AMMONIUM ION INHIBITION OF PINUS-STROBUS L CALLUS GROWTH

Effects of NO3- and NH4+ ions on growth of Pinus strobus L. callus were studied. Callus tissue was obtained from hypocotyl segments of P. strobus seedlings and was maintained on a modified Murashige and Skoog's medium supplemented with 0.2 mg/l alpha-napthaleneacetic acid (NAA). The tissue could be repeatedly subcultured on a medium containing KNO3 as the sole nitrogen source in the medium. However, the callus growth was totally inhibited when NH4Cl was used as the nitrogen source. The inhibitory effect of NH4Cl could be eliminated by providing appropriate amounts of KNO3 in the medium in addition to NH4Cl. The inhibitory effect on callus growth was found to be due to NH4+ and not Cl- ions; good callus growth occurred on media containing KCl. Experiments suggested that acidification of the medium due to NH4+ ions was not the cause of growth inhibition. Growth of P. strobus callus seems to be inhibited due to its inability to utilize NH4+ as a source of nitrogen, which results in accumulation of toxic levels of NH4+ in the tissue. Experimental evidence also suggests involvement of glutamine synthetase in NH4+-induced inhibition of P. strobus callus growth.