Germination Ecology of Four African mustard (Brassica tournefortii Gouan.) Populations in the Eastern Region of Australia

African mustard (Brassica tournefortii Gouan) is a problematic winter annual weed in Australia. Germination ecology of B. tournefortii may change in response to the maternal environments or habitats in which they grow. A study was conducted to evaluate the effect of environmental factors on germination and emergence of four populations of B. tournefortii that were collected from different fields. Averaged over populations, germination was stimulated by dark and was higher at 25/15 C (92%) as compared with 15/5 C (76%) and 35/25 C (45%). Averaged over light/dark regimes, at the lowest temperature regime (15/5 C), population A had higher germination than population D ; however, at the highest temperature regime (35/25 C), population D had higher germination than population A. Population B and C had higher germination in the temperature range of 25/15 C and 30/20 C compared with 15/5 C, 20/10 C, and 35/25 C. Seeds germinated at a wide range of alternating day/night temperatures (15/5 to 35/25 C), suggesting that seeds can germinate throughout the year if other optimum conditions are available. Population A was more tolerant to water and salt stress than population D. The sodium chloride concentration and osmotic potential required to inhibit 50% germination of population A was 68 mM and -0.60 MPa, respectively. Averaged over populations, seeds placed at 1cm soil depth had the highest emergence (54%), and burial depth of 8 cm resulted in 28% seedling emergence. Averaged over populations, wheat residue retention at 6000 kg ha-1 resulted in greater seedling emergence than the residue amount of 1000 kg ha-1. The results suggest that B. tournefortii will be favored in no-till systems and the seed bank of B. tournefortii could be managed by tillage regimes that bury its seeds below 8 cm depths and restrict seedling emergence and growth of new plants.

Auto-taxonomy of Brassica tournefortii Gouan. (Brassicaceae) in Egypt

Brassica tournefortii Gouan. (family Brassicaceae) is one of the five species in the Egyptian flora. Its populations showed notable morpho-plasticity with taxonomic debates, which were not yet resolved. The current study was carried out to assess the species morpho-plasticity and its molecular identity based on ISSR. The study was applied to 27 herbarium and fresh populations, representing all the species distribution ranges in Egypt. The taxonomic revision included 70 morphological characters, revealed five distinct Forms (1-5), radical leaf, and fruit provided the major distinguishable traits among the studied 70 morphological characters based on them the morphologic key is provided to delimit these forms. The pollen grain features using SEM are a pioneer at the infra-specific level, two shapes observed the subprolate (Forms 1& 3) and prolate (Forms 2, 4 & 5). Furthermore, the exine micro-features possess taxonomic value at the infraspecific level. The cluster analysis based on ISSR data revealed two clusters congruent to those developed by morphological and pollen traits. The ISSR results indicated that the species morpho-plasticity is genetically controlled. The study highlights the importance of the multidisciplinary approach to assess the taxonomic identity at the infra-specific level, for the auto-taxonomy of morpho-plastic species. Bangladesh J. Plant Taxon. 27(2): 233-250, 2020 (December)

Effect of seed size on germination in three species from arid Arabian deserts

In many species, seeds with different sizes show diverse responses in their requirements for temperature and light for optimal germination. In this study, the effect of seed size (i.e., small and large) on germination was tested in seeds of Brassica tournefortii Gouan., Lotus garcinii DC., and Salvadora persica L. from arid Arabian Desert habitats. The mean seed size of all three study species differs significantly between small and large-sized seeds. Differences were observed in seed germination between large and small seeds of B. tournefortii and L. garcinii. We found that large-size seeds of B. tournefortii and L. garcinii showed higher germination at all tested temperatures than that displayed by small-sized seeds. However, both small and large seeds of S. persica germinated equally (100%) well under all incubated conditions. Our results show significant differences in germination percentage between small and large-sized seeds of B. tournefortii at 25/35°C under a 12-hour light regime. However, L. garcinii has a significant difference in germination percentage at low temperatures (15/25°C) under a 12-hour light regime.

Effect of emergence time on growth and fecundity of Rapistrum rugosum and Brassica tournefortii in the northern region of Australia

Weeds from Brassicaceae family are a major threat in many crops including canola, chickpea, cotton and wheat. Rapistrum rugosum (L) All. and Brassica tournefortii Gouan. are two troublesome weeds in the northern region of Australia. In order to examine their phenology of these weeds, a pot study was conducted in 2018 at the Research Farm of the University of Queensland, Gatton campus with two populations sourced from high (Gatton) and medium (St George) rainfall areas of the northern grain region of Australia. Planting was carried out monthly from April to September, and the growth, flowering and seed production were evaluated. Maximum growth and seed production were observed in weeds planted in April, compared to other planting dates. Biomass of R. rugosum and B. tournefortii was reduced by 85% and 78%, respectively, as a result of the delay in planting from April to July. R. rugosum and B. tournefortii produced more than 13,000 and 3500 seeds plant−1, respectively, when planted in April and seed production was reduced by > 84% and > 76% when planted in July. No significant differences were observed between populations of both weeds for plant height, number of leaves and biomass, however, the medium rainfall population of R. rugosum produced more seeds than the high rainfall population when planted in April. The results of this study suggest that, although R. rugosum and B. tournefortii were able to emerge in a wider time frame, the growth and seed production were greatest when both weeds were planted in April and there was concomitant reduction in growth attributes when planted in the subsequent months, indicating that management of these weeds early in the cropping season is a prerequisite to population reduction and the mitigation of crop yield losses.

Biology of Brassica tournefortii in the northern grains region of Australia

Brassica tournefortii Gouan. (wild turnip, WT) has become a problematic weed in the no-till production systems of the northern grains region of Australia. Experiments were undertaken using different biotypes of B. tournefortii to examine its phenology, emergence and seedbank persistence. Biotypes were obtained from paddocks of barley (Hordeum vulgare L.) (WT1 and WT9) and chickpea (Cicer arietinum L.) (WT1/17 and WT2/17). Fresh seeds initially had high dormancy rates and persisted for a short period on the surface. Seedbank persistence increased with burial depth, with 39% of seeds remaining for WT1 and 5% for WT9 after 30 months at 2 cm depth. Persistence of buried seeds varied across biotypes; WT1/17 seedlings also emerged in the second growing season from 2 cm depth. Compared with buried seeds, seedlings readily emerged from the surface (in March–June following increased rainfall) within 6 months of planting. Emergence was greatest on the surface and varied between biotypes and tillage systems; the highest rate recorded was ~14%. Multiple cohorts were produced between February and October. No-till systems produced higher emergence rates than conventional tillage systems. Seedlings of B. tournefortii did not emerge from 5 cm soil depth; therefore, diligent tillage practices without seedbank replenishment could rapidly reduce the presence of this weed. A soil-moisture study revealed that at 25% of water-holding capacity, B. tournefortii tended to produce sufficient seeds for reinfestation in the field. Brassica tournefortii is a cross-pollinated species, and its wider emergence time and capacity to produce enough seeds in a dry environment enable it to become widespread in Australia. Early cohorts (March) tended to have vigorous growth and high reproduction potential. This study found B. tournefortii to be a poor competitor of wheat (Triticum aestivum L.), having greater capacity to compete with the slow-growing crop chickpea. Therefore, control of early-season cohorts and use of rotations with a more vigorous crop such as wheat may reduce the seedbank. The information gained in this study will be important in developing better understanding of seed ecology of B. tournefortii for the purpose of developing integrated management strategies.