Figure 1.
The germination process in X. viscosa.
(A) dry seed; (B) cross-section of an imbibed seed; (C) radicle emergence; (D) chlorophyll is present in the emerging cotyledon; (E) root hairs appear once the cotyledon is 1 to 3 mm in length; (Fi) cross section of seedling showing tip of the photosynthetic cotyledon in contact with the endosperm, and the pale meristem; (Fii) a central protoxylem connecting the meristem to the tip of the cotyledon is visible; (Gi) the cotyledon and root tip elongate rapidly, and a slit appears above the root through which the first true leaf emerges (Gii); (H) the emerging leaf is clearly visible and the cotyledon ceases to grow. A cross section at this stage (Ii and Iii) shows that the meristem is no longer connected to the endosperm via the protoxylem. Seedlings at this stage are no longer dependent on the endosperm for survival. The 0.5 mm scale bar at the top of the figure is applicable to images A to Fi, and the 1 mm scale bar to images Gi, H and Ii. Individual scale bars are provided for Fii, Gii and Iii.
Figure 2.
Desiccation tolerance in germinating seedlings of X. viscosa and A. thaliana.
(A) Survival of germinating X. viscosa seedlings after 48 h desiccation, grouped by cotyledon length. Data shows the mean survival rate ±SEM as determined by bootstrapping. Representative line drawings of germinating seedlings are shown above. (B) Incidence of chlorophyll degradation in the surviving seedlings from Figure 2A. Black bars: survived and did not lose chlorophyll. Grey bars: survived and lost chlorophyll. The numbers of seeds per bin are provided in Table S1.
Figure 3.
Recovery of X. viscosa seedlings from dehydration.
Representative images of three untreated X. viscosa seedlings 1 mm, 2.2 mm and 3 mm in length before and after dehydration and rehydration. The 1 mm seedling dehydrated without chlorophyll loss, but recovered and resumed growth rapidly after rehydration. A 2.2 mm seedling likewise failed to degrade chlorophyll, but did not survive the rehydration process. A 3 mm seedling displayed partial chlorophyll loss from its cotyledon during dehydration. The seedling survived with its meristem, primary leaf and leading end of the cotyledon intact; the primary root failed to survive, but two additional secondary roots emerged from the meristem. Anthocyanins accumulated in the photosynthetic tissue during rehydration of this seedling.
Figure 4.
Pre-treatment with PEG or sucrose improves desiccation tolerance in germinating seedlings of A. thaliana and X. viscosa.
(A) Mean survival rate (±SEM) of A. thaliana seedlings after 48 h desiccation at various stages of germination. Although survival improves with PEG and sucrose treatment, A. thaliana seedlings are unable to survive desiccation past the “root hairs visible” seedling stage. (B) Survival of germinating X. viscosa seedlings with or without PEG or sucrose pre-treatment, grouped by cotyledon size (±SEM). Seedlings pre-treated with either sucrose or PEG did not show a decline in desiccation tolerance compared to untreated seedlings. (C) Chlorophyll loss in X. viscosa sucrose-treated seedlings. Although the mean survival rate of sucrose-treated seedlings was relatively consistent across most stages, the incidence of chlorophyll loss increased steadily with increasing seedling size. Seeds per bin can be found in Table S1.
Figure 5.
Pre-treatment of X. viscosa seedlings with PEG results in chlorophyll degradation.
Representative images of an untreated, PEG-treated and sucrose-treated seedling undergoing dehydration and rehydration. An untreated seedling (MS media) lost chlorophyll and accumulated anthocyanins in response to dehydration, and survived rehydration. A PEG treated seedling completely degraded its chlorophyll in response to PEG priming (prior to dehydration), and recovered well after rehydration. A sucrose-treated seedling failed to lose chlorophyll during dehydration; however, the meristem, primary leaf and a small portion of the cotyledon recovered after rehydration nonetheless.