Polyploidy and cellular mechanisms changing leaf size: comparison of diploid and autotetraploid populations in two species of Lolium

Abstract

Background and aims: Growth and development of plant organs, including leaves, depend on cell division and expansion. Leaf size is increased by greater cell ploidy, but the mechanism of this effect is poorly understood. Therefore, in this study, the role of cell division and expansion in the increase of leaf size caused by polyploidy was examined by comparing various cell parameters of the mesophyll layer of developing leaves of diploid and autotetraploid cultivars of two grass species, Lolium perenne and L. multiflorum.

Methods: Three cultivars of each ploidy level of both species were grown under pot conditions in a controlled growth chamber, and leaf elongation rate and the cell length profile at the leaf base were measured on six plants in each cultivar. Cell parameters related to division and elongation activities were calculated by a kinematic method.

Key results: Tetraploid cultivars had faster leaf elongation rates than did diploid cultivars in both species, resulting in longer leaves, mainly due to their longer mature cells. Epidermal and mesophyll cells differed 20-fold in length, but were both greater in the tetraploid cultivars of both species. The increase in cell length of the tetraploid cultivars was caused by a faster cell elongation rate, not by a longer period of cell elongation. There were no significant differences between cell division parameters, such as cell production rate and cell cycle time, in the diploid and tetraploid cultivars.

Conclusion: The results demonstrated clearly that polyploidy increases leaf size mainly by increasing the cell elongation rate, but not the duration of the period of elongation, and thus increases final cell size.

Fig. 1.

Diagram representing leaf elongation and changes in cell length profile in the leaf growth zone during a given time interval. The division and elongation-only zones of the growth zone can be identified by the spatial distribution of cell length. Under steady-state condition of cell production, the number of cells produced in the division zone during a given time (Δt) is equal to the number of cells leaving the elongation-only zone. Under this assumption, a kinematic method quantifies various cell parameters in the division and elongation-only zone (see Materials and Methods).

Fig. 2.

Increase in blade length of leaf 4 as a function of time from leaf appearance for diploid and tetraploid cultivars of Lolium multiflorum and L. perenne. The mean leaf length averaged over three cultivars (n = 18) was shown in each ploidy of the two species. p2, p4, m2 and m4 represent the diploid and tetraploid of L. perenne and of L. multiflorum, respectively. Bars are ± s.e.

Fig. 3.

Cell length profiles of epidermal cells (A and B), cell length profile of mesophyll cells (C and D) and profile of CVs of cell length (E and F) in diploid (circles) and tetraploid (triangles) cultivars of L. multiflorum and L. perenne. Open symbols are mesophyll cells and closed symbols are epidermal cells. Data of a representative cultivar of each group are presented (n = 6). Bars are ± s.e.

Fig. 4.

Relationship between leaf elongation rate and mature cell length. p2, p4, m2 and m4 represent diploid and tetraploid of L. perenne and of L. multiflorum, respectively (n = 18). Bars are ± s.e.

Fig. 5.

Relationship between mature cell length and cell elongation rate (n = 18). p2, p4, m2 and m4 represent the diploid and tetraploid of L. perenne and of L. multiflorum, respectively.

Fig. 6.

Profiles of leaf width (A), file width (B) and number of files (C) along the leaf growth zone in the diploid (open circles) and tetraploid (closed circles) of L. multiflorum and the diploid (open triangles) and tetraploid (closed triangles) of L. perenne. Bars are ± 1 s.e.