Comparative uptake and assimilation of nitrate by excised roots and leaves from seedlings of C3 (Barley) and C4 (Corn) plants : effect of light, ambient CO2 and exogenously supplied sucrose
2002
Sehtiya, H.L. (Chaudhary Charan Singh Haryana Agricultural University, Karnal (India). Regional Research Stn.) | Goyal, S.S. (University of California at Davis, CA95616-8515 (USA). Dept. of Agronomy and Range Science))
Effect of light, ambient CO and exogenously supplied sucrose on in vivo NOJ- assimilation was studied in excised roots and leaves from 9-day-old CJ (barley) and C4 (corn) seedlings. The seedlings used were uninduced for NOJ- uptake and assimilation (i.e. had never seen nitrogen during germination and growth) and were exposed to continuous light for three days to avoid any diurnal variation, followed by 24 h dark pre-treatment to make the seedlings carbohydrate deficient. Excised roots of such carbohydrate deficient barley seedlings took up 5 times higher NOJ- than that of corn and assimilated 7umol g-lfw 6 h-l as compared to 2 ulmol g-lfw 6 h-1 assimilated by corn roots. Addition of 1% sucrose in the uptake medium increased NO3-uptake by 140% and in vivo assimilation by 203% in corn roots. The corresponding increase in barley roots were 26% in uptake and 73% in assimilation. In absolute terms, increase in uptake in response to sucrose feeding was more than increase in assimilation in both corn and barley roots. Excised leaves of such barley seedlings failed to show significant in vivo NO3- assimilation in darkness while corn leaves assimilated 10 ulmol g-lfw 24h-l constituting about 18% of uptake. Addition of 1% sucrose increased the NOJ- assimilation to 36 ulmol g-lfw 24h-l constituting 90% of uptake in corn and to 7 ulmol g-lfw 24h-1 constituting 22% of uptake in barley. In light and absence of CO2 also, barley leaves failed to show significant assimilation and corn leaves assimilated 15 ulmol g-lfw constituting about 15% of uptake. Addition of 1 sucrose under these conditions increased the assimilation to 57% of uptake in corn and 41% of uptake in barley. The data presented here and the discussion provide evidence that (i) Light perse is not obligatory for NO3- assimilation and that the carbohydrate supply may mimic light. (ii) Limiting factor for lower NO3- uptake by the roots during darkness may not be low assimilation/translocation but lower availability of carbohydrates to the roots. (iii) CJ and C 4 plants behave differently in terms of efficiency of NO3 uptake and assimilation. While roots of CJ plants are efficient in NO3- uptake, C4 leaf metabolism appears to have a built-in control that maintains an efficient assimilation of NO3 at all times, light as well as darkness.
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