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Although leaves are the primary sites of tropospheric ozone exposure, it has been postulated that ozone may cause greater and earlier disruption of below-ground growth with long-term consequences for productivity. As plant roots are hidden in soil, the time-course of ozone-induced below-ground changes remains poorly characterised. In this study, the in vivo root development before, during and after short-term ozone exposure of individual subterranean clover seedling shoots (Trifolium subterraneum L.) was studied non-destructively using a new experimental growth and exposure system. Plants were grown individually in thin root growth chambers, where the entire root system could be observed non-invasively for weeks. Seedlings were exposed individually in separate exposure cuvettes to either charcoal filtered air (n=3, similar to 1-3 ppb ozone) or charcoal filtered air supplemented with ozone giving a concentration of 75 10 ppb ozone (n=3) for 8 h during daytime for five consecutive days (AOT40 index 1400 ppb h) starting eleven days after sowing. 171 High-resolution digital images of the roots were captured every one-four days and were analysed to determine root length and root tip formation. Altered root growth occurred within the first week after exposure and persisted for three weeks. Root tip formation and elongation rates were reduced by 35-40%. Doubling of the number of root tips was delayed by one week. Roots of exposed plants were therefore shorter and less branched. Above-ground changes were apparent about one week after below-ground changes. The leaf development rate was reduced by about 25%, but the number of flower stems at harvest was unaffected. Ratios between below and above-ground growth were significantly reduced by similar to 60% the first week after exposure and remained lower for up to three weeks suggesting reduced carbon allocation to root growth. Nevertheless, biomass and biomass ratios at harvest five weeks after exposure were not significantly altered, indicating that traditional destructive sampling once, at harvest, would not have revealed any of the early below-ground changes observed in this study. Thus, ozone caused substantial changes to root architecture that may ultimately affect the root's function and thereby the plant's overall health and ability to cope with stress. (C) 2010 Elsevier By. All rights reserved. |
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