Physiological Integration Increases Sexual Reproductive Performance of the Rhizomatous Grass Hierochloe glabra

Abstract

Clonal plants usually reproduce asexually through vegetative propagation and sexually by producing seeds. Physiological integration, the translocation of essential resources between ramets, usually improves vegetative reproduction. However, how physiological integration affects sexual reproduction has been less studied in clonal grasses. Here, we chose Hierochloe glabra, a major early spring forage of the eastern Eurasian steppe, and conducted a series of field experiments, including sampling reproductive ramets connected by tillering nodes to different numbers of vegetative ramets and ¹⁵N leaf labeling of ramet pairs at the seed-filling stage. In the natural populations of H. glabra, vegetative ramets were taller, had more and larger leaves, and greater biomass than reproductive ramets. Except for reproductive ramet biomass, sexual reproductive characteristics significantly increased with an increase in the number and biomass of vegetative ramets connected to tillering nodes. ¹⁵N labeling showed that vegetative ramets supplied nutrients to reproductive ramets through tillering nodes. Overall, our results indicate that significant differences in morphological characteristics and biomass allocation underlie resources translocation from vegetative ramets towards reproductive ramets. Physiological integration between different functional ramets can increase sexual reproductive performance, which will be beneficial to population persistence in H. glabra.

Keywords: companion species; perennial herb; resource translocation; sexual reproduction; tillering node; vegetative ramet.

Figure 1

Effects of the number of vegetative ramets connected to tillering nodes on sexual reproductive characteristics (seed number, (A); floret number, (B); seed-setting rate, (C); seed biomass, (D); panicle biomass, (E); ramet biomass, (F)) in the natural populations of Hierochloe glabra over two consecutive years (data are represented as the means ± standard errors, n = 25). Different lowercase letters indicate significant differences among different numbers of vegetative ramets (p < 0.05); ns represents that there is no significant difference between different numbers of connecting vegetative ramets (p > 0.05).

Figure 2

Relationships between sexual reproductive characteristics (seed number, (A); floret number, (B); seed-setting rate, (C); seed biomass, (D); panicle biomass, (E); ramet biomass, (F)) and leaf biomass of vegetative ramets connected to tillering nodes in the natural populations of Hierochloe glabra over two consecutive years. The colored lines represent fitting lines in 2018 (n = 75) and 2019 (n = 100).

Figure 3

Relationships between sexual reproductive characteristics (seed number, (A); floret number, (B); seed-setting rate, (C); seed biomass, (D); panicle biomass, (E); ramet biomass, (F)) and total biomass of vegetative ramets connected to tillering nodes in the natural populations of Hierochloe glabra over two consecutive years. The colored lines represent fitting lines in 2018 (n = 75) and 2019 (n = 100).

Figure 4

Comparison of the leaf δ¹⁵N, stem δ¹⁵N, and panicle δ¹⁵N of reproductive ramets between the control and ¹⁵N labeling treatments in the natural populations of Hierochloe glabra (data are represented as the means ± standard errors, n = 5). ***, p < 0.001.

Figure 5

Schematic of the experiment design. (A) Schematic representation of the tagging manipulation at the early heading stage of Hierochloe glabra (the Arabic numerals represent the number of vegetative ramets connected to the reproductive ramet by tillering nodes. In each gradient, the panicle top of the reproductive ramet reaches approximately 2 cm over the flag leaf sheath). (B) Schematic representation of the stable-isotope (¹⁵N) labeling experimental design at the seed-filling stage of Hierochloe glabra. Each ramet pair consists of one reproductive ramet and one vegetative ramet connected by a tillering node.