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Abstract
Terrestrial plants may experience nutrient and oxygen stress when they are submerged, and increases in flooding are anticipated with climate change. It has been well reported that plants usually shift biomass allocation and produce more roots in response to nutrient deficiency. However, it is unclear whether plants experiencing oxygen deficiency stimulate biomass allocation to roots to enhance nutrient absorption, similar to how plants experiencing nutrient deficiency behave. We investigated the responses of the terrestrial species Alternanthera philoxeroides, upon root flooding, to nutrient versus dissolved oxygen deficiency in terms of plant growth, biomass allocation, root production, root efficiency (plant growth sustained per unit root surface area), and root aerenchyma formation. Both nutrient and dissolved oxygen deficiency hampered the growth of root-flooded plants. As expected, plants experiencing nutrient deficiency increased biomass allocation to roots and exhibited lower root efficiency; in contrast, plants experiencing dissolved oxygen deficiency decreased biomass allocation to roots but achieved higher root efficiency. The diameter of aerenchyma channels in roots were enlarged in plants experiencing dissolved oxygen deficiency but did not change in plants experiencing nutrient deficiency. The widening of aerenchyma channels in roots could have improved the oxygen status and thereby the nutrient absorption capability of roots in low oxygen environments, which might benefit the plants to tolerate flooding.
Highlights
Resource availability and disturbances have been regarded as the major factors driving plant functioning (Grime, 1979) and traits associated with these two factors define the range of strategies for plant growth and existence (Ackerly, 2004)
If we define root efficiency as the plant mass sustained per unit root surface area, we can hypothesize that plants living in nutrient-deficient environments have lower root efficiencies than plants growing in nutrient-rich environments
If the oxygen that the roots of a flooded plant need for absorbing nutrients is supplied by tissue photosynthesis or from air, it can be expected that the dissolved oxygen concentration in water will not influence the nutrient uptake of roots and plant growth will not be affected
Summary
Resource availability and disturbances have been regarded as the major factors driving plant functioning (Grime, 1979) and traits associated with these two factors define the range of strategies for plant growth and existence (Ackerly, 2004). If the oxygen that the roots of a flooded plant need for absorbing nutrients is supplied by tissue photosynthesis or from air (especially for waterlogged or partially submerged plants having aerial organs and structures above water), it can be expected that the dissolved oxygen concentration in water will not influence the nutrient uptake of roots and plant growth will not be affected. Taking Alternanthera philoxeroides (Mart.) Griseb., a floodingtolerant terrestrial plant (Gao et al, 2008) as a model, we studied the effects of nutrient as well as of dissolved oxygen availability on the growth, biomass allocation, and root traits of root-flooded plants. The following questions were addressed in this study: (1) Is the growth of root-flooded plants suppressed by nutrient and dissolved oxygen shortage? (2) Do root-flooded plants enhance their biomass allocation to root production under nutrient and dissolved oxygen shortage? (3) Is the root efficiency of plants decreased by shortage of nutrients as by dissolved oxygen? (4) Does the shortage of nutrients or of dissolved oxygen affect the formation and morphology of aerenchyma in roots ? The answers to these questions will expand and deepen our understanding of how plants respond to and tolerate flooding under different environmental regimes
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