RESPONSES OF GREENING BEAN SEEDLING LEAVES TO NITROGEN-DIOXIDE AND NUTRIENT NITRATE SUPPLY

Phaseolus vulgaris cv. Kinghorn Wax seedlings grown in darkness at 25- degrees-C for 7 days with half strength Hoagland's nutrient solution c ontaining no nitrogen, were transferred to lit continuous stirred tank reactors (CSTRs) in atmospheres containing 0 or 0.3 ppm NO2 and irrig ated with a nutrient solution containing 0 or 5 mM nitrate as sole nit rogen source and allowed to grow for a period of up to 5 days in a 14 h photoperiod. Exposure to NO2 increased total Kjeldahl nitrogen in th e leaves. Further, the exposure to NO2 increased chlorophyll content f rom day 3 onwards and inhibited the leaf dry weight substantially on d ays 4 and 5. The primary leaves of the seedlings exposed to 0.3 ppm NO 2 and supplied with nitrate accumulated some nitrite after 5 days of e xposure. Some of the seedlings were returned from CSTRs to growth cham bers and allowed to grow for a further period of 5 days in a 14 h phot operiod without NO2. The growth which developed after the NO2 exposure growth period, as measured by fresh and dry weights of the leaves, wa s significantly less in NO2-exposed plants than in nitrate-grown plant s. The experiments demonstrate that the leaves of greening seedlings a re able to assimilate NO2 and that a reduction in leaf dry weight by p rolonged NO2 exposure in the presence of nutrient nitrate can be assoc iated with nitrite accumulation, and that NO2 has a carry-over effect beyond the duration of NO2 exposure. It is apparent that NO2 induces s ome durable biochemical or cytological aberration in the presence of n utrient nitrate, which adversely affects subsequent leaf growth.