Page 192 - Plant Canada 2024 Proceeding

PLANT CANADA 2024

The ongoing expansion of global salt-affected land is a significant factor limiting crop growth and yield,
particularly for rice. This experiment explores the mitigation of salt-induced damage on rice (Oryza sativa
L. cv BRRI dhan100) by applying plant growth-promoting rhizobacteria (PGPR) cultures. Rice seedlings,
five- and six-weeks post-transplanting, were subjected to salt stress via two treatments with 50 and 100
mM NaCl at seven-day intervals. Bacterial cultures, comprising endophytic PGPR strains (Bacillus subtilis
and B. aryabhattai) and an epiphytic PGPR strain (B. aryabhattai), were administered at three critical
stages: during transplantation of 42-day-old seedlings, five weeks later at the vegetative stage at 35 days
after transplanting (DAT), and seven weeks later at 49 DAT during panicle initiation stage. Salt stress
prompted osmotic, ionic, and oxidative stress, in rice plants, causing a dose-dependent decrease in
relative water content, chlorophyll content, stomatal conductance, chlorophyll fluorescence, IAA
concentrations, and various growth parameters. Furthermore, osmotic stress escalated the hydrogen
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peroxide content and proline accumulation, while ionic stress disrupted ion balance by increasing Na and
reducing K content. Both types of stress generated reactive oxygen species, impairing the antioxidant
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defense system and causing oxidative damage, as well as methylglyoxal (MG) toxicity, which was visible
in heightened malondialdehyde levels, electrolyte leakage, and Glyoxalase I (Gly I) and Glyoxalase II (Gly
II) activities. PGPR treatment alleviated these negative effects by enhancing osmotic and ionic balance,
demonstrated by improved water balance and reduced Na content and Na /K ratio. Additionally, PGPR
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fortified the antioxidative defense system and MG detoxification in salt-exposed rice plants by increasing
ascorbate and glutathione levels, antioxidant enzymes (ascorbate peroxidase, dehydroascorbate
reductase, monodehydroascorbate reductase, glutathione reductase, catalase, peroxidase, superoxide
dismutase, lipoxygenases, glutathione peroxidase, glutathione S-transferase), and glyoxalase enzymes
(Gly I and Gly II). The introduction of PGPR led to enhancements in yield attributes (including effective
tillers per hill, panicle length, rachis per panicle, filled grains per panicle, and 1000-grain weight),
consequently boosting the grain yield per hill. In conclusion, this research highlights the potential of
PGPR to bolster physiological and biochemical functionality in rice, serving as an effective buffer against
salt stress-induced damage.

[O175] EFFECTS OF DEFOLIATION ON ROOT TRAITS, NITROGEN FIXATION, SOIL NITROGEN
AVAILABILITY, SOIL ENZYME ACTIVITIES AND SOIL BACTERIAL COMMUNITIES OF FORAGE
LEGUMES. Malinda Thilakarathna , Danielito Dollete , Rhea Amor Lumactud , Cameron Carlyle , and
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Krzysztof Szczyglowski . Department of Agricultural, Food and Nutritional Science, University of Alberta,
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Edmonton, Alberta, Canada, T6G 2P5; Department of Plant, Food, and Environmental Sciences,
Dalhousie University, Truro, Nova Scotia, Canada, B2N 5E3; and Agriculture and Agri-Food Canada,
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London Research and Development Centre, London, Ontario, Canada, N5V 4T3
Correspondence to: malinda.thilakarathna@ualberta.ca

Forage legumes fix atmospheric nitrogen through a mutualistic relationship with rhizobia bacteria.
However, frequent defoliation stress from grazing and mowing can alter the source-sink relationship
between above-ground and below-ground tissues, potentially impacting their nitrogen-fixing ability. In this
study, we evaluated the effects of defoliation intensities on nodulation, root phenotypic traits, plant
biomass, symbiotic nitrogen fixation, soil available nitrogen, soil enzyme activities, and soil microbial
community structure of alfalfa (Medicago sativa L.) and red clover (Trifolium pretense L.). Two defoliation
intensities were applied at flowering: mild (removing half of above-ground biomass) and severe (leaving
only 2 cm stubble), along with non-defoliated controls. Mild defoliation positively affected final shoot
biomass in both legumes but had negative effects on nodulation and non-symbiotic root traits, including
root biomass. The symbiotic nitrogen fixation capacity was reduced in red clover under severe defoliation
stress, whereas it was unaffected in alfalfa. The available nitrogen content in red clover was greater
following severe defoliation than in mild and non-defoliation, but no changes were observed in alfalfa
following defoliation. Severe defoliation significantly increased soil enzyme activities of β-1, 4-
glucosidase, β-D-cellobiosidase, and phosphatase enzymes in both legumes. Defoliation had no
significant effect on shifting soil bacterial diversity or taxonomic composition. Overall results suggest that
defoliation intensity had a deleterious effect on root traits, a positive influence on C and P extracellular
enzyme activities, but varied influence on the shoot growth, symbiotic nitrogen fixation, and soil available
nitrogen based on the forage legume.

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