Effect of ploidy level on fruit morphology, seed germination and juvenile growth in scurvy grass (Cochlearia officinalis L. s.l., Brassicaceae)

Abstract

AbstractThe biennial to short‐lived wintergreen perennial Cochlearia officinalis L. s.l. (basic chromosome number x = 6) encompasses a group of closely related taxa that is difficult to delimitate morphologically. Despite a lack of distinct sets of discontinuous morphological characters, at least six numerically different cytotypes have commonly been recognized, and they have different ecologic distributions. Evolution of the wide ploidy level was likely brought about by hybridization and polyploidization. Because plants are short‐lived, seed germination was established and was likely to favor subsequent establishment. Therefore, shoot growth of juveniles was also compared. Five ecologically allopatric populations of C. officinalis s.l., each supposedly representing a different cytotype, were sampled for flowering shoots and seeds to establish shoot morphological traits, germination capabilities and chromosome number. The sampled populations differed in chromosome number: 2n = 2x = 12 (diploid); 2n = 4x = 24 (tetraploid); 2n = 33; 2n = 6x = 36 (hexaploid); and 2n = 8x = 48 (octaploid), representing the taxa C. pyrenaica DC, C. officinalis L. s.s., C. danica L. ×C. officinalis L. s.s., C. x hollandica Henrard and C. anglica L., respectively. All cytotypes produced abundant seeds and seed weight was proportional to the ploidy level. Seed weight of the diploid C. pyrenaica did not follow this correlation. Germination capacity immediately after seed collection was high over a wide temperature range. Germination was non‐photoblastic. High germination capability accentuated the short‐lived life‐cycles of the five cytotypes. Seed germination of the coastal cytotypes (C. x hollandica and C. anglica) were less negatively affected by NaCl compared to the non‐coastal cytotypes (C. pyrenaica and C. officinalis s.s.) and the very rare coastal hybrid C. danica×C. officinalis s.s. The cytotypes differed markedly in growth characteristics but there was no simple relationship between them. Increases in ploidy level were not paralleled by increased shoot growth. The hexaploid hybrid C. x hollandica indicated hybrid vigor in both shoot growth (yield) and petiole length of the strongly ascending leaves. Leaves of C. anglica were arranged in a prostratous rosette and the difference in shoot architecture between both C. x hollandica and C. anglica may be one of the reasons why this species is being replaced by the hexaploid hybrid, if sympatric, in coastal regions of The Netherlands. The 2n = 33‐hybrid C. danica×C. officinalis s.s. showed no such heterosis effects in shoot growth characteristics.