Physiological responses of biomass allocation, root architecture, and invertase activity to copper stress in young seedlings from two populations of Kummerowia stipulacea (maxim.) Makino

In the current study, we hypothesize that mine (metallicolous) populations of metallophytes form a trade-off between the roots and shoots when under copper (Cu) stress to adapt themselves to heavy metal contaminated habitats, and thus, differ from normal (non-metallicolous) populations in biomass allocation. To test the hypothesis, two populations of the metallophyte Kummerowia stipulacea, one from an ancient Cu mine (MP) and the other from a non-contaminated site (NMP), were treated with Cu(2+) in hydroponic conditions. The results showed that MP plants had higher root/shoot biomass allocation and more complicated root system architecture compared to those of the NMP plants when under Cu stress. The net photosynthetic capacity was more inhibited in the NMP plants than in the MP plants when under Cu stress. The sugar (sucrose and hexose) contents and acid invertase activities of MP plants were elevated while those in NMP plants were inhibited after Cu treatment. The neutral/alkaline invertase activities and sucrose synthase level showed no significant differences between the two populations when under Cu stress. The results showed that acid invertase played an important role in biomass allocation and that the physiological responses were beneficial for the high root/shoot biomass allocation, which were advantageous during adaptive evolution to Cu-enriched mine soils.

Seedling disease in an annual legume: consequences for seedling mortality, plant size, and population seed production

The effect of seed and seedling mortality on plant population dynamics depends on the degree to which the growth and reproduction of surviving individuals can compensate for the deaths that occur. To explore this issue, we sowed seeds of the annual Kummerowia stipulacea at three densities in sunken pots in the field, which contained either field soil, microwaved field soil, or microwaved field soil augmented with oospores of three Pythium species. High sowing density reduced seedling establishment and seedling size, but these effects were independent of the soil treatment. In the oospore-augmented soil, seed and seedling survival was low. The surviving plants were initially smaller but, at maturity, average plant size was greatest in the oospore-augmented soil, compared to the other treatments. Total population seed production was unaffected by soil treatment, suggesting that the effect of disease was limited to the seedling stage, with surviving plants released from intraspecific competition. To test the hypothesis that the surviving plants in the oospore-augmented soil were more disease-resistant, seeds from each of the sowing density-soil type treatments were sown in a growth chamber inoculation study. No evidence for selection for resistance was found. A second inoculation experiment revealed that oospore inoculum reduced plant numbers and mass regardless of whether field or microwaved soil was used, suggesting that results from the field experiment were not dependent on the use of microwaved soil. The findings of this study indicate that the ecological effects of disease on individual plants and on plant populations are not necessarily equivalent.