Alternaria brassicicola‐induced ROS accumulation during black spot disease differentially affects antioxidant efficiency, phenolic content and susceptibility of Brassica species

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Alternaria brassicicola-induced ROS accumulation during black spot disease differentially affects antioxidant efficiency, phenolic content and susceptibility of Brassica species

Alternaria brassicicola triggers reactive oxygen species generation, causing black spot disease, following diverse patterns of antioxidative enzyme activity in susceptible Brassica juncea, B. napus and B. oleracea.


Abstract

Early-stage responses of Brassica juncea, B. napus and B. oleracea during black spot disease and the development of Alternaria brassicicola were evaluated. Infection of plant cells by the fungus occurred mainly through direct penetration or by appressoria and, rarely, stomata in B. juncea and B. oleracea; in B. napus, penetration was mostly direct. The process of conidial germination, germ tubes and appressoria formation on a leaf surface was correlated with the post-inoculation time and host species (p < 0.05). Changes in the leaf surface were observed at successful infection sites as bright, wax-deprived areas. Significant differences in progression of infection between Brassica species were observed, with B. oleracea being the most susceptible and B. napus the least. Accumulation of superoxide anion radicals and hydrogen peroxide was not only observed in plant cells, but also in A. brassicicola germ tubes and appressoria at 8 and 12 h post-inoculation (hpi). Enhanced production of reactive oxygen species (ROS) and lipid peroxidation during infection triggered diverse patterns of enzymatic and non-enzymatic antioxidant activities and phenolic compound contents in response to A. brassicicola. In each Brassica species, a different antioxidant was the most active. With their antioxidant properties, phenolic compounds also played an essential role in these interactions. This phenomenon could be related to the disparate levels of susceptibility of the Brassica species to A. brassicicola; for example, the most susceptible, B. oleracea, showed high ROS accumulation with decreasing tendency during disease progression, unchanged phenolics content, and the highest catalase activity, differentiating it from B. juncea and B. napus.

Annual species’ experimental germination responses to light and temperature do not correspond with their microhabitat associations in the field

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Annual species' experimental germination responses to light and temperature do not correspond with their microhabitat associations in the field

Species may “select” microsites via germination cues that predict future habitat quality. We investigated germination responses to light and temperature as drivers of species' associations with tree and litter cover in a guild of winter annuals in Western Australia. Species responded differently to experimental light and temperature treatments, but these responses did not explain their microsite associations in the field.


Abstract

Questions

Annual species have evolved sets of germination cues that are thought to be predictive of the post-germination environment. In naturally patchy environments, germination microsites often vary considerably in the amount of light they receive and in the diurnal temperature fluctuations they experience. However, whether species' differential germination responses to light and temperature are associated with their spatial patterns of occurrence remains largely untested.

Location

Mediterranean-climate woodlands in Southwest Western Australia.

Methods

We surveyed species' occurrences in annual plant communities in 150 quadrats across gradients of canopy cover and litter cover. Nineteen species recorded in this survey were then included in a germination experiment that manipulated (1) Light vs Dark (12 h light or continuous dark) approximating seeds near the soil surface vs those covered by litter and (2) Cold vs Warm temperature regimes (7/18°C and 7/24°C) approximating diurnal fluctuations experienced in shaded vs sun-exposed microsites, respectively.

Results

In the germination experiment, six species had highest germination probabilities in the Light treatment (regardless of temperature), five in Cold + Light, one in Warm + Light, two were indifferent to the treatments, and four did not germinate at all. Binomial linear mixed-effects models showed that species' maximum responses to light and temperature did not explain their spatial distributions along canopy cover and litter cover gradients, contrary to theoretical expectations of germination being a strong driver of species' occurrences.

Conclusions

Despite variation in species' responses to experimental treatments, no association was found with their field microsite associations. Germination strategies in our system were wider than expected for Mediterranean systems. Our results support that germination cues are not strong drivers of microhabitat associations in this system.