Comparative uptake of nitrate by intact seedlings of C-3 (barley) and C-4 (corn) plants: Effect of light and exogenously supplied sucrose

The effect of light and exogenously supplied sucrose on NO3- uptake was studied in 9-day-old intact C-3 (barley) and C-4 (corn) seedlings. The seedlings used were uninduced for nitrate uptake system (i.e. had never seen nitrogen during germination and growth) and were exposed to continuous light for 3 days to avoid any diurnal variation and to load the seedlings fully with photosynthates. The uptake assay was conducted either in light or in darkness. Prior to assay, seedlings were treated with darkness or light for 24 h. Accordingly, four sets of seedlings, i.e. pretreated with light and assayed in light (LL); pretreated and assayed in darkness (DD); pretreated with light and assayed in darkness (LD); and pretreated with darkness and assayed in light (DL) were formed. Barley exhibited 55% higher NO3- uptake than corn during light (LL) and 91% higher during darkness (DD). Shifting barley seedlings from light to dark (LD) or dark to light (DL) for uptake assay, did not affect NO3- uptake, i.e. in LD the uptake was similar to LL and in DL it was similar to DD. However, in corn, the light conditions during the assay determined the uptake regardless of the conditions during the period preceding the assay. One percent sucrose in the medium increased NO3- uptake by 31% in barley and 70% in corn during light (LL). The corresponding increase during darkness (DD) was 38% in both barley and corn. Removal of the corn residual endosperm decreased NO3- uptake by 40% during darkness. Etiolated seedlings (those having never seen light) of both barley and corn were able to take up significant amount of NO3- during darkness. Externally supplied sucrose in the assay medium of etiolated seedlings increased the NO3- uptake to about 4 and 2 fold in barley and corn, respectively. The data presented here provide evidence that: 1. In intact seedlings, light per se is not obligatory for NO3- uptake and that the carbohydrate supply may mimic light. 2. Light affected the NO3- uptake differently in barley and corn.