NITRATE ASSIMILATION IN TOBACCO-LEAVES TREATED WITH MALEIC HYDRAZIDE TO CONTROL AXILLARY BUD GROWTH

After removal of the upper stalk containing the developing inflorescence and small expanding leaves (<20 cm) of burley tobacco (Nicotiana tabacum L.), maleic hydrazide (MH) is used to control axillary bud growth (suckers) and enhance economic yield. Maleic hydrazide has numerous physiological effects on tobacco; many represent delayed or secondary effects on plant metabolism and may affect chemical composition of leaves, The effects of MH level (0, 0.17, 0.34, and 0.68 g plant(-1)) on the in situ assimilation of nitrate and other selected physiological traits of the second leaf from the top in plants grown in 1989 and 1990 were determined at weekly intervals for 3 wk beginning approximate to 10 d after the MH application. Total chlorophyll in laminal tissue decreased as MH level increased; this response to MH became increasingly pronounced with time. The dry weight of the second leaf from the top increased with time, but differences in leaf weight due to treatment were not significant. Leaf total N and nitrate concentrations of the lamina averaged across both years and three sampling dates decreased with increasing MH level. However, in situ assimilation of nitrate was unaffected by MH. Differences in leaf N concentrations due to MH are partly the consequence of greater leaf dry weight accumulation and possibly to decreased uptake and partitioning of nitrate rather than nitrate assimilation. The slightly lower levels of nitrate in the upper leaves of MH-treated burley tobacco should be a desirable benefit of chemical suppression of suckers, because lower levels of tissue nitrate are often associated with decreased production of nitrosamines in cigarette smoke.