Forschung

Photosynthesis is a highly coordinated process in plants which provides the plant cells with metabolic energy and carbon molecule and generates oxygen as a byproduct.
Photosynthesis takes place in the chloroplast. Inside the chloroplasts is an internal system of interconnected membranes, the thylakoids. On these thylakoid membranes, light harvesting complexes which consist of pigments such as chlorophyll and proteins of the photosystem I and II, are bound. The pigments absorb the incident light, whose energy drives an electron transport through the photosystems which induces the synthesis of the energy-rich molecules ATP and NADPH. Part of the energy is used for the fixation of CO₂ by the enzyme Rubisco which takes place in the stroma and generates sugar. The processes of photosynthesis are affected by numerous environmental conditions such as nutrient and water availability. Under insufficient and excess nutrient and water supply, the CO₂ assimilation rate decreases and the photosynthetic capacity is limited. For instance, under drought conditions the stomata aperture is reduced restricting CO₂ diffusion into the internal leaf spaces and thereby CO₂ supply to Rubisco.
Understanding the responses in the photosynthetic capacity to environmental stress factors, to mainly Mg and K deficiency and drought, is one of the main research focuses at IAPN. In order to assess the photosynthetic capacity, measurements of leaf gas exchange and chlorophyll fluorescence are used. The latter is a measure of re-emitted light from photosystem II as the absorbed light energy in the chlorophyll molecules does not only drive photosynthetic processes but is partly re-emitted as fluorescence. Hence, measurements of chlorophyll fluorescence give valuable information about the efficiency of photochemistry and thereby about the plants' productivity. Leaf gas exchange measurements assess the CO₂ influx from the atmosphere into the leaf by infra-red gas analysers. Both techniques, chlorophyll fluorescence and leaf gas exchange, are non-invasive, hence allowing in vivo observations in plants which are useful in monitoring the photosynthetic alterations under ongoing stress situations.

Unfavorable environmental conditions such as nutrient deficiency, lead to restrictions in the photosynthetic capacity. Under this condition, the light absorption in the light harvesting complexes of the photosystems is excessive, i.e. the available energy cannot be consumed within the photosynthetic processes. Therefore, the excessive energy might react with ground state molecular oxygen which leads to production of reactive oxygen species inside the photosynthetic machinery. Reactive oxygen species, mainly singlet oxygen and hydrogen peroxide, mediate photodamage in the photosystems. When excess energy conditions persist over a prolonged time period, the photosynthetic apparatus is destroyed. This might lead to photoinhibition which is a sustained reduction of the photosynthetic capacity.
To prevent photoinhibition, photoprotective mechanisms have evolved in plants. These mechanisms comprise movement of leaves and chloroplasts, photorespiration, cyclic electron flow around photosystem I, screening of photoradiation, scavenging of reactive oxygen species, and thermal energy dissipation of absorbed light energy. The latter two mechanisms are in focus of the research. By chlorophyll fluorescence, the thermal energy dissipation of absorbed light energy can be assessed as non-photochemical quenching (NPQ). NPQ operates in the photosystem II on thylakoid membranes in chloroplasts and involves the pigments of xanthophyll cycle and the protein PsbS, which acts as a pH sensor. The scavenging of reactive oxygen species not only takes place in the chloroplasts, but also in peroxisomes and mitochondria of plant cells. The reactive oxygen species are converted to non- or less-toxic molecules by enzymes such as superoxide dismutase, ascorbate peroxidase, glutathione reductase and catalase. Evaluation of the scavenging capacity in plants is achieved by in vitro analysis of the enzyme activities by photospectrometry, and transcript abundance of the respective gene by molecular methods.
The plant nutrient Mg plays crucial roles in photosynthetic processes. Being the central atom in chlorophyll molecules, it contributes to light harvesting. The CO₂ fixation is strictly dependent on the presence of Mg as it activates the enzyme Rubisco. Furthermore, Mg is inevitable for ultrastructure formation of chloroplasts which is necessary for efficient functioning of the light reactions in the photosynthesis. Besides these functions, Mg is involved in many more cellular functions like enzyme activation and energy metabolism. Plants with Mg deficiency suffer photooxidative stress as light energy reaching the photosystems is excessive and photoprotective mechanisms are enhanced. The research aims at understanding how the photosynthetic processes are affected at a range of Mg concentrations and corresponding how photoprotection is affected by Mg supply.

Water shortage affects yield considerably depending upon timing and severity of stress. However, the water relations at the whole plant and canopy levels and the way plants respond to water stress by distributing water, and consequently nutrients, between its different organs at specific times of plant development is not fully understood. This is more relevant as terminal drought is the most detrimental to yield production, and a greater understanding of the impact of terminal drought may help in developing more drought resistant cultivars, thus with higher water-use efficiency.
Water-use efficiency can be considered a measure for the efficiency in optimizing carbon assimilation while minimizing the water use and is generally described by the ratio of assimilated carbon or biomass per unit of water loss via transpiration. Stomata play a crucial role in regulating the water-use efficiency as they determine the rate of water loss by transpiration and CO₂ uptake for photosynthesis and thus, plant growth. Water-use efficiency can be assessed on different temporal and spatial scales, i.e. from seconds to years and from small leaf areas on a single plant to whole ecosystems. Research at IAPN focuses on the evaluation of water-use efficiency on a leaf and plant scale by measurements of photosynthetic leaf gas exchange, hence assessing the carbon and water fluxes in the air-leaf-interphase. Water-use efficiency is very dynamic as both photosynthesis and transpirational processes react to environmental and plant internal responses and thereby ultimately affect water-use efficiency. Transpirational processes are affected by water relations within different plant parts and crop canopies. In this respect, the non-destructive and continuous monitoring of water relations in different plant parts can complement measurements of transpiration and provide insight into how plants respond to their environment and water-use efficiency is affected.
The research conducted at IAPN on this field aims to investigate the hydraulic mechanisms used in complex spatial and temporal dynamics of plant water relations, in particular plant responses to drought stress, across multiple scales from within-leaf to canopy level using the leaf patch clamp pressure probe (LPCP). This non-invasive method allows continuous monitoring of cell turgor (thus plant water status) for extensive periods of time and comparing it with changes in environmental conditions. The patch clamp pressure output is inversely related to the leaf patch turgor pressure, Pc, which in its turn relates to the cell water content. In this way, it is possible to follow the dehydration/rehydration kinetics, i.e. the internal redistribution of water, over diel scales and in response to treatments, e.g. through closure of stomata at midday or in response to drought, light intensity or CO₂. Additionally, comparison between different plant species, genotypes or parts of the plant could also identify different mechanisms of tolerance to water stress.

Remote sensing generally refers to the use of satellite- or aircraft-based sensor technologies to detect and classify objects on Earth. When applied to agriculture or forestry, it makes use of the optical properties of leaves, through the interaction between light and the different components in plant tissues, e.g. pigments, water, cellulose, proteins.
Plant development and physiology, including how plants react to nutrient deficiencies, diseases, water stress, or environmental changes, can be expressed by changes in how these components interact with light, i.e. leaf optical properties. By determining at which wavelength and how much of the incident light is absorbed while reflecting the remaining spectrum, it is possible to assess several physiological and anatomical parameters and thus evaluate plant health. To this end, remote sensing methodology, including field spectrometry and satellite imagery analysis, is used and compared with parallel field sampling data collection. Apart from its non-invasive aspect, which allows in vivo observations of plants, this methodology is particularly useful as it enables continuous monitoring of plant development with time with high spatial resolution. Therefore, it is possible to follow the whole life cycle of crops completely.
Early detection of nutrient deficiencies is one of the main focuses of research at IAPN and, particularly, special attention is given to the role of magnesium (Mg²⁺) in plant physiological processes.
Being one of the plant essential macronutrients, magnesium participates in many physiological processes by being a cofactor of many enzymes involved in respiration, membrane transport, photosynthesis, and the synthesis of DNA and RNA, a structural stabilizer of several nucleotides, and part of the ring structure of the chlorophyll molecule. Its deficiency in crops can affect plant biomass and yield formation severely. Under magnesium deficiency, due to the mobility of magnesium in the phloem and its recycling from chlorophyll degradation, the chlorophyll in the vascular bundles remains unaffected for longer periods than the chlorophyll in the cells between the bundles. This situation causes the typical intervenous pattern of chlorosis that occurs first in older leaves, leading to senescence and premature abscission eventually. However, similarly to other nutrient deficiencies, when plants display visible symptoms or even before they appear, plant growth and development has already been severely compromised, thus decreasing yield considerably. Therefore, the presymptomatic and non-invasive detection of magnesium deficiencies in plants, e.g. through remote sensing, is crucial for crop monitoring and to alleviate stress at early stages of plant development, thus avoiding irreversible damage and mitigate yield losses substantially.

As population increases worldwide, the arable land per caput also decreases, while more land is becoming degraded. It is estimated that about 15% of the total land area of the world has been degraded by soil erosion and physical and chemical degradation, including soil salinization. Drought and salt stresses are important abiotic factors which are the more severe and wide-ranging environmental stresses that significantly affect crop growth and productivity.
Salt stress is the accumulation of excessive salts in the soil which eventually results in the inhibition of plant growth (phase I) and leads to plant death (phase II), decreasing agricultural productivity. In phase I, the availability of water for plants decreases due to negative water potential in the rhizosphere inhibiting plant growth. In phase II, the ion toxicity takes place when the exclusion mechanisms and internal compartmentalization processes are overloaded. Salt-affected soils occur in all continents and under almost all climatic conditions. Worldwide, the major factor in the development of saline soils is the lack of precipitation. Most salt-affected soils are found in the arid and semiarid regions compared to the humid regions. Drought stress is another major abiotic stress, which adversely affects crop growth and productivity. Agricultural drought stress is imposed when the required amount of moisture is not provided to the plant for its growth and development. Water scarcity, mainly due to the shortage of rain which is often erratic and poorly-distributed, heavier rainfalls leading to lower storage capacity and improper water resource management practices will increase droughts. These abiotic stresses affect plants in multiple ways such as nutritional disorders, alteration of metabolic processes, synthesis of photosynthetic pigments, oxidative stress, reduction of cell division and expansion.
Due to water scarcity and salinization, new cultivation techniques and the use of other potential crops to exploit areas that are not suitable for traditional crops are necessary. Halophytes play an important role as they are salt-tolerant plants and can withstand water stress. The halophyte Chenopodium quinoa is a very important crop due to its edible seeds and its ability to grow in highly saline environments. It has been selected by the Food and Agriculture Organization of the United Nations (FAO) as one of the crops destined to offer food security in the century.
At the IAPN we are conducting research with C. quinoa working with different salinities, and the importance of potassium (K⁺) and magnesium (Mg²⁺) in abiotic stress is being investigated. K⁺ and Mg²⁺ are cations that can compete with sodium (Na⁺), the latter being in high concentrations in saline soils. Since Na⁺ interferes with K⁺ homeostasis, particularly given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na⁺/K⁺ ratio has become a key mechanism for salinity tolerance. The aim of this work is to determine the effect of nutrients, mainly K⁺ and Mg²⁺, on plant growth and different physiological parameters under saline and non-saline conditions.