Επίδραση αζωτούχου και θειικής λίπανσης στην παραγωγή γλυκοσινολικών στους ιστούς της ρόκας (Eruca sativa) και του μπρόκολου (Brassica oleracea var. italica) και επιπτώσεις της ενσωμάτωσης των υπολειμμάτων του μπρόκολου και των γλυκοσινολικών στις μικροβιακές κοινότητες του εδάφους

Abstract

“Biofumigation” is an alternative method to chemical fumigants for controling soil borne pathogens and pests. The term is specifically used to describe the utilization of naturally occurring biotoxic isothiocyanates (ITC) that evolve as hydrolysis products from glucosinolate (GSL)-containing plants or plant products applied to soil. In Mediterranean countries, Brassica species are particularly important vegetables since they are major winter crops and are considered to be rich sources of health promoting substances. After harvesting, the plant residues may be incorporated into the soil. The “biofumigation” potential of these residues depends on various factors that affect the toxicity of the GSL hydrolysis products to soil microorganisms, such as the size of biomass incorporation, the GSL concentration and type and the environmental conditions, . GSL content and profile in Brassica species are influenced by plant factors, such as species and developmental stage, as well as by environmental conditions. The later include nutrient availability, especially nitrogen (N) and sulfur (S) supply. Sulfur is a key element for the GSL biosynthetic pathway, since it is assimilated by plants into organic sulfur-rich molecules as methionine and cysteine which are precursor molecules of GSLs. N is an essential element highly related with plant growth, protein-synthesis and agricultural production; moreover it is co-related with S in several biosynthetic processes within plant including GSL biosynthesis. The aims of this study was (1) to investigate the effects of S and N fertilization on the distribution of different GSL in different plant organs of Brassica species and varieties with emphasis to the plants broccoli (Brasica oeracea var. italica) and rocket (Eruca sativa) and (2) to examne the effects of the GSLs to the soil microbial community when they are introduced into the soil with the incorporation of the respective plant residues. The extend of S assimilation in GSL as a percentage of total plant-S was examined in Chapter 3. Moreover, different N and S fertilization doses were applied in order to examine the response of GSL concentration in two different Brassica species, broccoli (Brassica oleraceae var. Italica) and rocket (Eruca sativa M.). In addition a relationship between N-NO3 - and biomass production in terms of critical levels was established in both species in order to optimize N use (Chapter 4 and 5). The impact of the incorporation of broccoli residues as a “biofumigant” on the soil microbial community and activity was investigated in comparison with chemical fumigation (Chapter 6); whereas a more detail examination of ascomycetes and ammonia oxidizers community structure using molecular techniques was examined in Chapter 7. Finally, GSL dissipation in soils under two different %WHC regimes was examined using pure GSL and broccoli plant residues (Chapter 8). The results of the current study clearly showed that GSL profile and concentration depends on Brassica species, variety and plant organ. Moreover N and S supply rate had a significant impact on GSL concentration found in broccoli and rocket tissues. It was evident that N influenced the individual GSL profile in the various organs that were examined in both species whereas S had a pronounced effect on their concentration. Furthermore, besides N and S fertilization, plant developmental stage had a detrimental effect on GSL concentration. Therefore it was concluded that GSL profile and content of the plant organs, of these species could be optimized and regulated (increased) using appropriate fertilization schemes and harvesting times. However it was showed that the response of different Brassica species in N and S fertilization differs, therefore, specific research is needed for other species in order to optimize their GSL content through fertilization. Incorporation of broccoli residues in soil didn’t reduced microbial activity in comparison with conventional soil fumigation practices such as fumigation with metham sodium. Instead an increase of soil microbial activity was noticed. The differences that were observed after broccoli residues incorporation in the soil are not associated with the GSL content of the residues and their subsequent hydrolysis products (ITC). It seems that the impact of broccoli residues on the soil microbial activity and community is related with their general role in soils as carbon sources. The persistence of GSL in soil is very short and the increase of soil water content is negatively related with their persistence in the medium. This was noticed when they were applied either as pure substances or through incorporation of broccoli residues. Dissipation of pure GSL in soils seems to be related with microbial myrosinase activity or other GSL degradating enzymes (probably sulfatases); whereas plant derived GSL dissipation is more related with plantmyrosinase activity. In conclusion, this work indicates that the suppressive impact of broccoli residues to various soil borne pathogens, which is mentioned in literature, is not related with their GSL content. Incorporation of plant residues belonging to a different species than broccoli, with higher GSL content, could be more effective and this effect could perhaps be attributed in GSL content. Under these circumstances, optimization of plant GSL content through fertilization with N and S could be accomplished. Higher “biofumigation” potential could be also increased by incorporation of plant material at the appropriate developmental stage and by optimization of abiotic factors during incorporation that are involved in GSL hydrolysis and ITC production such as soil water content. High GSL containing pre-selected plant species, incorporated under high soil water conditions, could be combined with other alternative techniques such as soil solarization for the control of soil borne pathogens and pests. These issues will be the target of future research activities for the development, optimization and evaluation of sustainable agricultural practices.