Impact of ozone on the reproductive biology of Brassica campestris L. and Plantago major L.
thesisposted on 16.09.2011, 13:30 by Caroline A. Stewart
Although sexual reproductive development is a critical phase in the life cycle of plants, the effects of air pollutants on the processes involved have not been investigated extensively. This thesis describes experiments undertaken to determine the direct effects of short-term exposures to ozone on the reproductive biology of species with contrasting reproductive growth habits, Brassica campestris L., an indeterminate species, and Plantago major L., a more determinate species. Two purpose-designed exposure chambers were constructed, each of which permitted the reproductive structures of up to twelve plants to be isolated from the vegetative parts and exposed simultaneously to ozone-enriched or charcoal-filtered air. The design of these chambers also permitted whole plant exposures. The occurrence of significant decreases in stomatal conductance and the net rate of photosynthesis following exposure to 70 ppb ozone established that the vegetative structures of both B. campestris and P. major were sensitive to ozone. Visible leaf injury also developed in B. campestris, the extent of which varied with the stomatal conductance of plants prior to fumigation and therefore ozone uptake. The growth of B. campestris and P. major was also reduced following 10 and 14 days of exposure respectively to 70 ppb ozone for 7 h d-1. The effects of ozone on growth differed between the four populations of P. major examined in this study. While a single 6 h exposure of the terminal inflorescence of B. campestris to 100 ppb ozone had no significant effect on reproductive development, repeated exposures over four consecutive days increased the abortion of seeds in apical pods and the precocious germination of seeds in older pods. However, the impact on final seed yield was dependent upon the timing of exposure; thus seed yield was significantly reduced following exposure during the early flowering phase, but was unaffected following exposure during the later stages of reproductive development. The compensation for seed losses in more mature plants resulted from their ability to retain naturally aborting seeds in older pods. Nonetheless, single and multiple exposures to ozone both reduced the rate of germination of harvested seeds and multiple exposures affected seed colour. In P. major, exposure of the fIrst spike to 120 ppb ozone for 7 h d-1 during flowering primarily affected seed number per capsule. Both significant increases and decreases in seed number were observed in the four populations examined, and seed number was also affected in spikes which had not been exposed to ozone, indicating the existence of possible compensation mechanisms. Pollen from both species was also shown to be sensitive to a 6 h in vivo exposure to ozone. The complex responses of B. campestris and P. major following direct exposure of the reproductive structures to ozone suggest that factors other than the type of reproductive growth habit are important in determining the extent of ozone damage and the ability of plants to compensate for reproductive site losses.