Intra- and interspecific variation in salt tolerance of the sunflowers Helianthus annuus and H. paradoxus

Abstract

Environmental salinity is an abiotic stressor to most plants, and human activity is predicted to increase land salinization worldwide. Some ‘halophytic’ plants have evolved specialized mechanisms to tolerate high salinity. However, most plants (glycophytes) are limited in ability to tolerate salt. Little is known about intraspecific variation of salt tolerance in glycophytes. Here, I use a growth chamber experiment to investigate intraspecific variation in phenotypic traits of Helianthus annuus in response to salinity, and interspecific variation between H. annuus and related halophyte, H. paradoxus. H. annuus seeds were collected from saline areas around Great Salt Lake (Utah), from low-salinity areas in Utah, Colorado, and Nevada; and H. paradoxus seeds from New Mexico. Soil salinity data from 26 Great Salt Lake H. annuus population sites (10 of which were included in this experiment) were used to quantify environmental salinity associated with populations in this region. Juvenile plants received three salt treatments: control (non-saline), moderate (90 milliequivalent NaCl), and high salinity (180 mEQ NaCl). Plants were phenotyped for above- and below-ground biomass, root/shoot ratio, height and leaf growth rate, and leaf succulence. A subset of plants were analysed for leaf ion content. Moderate salinity treatment reduced the size and growth rate of H. annuus but not H. paradoxus, whereas high salinity reduced the size and growth rate of both species. Saline-origin H. annuus plants exhibited lower vigour but lower proportional decrease in above-ground biomass and leaf growth rate than non-saline-origin plants. Quantitative analysis revealed that environmental sodium-sulfur interaction predicts lower biomass, height growth rate, and leaf growth rate of H. annuus. H. annuus exhibits a salt-exclusion strategy, maintaining low leaf sodium, high potassium/sodium ratio, and high potassium-sodium selectivity. Meanwhile, H. paradoxus phenotype is divergent, with low biomass, low growth rate, high leaf succulence, and extreme accumulation of leaf sodium and sulfur. While plasticity of growth-defense trade-offs appears to account for most of H. annuus’ salt tolerance, low biomass plasticity of saline-origin populations may represent an evolutionary shift toward a stress-tolerant life history strategy. Salt tolerance appears to lack rapid evolvability in glycophytes, therefore salinization threatens plant ecosystems.