The environmental adaptation strategy of seed germination, and roles of the seed pappus on dispersal and hypocotyl hairs on seedling anchorage in Tamarix ramosissima

Abstract

Seed dispersal, germination and seedling establishment are affected by various ecological factors in desert plant species. Tamarix ramosissima has evolved multiple strategies to facilitate its survival in harsh environments during the early stages of development. In this study, we investigated the effects of different ecological factors on seed germination and seedling growth, the function of the seed pappus in seed dispersal, as well as the function of the hypocotyl hairs in seedling establishment. We found that the seed germination of T. ramosissima was rapid and could occur under a wide range of temperatures (5-30 °C), after long periods of storage (at least 12 months on dispersal), under high concentrations of salts (700-900 mmol·L^-1) and polyethylene glycol (PEG) 6000 (500 g·L^-1) and under medium concentrations of alkalis (300-500 mmol·L^-1). Lower concentrations of salts and PEG promoted seedling growth. The seed pappus had no effect on seed germination, but it might function as an accessory structure that provides a buoyancy force and promotes long-distance seed dispersal. The hypocotyl hairs located on the edge of the hypocotyl end might aid the upright positioning of the seedlings during early development, especially when seed germination occurs under floating or flooding conditions. In conclusion, the germination of T. ramosissima seeds and seedling development can occur under diverse types of abiotic stress, and the seed pappus and hypocotyl hairs played an important role in seed dispersal and seedling establishment.

Keywords: Hypocotyl hairs; Tamarix ramosissima; seed dispersal; seed germination; seed pappus.

Figure 1.

The morphology and micro-structure of T. ramosissima seed. (A–C) Seed under stereomicroscope; (D–G) seed under scanning electron microscope. (A) Dry intact seed; (B) seed without pappus; (C) seed pappus; (D) seed without pappus; (E) seed coat surface; (F) the joint part of the pappus connected with seed; (G) the upper part of the pappus. The scale bar in (A, C) is 200 μm, in (B) is 100 μm, in (D, G) is 50 μm and in (E, F) is 20 μm.

Figure 2.

The effect of seed pappus on seed dispersal and germination. (A) Dispersal test of intact seeds indoors; (B) dispersal test of de-haired seeds indoors; (C) the falling speed of intact and de-haired seeds; (D) the GP of intact and de-haired seeds. In (A) and (B), different lowercase letters indicate significant difference (P < 0.05) of the seed number between different release heights within the same dispersal radius; in (C), different lowercase letters indicate significant difference (P < 0.05) of the falling speed between different release heights within the same seed type; in (D), the same lowercase letter represents no significant difference (P < 0.05) of the GP between intact and de-haired seeds. Values are means ± SE of four replicates.

Figure 3.

The morphological and structural changes of seeds in early germination of T. ramosissima. (A–H) The seed/seedling morphology at 1, 3, 12, 24, 34, 48, 72 and 120 h after the imbibition. The scale bar in (A–D) is 200 μm and in (E–H) is 100 μm.

Figure 4.

Effects of different conditions on seed germination of T. ramosissima. (A, B) temperature; (C) light quality. Wh, Re, Ye, Gr, Bl and Da represent the white, red, yellow, green, blue and dark light, respectively; (D) the storage time. J: January; F: February; M3: March; A4: April; M5: May; J6: June; J7: July; A8: August; S: September; O: October; N: November; D: December. RT: room temperature. 2020 and 2021 indicate the years of seed germination. In (A) and (C), the same lowercase letter indicates no significant difference (P < 0.05) of the GP between different temperatures or light qualities. Values are means ± SE of four replicates.

Figure 5.

Effects of salts, alkalis and PEG stress on seed germination of T. ramosissima. (A) NaCl (0, 100, 300, 500, 700, 900 mmol·L⁻¹); (B) Na₂SO₄ (0, 100, 300, 500, 700, 900 mmol·L⁻¹); (C) NaHCO₃ (0, 100, 300, 500, 700, 900 mmol·L⁻¹); (D) Na₂CO₃ (0, 100, 300, 500, 700, 900 mmol·L⁻¹); (E) PEG 6000 (0, 100, 200, 300, 400, 500 g·L⁻¹). Different lowercase letters indicate significant difference (P < 0.05) of the GP between different concentrations of salts or alkalis or PEG. Values are means ± SE of four replicates.

Figure 6.

Effects of different stresses on seedling growth of T. ramosissima. (A–F) The cotyledon width, hypocotyl length and root length under NaCl, Na₂SO₄, NaHCO₃, Na₂CO_3, PEG 6000 and light quality treatments. CW: cotyledon width; HL: hypocotyl length; RL: root length. The legend of (B–D) is the same as (A). *The value of zero. Different lowercase letters indicate significant differences between different concentrations of salts, alkalis and PEG (or different light qualities) of cotyledon width or hypocotyl length or root length (P < 0.05). Values are means ± SE of four replicates.

Figure 7.

Observations on the developmental structure of hypocotyl hairs of T. ramosissima. (A–D) Micro-structure at 12, 18, 24 and 30 h after imbibition; (A1–D1) the enlarged micro-structure corresponding to (A–D); (E–H) the ultra-microstructure under the scanning electron microscope corresponding to (A–D); (I, J) seeds germinated on different substrates. (I) filter paper; (J) fine sands; (K) ground perlite; (L) vermiculite. The scale bar in (A–D) and (I–L) is 100 μm and in (A1–H) is 50 μm.

Figure 8.

The proposed model of the seed dispersal of T. ramosissima.