Root porosity differed among species (two-way anovas, F1,16 > 165·21, P < 0·001). Control plants of P. dilatatum presented higher values of root porosity than L. glaber (Table 1). In the grass, porosity corresponded to an extensive system of lysigenous aerenchyma tissue arranged radially in the root cortex, separated by rows of parenchymatic cells and surrounded by a ring of sclerenchymatic cells in the exodermis (Fig. 3a,b). In the dicot, constitutive porosity was related mainly to intercellular air spaces because of the cubic configuration of the cells in the medium and outer cortex, with few lysigenous lacunae. Also, L. glaber did not present a ring of thicker cells in the outer layers of the cortex (Fig. 3e,f). Flooding increased root porosity in both species (F1,16 = 41·30, P < 0·001; species × flooding: F1,16 = 2·64, P = 0·12; Table 1). In P. dilatatum the higher value of root porosity under flooding conditions was achieved without major changes in the root structure (Fig. 3a,bvs c,d). In L. glaber the higher root porosity was achieved by the development of lysigenous aerenchyma lacunae (Fig. 3g). Sheath porosity of P. dilatatum and stem porosity of L. glaber were also higher in flooding conditions (F1,16 = 28·37, P < 0·001; species × flooding: F1,16 = 2·67, P = 0·12; Table 1). Trampling reduced root porosity in both species (F1,16 = 8·64, P < 0·01; species × trampling: F1,16 = 0·11, P = 0·74; Table 1), while porosity of the aerial organs remained unchanged (F1,16 = 0·34, P = 0·56; Table 1).
Figure 3. Transverse sections of roots of Paspalum dilatatum (a–d) and Lotus glaber (e–h) plants grown for 15 days under different treatments: control (a,e); trampling (b,f); flooding (c,g); trampling × flooding (d,h). Simulated trampling was applied at the beginning of the experiment. Plants of L. glaber subject to trampling in flooded soil did not survive. Arrows indicate the ring of sclerenchymatic tissue for mechanical protection in P. dilatatum. L., aerenchyma lacunae for root oxygenation. Bar, 100 µm for P. dilatatum; 80 µm for L. glaber.
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Root mechanical strength differed among species (two-way anova, F1,16 = 38·33, P < 0·001). The grass P. dilatatum presented higher values for this parameter than the dicot L. glaber (Fig. 4). The effect of flooding on this parameter differed between species, as indicated by the significant species × flooding interaction (F1,16 = 7·29, P < 0·01). For the grass, the root structure resembled a bicycle wheel, and required pressures higher than 380 kPa to cause collapse (Fig. 4). In this species, root structure was not weakened by the generation of new aerenchyma tissue (Tukey's test, P = 0·68), as 423 kPa radial pressure was required to cause root collapse (Fig. 4). Roots of the dicot L. glaber cracked at pressures of 260 kPa (Fig. 4). Under flooding conditions, the root structure of L. glaber was considerably altered and weakened by flood-induced anatomical changes (Tukey's test, P < 0·001), as only 115 kPa pressure was required to cause root collapse (Fig. 4).