Response of wheat cultivars to different soil nitrogen and moisture regimes. III. Leaf water content, conductance, and photosynthesis

Two greenhouse studies were conducted with contrasting patterns of soil water availability and nitrogen (N) supply to extend our knowledge of the combined effect of these environmental factors on cultivar response of wheat (Triticum aestivum L.). Several parameters of water relations and photosynthesis were measured. Results showed that at the beginning of stem elongation and before imposing water stress, the low N treatment decreased leaf conductance and carbon dioxide (CO2) exchange rate (CER). This effect was more significant in the case of the cultivar Nesma than the other cultivars. Leaf conductance was a much better indicator of CER than chlorophyll content. Overall, water stress decreased leaf conductance. Under mild stress, Nesma and Merchouch 8 behaved similarly, but Nesma was more sensitive to water stress. Increased N decreased CER under water stress but tended to have no effect on chlorophyll concentration. Under well watered conditions, N application helped the plants maintain greater leaf conductance, CER, and chlorophyll concentration than low N supply. Cultivar and N effects on relative water content (RWC) were not significant. Under severe water stress, Nesma tended to keep its stomata more open under N stress, while stomata of Merchouch 8 were not sensitive to N level variation. High N decreased leaf RWC of Nesma and Merchouch 8 but had no effect on Saada. From this study we concluded that under non-water stressed conditions, added N increased leaf water conductance, CER, and chlorophyll. Under water stress, the cultivars responded differently to N application. High N decreased leaf conductance and relative water content. These results indicated that careful cultivar selection can complement good N management of wheat in areas prone to soil moisture stress.