ABSTRACT
Scheible et a\. (1997) used partially NR-deficient tobacco lines (Nia30 (145)) to examine specifically the role of N 0 3 on various metabolic and developmental aspects and particularly shoot/root allocation of assimilates. Nia30 (145) resembles nitrate-limited wild-type plants with respect to growth rate and protein and amino acid content, but they accumulate large amounts of N 0 3 when grown on high nitrate. Among the effects attributed to high levels of tissue N 0 3 are a dramatic increase in shoot/root ratio due at least in part to a strong inhibition of root growth and stimulation of shoot growth. Leaf nitrate would be more effective in the regulatory effect exerted by nitrate. Leaf N 0 3 content correlated highly with shoot/root ratio whereas root nitrate content did not provide as good or significant correlation with shoot/root ratio. Split-root experiments in which plants with decreased nitrate reductase were grown with part of their root system in high nitrate and the other part in low nitrate, showed that root growth is inhibited by nitrate in the leaf. The authors suggest that high levels of nitrate in the leaves might inhibit carbon transport to the root due to modifications in loading or transportprocesses in specific sectors of the phloem. N 0 3 content seems to be monitored in the shoot and from there a signal is sent to the root. However, as yet there is no clear indication of how N 0 3 content is monitored, nor the nature of the signal transmitted to regulate root development. What may be a closely related phenomenon has been observed in Arabidopsis roots (Zhang and Forde, 1998; Zhang et al, 1999). High N 0 3 contents (from 1 mM) in the external medium inhibited root growth due specifically to the inhibition of lateral roots; at 50 mM N 0 3 lateral root growth was completely blocked. Localised N 0 3 supply, however, resulted in localised proliferation of lateral roots, probably as an adaptation of plants to maximise their ability to capture the available nutrients. Regulation of lateral root development by N 0 3 involves then two antagonistic pathways. While the stimulatory effect of N 0 3 is localised to those lateral roots directly exposed to the N 0 3 , its inhibitory effect was shown to be systemic.
