Effects of Altered Carbohydrate Availability on Whole-Plant Assimilation of ¹⁵NO₃⁻

Abstract

An experiment was conducted to investigate the relative changes in N03-assimilatory processes which occurred in response to decreasing carbohydrate availability. Young tobacco plants (Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were exposed to 1.0 millimolar 15N03-for 6 hour intervals during a normal 12 hour light period and a subsequent period of darkness lasting 42 hours. Uptake of 15NO3-decreased to 71 to 83% of the uptake rate in the light during the initial 18 hours of darkness; uptake then decreased sharply over the next 12 hours of darkness to 11 to 17% of the light rate, coincident with depletion of tissue carbohydrate reserves and a marked decline in root respiration. Changes also occurred in endogenous 15NO3-assimilation processes, which were distinctly different than those in '5NO3-uptake. During the extended dark period, translocation of absorbed '5N out of the root to the shoot varied rhythmically. The adjustments were independent of 15NO3-uptake rate and carbohydrate status, but were reciprocally related to rhythmic adjustments in stomatal resistance and, presumably, water movement through the root system. Whole plant reduction of '5NO3-always was limited more than uptake. The assimilation of '5N into insoluble reduced-N in roots remained a constant proportion of uptake throughout, while assimilation in the shoot declined markedly in the first 18 hours of darkness before stabilizing at a low level. N03-uptake (6, 11, 25), and reduction (1, 27) observed in darkness. Accordingly, our experiment involved exposure of young tobacco plants to '5NO3-for 6 h intervals during a normal 12 h light period and during a following 42 h period of extended darkness. The approach allowed assessment of the relative sensitivities of NO3-assimilation activities in plants with widely varying carbohydrate availabilities.Of particular interest in this experiment was the regulation of NO3-transport from the root into the xylem. Previous studies have revealed that a larger proportion of the NO3-taken up by roots is translocated to the shoot in light than in darkness (22,24,29). The observation raises the possibility that xylem transport of NO3-is highly sensitive to a carbohydrate limitation. An alternative explanation is that the lower rate of xylem transport in darkness is closely associated with decreased water movement through the root and vascular system. Since stomatal opening and closure, and transpiration, continue to oscillate rhythmically in periods of extended darkness (13,16)

Regulation of Root Ion Transporters by Photosynthesis: Functional Importance and Relation with Hexokinase

“…Second, a strong correlation was found between the responses of transcript accumulation to illumination of the plant on the one hand and to sucrose supply to the roots on the other hand (Figure 3). This finding suggests that the diurnal changes in gene expression result from the decreased downward transport of sucrose to the roots at night (Riens et al, 1994) and the concomitant decrease in the root concentrations of soluble sugars (Figure 4) (Kerr et al, 1985;Rufty et al, 1989). The effect of light seen in this study probably relates to a regulation exerted by photosynthates.…”

“…On a functional basis, the regulation of root ion transport by photosynthesis has been documented particularly in the case of NO 3 Ϫ , the uptake of which is regulated by the CO 2 concentration in the atmosphere (Gastal and Saugier, 1989;Delhon et al, 1996), and by the availability in the roots of various C metabolites, such as sugars (Hänisch Ten Cate and Breteler, 1981;Rufty et al, 1989, Delhon et al, 1996 and carboxylic acids (Touraine et al, 1992). Furthermore, root NO 3 Ϫ uptake is almost independent of the variations in transpiration and is not under the control of an endogenous circadian rhythm (Delhon et al, 1995b).…”

Effect of Light/Dark Cycles on Expression of Nitrate Assimilatory Genes in Maize Shoots and Roots

“…However, the phytochrome response is seen only with etiolated plants, suggesting that light has an effect at some other level (18). For instance, light may have an effect on the level of reductant, uptake of nitrate, transfer of nitrate to the xylem, or the release of nitrate from vacuoles in the leaves (9,25).…”

The effect of light on nitrate uptake by wheat roots

“…This is comparable to but slightly lower than the changes of 20-50% reported in some other species [4][5][6]17].…”

“…For instance, nitrate uptake increases 20% to 40% within a few hours and then becomes steady when plants are transferred from dark to light in barley [4] and decreases by a similar amount on transfer from light to dark in tobacco and soybean [5,6]. In prolonged darkness, the nitrate uptake rate may subsequently decline even further [7,8].…”