Nicole Geissler

Elevated atmospheric CO2 concentration ameliorates effects of NaCl salinity on photosynthesis and leaf structure of Aster tripolium L.

This study investigated the interaction of NaCl-salinity and elevated atmospheric CO2 concentration on gas exchange, leaf pigment composition, and leaf ultrastructure of the potential cash crop halophyte Aster tripolium. The plants were irrigated with five different salinity levels (0, 25, 50, 75, 100% seawater salinity) under ambient and elevated (520 ppm) CO2. Under saline conditions (ambient CO2) stomatal and mesophyll resistance increased, leading to a significant decrease in photosynthesis and water use efficiency (WUE) and to an increase in oxidative stress. The latter was indicated by dilations of the thylakoid membranes and an increase in superoxide dismutase (SOD) activity. Oxidative stress could be counteracted by thicker epidermal cell walls of the leaves, a thicker cuticle, a reduced chlorophyll content, an increase in the chlorophyll a/b ratio and a transient decline of the photosynthetic efficiency. Elevated CO2 led to a significant increase in photosynthesis and WUE. The improved water and energy supply was used to increase the investment in mechanisms reducing water loss and oxidative stress (thicker cell walls and cuticles, a higher chlorophyll and carotenoid content, higher SOD activity), resulting in more intact thylakoids. As these mechanisms can improve survival under salinity, A. tripolium seems to be a promising cash crop halophyte which can help in desalinizing and reclaiming degraded land.

Abiotic Stress Responses in Plants: An Overview

Plants are more and more affected by environmental stresses, especially by the devastating consequences of desertification and water scarcity which can be seen and felt all over the world. About 3.6 billion of the world’s 5.2 billion hectares of dryland used for agriculture have already suffered erosion, soil degradation, and salinization. Desertification can hinder efforts for sustainable development and introduces new threats to human health, ecosystems, and national economies. This problem is catalyzed by global climate change which exacerbates desertification and salinization. Therefore, solutions are desperately needed, such as the improvement of drought and salinity tolerance of crops, which in turn requires a detailed knowledge about tolerance mechanisms in plants. These mechanisms comprise a wide range of responses on molecular, cellular, and whole plant levels, which include amongst others the synthesis of compatible solutes/osmolytes and radical scavenging mechanisms. Regarding global change, elevated atmospheric CO2 concentrations can enhance salt and drought tolerance because oxidative stress is alleviated and more energy can be provided for energy-dependent tolerance mechanisms such as the synthesis of compatible solutes and antioxidants, thus increasing the suitability of plants as crops in future. A detailed knowledge of the physiological and biochemical basis of drought and salt tolerance and its interaction with elevated CO2 concentration can provide a basis for the cultivation of suitable plants in regions threatened by desertification and water scarcity under sustainable culture conditions. Even the drylands could offer tangible economic and ecological opportunities.

Effects of Salt Stress on Photosynthesis Under Ambient and Elevated Atmospheric CO2 Concentration

Soil salinization is one of the most important factors which limit plant productivity. About 3.6 billion of the world’s 5.2 billion hectares of dryland used for agriculture have already suffered erosion, soil degradation, and salinization. Global climate change caused by rising atmospheric trace gases such as CO2 and forced migration add to the urgency of this global problem. Therefore, solutions are desperately needed, such as the improvement of drought and salinity resistance of crops or the use of (xero-) halophytes instead of glycophytic crops. As photosynthesis is a prerequisite for biomass production, this chapter focuses on information related to this essential sequence of reactions, thereby discussing the different levels of photosynthesis. At first, there are primary reactions of photosynthesis, namely absorption of light energy and (1) its conversion to redox energy, conserved in the coenzyme NADPH, and (2) energy of chemical bounds, conserved in the coenzyme ATP. On the second level, we find reactions of the Calvin cycle, nitrate and sulfate reduction as well as sugar, lipid, and amino acid metabolism. Typical reactions on the third level are transmembrane and inter tissues transport of metabolites. The fourth level of photosynthesis relates to physiological aspects of gas exchange and water relations.

Survival at Extreme Locations: Life Strategies of Halophytes

Seven percent of the lands surface and fi ve percent of cultivated lands are affected by salinity. There are often not suffi cient reservoirs of freshwater available and most of the agronomically used irrigation systems are leading to a permanent increase in the soil-salinity and step by step to growth conditions in-acceptable for most of the conventional crops. Signifi cant areas are becoming unusable each year. Although it is a world-wide problem, most acute is in Australasia, the Near East and Africa, North and Latin America and to an increasing degree also in Europe. This large extent of salinity problem reduces crop productivity. In contrast to crop plants, there exist specialists that thrive in the saline environments along the sea shore, in estuaries and saline deserts. These plants, called halo-phytes, have distinct physiological and anatomical adaptations to counter the dual hazards of water defi cit and ion toxicity. The sustainable use of halophytic plants is a promising approach to valorize strongly salinised zones unsuitable for conventional agriculture and mediocre waters. There are already many halophytic species used for economic interests (human food fodder) or ecological reasons (soil desalinisation, dune fi xation, CO2-sequestration). However, the wide span of halophyte utilisation is not jet explored even to a small degree. For economic utilisation of potential halophytes ecological studies should be complemented with comparative physiological studies about salinity tolerance in halophytes are essential.

Cash Crop Halophytes: The Ecologically and Economically Sustainable Use of Naturally Salt-Resistant Plants in the Context of Global Changes

Global climate change—caused by rising atmospheric concentrations of trace gases such as CO2—brings about increased desertification, soil salinisation and degradation, which will lead to unhealthy living conditions. There will be a shortage of freshwater, especially in arid regions. Furthermore, artificial irrigation with saline water in an unprofessional manner leads to an increasing salinisation of usable areas and to economic damage. Increased soil salinity poses a serious threat to agriculture because most conventional crops are salt-sensitive glycophytes. A promising solution to these problems is the sustainable cultivation of halophytes (naturally salt-resistant plants) on salt-affected soils under saline irrigation.