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The aim of this research was to verify, on whole plant level and during all the plant cycle, the hypothesis that nitrogen deficiency reduces root hydraulic conductivity through the water channels (aquaporins) activity, and that the plant reacts by changing root/shoot ratio. Root hydraulic conductivity, plant growth, root/shoot ratio and plant water status were assessed for durum wheat (Triticum durum Desf.) and barley (Hordeum vulgare L.), as influenced by nitrogen availability and HgCl2 treatment. On both species during the plant cycle, nitrogen deficiency induced lower root hydraulic conductivity (-49 and -66% respectively for barley and wheat) and lower plant growth. On wheat was also observed cycle delay, lower plant nitrogen content, but not lower leaf turgor pressure and epidermic cell dimension. The lower plant growth was due to lower plant dimension and lower tillering. Root /shoot ratio was always higher for nitrogen stressed plants, whether on dry matter or on surface basis. This was due to lower effect of nitrogen stress on root growth than on shoot growth. On wheat HgCl2 treatment determined lower plant growth, and more than nitrogen stress, cycle delay and higher root/shoot ratio. The mercury, also, induced leaf rolling, lower turgor pressure, lower NAR, higher root cell wall lignification and lower epidermic cell number per surface unity. In nitrogen fertilized plants root hydraulic conductivity was always reduced by HgCl2 treatment (-61 and 38%, respectively for wheat and barley), but in nitrogen unfertilized plants this effect was observed only during the first plant stages. This effect was higher during shooting and caryopsis formation, lower during tillering. It is concluded that barley and durum wheat react to nitrogen deficiency and HgCl2 treatment by increasing the root/shoot ratio, to compensate water stress due to lower water root conductivity probably induced by lower aquaporin synthesis or inactivation. However, this compensation is not enough to keep a normal water status and this leads to lower plant growth. At the same time some doubts persist about mercury toxicity on other plant functions besides the aquaporin blockage