Growth and stress conditions cause similar changes in xylem amino acids for different legume species

The composition of nitrogenous compounds transported in the xylem to the shoot can reflect important changes in root N metabolism. Stress conditions that impair N assimilation in soybean (N2 fixation in symbiotic plants and nitrate assimilation in non-symbiotic plants) lead to specific changes in the amino acid composition of the xylem. In this study, it is shown that these changes also occur in other legume species (Phaseolus vulgaris, Vigna unguiculata, Pisum sativum, Lupinus albus and Crotalaria juncea) of both the ureide- and amide-exporting categories, when subjected to the same stress conditions. Waterlogging of nodulated plants led to a large and specific reduction in xylem glutamine levels, attributed to impaired N2 fixation. Transfer of nodulated plants to aerated hydroponics with N-free nutrient solution, which also impairs N2 fixation, resulted in elevated aspartate/asparagine ratios, besides the reduction in glutamine. Non-symbiotic plants grown on nitrate and transferred to a N-free hydroponic nutrient solution (impaired nitrate assimilation) likewise showed a marked increase in aspartate/asparagine ratios of the xylem bleeding sap. Three non-legume species studied (Lycopersicon esculentum, Zea mays and Helianthus annuus) produced a similar response to N deficiency. It is concluded that xylem bleeding sap amino acids can be useful indicators of the response of assimilatory processes in the root system to stress, for both ureide- and amide-exporting legumes, as well as non-legume plants.