Iron toxicity to fen plant species

The toxicity of ferrous iron to seedlings of 44 fen plant species was examined using 2-week screening experiments in solution culture. These were found to be more practicable than alternative methods which were tested. Growth measurements (including shoot and root length and dry weight, leaf size, and numbers and health of leaves) were made. Various tolerance indices were derived and compared, and one based on summation of standardized relative growth rates over a range of iron concentrations (sigma%RGR) was selected as being most appropriate. Standardization was achieved by expressing mean RGR for each treatment as a percentage of that in a low-iron control. An iron-tolerance species ranking was derived from this. Ward's classification (multivariate analysis) grouped the species into four main clusters on their standardized relative growth rate response. The very iron-tolerant species (cluster 1) were almost entirely monocotyledons, cluster 2 contained both monocotyledons and dicotyledons; all except one species in cluster 3 were dicotyledons: while the most sensitive species (cluster 4) were exclusively dicotyledons. Evidence suggests that the greater iron tolerance of monocotyledonous species may be due to an inherently superior oxidative detoxification system reducing direct toxicity, and to a generally lower growth rate minimizing indirect toxicity (nutritional effects associated with high iron concentrations). Analysis by Ward's method of shoot:root dry weight ratio trends revealed that for most monocotyledons shoot:root ratio was little affected by iron concentration. Conversely, root growth of most dicotyledonous species was affected more severely than shoot growth by increased iron supply. Assessed tolerance of species correlated positively with mean and maximum field measurements of iron concentrations in sites where they occurred. Iron-sensitive species also tended to grow where substrate pH, extractable calcium concentration and titratable alkalinity were high, and where mean summer water table height, extractable manganese, aluminium and nitrogen concentrations were relatively low. For those fen species tested, an inverse relationship was found between a species iron tolerance and the fertility and extractable phosphorus concentration of sites in which it grew. Iron tolerance and relative growth rate (control treatment) were negatively correlated.