Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth, transpiration, and water and potassium uptake and flow in tobacco

NH4+-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO3 -, NH4NO3, NH4+ or NH4++6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO3--N by NH4+-N (NH4NO3) did not change growth, transpiration, uptake and flow of water and K+ compared with the NO3--N treatment. However, NH4+-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO 3--N and NH4NO3 treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (δ13C) compared with the NO3 - treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K+ uptake to 10%. Under NH4+ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K+ currently taken up, indicating massive mobilisation of K+ from lower leaves and even the roots. Phloem retranslocation of K+ from the shoot and cycling through the root contributed 67% to the xylem transport of K+, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K+) caused by NH4+-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10-8M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH 4+-fed plants are discussed.