Harnessing the biodiversity of Italian Tamarix species: populations, plants and leaf responses to extreme environmental constraints
Author(s)
Abou Jaoudé, Renée
Date Issued
March 10, 2011
Type
Doctoral Thesis
Abstract
Mediterranean countries are expected to experience a pronounced increase in the average air temperatures and an alteration of the precipitation patterns (distribution, intensity and duration) as a result of the global climate change, which might damage vegetation and soil structure. In this region, wetlands and coastal areas are increasingly at risk, as they are particularly exposed to a range of hazards connected to climate change, such as drought, flooding, and salinization caused by the increase in the average seawater level. In these areas, it is possible to find plants adapted to survive under extreme environmental fluctuations, such as Tamarix spp. This genus is composed of about ninety species, part of which are naturally distributed in the Mediterranean region, and are characterised by high endurance of adversities. In Italy, Tamarix species are eleven, and occupy coastal dunes and on riverbanks of Southern regions. The most widespread species are Tamarix gallica and Tamarix africana. Although they play a fundamental ecological role in dunes fixation and in inhabiting salinized areas that would otherwise be subjected to desertification, their distribution, ecology and physiology are not well known. The knowledge of the specific and intra-specific diversity of any endangered species is the first step for conservation practices, as some of them may be selected for their natural tolerance to a particular stress, and may be conserved and used in the future for the restoration of damaged ecosystems in which these species are already present. Here, we present the results of an overall study which aims to describe natural Italian Tamarix spp. populations and their structure, and to characterise plant and leaf responses under natural and stress-induced conditions, with particular regard to extreme events like flooding and water salinity.
Populations and plants structure significantly differed according to these conditions. Particularly, plants growth and dimensions seemed to be lower in dune stands, where a high soil or water NaCl concentration can limit plants photosynthesis and energy production, as also demonstrated by the gas exchanges experiments. Among the populations distributed in the same environment, the highest plants growth and age were related to lower soil NaCl salinity and/or to adequate and constant fresh water availability. Moreover, although Tamarix gallica seems to be more frequent in the dune environment and more tolerant to salinity compared to Tamarix africana, probably as a result of its capacity to extrude more salt at increasing NaCl concentration, its gas exchanges capacity is also reduced under high saline conditions (200-400 mM), which could occur in particularly dry periods (summer) leading to an increased salt concentration in the soil solution. Salinity also seems to differently affect provenances, although the analysed plant material was collected from sites characterised by a similar salinity. Moreover, salinity tolerance depends on leaf age, older leaves being characterised by a higher tolerance compared to young leaves. Gas exchanges reduction under saline conditions (200 mM) is primary due to non stomatal limitations, independently of flooding, and particularly to a decreased RuBisCO activity, probably caused by the increase in leaf salt accumulation over a tolerated threshold, which induces a reduction of the net assimilation rates. At higher salinity, photosystems damage is observed, coupled with an increase in salt extrusion, and with a reduction of stomatal conductance probably due to (1) a direct harmful effect of Na+ on guard cells functioning, (2) a top-down regulation mechanism to prevent additional salt to be transport through the transpiration stream, (3) a signal transduction pathway which causes an increase in the cytosolic concentration of Ca2+, acting in a similar way to an increase in abscisic acid, (4) the feed-back controls on stomata in response to the accumulation of intermediates or products of the Calvin cycle, (5) an increase in intercellular CO2 concentration as a result of photosynthesis inhibition, (6) a lower efficient in water transport of the vessels characterised by a higher cavitation safety, (7) an hygroscopic salt effect which may trap the water vapour lost by transpiration, leading to an alteration of the measure, or (8) a physical obstruction of stomata by salt particles accumulation, which reduces gas
exchanges. Tamarix spp. are very tolerant to continuous flooding. Particularly, photosynthesis did not decrease in plants subjected to this treatment. This tolerance is the result of adventitious roots emissions, which enable gases (particularly oxygen) to be transport to/from the roots from/to the atmosphere. Thus, complete or partial continuous soil submergence expected in the future years could probably not affect Tamarix growth.
These results, coupled with the high rates of transpiration due to a constant stomatal aperture, even during the night, suggest a positive role of Tamarix spp. in the restoration of areas which could be affected by flooding, where these species are already naturally distributed. Tamarix spp. could also be used for the desalinisation of salinized areas, as about 90% of the salt present in the soil solution is extruded through the leaf salt glands. However, the desalinisation capacity of Tamarix spp. could be more rapid under non-flooded conditions, as flooding significantly reduced salt extrusion through salt glands, probably as a result of the reduced ions absorption by the adventitious roots; moreover, soil desiccation would also be reduced as salinity causes a decrease in stomatal conductance.
Additional information
Dottorato di ricerca in Ecologia forestale
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