Institut für Tropische Agrarwissenschaften (Hans-Ruthenberg-Institut)

Permanent URI for this collectionhttps://hohpublica.uni-hohenheim.de/handle/123456789/21

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Drought affects the synchrony of aboveground and belowground phenology in tropical potato
(2023) Hoelle, Julia; Khan, Awais; Asch, Folkard
The literature describes the belowground and aboveground phenology of potato to be linearly related. Bud formation is synchronous with tuber initiation and flowering with tuber filling. Many agronomic and breeding studies on potato use non‐destructive aboveground phenology to assess belowground development. No information is currently available on the influence of water deficit on the synchrony of above‐ and belowground development in potato. Five contrasting potato genotypes were subjected to four irrigation treatments on two different soil types. The irrigation treatments were as follows: fully watered, early drought, intermediate drought, and late drought. In 5‐day intervals after withholding water, detailed belowground and aboveground development was recorded. Results showed that the synchrony between aboveground and belowground development is strongly influenced by both water deficit and development stage at drought initiation. Under early drought, the aboveground development was hastened and belowground development was delayed. The opposite was found in later development stages. The earlier the drought was initiated, the longer the tuber filling phase was, while the bulking phase was shortened. We concluded that under terminal drought conditions aboveground development and belowground development need to be evaluated separately and cannot follow the standard evaluation system that uses aboveground phenology as a proxy for tuber formation belowground development rates.
Genotype specific responses to Bacillus spp. inoculation in lowland rice (Oryza sativa L.) under iron toxicity
(2023) Weinand, Tanja; Asch, Folkard
Amidst a growing global population, limited arable land, and higher pressure from both abiotic and biotic stressors in a shifting climate, there is a need for enhancing yields through sustainable agricultural practices, and new, more tolerant cultivars. In recent decades, employing microbial inoculants as biofertilizers and biopesticides has gained growing popularity. Yield reductions ranging from 16-78%, and sometimes complete crop failure, can occur in in lowland rice cultivation systems where high iron concentrations in the soil solution lead to excess iron uptake by the plants. Twenty to 60% of the rice growing area of sub-Saharan Africa is affected by iron toxicity. Development of iron-tolerant cultivars has lagged, largely due to gaps in understanding the genotypic adaptation mechanisms to this stress. Furthermore, effects of the microbiome on such stress responses are often overlooked. Although there have been previous reports on growth promoting effects of bacteria inoculation in lowland rice under iron toxicity, these studies were focused on plant growth promotion and mineral nutrient uptake. The primary aim of this dissertation was to assess the effects of Bacillus spp. inoculation on different lowland rice cultivars under iron toxicity, with emphasis on genotypic shoot tolerance strategies. Physiological, biochemical, and molecular mechanisms underlying genotypic responses to Bacillus inoculation were investigated and potential overlaps with responses to biotic stressors explored. Within the framework of this dissertation, three lowland rice cultivars, were inoculated with three Bacillus isolates (two B. pumilus isolates, one B. megaterium isolate) and exposed to 1000 ppm Fe2+ in the nutrient solution. The three cultivars were selected because they differ in their tolerance against iron toxicity, with one being sensitive, one a tolerant excluder (tolerance through minimizing iron uptake), and one a tolerant includer (tolerance of high iron concentrations in the plant). At day eight of stress exposure, the effects of Bacillus inoculation on tolerance against iron toxicity were evaluated by leaf symptom scoring. The effects of bacteria on the progression of leaf bronzing were then related to specific tolerance mechanisms, such as shoot iron content, iron allocation within the shoot, ROS scavenger enzyme activity, and the expression of genes related to iron toxicity tolerance. Furthermore, the effects of inoculation on brown spot disease development seven days after infection with Bipolaris oryzae were also assessed by leaf symptom scoring. All three Bacillus isolates were characterized for their ability to solubilize Zn and P, production of auxin, siderophores, and HCN, the presence of ACC deaminase activity, and in vitro inhibition of fungal growth. Effects of Bacillus inoculation on iron toxicity tolerance were found to depend on the cultivar x Bacillus isolate combination. While leaf symptom expression was ameliorated in the inoculated sensitive cultivar, the tolerant excluder cultivar generally developed stronger symptoms of iron toxicity when inoculated with Bacillus. No significant effects of Bacillus inoculation on the tolerance against iron toxicity were found in the tolerant includer cultivar. The beneficial outcomes of bacterial inoculation on plant stress tolerance are often credited to the bacteria's plant growth promoting properties. However, we did not find a clear association between plant growth and tolerance to iron toxicity. Furthermore, the Bacillus isolates did not display ACC deaminase activity nor the ability to solubilize Zn or P. Auxin production was only notable in B. megaterium, the isolate with least effects on both tolerance to iron toxicity and brown spot disease development. Siderophore production was found in B. pumilus D7.4 but only under low iron supply. Instead of plant growth, iron homeostasis as well as the interconnection between iron homeostasis and the immune response of lowland rice seem to be affected by bacterial inoculation. For the first time it was shown that Bacillus inoculation can directly affect tolerance against iron toxicity in lowland rice through inducing the production of ferritin in the young leaf blades of the sensitive cultivar. NO produced by B. pumilus Ni9MO12 is hypothesized to be involved in the signaling cascade leading to OsFER expression. Activity of ROS scavenger enzymes of the ascorbate-glutathione redox cycle were not affected by Bacillus inoculation in the leaf blades. In the tolerant excluder cultivar, an alteration in iron distribution within the shoot of B. pumilus Ni9MO12 inoculated plants, is assumed to cause the decline in tolerance. In conclusion, it was shown that Bacillus inoculation can influence iron toxicity tolerance in lowland rice. The results underline the significance of the interaction between rice genotypes and bacteria isolates. Furthermore, Bacillus inoculation did not promote plant growth, instead, distinct adaptation mechanisms within the shoot tissue were triggered to allow for increased tolerance of high iron concentrations in the leaves. The signaling cascades involved might be linked with biotic stress responses. Understanding such intricate mechanisms is vital for improving plant productivity. While inoculants composed of single microbial isolates may not meet the anticipated outcomes for practical application in sustainable agriculture, they offer a valuable laboratory tool for investigating genotypic plant tolerance to various abiotic and biotic stresses and the role of the microbiome within. New breeding approaches that consider genotypic traits essential for obtaining a beneficial microbiome might accelerate the creation of more tolerant cultivars.

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