Abstract
The carbon assimilated by photosynthesis in plants can be partitioned into starch, soluble sugars, and cell wall polymers. Higher levels of starch accumulation in leaves are usually correlated with a lower growth capacity. Duckweeds are fast-growing aquatic monocot plants that can accumulate high levels of starch. They are an unusual group because their cell wall has very low levels of lignin while accumulating apiogalacturonan, a pectic polysaccharide that could be involved with boron assimilation. In this work, five duckweed species from different genera (Spirodela polyrhiza, Landoltia punctata, Lemna gibba, Wolffiella caudata, and Wolffia borealis) were cultivated under two light intensities (20 and 500 μmoles of photons m−2 s−1) to evaluate the effects of growth rate on carbohydrate metabolism. A comparative analysis was performed by measuring their relative growth rates (RGR), and their content for starch, as well as soluble and cell wall carbohydrates. We found that the faster-growing species (the Lemnoideae) accumulate lower starch and higher soluble sugars than the slower-growing species within the Wolffioideae. Interestingly, analysis of the cell wall monosaccharides revealed that the slower-growing species displayed lower content of apiose in their walls. Our results indicate that higher accumulation of apiose observed in cell walls of the Lemnoideae species, which likely correlates with a higher proportion of apiogalacturonan, may lead to higher efficiency in the assimilation of boron. This is consistent with the increased RGR observed under conditions with higher apiose in the cell wall, such as higher light intensity. Consistent with their lower growth capacity, the Wolffioideae species we studied shows higher starch accumulation in comparison with the Lemnoideae species. We suggest that apiose levels could be good biomarkers for growth capacity of duckweeds and suggest that boron uptake could be an important factor for growth control in this aquatic plant family.
Highlights
Duckweeds are the smallest monocots and live as free-floating aquatic plants (Landolt, 1992; Appenroth et al, 2013)
Using either dry weight (RGR-D, Figure 2A) or frond number (RGR-F, Figure 2B) as the basis for relative growth rates (RGR) calculation, we found that the rates for the two Wolffioideae species (W. caudata and W. borealis) were significantly lower (0.1–0.14 day−1) than the other species of the Lemnoideae (0.14–0.24 day−1)
We found significant differences between Lemnoideae and Wolffioideae regarding growth, starch content and cell wall components
Summary
Duckweeds (the family Lemnaceae) are the smallest monocots and live as free-floating aquatic plants (Landolt, 1992; Appenroth et al, 2013). The 37 species of Lemnaceae have been classified into five genera (Spirodela, Landoltia, Lemna, Wolffiella, and Wolffia) based on their morphology and physiology (Borisjuk et al, 2015). They are further subdivided into two subfamilies, the Lemnoideae (Spirodela, Landoltia, and Lemna) and Wolffioideae (Wolffiella and Wolffia) (Les et al, 2002), the latter being the rootless duckweeds. Starch levels are found to be related to the nutrient status (Xiao et al, 2013). These authors found that growth of Landoltia punctata, Spirodela polyrhiza, and L. aequinoctialis is boosted with a concomitant decrease of starch under higher availability of P and N. Lower concentrations of starch can be a sign for higher growth rates and vice-versa
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