Role of adventitious roots in water relations of tamarack (Larix laricina) seedlings exposed to flooding

Abstract

BackgroundFlooding reduces supply of oxygen to the roots affecting plant water uptake. Some flooding-tolerant tree species including tamarack (Larix laricina (Du Roi) K. Koch) produce adventitious roots in response to flooding. These roots were reported to have higher hydraulic conductivity under flooding conditions compared with non-adventitious roots. In the present study, we examined structural and functional modifications in adventitious roots of tamarack seedlings to explain their flooding tolerance.ResultsSeedlings were subjected to the flooding treatment for six months, which resulted in an almost complete disintegration of the existing root system and its replacement with adventitious roots. We compared gas exchange parameters and water relations of flooded plants with the plants growing in well-drained soil and examined the root structures and root water transport properties. Although flooded seedlings had lower needle chlorophyll concentrations, their stomatal conductance, net photosynthesis rates and shoot water potentials were similar to non-flooded plants, indicative of flooding tolerance. Flooded adventitious roots had higher activation energy and a higher ratio of apoplastic to cell-to-cell water flow compared with non-flooded control roots as determined with the 1-hydroxypirene 3,6,8-trisulfonic acid apoplastic tracer dye. The adventitious roots in flooded plants also exhibited retarded xylem and endodermal development and accumulated numerous starch grains in the cortex. Microscopic examination of root sections treated with the PIP1 and PIP2 antibodies revealed high immunoreactivity in the cortex of non-flooded roots, as compared with flooded roots.ConclusionsStructural modifications of adventitious roots suggest increased contribution of apoplastic bypass to water flow. The reduced dependence of roots on the hypoxia-sensitive aquaporin-mediated water transport is likely among the main mechanisms allowing tamarack seedlings to maintain water balance and gas exchange under flooding conditions.

The Physiology of Adventitious Roots

“…Flooded tamarack seedlings possess fewer but longer adventitious roots compared with seedlings grown in air (Calvo-Polanco et al, 2012). Interestingly, the endodermis of these longer adventitious roots is poorly developed, cell walls are less suberized, tracheids possess a smaller diameter, and cortex cells are filled with starch (Calvo-Polanco et al, 2012).…”

Etiolation and flooding of donor plants enhance the capability of Arabidopsis explants to root

“…The replacement of the original root system with ARs from the stems in flooded plants has also been observed in other species such as Rumex palustris Sm. (Visser et al 1996), deepwater rice (Mergemann and Sauter 2000), the perennial wetland species Cotula coronopifolia and Meionectes brownii (Rich et al 2012) and in Larix laricina (tamarack) (Calvo-Polanco et al 2012). Thus, initiating organogenesis to replace the original root system with ARs seems to be an adaptive response to the stress situation and is related to ethylene accumulation in the stem (Visser et al 1996).…”

Effect of waterlogging on boreal forest tree seedlings during dormancy and early growing season

“…Root hydraulic conductance (water flux versus driving force, K r ) or conductivity (hydraulic conductance expressed on the root surface or volume basis, L p ) has been used widely for the estimation of water uptake capacity (Aroca et al 2012). The change of L p in hypoxic plants has also been linked to the radial water transport pathway of roots (Bramley et al 2010;Calvo-Polanco et al 2012). The reduction of L p under flooding stress has been attributed to resistance to transmembrane water flow resulting from cytosolic acidification and inhibition of aquaporin activity (Tournaire-Roux et al 2003).…”

“…Flooding did not change L p in lupins (Lupinus angustifolius L., Lupinus luteus L.) in which radial water transport is dominated by the apoplastic path, whereas it led to decreased L p in wheat (Triticum aestivum L.) in which the cell-to-cell path dominates radial water transport (Bramley et al 2010). The root hydraulic conductivity of adventitious roots in tamarack (Larix laricina (Du Roi) K. Koch) subjected to flooding stress was higher than non-adventitious roots, in which flooded adventitious roots was found to have a higher ratio of apoplastic to cell-tocell water flow (Calvo-Polanco et al 2012). Changes in K r and L p are related to root structure and anatomy.…”

“…The decreased L p of roots of pubescent birch seedlings caused by GW at week 8 may be explained by a temporary increase in fine root surface area. Some flood-tolerant species like tamarack are able to develop new adventitious roots with high L p values when exposed to flooded conditions (Islam et al 2003;Calvo-Polanco et al 2012). It seems that the L p of temporary fine roots is not very high, however, and their role may be greater for nutrient uptake.…”