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3. Doktoritööd

Selle kollektsiooni püsiv URIhttp://hdl.handle.net/10492/2490

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Sirvi 3. Doktoritööd Märksõna "abiootiline stress" järgi

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    Effects of abiotic stress on foliage photosynthetic characteristics and volatile organic compound emissions in tropical agricultural species
    (Estonian University of Life Sciences, 2022) Okereke, Chikodinaka Nkechinyere; Niinemets, Ülo; Institute of Agricultural and Environmental Sciences; Fares, Silvano (opponent)
    Plant species emit a wide varieties of biogenic volatile organic compounds (BVOCs) when they are stressed. Plant stressors can be either biotic such as pathogens, viruses and insect infestations or abiotic such as ozone, high and low temperatures, drought and salinity. Furthermore, volatile organic compound emission is one of the major stress indicators that reveals the magnitude of stress and critical threshold in plants. In addition, photosynthetic characteristics - net assimilation rate, stomatal conductance to water vapour, intercellular CO2 concentration and changes in maximum quantum yield of photosystem II are altered during plant stress episode simultaneously with emission of volatiles. During stress episodes, plant BVOCs play a key role in plant protection by enhancing their ability to withstand stress thereby reducing cellular membrane damage. Extreme temperature is one of the major abiotic stressors negatively affecting plant physiological and biochemical characteristics leading to improper development, reduction in plant growth, yield and quality of crop. Hence, reduced plant productivity and overall yield as a result of high temperatures and heat waves poses as threat to plants in the tropical “hot” regions of the world. In addition, high temperatures lead to alteration in biosynthesis, oxidative stress and emission of BVOCs plants. In particular, negative influence of extreme temperatures is not limited only to global crop yield, but also to global atmospheric chemistry negatively affecting human existence and activities. Furthermore, wounding is a mechanical common mechanical injury on plants caused by wind, piercing or chewing insects, precipitation and falling debris. Wounding causes damage to leaf storage structures resulting in excess water loss, damage to cellular structures and rapid emission of BVOCs. Therefore, exposure of plant to heat and wounding treatments most likely would result in modification of plant major biosynthesis and physiological activities. Tropical agricultural species largely contributes to the world vegetation cover, yet they are underutilized and understudied. There is less information on how tropical plants tolerate and respond to abiotic stress, and the relationship between plant BVOCs emission and atmospheric chemistry. On that account, having a full knowledge and providing important insights on the effects of stress on various tropical plant species through modifications of their eco-physiological activities is necessary. AIMS OF THE STUDY The overall objective of the thesis were to: 1. Investigate the impacts of varying magnitudes of heat stress on foliage photosynthetic characteristics and emission of BVOCs in tropical crop species from stress application through various modifications and recovery in order to show species resistance to heat stress (Papers I, II). 2. Study the modifications of different BVOC groups upon heat stress from major plant biochemical pathways through volatile emission responses and stress recovery (Papers I, II). 3. Assess the effects of heat and wounding stress on leaf constitutive and stress-induced BVOC emission response upon stress through recovery (Papers I, II, III). 4. Study the various modifications in BVOC composition in the studied species in response to heat and wounding stress (Papers I, II, III). MAIN RESULTS In this thesis, a three model system was developed in our laboratory to demonstrate the effect of abiotic stresses on photosynthetic activities and secondary metabolism through recovery in tropical agricultural crop species.  In Amaranthus cruentus, Amaranthus hybridus, Solanum aethiopicum, Telfairia occidentalis and Vigna unguiculata leaves, severe heat stress of 49 °C led to species-specific modification in photosynthetic and BVOC emission rates, and a major inhibition in leaf net assimilation rate. Exposure of species to heat stress resulted in the elicitation of key volatile groups in individual species, hence, species heat resistance and physiological activity largely contributed in variations in volatile emission composition. Species possessing a greater heat stress resistance stand a better chance at photosynthetic recovery and a much lower degree of immediate stress response whereas, species with a lower heat stress resistance were more affected by heat stress exposure, thereby resulting to a greater emission of stress-induced BVOCs. Although the BVOC emissions were generally low, a greater species constitutive BVOC emission capacity was associated with a greater heat stress resistance (Paper I).  In leaves of Carica papaya, varying degrees of heat stress from mild to severe led to a decline in leaf photosynthetic traits with no recovery observed after severe heat treatment, moderate heat stress resulted in partial recovery and full recovery was attained after mild heat treatment. Leaf exposure to moderately severe heat treatment, resulted in enhanced emission of stress-induced volatiles especially oxygen-containing terpenes indicating increased leaf oxidative status. Severe heat stress in comparison with moderately severe heat stress, resulted in decreased BVOC emissions, indicating overall inhibition of leaf physiological activity. Different heat stress treatments resulted in modifications of volatile pool in C. papaya. (Paper II).  In Abelmoschus esculentus, Amaranthus cruentus, Amaranthus hybridus, Solanum aethiopicum and Telfairia occidentalis leaves, wounding stress (12-mm cuts) led in quick emission of major stress volatiles such as methanol (40% of total emissions), hexenal (24%), and acetaldehyde (11%). Hence, interspecific differences in emission rate and emission magnitude was at a great level. However, an inverse relationship scaled between emission strength and emission signal specificity due to negative relationships between LOC and terpenoids and benzenoid and LOX volatiles and terpenoids and benzenoid. Net assimilation rate and stomatal conductance recovered fully after wounding stress although, leaf foliage anatomical and chemical characteristics varied among species. Overall, emission oxygenated volatile organic compounds was more pronounced in the study, hence emissions tropical agricultural crop species are a major contributor of oxygenated volatiles in the atmospheric chemistry (Paper III).