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SUM M A R YFoliar uptake of peroxyacetyl nitrate (PAN) was measured by the gas exchange method on nine herbaceous species. Susceptibility to PAN was also examined in the tested species in order to seek correlations with the uptake rate of PAN. PAN was synthesized by the nitration of peracetic acid in /;-tridecane.Uptake rate of PAN by the leaves was kept at a steady level during the 90 min light period, w-hile in the dark it declined rapidly and reached almost zero within 30-^5 min. This suggests that adsorption of PxiXN on to the surface of the leaves is very small and most of it enters the leaf through open stomata.Among the species tested, sunflower showed the highest rate of PAN uptake, followed by radish, tomato and spinach. Maize and soyahean exhibited the lowest rate. .\ highly significant correlation was observed between the rate of PAN uptake and stomatal conductance among the tested species.Petunia was most susceptible to PAN, followed by radish and tobacco, but maize and peanut were tolerant. There was no significant correlation between the uptake rate of PAN and susceptibility to it among the species. A higher susceptibility of petunia to PAN was not characterized by a higher rate of pollutant uptake. These results show that the main factor determining the differences between species in susceptibility to PAN is not the amount that gains access to the tissues, but some internal metabolic processes.

Absorption of nitrogen dioxide (N(),J iiy u\^h\ hcrhncfous spocios was detcrniincd hy tlu' ''N diitition mctiiod. The piants were continuously exposed to NO., at OS //I 1 ' lor 2 weeks. Siiseeptibiiity to NO., was siniultaneousiy examined in the tested species to see li this was correlated with the rate of absorption.The total amount of NO.,-nitroRen absorbed b\ a plant during the 2-week exposLue period primariK tiepended on the size of tlie plant. Amon^ the tested species, stuiHower showed the highest rale of NO., absorption per unit leaf area, reaciiinf» as niuch as 0 . S. S mu; N dm -d ', which was about four times .greater than that for sorRlium which had the lowest rate. A liifjhly significant correlation was observed between the rate of absorption and the stomatal conductance among the species. A proportion of tiie NO^-nirrogen aecumuiated in the plan! might be absorbed via the air-soil-root route.Sunflower and radish which had higher rates of NO., absorption than the otiier species were more susceptible to NO.,. On the eontrary, sorghum and maize whicli showed a lower rate of NO., absorption were tolerant. Consequently, the higher susceptibility to NO., of radish and stmHower is considered to be directly related to their superior ability to absorb NO.^.

SUMMARYThe effect of varied levels of nitrate on the absorption of atmospheric nitrogen dioxide (NOj) by sunflower {Helianthus annus L.) plants was investigated by using a ^^N dilution method. Growth of the plants was greatest at 5 mM nitrate, with less growth at 15 mM nitrate and a considerable reduction at zero nitrate. The total amount of NOj-nitrogen absorbed by the plant depended on the plant size and total leaf area.Nitrate supply did not influence the rate of absorption of NOg by the plants when exposed to 0-3 /i\ r^ NOg. On the contrary, when the plants were exposed to 2-0 /il T^ NOg, the absorption rate was significantly lower for the plants grown at zero nitrate compared with those grown at 5 or 15 mM nitrate. The lower rate of NOg absorption in the nitrate-deficient plants was not due to the changes in stomatal resistance or stomatal density of the leaves, but could have been due to an increase in mesophyll resistance resulting from accumulation of high levels of nitrite in the leaf tissues.Nitrogen dioxide at 0-3 /i\ \'^ exerted a nutritional effect on the nitrate-deficient plants, while NO2 at 2-0 ju,\ T^ acted as a toxic air pollutant on the plants at all the levels of nitrate supplied.

Summary A quantitative tracer experiment was conducted to elucidate the effects of continuous exposure to ozone (O3) at 0.2 μl 1−1 on the translocation and the distribution of 13C‐labelled photosynthetic assimilates from the individual source leaves in 14‐d‐old plants of Phasealus vulgaris. The amount of labelled assimilates exported from the primary leaf, which acted as a main source of photosynthates for growth of the roots, decreased remarkably in the presence of O3 as a result of both a considerable reduction of 13CO2 fixation (62%) and the inhibition of translocation. On the other hand, that from the first trifoliate leaf, which mainly nourished the immature growing leaves, was not decreased in the same way by O3 because a smaller reduction of 13CO2 fixation (24%) was almost compensated by an acceleration of translocation. The pattern of assimilate distribution was altered by O3, so that a greater proportion of assimilates was partitioned to the growing leaves at the expense of the root and the stem. Consequently, the amount of labelled assimilates translocated to the non‐photosynthetic organs (stem and root) decreased by 53 %, while that to the photosynthetic organs (leaves) was reduced by only 28%. These results suggest that the plants might have adapted themselves to an environment containing O3 so that the reduction of growth efficiency caused by O3 could be minimized.