remained green and apparently active. Nevertheless, to assess whether wounding had affected carbon assimilation, the content of NSCs (i.e. the sum of glucose, fructose, sucrose and starch) was measured in unscathed and in wounded leaflets (Table ). Contrary to what had been expected, carbohydrate levels were higher in wounded leaflets than in unscathed ones at all levels of phosphorus availability (P < 0·01). Furthermore, the content of NSCs in unscathed leaves tended to decrease with increasing phosphorus availability, although this result was not significant.
Non-structural carbohydrates (mg g−1 d. wt) in unscathed and wounded leaflets at four levels of phosphorus concentration (mm) in the nutrient solution (mean ± s.e., n = 6)
Concentrations of condensed tannins
The relationship between phosphorus availability and CT concentrations in the absence of any wounding or elicitor treatment was estimated using the mean data for the unscathed control plants (in which no distinction can be made between local and systemic effects). The results show a strong and highly significant decline in tannin concentrations with increasing phosphorus availability (94·9 ± 5·0, 84·9 ± 6·3, 75·5 ± 6·0 and 69·0 ± 3·6 mg g−1 d. wt, respectively; ANOVA: P < 0·001).
Wounding and the application of elicitors affected tannin concentrations locally (i.e. in the damaged leaflets; ANOVA local: P < 0·001) but not systemically (i.e. in leaflets of undamaged neighbouring leaves; ANOVA systemic: P = 0·964; Fig. ). Tannin concentrations in leaflets that were wounded but not treated with an elicitor were lower than those in leaflets of unscathed plants (P < 0·01). However, tannin concentrations in leaflets that had also been treated with one of the elicitors were significantly higher than those in leaflets that had only been wounded (P < 0·05, P < 0·05 and P < 0·001 for the fungal, the bacterial and the insect elicitor, respectively; Table ). For plants treated with either the bacterial or the fungal elicitor, this difference was not large and the CT concentrations were similar to those of unscathed plants. The only elicitor that increased tannin concentrations above the level of unscathed leaves was saliva of Spodoptera littoralis larvae. There was no evidence of a stronger stimulation of tannin concentrations by the wounding or elicitor treatments at low than at high nutrient availability (no significant Ph × T interaction; Table ). All of these results remain essentially the same whether the tannin concentration is expressed as a percentage of the leaflet dry weight (CTL) or of the NSC-free dry weight (CT′L; Table ).
Fig. 4. (A) Local and (B) systemic tannin concentrations of unscathed (white), wounded (black), and elicitor-treated plants (hatched columns, from left to right: bacteria, fungi and insects) at the end of the experiment (n = 6). For statistical details see Table (more ...)
It is commonly supposed that, at moderate to high levels of nutrients, tannin synthesis is ‘costly’ because it diverts carbon resources from plant growth (growth–defence trade-off), whereas at low nutrient availability producing tannins is ‘cheap’ because the availability of carbon exceeds growth demands (nutrient limitation of growth). Based on this concept, it was expected that, as phosphorus availability increased, an increasingly negative correlation between plant biomass and tannin concentrations of unscathed plants would be found. However, Pearson regression coefficients for this relationship – ranked from low to high phosphorus availability – were (−0·39), 0·50, 0·32 and 0·49, and none of them was significant.