Synergic effect of salinity and CO2 enrichment on growth and photosynthetic responses of the invasive cordgrass Spartina densiflora

Abstract

Spartina densiflora is a C(4) halophytic species that has proved to have a high invasive potential which derives from its clonal growth and its physiological plasticity to environmental factors, such as salinity. A greenhouse experiment was designed to investigate the synergic effect of 380 and 700 ppm CO(2) at 0, 171, and 510 mM NaCl on the growth and the photosynthetic apparatus of S. densiflora by measuring chlorophyll fluorescence parameters, gas exchange and photosynthetic pigment concentrations. PEPC activity and total ash, sodium, potassium, calcium, magnesium, and zinc concentrations were determined, as well as the C/N ratio. Elevated CO(2) stimulated growth of S. densiflora at 0 and 171 mM NaCl external salinity after 90 d of treatment. This growth enhancement was associated with a greater leaf area and improved leaf water relations rather than with variations in net photosynthetic rate (A). Despite the fact that stomatal conductance decreased in response to 700 ppm CO(2) after 30 d of treatment, A was not affected. This response of A to elevated CO(2) concentration might be explained by an enhanced PEPC carboxylation capacity. On the whole, plant nutrient concentrations declined under elevated CO(2), which can be ascribed to the dilution effect caused by an increase in biomass and the higher water content found at 700 ppm CO(2). Finally, CO(2) and salinity had a marked overall effect on the photochemical (PSII) apparatus and the synthesis of photosynthetic pigments.

Fig. 1.

Pot of Spartina densiflora with fully expanded penultimate leaves marked in red. (This figure is available in colour at JXB online.)

Fig. 2.

Growth analysis of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO₂ concentration over 90 d. Total dry mass (A), relative growth rate (B), total leaf area (C), specific leaf area (D), unit leaf rate (E), leaf elongation rate (F), and number of tillers (G). Values represent mean ±SE, n=7 (n=14 for total leaf area). The analysis was carried out on an ash-free dry mass basis. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO₂×salinity; Tukey test, P <0.05).

Fig. 3.

Net photosynthetic rate, A (A–C), stomatal conductance, G_s (D–F), and intercellular CO₂ concentration, C_i (G–I) in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of NaCl concentrations at ambient and elevated CO₂ concentration after 7 d (A, D, G), 30 d (B, E, H), and 90 d (C, F, I). Values represent mean ±SE, n=10.

Fig. 4.

Water use efficiency, WUE (A) and leaf water content (B) in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of NaCl concentrations at ambient and elevated CO₂ concentration over 90 d. Values represent mean ±SE, n=10 and n=7 for WUE and water content, respectively.

Fig. 5.

Maximum quantum efficiency of PSII photochemistry, F_v/F_m (A–C) and quantum efficiency of PSII, ΦPSII (C–E) at midday in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO₂ concentration after 7 d (A, C), 30 d (B, D), and 90 d (C, E). Values represent mean ±SE, n=10. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO₂×salinity; Tukey test, P <0.05).

Fig. 6.

Chlorophyll a, Chl a (A), chlorophyll b, Chl b (B), carotenoid, Cx+c (C), and leaf nitrogen (D) concentrations in randomly selected, fully expanded penultimate leaves of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO₂ concentration over 90 d. Values represent mean ±SE, n=5. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO₂×salinity; Tukey test, P <0.05).

Fig. 7.

PEPC activity (A) and IC₅₀ values for L-malate (B) in crude extracts of illuminated leaves of Spartina densiflora in response to treatment with a range of salinity concentrations at ambient and elevated CO₂ concentration over 90 d. Values represent mean ±SE, n=5. Different letters indicate means that are significantly different from each other (two-way ANOVA, CO₂×salinity; Tukey test, P <0.05).