Arbuscular mycorrhizal (AM) symbiosis is known
to help the host plant to overcome environmental stresses as
drought by a combination of multiple mechanisms including
enhancing of root water uptake capacity. On the other hand,
Nitric oxide (NO) is involved in regulating the response of
plants to environmental stresses and colonization process of
AM fungi. The objective of this research was to study how
AM and non-AM lettuce plants responded to a NO donor
(sodium nitroprusside; SNP) or to a NO synthesis inhibitor
(Nω-nitro-L-arginine methyl ester hydrochloride; L-NAME)
under well watered and drought conditions. Most remarkable
results were that L-NAME increased the percentage of AM
colonized roots under both water regimes and AM plants
modified the shoot:root ratio by both chemicals under well
watered conditions. Also, the deleterious effects of SNP treat-
ment were partially prevented by AM symbiosis. Moreover,
NO could be involved in the diminution of leaf water content
under drought conditions, and SNP treatment seems to favor
apoplastic water path inside roots. Therefore, different out-
comes of relative water content, stomatal conductance and
root hydraulic conductivity observed between AM and non-
AM plants could be mediated by NO.

• Arbuscular mycorrhizal symbiosis modifies the effects of a nitric oxide donor (sodium nitroprusside;SNP) and a nitric oxide synthesis inhibitor (Nω-nitro-L-arginine methyl ester;L-NAME) on lettuce plants under well watered and drought conditions