Nitrogen source and water regime effects on durum wheat photosynthesis and stable carbon and nitrogen isotope composition

Water stress and nitrogen (N) availability are the main constraints limiting yield in durum wheat (Triticum turgidum L. var. durum). This work investigates the combined effects of N source (ammonium-NH4+, nitrate-NO3- or a mixture of both-NH4+:NO3-) and water availability (well-watered vs. moderate water stress) on photosynthesis and water-use efficiency in durum wheat (cv. Korifla) flag leaves grown under controlled conditions, using gas exchange, chlorophyll fluorescence and stable carbon isotope composition (delta C-13). Under well-watered conditions, NH4+-grown plants had lower net assimilation rates (A) than those grown with the other two N forms. This effect was mainly due to lower stomatal conductance (g(s)). Under moderate water stress, differences among N forms were not significant, because water regime (WR) had a stronger effect on g(s) and A than did N source. Consistent with lower g(s), delta C-13 and transpiration efficiency (TE) were the highest in NH4+ leaves in both water treatments. These results indicate higher water-use efficiency in plants fertilized with NH4+ due to stomatal limitation on photosynthesis. Moreover, leaf delta C-13 is an adequate trait to assess differences in photosynthetic activity and water-use efficiency caused by different N sources. Further, the effect of these growing conditions on the nitrogen isotope composition (delta N-15) of flag leaves and roots was examined. Water stress increased leaf delta N-15 in all N forms. In addition, leaf delta N-15 increased as root N decreased and as leaf delta C-13 became less negative. Regardless of WR, the leaf delta N-15 of NO3--grown plants was lowest. Based on stepwise and canonical discriminant analyses, we conclude that plant delta N-15 together with delta C-13 and other variables may reflect the conditions of N nutrition and water availability where the plants were grown. Thus well-watered plants grown with NH4+:NO3- resembled those grown with NO3(-), whereas under water stress they were closer to plants grown with NH4+.