Modelling of the Uptake, Flow and Utilization of C, N and H 2O within Whole Plants of Ricinus communis L. Based On Empirical Data

Abstract

Summary An empirically based modelling technique was used to quantitatively depict uptake, flow and utilization of C, N and H2O for a 9 day period in mid vegetative growth of NO3-fed castor bean (Ricinus communis L.) exposed to a mean salinity stress of 128 mol m-3 NaCl. The models incorporated data on C : N weight ratios of solutes of phloem sap and pressure-induced xylem exudates of leaves, stem internodes and petioles, net increments or losses of C, N and H2O in plant parts, transpirational losses of shoot organs and respiratory losses of C from shoot parts and root. A computational technique was developed to assess the extent of xylem to phloem transfer of C and N within internally sited organs (stem segments and petioles) in addition to traffic of C and N through these organs to terminal organs (root, shoot apex and leaves). Molar ratios of inputs of H2O: C : N were 4803: 22: 1. Half of the net daytime photosynthetic gain of C by the shoot was translocated initially to the root, 33 % was lost in root respiration, 17% in shoot night respiration, 8% cycled through the root system and 40% was finally incorporated into shoot dry matter, 10% into root dry matter. The corresponding N budget showed 93% initial transfer in xylem from root to leaf laminae, 32% backflow in phloem from shoot to root and 15 % cycled through the root. The water budget involved a 98% loss in transpiration, 2% incorporation into tissues and 2% commitment to phloem transport. Xylem to phloem exchanges of C and N in shoot mostly involved transfer from xylem to phloem. The models predicted a steep upward gradient in decreasing water use efficiency with leaf age, due principally to poor CO2 fixation in relation to water loss by youngest leaves. Comparison of fluxes of N and H2O between stem internode segments with those passing out to leaves indicated progressive lateral abstraction of N from leaf traces serving lower leaves and subsequent passage of this N to the xylem of cauline traces serving upper regions of the shoot. Data were compared with earlier obtained information on C, N and H2O partitioning in nodulated white lupin (Lupinus albus L.).