Regulation of Assimilate Partitioning in Soybean

Abstract

Increased concentrations of nitrate in a nutrient solution (2, 5, and 10 millimolar KNO3) were correlated with increased shoot:root ratios of non-nodulated soybeans (Glycine max [L.] Meff.) grown in sand culture.While altering the pattern of C and N partitioning, the N treatments did not affect whole plant photosynthesis over the study period. To determine the mechanism responsible for the observed changes in assimilate partitioning, detailed C and N budgets were worked out with plants from each N treatment over three consecutive 4-day periods of midvegetative growth. The information for the C and N budgets from the 2 and 10 millimolar N03-treatments was combined with data on the composition of xylem and phloem exudates to construct a series of models of C and N transport and partitioning. These models were used to outine a 'chainreaction' of cause-and-effect relationships that may account for the observed changes in assimilate partitioning in these plants. The proposed mechanism identifies two features which may be important in regulating the partitioning of N and other nutrients within the whole plant. (a) The concentration of N in the phloem is highly correlated with the N concentration in the xylem. (b) The amount of N which cycles through the root-from phloem imported from the shoot to xylem exported by the root-is regulated by the root's requirement for N: only that N in excess of the root's N requirements is returned to the shoot in the xylem. Therefore, roots seem to have the highest priority for N in times of N stress.The interdependence ofphotosynthate supply from shoots and mineral N acquisition by roots has been recognized as an important relationship in determining the rate and pattern of whole plant growth in both empirical studies (23) and in theoretical models (5). In general, a decrease in the availability of combined N to roots decreases the shoot to root ratio and the overall rate of plant growth (25). However, the mechanism responsible for altering the pattern of C partitioning in response to decreased concentrations of combined N has not been clearly identified. Brouwer (4) suggested that under N deficiency enhanced root growth relative to shoot growth was the result of a greater proportion of absorbed N being retained by roots and never being translocated by the xylem to the shoots. Alternatively, other researchers have suggested that the decrease in shoot to root ratio due to suboptimal concentrations of combined N was due to the redistribution of shoot N to the roots as mediated by the effects of growth regulators (25). In particular, the decrease in root cytokinin synthesis and shoot cytokinin levels in response to N stress has been associated with this hypothesis (26).The present study describes the effects ofchanges in the nitrate supply on C and N transport and partitioning over a 12 d period of growth in non-nodulated soybeans. Budgets of C and N partitioning were combined with information of xylem and phloem sap composition to construct models of C and N transport similar...

Influence of the tobacco mosaic virus 30-kDa movement protein on carbon metabolism and photosynthate partitioning in transgenic tobacco plants

“…Increasing nitrogen availability results in a reduction in carbon allocation to the roots and this causes a decrease in the root-toshoot ratio (/~gren and Ingestad 1987;Vessey and Layzell 1987;Kachi and Rorison 1989;Mooney and Winner 1991). Imposition of water stress has the opposite effect on the root-to-shoot ratio (Sharpe and Rykiel 1991); presumably an enlarged root biomass (increased surface area) assists in water uptake.…”

Charge Balance in NO3−-Fed Soybean

“…The pH of fresh solutions was 5.5. The N03-medium contained 2 mm KNO3, 1 mm Ca(NO3)2, 1 mM MgSO4, 1 mM KH2PO4, 0.1 mM 50 ,uM KCI,30 ,M H3BO3,5 ,uM MnSO4,1 M ZnSO4, 1I M CuSO4 and 0.1 M (NH4)6Mo7024.…”

“…The pH of fresh solutions was 5.5. The N03-medium contained 2 mm KNO3, 1 mm Ca(NO3)2, 1 mM MgSO4, 1 mM KH2PO4, 0.1 mM 50 ,uM KCI,30 ,M H3BO3,5 ,uM MnSO4,1 M ZnSO4, 1I M CuSO4 and 0.1 M (NH4)6Mo7024.In the urea medium KNO3 and Ca(NO3)2 were replaced by 2 mm urea, 1 mm K2SO4, and 1 mM CaCl2. Solutions were renewed every 3 or 4 d, in order to minimize pH shift (less than 0.4 pH unit in urea medium, and less than 0.3 pH unit in NO3-medium) and nutrient depletion (maximum decrease of NO3-concentration: 15%).…”

Nitrate Effects on Nodule Oxygen Permeability and Leghemoglobin (Nodule Oximetry and Computer Modeling)

“…Birdsfoot trefoil (Lofus corniculafus L. cv F2rgus) and alfalfa (Medicago safiva L. cv Weevlchek) were grown in growth pouches under controlled conditions as previously described (Denison and Layzell, 1991;Denison et al, 1992a), except that the nitrogen-free nutrient solution of Vessey and Layzell (1987) was modified by substituting N,N'-ethylenebi~-2,2'-hydroxyphenylglycine (C'haney and Bell, 1987) for Fe-Sequestrene 330.…”