Root and shoot traits associated with acidity and drought tolerance in Vicia faba L. plants

Background. Abiotic stresses affect the productivity and the evolutionary pathway of adaptation in crops in different agricultural regions. Soil acidity and drought are two major abiotic stresses, when severe, reduce the suitability of fertile lands for crop production, and when moderate, reduce yield and often quality. Faba bean (Vicia faba L.) is sensitive to acidity, aluminium toxicity and limitation of soil moisture, and these stresses greatly reduce the yield potential and stability of the crop. The overall objectives of this study were to investigate complementarity in shoot and root morphological and physiological phenotypic markers to acid soil and drought adaptation in pre-flowering faba bean plants, and to identify sources of tolerance for further breeding work. These objectives were tested in light of four hypotheses: acid zone germplasm would have higher acid and Al3+ tolerance index than other germplasm (publication I); dry-zone germplasm would have more prolific root systems than wet-zone germplasm (publication II); dry-zone germplasm would maintain its root system growth better in drought than wet-zone germplasm would (publication II); and drought avoidance is based on a combination of leaf gas exchange and exploitation of soil water (publication III). Materials and Methods. Multiple sets of faba bean accessions were chosen based on their expected exposure to acidity, aluminium, or drought stresses in their regions of provenance, and based on previous research data and reports. Experiments were established in aquaponic, peat and perlite media in controlled/environment growth chambers, greenhouses and a robotic phenotyping facility to evaluate the performance of a range of faba bean accessions in acid, aluminium, and watering treatments. Key root and shoot data were collected and analysed. Results and Discussion. Acidity and Al3+-toxicity treatments were sufficiently strong to initiate detectable variation in root length, stain score and Al3+ tolerance index, SPAD value, stomatal conductance, biomass and leaf area in solution culture, peat, and perlite experiments. Roots behaved differently in response to pH and Al3+ treatment differences. Al-tolerant accessions showed contrasting shoot Al content, indicating multiple Al tolerance mechanisms in faba bean. The results of acid tolerance index in aquaponic and perlite media experiments were positively correlated. Trait expression complementarity and variability were observed across the experiments owing to differences in growth media. Accessions differed in root regrowth length in solution culture and in SPAD values and taproot length in perlite medium to changing pH and Al3+ concentrations as shown by accession by treatment interactions. Root tolerance index, root regrowth length, and SPAD values were found to be largely informative traits in solution culture, and peat and perlite pot experiments. In aquaponics experiment, 41 µmol/l Al3+ was not informative, 82 µmol/l Al3+ was informative, 123 µmol/l Al3+ was severe. As a result, 82 µmol/l Al3+ was used in the next peat and perlite experiments. However, 82 µmol/l Al3+ was found to be less informative in peat experiment, hence 123 µmol/l Al3+ could be recommended for selection of outstanding accessions in solid media. Overall, accessions responded to acid and Al3+ treatments independently. Cultivars Aurora and Messay were found to be Al3+ tolerant but acid sensitive; Kassa and GLA 1103 acid tolerant, but Al3+-sensitive; NC 58 and Dosha were tolerant to both Al3+ and acidity, while Babylon was sensitive to both. Aquaponic media for mass screening and perlite media for verification experiments were found to be convenient (publication I). Screening of germplasm for drought was successfully conducted in a perlite-based pot experiment, which allowed quicker screening of a large set of materials and enabled detection of variation in constitutive traits among accessions. Use of the GROWSCREEN Rhizo phenotyping facility allowed detection of useful differences between treatments and among accessions. In both the screening and phenotyping drought experiments, accessions originating from the drier regions of the world showed drought avoidance behaviour thereby confirming FIGS as a valuable strategy (publication II and III). In germplasm screening, root and shoot dry mass and their fractions, along with SPAD value provided useful information in discriminating accessions with potential drought-avoidance characteristics. In the phenotyping experiment, root traits were strongly and positively correlated with each other and with shoot traits, but these correlations indicated specific plasticity of traits with watering treatments (publication III). In the well watered treatment, total dry mass was correlated with root length traits, whereas in the water-limited treatment, it was correlated with root width and convex hull area. Apparent root length density was positively correlated with second order lateral root length in the well watered treatment and with apparent specific root length in water limited treatment, indicating high surface area to volume ratio to maximize water absorption is a key strategy in droughted condition. In the water-limited treatment, root traits contributing to drought avoidance such as lateral root length and root system depth, convex hull area and root system width, and apparent root length density (publication II and III) were positively associated with shoot traits such as total dry mass, leaf number, and leaf mass fraction reported in publication III. Accession DS70622 exhibited deeper and wider growing roots that filled the root system volume with long and thin laterals. The larger root system combined with moderately high total dry mass and stomatal conductance endorsed this accession as a potential drought-avoiding candidate by effective use of water suitable in transient droughts. Accessions such as DS11320 and ILB938/2 that combined a large and thick root system with low root length density, low specific root length and low stomatal conductance can be recommended as potential sources of drought-avoiding traits by improved water use efficiency suitable in terminal droughts. Future research directions on the development of multiple abiotic stress tolerant cultivars enables climate change resilience in crops. Most acid soils are subject to Al3+-toxicity, and drought can occur on this soils as it occurs in others. Hence, multiple stress tolerance traits, mechanisms and QTLs need to be investigated in faba bean to identify host accessions with multiple tolerance to Al3+ and drought stresses for breeding of high yielding materials.

Kaksi suurinta abioottista rasitetta ovat maaperän happamuus ja kuivuus, jotka alentavat tuotantoa ja rajaavat maankäyttöä viljelyssä. Härkäpapu (Vicia faba L.) on herkkä maan happamuudelle, korkeille alumiini (Al) pitoisuuksille ja kuivuudelle. Tämän tutkimuksen tavoitteena oli löytää versojen ja juurten morfologisia ja fysiologisia piirteitä, jotka liittyvät härkäpavun kuivuuden ja happamuuden sietokykyyn ja tunnistaa parhaat lajikkeet jalostusta varten. Tätä varten valittiin useita lajikkeita Etiopiasta ja Euroopasta. Kokeissa kasvualustana käytettiin vesiviljelyä, turvetta ja perliittiä. Kokeet tehtiin kasvatuskammioissa ja kasvihuoneissa Helsingin yliopistolla ja Jülich Research Centre Saksassa. Juuret reagoivat eri tavalla pH- ja Al käsittelyihin. Al kestävillä lajikkeilla löytyi eri Al pitoisuuksia versoista, mikä viittaa useisiin Al sietokyvyn mekanismeihin. Juuren sietokyvyn indeksin, juurikasvun pituuden ja SPAD-arvojen havaittiin olevan suurelta osin informatiivisia piirteitä vesiviljelmässä sekä turve ja perliitti kokeissa. Vesiviljelykokeissa käytettiin 41, 82 ja 123 µmol/l Al. 42 µmol/l pitoisuus ei tuottanut tuloksia, 82 µmol/l taas tuotti eroja lajikkeissa ja 123 µmol/l vaikutus oli voimakas. Tämän takia 82 µmol/l käytettiin turve ja perliitti kokeissa. 82 µmol/l ei kuitenkaan tuottanut tuloksia turvekokeessa, joten 123 µmol/l tulee käyttää kiinteissä kasvualustoissa. Yleisesti lajikkeet reagoivat happamuuteen ja Al pitoisuuksiin erikseen. Aurora ja Messay lajikkeet kestivät Al, mutta olivat herkkiä maan happamuudelle. Kassa ja GLA 1103 kestivät happamuutta, mutta eivät Al. NC 58 ja Dosha kestivät molempia, kun taas Babylon ei kumpaakaan. Vesiviljely toimi lajikkeiden massaseulonnassa ja perliitti kasvualusta tulosten vahvistamiseksi. Lajikkeiden seulonta kuivuuden suhteen tehtiin perliitti kokeessa. GROWSCREEN Rhizo –fenotyyppi laitteistolla määritettiin hyödyllisiä eroja lajikkeissa. Lajike DS70622 kasvatti suuren juuriston. Lajike tuottaa myös kohtalaisen suuren biomassan, mikä tekee siitä hyvän kandidaatin jalostettaessa kuivuuden sietokykyä. Lajikkeet DS11320 ja ILB938/2, joissa yhdistyy suuri juuristo, vähäinen hienojen juurien määrä ja alhainen ilmarakojen johtavuus, soveltuvat myös hyvin kuivuuden sietokyvyn jalostamiseen. Happamassa maassa on usein myös korkeat Al pitoisuudet, ja kuivuus voi tapahtua kaikkialla. Täten härkäpavun jalostamisessa pitäisi luoda lajikkeita joilla on sietokykyä useille abioottisille rasitteille.

ei saavutettava