Construction costs and physico-chemical properties of the assimilatory organs of Nepenthes species in Northern Borneo

Abstract

Background and aims: Species of the Nepenthaceae family are under-represented in studies of leaf traits and the consequent view of mineral nutrition and limitation in carnivorous plants. This study is aimed to complement existing data on leaf traits of carnivorous plants.

Methods: Physico-chemical properties, including construction costs (CC), of the assimilatory organs (leaf and pitcher) of a guild of lowland Nepenthes species inhabiting heath and/or peat swamp forests of Brunei, Northern Borneo were determined.

Key results: Stoichiometry analyses indicate that Nepenthes species are nitrogen limited. Most traits vary appreciably across species, but greater variations exist between the assimilatory organs. Organ mass per unit area, dry matter tissue concentration (density), nitrogen (N), phosphorus (P), carbon, heat of combustion (H(c)) and CC values were higher in the leaf relative to the pitcher, while organ thickness, potassium (K) and ash showed the opposite trend. Cross-species correlations indicate that joint rather than individual consideration of the leaf and the pitcher give better predictive relationships between variables, signalling tight coupling and functional interdependence of the two assimilatory organs. Across species, mass-based CC did not vary with N or P, but increases significantly with tissue density, carbon and H(c), and decreases with K and ash contents. Area-based CC gave the same trends (though weaker in strength) in addition to a significant positive correlation with tissue mass per unit area.

Conclusions: The lower CC value for the pitcher is in agreement with the concept of low marginal cost for carnivory relative to conventional autotrophy. The poor explanatory power of N, P or N : P ratio with CC suggests that factors other than production of expensive photosynthetic machinery (which calls for a high N input), including concentrations of lignin, wax/lipids or osmoregulatory ions like K(+), may give a better explanation of the CC variation across Nepenthes species.

Fig. 1.

Relationships between mass-based construction costs (g glucose g⁻¹) and physico-chemical properties of the leaf (closed circles) and the pitcher (open cycles) of eight Nepenthes species from Brunei, Borneo. Each point is the mean values from six sites, except for carbon where data are from three sites only. Continuous lines indicate significant correlation across species and assimilatory organs at P ≤ 0·05; dashed lines indicate non-significant trends of P > 0·05. For significance of trends within each organ type, see Table 3.

Fig. 2.

Ordination on axes I and II of eight Nepenthes species based on 11 primary properties and three derived nutrient ratios (N : P, C : N and N : K) of their leaves and pitchers (see Tables 1 and 2) and using a principal component analysis. Variables that load significantly are indicated, as well as the proportion of variation captured by each axis. Organ CC mass, heat of combustion, N : K ratio, carbon and tissue concentration (density) were strongly positively correlated with Axis I (r = + 0·91, + 0·89, +0·85, +0·81 and + 78, respectively), while K and ash contents gave significant negative trends with this axis (–0·86 and –0·74, respectively). On Axis II, N and P (with r = + 0·83 and + 0·74, respectively), and C : N ratio and TMA (with r = –0·84 and –0·62, respectively) were the main drivers. 1, Nepenthes albomarginata; 2, N. ampullaria; 3, N. bicalcarata; 4, N. gracilis; 5, N. mirabilis; 6, N. rafflesiana var. typical; 7, N. rafflesiana var. elongata, 8, N. rafflesiana var. gigantia.