Ammonium and NO3- can be utilized as nitrogen sources by most plant species
although the plant response to continuous ammonium nutrition is species de
pendent. The effect of the nitrogen source (nitrate and ammonium) on growth
, photosynthetic parameters and nitrogen content in spinach (Spinacea olera
cea L.), sunflower (Helianthus annuus L.) and pea (Pisum sativum L.) was st
udied. Results showed spinach to be highly sensitive, sunflower moderately
sensitive and pea tolerant to ammonium nutrition. Ammonium accumulation in
shoots was closely correlated to growth reduction. Moreover, ammonium accum
ulation was correlated to an increase of organic nitrogen content. The data
suggest that the site of ammonium assimilation is a key factor controlling
tolerance to ammonium nutrition in the different plant species. with plant
s being more tolerant when ammonium is assimilated in roots. (C) 2001 Elsev
ier Science B.V. All rights reserved.