Changes in gas exchange versus leaf solutes as a means to cope with summer drought in Eucalyptus marginata

Abstract

Two of the ways in which plants cope with water deficits are stomatal closure and "osmotic adjustment". We sought to assess the contributions of these processes to maintenance of leaf hydration in field-grown, 7-year-old Eucalyptus marginata. Plants were exposed to their normal summer drought (controls) or supplied with additional water (irrigated). Irrigation increased photosynthesis by 30% in E. marginata. These increases in photosynthesis were related to an 80% increase in g (s). However, there was no difference in substomatal CO(2) concentrations between treatments, or in chloroplast CO(2) concentrations, as indicated by carbon isotope composition of leaf soluble sugars. This suggests that impaired mesophyll metabolism may partially explain slower rates of photosynthesis in plants exposed to their normal summer drought. There was no difference in concentrations of solutes or osmotic potential between non-irrigated and irrigated individuals, perhaps because relative water content was the same in non-irrigated and irrigated plants due to stomatal sensitivity to water deficits. Irrespective of the absence of osmotic adjustment, analysis of leaf solutes gave a clear indication of the major groups of compounds responsible for maintaining cell osmotic potential. Soluble sugars were three times as abundant as amino acids. Proline, a putatively osmotically active amino acid, contributed less than 1% of total solutes. These patterns of solutes in E. marginata are consistent with a growing body of literature arguing a greater role for carbohydrates and cyclitols and lesser role for amino acids in maintaining osmotic potential. Our data suggest the primary mechanism by which E. marginata coped with drought was partial stomatal closure; however, we cannot discount the possibility of osmotic adjustment under more severe water deficits.

Metabolomics demonstrates divergent responses of two Eucalyptus species to water stress

“…The shift in carbohydrate partitioning is likely an adaptive response that helps plants cope with water stress via osmotic and non-osmotic mechanisms. For example, increases in fructose and glucose made a large contribution to osmotic adjustment (Merchant et al 2006b;Warren et al 2007;Foito et al 2009), while the increases in acyclic polyols are consistent with studies showing increases in acyclic polyols under water stress due to their non-osmotic protective functions (Karakas et al 1997;Shen et al 1999;Abebe et al 2003;Li and Li 2005). Increases in some of the key components of cell walls (Keegstra et al 1973), viz., four minor monosaccharides (xylose, arabinose, rhamnose) and two sugar acids (galactonic acid and galactaric acid) suggest that an additional way E. dumosa may have coped with water stress was by increasing synthesis or changing the composition of cell walls, as has been found in other species subjected to water stress (Joly and Zaerr 1987;Zwiazek 1991).…”

“…pinitol, ononitol) tend to increase under salt stress and lead to osmotic adjustment (Sanchez et al 2010). In contrast, in many Eucalyptus concentrations of proline are very small, do not necessarily increase under drought and never account for more than a few percent of total osmotic potential (Marsh and Adams 1995;Adams et al 2005;Merchant et al 2006b;Warren et al 2007). Instead, leaves of some (but not all) Eucalyptus contain large constitutive amounts of the cyclohexanepentol proto-quercitol Merchant et al 2006b;Arndt et al 2008), but increases in proto-quercitol due to drought tend to be small and thus its role (if any) in osmotic adjustment is unclear.…”

“…Instead, leaves of some (but not all) Eucalyptus contain large constitutive amounts of the cyclohexanepentol proto-quercitol Merchant et al 2006b;Arndt et al 2008), but increases in proto-quercitol due to drought tend to be small and thus its role (if any) in osmotic adjustment is unclear. Irrespective of whether a Eucalyptus species contains proto-quercitol, in Eucalyptus species the compounds most consistently associated with osmotic adjustment tend to be mono-and di-saccharides Merchant et al 2006b;Warren et al 2007). …”

“…Adams et al 2005;Merchant et al 2006b;Warren et al 2007). Here we build on this earlier literature by using a metabolomic approach to examine the response of leaf metabolites and water relations to a 2-month water stress treatment and subsequent re-watering.…”

Effect of Climate-Related Change in Vegetation on Leaf Litter Consumption and Energy Storage by Gammarus pulex from Continental or Mediterranean Populations

“…Nevertheless, the initial toughness of northern C. betulus leaves did not differ from that of southern O. carpinifolia leaves. O. carpinifolia leaves might have developed a different resistance strategy for desiccation, which does not entail tougher tissues, such as stomatal control, osmotic adjustment, or cell-wall storage [56]. Thus, our first hypothesis was partially validated, although leaf toughness might depend on the leaf species, despite literature showing that vegetation in a Mediterranean climate (lower mean annual precipitation and higher summer temperatures), such as the southern vegetation examined in this study, generally has more resistant tissues [29], [31].…”