The Underwater Life of Secondarily Aquatic Plants: Some Problems and Solutions

Abstract

The freshwater secondarily aquatic plants, most of which are higher plants, are those returned to the water environment after spending a period of time living on land. The readaptation to living underwater has made it necessary for these plants to put in place morphological and functional strategies to cope with some major problems due to features of the aquatic environment, but also deriving from the specialized organization of their "terrestrial" bodies. The poor O 2 availability underwater accounted for the evolution of wide aerenchyma tissues throughout the plant organs to improve the photosynthetic O 2 flux from the shoot to the roots buried in anoxic sediments and to the neighboring rhizosphere. This favors sediment oxygenation, sustains the aerobic metabolism of roots, and improves the availability and uptake of mineral nutrients, whose delivery to the entire plants, without a transpirational flux, is ensured by an acropetal mass transport depending on root pressure, guttation from hydathodes and channeling by apoplast closure around the vascular tissues. A great expansion of leaf surfaces and an enhanced surface:volume ratio of chloroplast-rich photosynthetic cells help to contact the water medium and to increase the cell/environment exchanges to gain inorganic carbon. Furthermore, different physiological mechanisms operate to cope with the scarce availability of CO 2 and the prevalence of HCO 3 -as inorganic carbon form in water. Some of them, like cell wall acidification through H + extrusion by a light-dependent APTase or activation of an apoplastic carbonic anhydrase, operate outside the cells, leading to a conversion of HCO 3 -to CO 2 , which then diffuses into the cells. Others, on the contrary, act inside the cells to load the active site of Rubisco with CO 2 , thus favoring photosynthesis and lowering photorespiration. Aquatic macrophytes with isoetid life form, moreover, can obtain most ot the fixed CO 2 from sediments. In submerged species, in additin to the C 3 cycle, the C 4 and CAM-like photosynthetic metabolisms can also operate, and are modulated by the environmental inorganic carbon availability and the plant photosynthetic demand. Interestingly, in the aquatic plants the C 4 pathway, which can be concomitant with the C 3 one, does not depend on the Kranz anatomy of leaves, but relies on the intracellular compartmentation of carboxylative and decarboxylative enzymes. The CAM-like pathway, defined AAM, which also coexists with the C 3 , allows the submerged plants to fix CO 2 in the dark, thus exploiting the higher CO 2 availability in the water medium during the night, and extending to 24 h the period of inorganic carbon assimilation. In almost all the aquatic macrophytes the AAM is only expressed in the submersion state, whereas it is quickly inactivated in emerging leaves in a cell by cell way.

Predicting aquatic macrophyte occurrence in soft-water oligotrophic lakes (Pyrenees mountain range)

“…The isoetid I. lacustris grows at the lowest conductivity and pH values, and occurs within a very narrow range of water conductivity, featuring a specialized and very vulnerable species (Pip, 1988). I. lacustris behaves similarly throughout its biogeographical distribution range (Toivonen and Huttunen, 1995;Heegaard et al, 2001;Pulido et al, 2012b) and holds particular structural and physiological adaptations to ensure an efficient use of CO 2 for the photosynthesis (Rascio, 2002). At the upper limit of the conductivity range within the Pyrenean context, we encounter two Potamogeton species (P. alpinus and P. berchtoldii) and R. trichophyllus, which, as expected, also show a broader tolerance range.…”

IS Annona emarginata CAPABLE OF ACCUMULATE ESSENTIAL HEAVY METALS WITHOUT DAMAGES IN THE METABOLISM?

“…4.2.1.1). In this enzyme the Zn is the active site, holding prime importance in plants for carbon assimilation and photosynthesis (RASCIO, 2002). Hence Zn might play a very important role in protecting CA from ROS-mediated oxidation and modifications to its structural conformation, due to its association as a co-factor of the enzyme (ARAVIND; PRASAD, 2005).…”

Macrophyte-flow interactions in aquatic plants.

“…Another strategy used by plants to maximize photon capture under poor light conditions is to have longer branches and taller plants (associated with a decreased number of branches and below-ground biomass); this is also a physical response to flow (Strand and Weisner, 2001). The most basic morphological response however, is to increase the size of the leaf surface and the number of photosynthetic cells on that surface (Rascio, 2002).…”