Changes in physico-chemical properties of floodwater during the submergence treatment
Shaded-submerged treatment decreased the pH value of floodwater (Table ). Accordingly, the HCO3− concentration in the floodwater decreased slightly; by contrast, the CO2 concentration increased. When CO2 and HCO3− levels were added together, there was a minor decrease in the total inorganic carbon from day 0 to day 20 (from 467 to 440 µmol L−1). Amounts of CO32− detected in the floodwater were negligible (<1 µmol L−1, data not shown). The dissolved O2 concentration changed little in the submergence tanks.
Shoot and root anatomy
Both A. philoxeroides and H. altissima are terrestrial plants which can grow well in wetland. Transverse sections prepared from shoot/culm and root tissues of Shaded-submerged plants showed a well-developed pith cavity and aerenchyma (Fig. ), suggesting structural adaptation to wetland habitats. The samples were taken at 30 mm above and 30 mm below the soil surface for stems/culms and roots, respectively. These tissues were formed during cultivation in the glasshouse, i.e. they are not aquatic shoots/culms or roots which grew during Shaded-submerged treatment. In fact, different flooding treatments did not visibly alter the number and/or size of pith cavity and aerenchyma during the 30-d flooding treatments, and similar anatomy was also observed in well-drained Control plants (data not shown). Thus, it seems that these plants have porous shoots and roots constitutively.
Fig. 1 Transverse sections of stem/culm (A, B) and root tissues (C, D) of Shaded-submerged plants of A. philoxeroides (A, C) and H. altissima (B, D). Sections were made at 30 mm above and 30 mm below the soil surface for stems/culms and roots, respectively. (more ...)
Growth responses to flooding with or without shading
Growth responses to different flooding treatments with or without shading were compared in A. philoxeroides
and H. altissima
during 50 d of the flooding and recovery experiments. Above- and below-ground dry mass increased in Control plants of A. philoxeroides
>20-fold during the experiment (Fig. A and B, respectively). Waterlogged treatment resulted in a significantly smaller dry mass increase compared with Control. Plants of A. philoxeroides
have a repent growth form, and the contact of shoots with the floodwater in the waterlogging tank stimulated the formation of adventitious roots at stem nodes. On day 30 these adventitious roots amounted to 7–10 % of the total root fresh weight for Waterlogged plants (data not shown). When shaded (Shaded-control, Shaded-waterlogged and Shaded-submerged), A. philoxeroides
virtually did not accumulate dry mass, with or without flooding. Yet the dry mass increase started between day 30 and 50, with the recovery in Shaded-submerged plants being slower than in other shaded plants both above and below ground. The root-to-shoot ratio increased in Control plants of A. philoxeroides
with increasing plant size (Fig. C), presumably due to ‘ontogenic drift’ (Geng et al., 2007
). The ratios strongly decreased in Shaded-control and Shaded-waterlogged plants from day 0 to 30, whereas they did not change significantly in Waterlogged and Shaded-submerged plants. The ratio then increased during the recovery period in all treatments (except for Control) to reach Control-like values by day 50 in all but Shaded-submerged plants.
Fig. 2 Changes in above-ground dry mass (A, D), below-ground dry mass (B, E) and root-to-shoot ratio (C, F) of A. philoxeroides (A–C) and H. altissima (D–F) following the different flooding treatments with or without shading. Each value is a (more ...)
The Control plants of H. altissima accumulated much less dry mass than Control plants of A. philoxeroides: <10-fold for above-ground and <6-fold for below-ground (Fig. D and E, respectively) dry mass. As in A. philoxeroides, dry mass accumulation of Waterlogged plants was less than in Control plants, and shading allowed no increment of above- and below-ground dry mass, with or without flooding. Accumulation of dry mass started in shaded plants after day 30, but the major increase occurred between day 40 and 50. The apparent lack of change in above-ground dry mass of Shaded-submerged plants between day 30 and 40 (Fig. D) actually contained growth of some new leaves which replaced mature leaves lost shortly after de-submergence. Unlike in A. philoxeroides, the root-to-shoot ratio decreased in Control plants of H. altissima from day 0 to 30 (Fig. F). While retarding the dry mass increase, Waterlogged treatment did not affect the root-to-shoot balance in H. altissima. Shading slowed the decrease in root-to-shoot ratio, but the values for Shaded-control and Shaded-waterlogged plants approached those of Control and Waterlogged plants within the first 10 d of recovery. Only Shaded-submerged plants maintained higher values throughout the experiment. In general, the root-to-shoot ratio of H. altissima was about half that of A. philoxeroides on day 50 (Fig. C, F), indicating that relative dry mass allocation to shoots was greater in H. altissima than in A. philoxeroides, and vice versa for relative allocation to roots, during the experiments.
The responses of above-ground growth patterns to different treatments were further characterized by comparing the number of branches (A. philoxeroides) or tillers (H. altissima). The total number of branches increased from day 0 to 30 in Control and Waterlogged plants of A. philoxeroides by about four- and three-fold, respectively (Fig. A). Shading suppressed the formation of new branches, but Shaded-control and Shaded-waterlogged plants were able to produce many new branches after the end of the treatments. These plants had as many branches as Control and Waterlogged plants by day 50 even though their above-ground dry mass was still much less (Fig. A). Recovery of branching was far slower in Shaded-submerged plants, having only half as many branches as Control plants at the end of the recovery period (Fig. A). The above-ground growth of H. altissima was characterized by a large increase (about eight-fold in Control plants) in total tiller number (Fig. B). Tiller formation in this grass species responded very sensitively to flooding as well as to shading treatments; it was strongly impaired in Waterlogged treatment and was completely halted until day 40 in Shaded-control and Shaded-waterlogged treatments. Shaded-submerged plants did not grow new tillers during 20 d of the recovery period.
Fig. 3 Changes in the total branch/tiller number of A. philoxeroides (A) and H. altissima (B) following the different flooding treatments with or without shading. Each value is a mean of 8–10 plants (±s.e.). All plants were under the Control (more ...)
Although suppressing the accumulation of dry mass and formation of new branches, 30 d of submergence strikingly enhanced elongation of internodes in A. philoxeroides (Fig. A). Even though dry mass accumulation was similarly inhibited in all shaded plants, only Shaded-submerged plants had longer internodes than Control plants (by approx. 20 %) on day 30. By elongating the existing internodes (i.e. without formation of new internodes) Shaded-submerged plants increased the total length of the main stem by about 40 cm (or 200 % of the initial length on day 0), while the corresponding increment in Shaded-control and Shaded-waterlogged plants was only about 15 cm (Fig. B). The main stems of Control and Waterlogged plants grew much longer than those of shaded plants, but this was achieved by making new internodes (data not shown). In marked contrast, variations in the internode length among different treatments were minimal for H. altissima (Fig. C). The main culms grew longer only in Control and Waterlogged plants, which produced several new internodes during the period (Fig. D).
Fig. 4. Length of the longest internodes (A, C) and the main stem/culm (B, D) for A. philoxeroides (A, B) and H. altissima (C, D) at the end of the different flooding treatments with or without shading. Each value is a mean of 8–10 plants (±s.e.). (more ...)
Photosynthetic responses to flooding with or without shading
The above results demonstrated the ability of A. philoxeroides and H. altissima to resume growth and dry mass accumulation relatively quickly after de-submergence. According to the present hypothesis, the photosynthetic apparatus of these plants must be able to cope with extreme changes in O2 and light. However, growth recovery was retarded in Shaded-submerged plants compared with Shaded-control and Shaded-waterlogged plants (Figs and ), which may reflect slower acclimation and recovery of photosynthesis due to detrimental effects of submergence on the photosynthetic apparatus.
The maximal quantum yield of PSII (Fv/Fm) indicated no significant photoinhibition in Waterlogged plants of A. philoxeroides throughout the experiment (Fig. A). Shaded-submerged plants had slightly lower Fv/Fm values shortly after de-submergence, but Fv/Fm fully recovered in these plants on day 39. Shading quickly increased Fv/Fm to the maximal level (approx. 0·84) in both Shaded-control and Shaded-waterlogged plants, whereas the transfer back to the growth conditions transiently decreased Fv/Fm to the values found in Shaded-submerged plants (Fig. B). Thereafter, Fv/Fm fully recovered in Shaded-control and Shaded-waterlogged plants by day 37, i.e. 2 d earlier than in Shaded-submerged plants. In contrast to the situation in A. philoxeroides, Waterlogged treatment markedly decreased Fv/Fm in H. altissima in the first 20 d, suggesting some photoinhibitory damage to PSII (Fig. C). Yet Fv/Fm started to recover in the last 10 d of Waterlogged treatment to become comparable with that of Control plants 3 d after draining the soil. Fv/Fm values of Shaded-submerged plants during the recovery period were as high as those of Control plants. The shading responses of Fv/Fm in H. altissima were generally the same as described for A. philoxeroides, albeit with less pronounced changes (Fig. D). Full recovery of Fv/Fm was found on day 37 in Shaded-control and on day 39 in Shaded-waterlogged treatment.
Fig. 5. Changes in the maximal quantum yield of photosystem II (Fv/Fm) in dark-adapted leaves of A. philoxeroides (A, B) and H. altissima (C, D) during and after different flooding treatments with or without shading. Data from Shaded-submerged plants are shown (more ...)
Whereas Fv/Fm indicated little PSII photoinhibition in all but Waterlogged plants of H. altissima, the effective quantum yield of PSII (ΔF/Fm′) revealed a strikingly reduced capacity of shaded plants to utilize light energy under illumination (Fig. ). Waterlogged treatment had no significant effect on ΔF/Fm′ in A. philoxeroides (Fig. A). The shading treatment caused a rapid and dramatic decrease in ΔF/Fm′ in Shaded-control and Shaded-waterlogged plants to a level as low as that measured in Shaded-submerged plants (Fig. B). Recovery of ΔF/Fm′ started in all shaded plants of A. philoxeroides within 3 d after the end of the treatments, although it took approx. 10 d to recover fully. In comparison, H. altissima generally had much lower photochemical efficiency than A. philoxeroides (Fig. C, D). The pronounced negative effect of Waterlogged treatment was also visible in ΔF/Fm′ of this species (Fig. C). Unlike Fv/Fm, ΔF/Fm′ of H. altissima was substantially reduced to approx. 60 % of Control plants at the beginning of Shaded-control and Shaded-waterlogged treatments, or at the end of Shaded-submerged treatment (Fig. D). For both species, recovery of ΔF/Fm′ in Shaded-submerged treatment was not slower than in Shaded-control and Shaded-waterlogged treatments.
Fig. 6. Changes in the effective quantum yield of photosystem II (ΔF/Fm′) in leaves of A. philoxeroides (A, B) and H. altissima (C, D) during and after different flooding treatments with or without shading. Values were obtained after 4·5 (more ...)
When a large fraction of absorbed light energy becomes excessive due to low ΔF/Fm′, leaves may up-regulate NPQ to protect the photosynthetic apparatus and minimize photoinhibition. Shading substantially diminished the NPQ capacity in both A. philoxeroides and H. altissima, but rapid recovery was observed after day 30 (Fig. ). The changes in NPQ paralleled the large variations in Fv/Fm and ΔF/Fm′ in Waterlogged plants of H. altissima (Fig. C). Again, the NPQ recovery of Shaded-submerged plants did not differ from that in other shaded plants for both species (Fig. B, D).
Fig. 7. Changes in non-photochemical energy quenching (NPQ) in leaves of A. philoxeroides (A, B) and H. altissima (C, D) during and after different flooding treatments with or without shading. Values were obtained after 4·5 min of illumination at 800 (more ...)