Effects of different nitrogen forms on the growth and cytokinin content in xylem sap of tomato (Lycopersicon esculentum Mill.) seedlings

In order to investigate the effects of homogeneous and localized supply of different nitrogen forms (nitrate, NO3− vs ammonium, NH4+) on the growth of tomato seedlings, root morphology and six cytokinin (CTK) fractions in xylem sap were analyzed. Whole roots were supplied with different ratios of NO3− to NH4+ (100:0, as 100-0NA; 75:25, as 75-25NA; 50:50, as 50-50NA) under homogeneous supply. In split-root experiments, three treatments were compared: a sole NO3− supply (N|N), a spatially separated supply of NO3− and NH4+ (N|A), and a spatially separated supply of NO3− and a mixture of NO3− and NH4+ nutrition at a ratio of 75:25 (N|AN). All concentrations of total N were set at 5 mM. The results showed that (1) homogeneous 75% NO3− plus 25% NH4+ supply to the whole root zone led to maximum shoot and root dry matter (DM), root surface area (RS) and root volume (RV). The spatially separated supply of NO3− and NH4+ (N|A) resulted in a contrasting effect on root morphology: in comparison to N|N, root DM in the NO3−-containing pot was increased by 50% whereas it was depressed by 50% in the NH4+-containing pot. The 75% NO3− plus 25% NH4+ supply in the split-root experiment led to no significant effects either on shoot DM and root DM, or on RS and RV when compared to N|N. (2) The presence of NH4+ in the external medium led to a significantly reduced total xylem-CTK concentration, and a close negative correlation was found between xylem NH4+ and total CTK concentration irrespective of culture mode. A relatively high level of zeatin riboside (ZR) was maintained both in 75-25NA and N|A treatments. It was concluded that, in addition to the percentage of NH4+ to NO3− in the nutrient solution, whether NH4+ was supplied to the whole root system or to only part of the root system was also an important factor affecting plant growth. The fact that the 75-25NA and N|A treatments resulted in optimal growth of tomato seedlings might be attributed to the higher ZR concentration in xylem.