The effects of different doses of metribuzin on net photosynthetic rates and tolerance of soybean (Glycine max (L. ) Merrill, 'Alvorada' e Uberaba') pla nta was studiet in glasshouse conditions at Centro de Pesquisa do Cacau (CE-PEC), Ilhéus, BA. No significant differences were observed either among the varieties or the interation varieties x doses for all parameters analysed. The aver age of the net phot osyn thetic rates for both va rietie s were 22,9 2, 19,87 and 18,20 mg CO2.dm- 2.h-1 tre ated with 0,0, 0,4 and 0,8 kg a.i. ha-1 of metribuzin, respectively. The negative effect of metribuzin on net photosynthetic rate was evident only in the soybean treated with 0,8 kg a.i./ha-l. The herbicide did not affect the stomatal resistance and transpiration rate. As the dose s of metribuzin increased the dry weight of plants parts and leaf area decreased. The effect was more prominent on root dry weight than shoot biomass. Subletals doses of metribuzin enhanced the shoot and root relative dry weights and leaf area in soybean Uberaba. By stimating the dose which inhibit growth in 50% (I50), it was shown that metribu zin had an accentuated phytotoxics effect on roots of Alvorada soybean, suggesting that this variety has a lower capacity of metabolizing metribuzin in non-phyto toxic compounds than the Uberaba.
Stomatal resistance; transpiration; biomass; growth inhibition