Effects of nitrogen supply on xylem cytokinin delivery, transpiration and leaf expansion of pea genotypes differing in xylem-cytokinin concentration
Ian C. Dodd A C , Chuong Ngo A , Colin G. N. Turnbull A B and Christine A. Beveridge AA Department of Botany, University of Queensland, St Lucia, Qld 4072, Australia.
B Current address: Department of Agricultural Sciences, Imperial College London, Wye Campus, Wye, Kent TN25 5AH, UK.
C Corresponding author; email: I.Dodd@lancaster.ac.uk
Functional Plant Biology 31(9) 903-911 https://doi.org/10.1071/FP04044
Submitted: 24 February 2004 Accepted: 19 May 2004 Published: 27 September 2004
Abstract
The rms2 and rms4 pea (Pisum sativum L.) branching mutants have higher and lower xylem-cytokinin concentration, respectively, relative to wild type (WT) plants. These genotypes were grown at two levels of nitrogen (N) supply for 18–20 d to determine whether or not xylem-cytokinin concentration (X-CK) or delivery altered the transpiration and leaf growth responses to N deprivation. Xylem sap was collected by pressurising de-topped root systems. As sap-flow rate increased, X-CK declined in WT and rms2, but did not change in rms4. When grown at 5.0 mm N, X-CKs of rms2 and rms4 were 36% higher and 6-fold lower, respectively, than WT at sap-flow rates equivalent to whole-plant transpiration. Photoperiod cytokinin (CK) delivery rates (the product of transpiration and X-CK) decreased more than 6-fold in rms4. Growth of plants at 0.5 mm N had negligible (< 10%) effects on transpiration rates expressed on a leaf area basis in WT and rms4, but decreased transpiration rates of rms2. The low-N treatment decreased leaf expansion by 20–25% and expanding leaflet N concentration by 15%. These changes were similar in all genotypes. At sap-flow rates equivalent to whole-plant transpiration, the low N treatment decreased X-CK in rms2 but had no discernible effect in WT and rms4. Since the low N treatment decreased transpiration of all genotypes, photoperiod CK delivery rates also decreased in all genotypes. The similar leaf growth response of all genotypes to N deprivation despite differences in both absolute and relative X-CKs and deliveries suggests that shoot N status is more important in regulating leaf expansion than xylem-supplied cytokinins. The decreased X-CK and transpiration rate of rms2 following N deprivation suggests that changes in xylem-supplied CKs may modify water use.
Keywords: cytokinins, leaf growth, mutant, pea.
Acknowledgments
ICD and CAB thank the Australian Research Council for financial support. Dr John Passioura (CSIRO Plant Industry, Canberra, ACT) is thanked for construction of the root pressure chamber, Mr Grant Andrews for construction of additional pressure pots, and Dr Alun Jones and Mr Gordon Moss for mass-spectrometry of CK and N, respectively.
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