Research by Tim Harris at the Institute of Botany of the Czech Academy of Sciences.
In temperate environments, many of the herbaceous plants are perennial. In contrast to trees and shrubs, winter conditions lead to the dieback of aboveground stems of herbs. But when favourable conditions return, perennial herbs can replace their shoots aboveground using stored resources. Perennial herbs can have specialised organs belowground that can facilitate this resprouting. Such organs include bulbs, tubers and rhizomes. In fact, more than half of the herbaceous plant species in central Europe have some sort of storage organ belowground. Our recent study has focused on the storage provided by rhizomes. Rhizomes are plant stems that, rather than growing towards the light, grow belowground and produce roots and shoots at their nodes. Rhizomes provide the plant with more than just stored energy – they also allow the plant to replicate vegetatively and spread in the soil. Our study has considered what might be driving differences between species in the size of their rhizomes. Thus, our main question was: what is driving differences in relative rhizome biomass?
To answer our question, we collected flowering plant individuals of 20 species with their whole connected rhizome system (some of these plants pictured above) and measured the biomass of rhizomes and aboveground plant parts – including leaves and shoots separately. Because we wanted to know whether the proportion of a plant’s biomass in its rhizome is related to its flowering phenology, we collected plants flowering at different times of the year. In March, before the leaves on the oak trees have emerged, we found early flowering woodland species, such as Anemone nemorosa and Pulmonaria obscura. By May the flowering season of these early species is finishing, but we found rhizomatous meadow species such as Alchemilla glaucescens.July marks the end of flowering for this meadow species, just as the late summer species are beginning to flower, including species such as Agrimonia eupatoria and Succisa pratensis.
When we analyzed investment by the plants into rhizomes in relation to aboveground parts, we found substantial differences between the herbaceous species in how much of their biomass they kept in their rhizome. For example, plants of Anemone nemorosa had rhizomes about ten times heavier than the weight of all leaves. At the other extreme, plants of Agrimonia eupatoria tended to have total leaf mass twice as heavy as their rhizomes. This spectrum also aligned with the different flowering phenologies of these species – species that flowered earliest allocated more of their biomass to rhizomes than species that flower later in the year (Figure 1). We think that having a substantial proportion of a plant’s biomass belowground in a rhizome allows a plant to flower when conditions are not optimal for photosynthesis. The idea of stored capital has led to this strategy being referred to as ‘capital breeding’. In contrast, we found that late flowering species had less of their biomass belowground in rhizomes and rely less on that storage and more on current photosynthesis to support their flowering, referred to as ‘income breeding’. Our study shows that the morphologies of plants in a seasonal climate reflects the need to time reproduction in relation to seasons favorable for obtaining new resources (summer) and seasons that are unfavorable (winter).
While meadows and forest floors may appear to us as lush, verdant places, they are environments that present several challenges to the perennial herbaceous plants, like mowing, grazing, shading by canopy, drought in summer, or frost in winter. There are many questions still to be answered about how plants establish and how they use their storage to persist in such challenging conditions, but our work indicates that plants that flower early in spring have relatively more of their biomass in rhizomes belowground than late flowering species.
Reference
Harris, T., A. Klimeš, J. Martínková, and J. Klimešová. 2023. Herbs are not just small plants: What biomass allocation to rhizomes tells us about differences between trees and herbs. American Journal of Botany 110(7): e16202. Online
Plant Life Belowground 