Abstract
Polyploid species generally occupy harsher habitats (characterized by cold, drought and/or high altitude) than diploids, but the converse was observed for Chamerion angustifolium, in which diploid plants generally inhabit higher altitudes than their polyploid derivatives. Plants at high altitudes may experience cold-induced water stress, and we therefore examined the physiological responses of diploid and hexaploid C. angustifolium to water stress to better understand the ecological differentiation of plants with different ploidy levels. We conducted a common garden experiment by subjecting seedlings of different ploidy levels to low, moderate, and severe water stress. Fourteen indicators of physiological fitness were measured, and the anatomical characteristics of the leaves of each cytotype were determined. Both cytotypes were influenced by drought, and diploids exhibited higher fitness in terms of constant root:shoot ratio (R:S ratio) and maximum quantum yield of PS II (Fv/Fm), less reduced maximal photosynthetic rate (Amax), transpiration rate (E), intercellular CO2 concentration (Ci) and stomatal conductance (gs), and higher long-term water use efficiency (WUEL) under severe water stress than did hexaploids. Analysis of leaf anatomy revealed morphological adjustments for tolerating water deficiency in diploids, in the form of closely packed mesophyll cells and small conduits in the midvein. Our results indicate that diploid C. angustifolium is more tolerant of drought than hexaploid plants, ensuring the successful survival of the diploid at high altitudes. This eco-physiological divergence may facilitate the species with different cytotypes to colonize new and large geographic ranges with heterogeneous environmental conditions.
Highlights
Polyploidy, the state of having more than two complete chromosome sets per nucleus, has played a key role in the evolution and diversification of the plant kingdom (Leitch and Bennett, 1997; Soltis et al, 2009)
The differences between cytotypes were significant for Total dry mass (TDM) accumulation and aboveground parts dry mass (ADM) accumulation under high water stress, and for root dry mass (RDM) accumulation under low water stress (Table 1)
TDM apparently declined in both cytotypes as the available soil water decreased, both ADM and RDM decreased in diploids, whereas hexaploids only showed a reduction in ADM
Summary
Polyploidy, the state of having more than two complete chromosome sets per nucleus, has played a key role in the evolution and diversification of the plant kingdom (Leitch and Bennett, 1997; Soltis et al, 2009). Polyploidization can be accompanied by considerable cytological, morphological, and physiological alterations, meaning that ecological requirements can differ significantly between diploids and their polyploid derivatives (Ramsey and Schemske, 2002; Soltis et al, 2014) This may result in different adaptations in the different cytotypes and in habitat segregation (Levin, 2004; te Beest et al, 2011), and it has long been hypothesized that polyploids may be able to occupy harsher environments relative to diploids because of the advantages of polyploidy (Grant, 1981; Levin, 1983). Many examples suggest that the frequency of diploids tends to increase with altitude (Hardy et al, 2000; Sonnleitner et al, 2010) or latitude (Ricca et al, 2008), indicating that diploids may be more tolerant of certain stressful conditions (Buggs and Pannell, 2007; Visser and Molofsky, 2015). These conflicting results suggest that polyploids may not necessarily occupy more extreme habitats than their diploid parents, but rather can be regarded as ‘fill-in’ taxa that occupy habitats which become available for them
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