Differences in growth and nitrogen productivity between a stay-green genotype and a wild-type of Lolium perenne under limiting relative addition rates of nitrate supply

James Macduff, Neil Raistrick and Mervyn Humphreys (2002). Differences in growth and nitrogen productivity between a stay-green genotype and a wild-type of Lolium perenne under limiting relative addition rates of nitrate supply. Physiologia Plantarum, 116 (1), 52-61. Sponsorship: BBSRC RAE2008

The stay-green mutation of the nuclear gene sid inhibits chlorophyll degradation during leaf senescence in grasses. Decreased productivity is expected under conditions of limited external N availability, due to the higher retention of N in senescent tissues. However, this has not been reported when plants are grown at limiting low external concentrations of N. In the present study a different approach was taken, based on the relative addition rate method for defining limiting N supply. Wild-type and stay-green genotypes of Lolium perenne L. were grown for 30 days in flowing solution culture and then supplied with NO3? on an hourly basis over 35 days at relative addition rates (RARs) of 0.03, 0.06, 0.09 and 0.12 day?1, ranging from severe N limitation to optimal supply. Plants were steady-state labelled with 15NO3? prior to RAR treatments, then switched to 14NO3? to allow measurement of the re-distribution of N absorbed prior to RAR control. Following acclimation, relative growth rates (RGRs) approached the corresponding RAR, but were significantly lower for stay-green than wild-type at RARs of 0.03 and 0.06 day?1. Tiller numbers were lower in stay-green plants after 35 days at all RARs except 0.12 day?1. Concentrations of total N in senescent laminae of stay-green plants exceeded those in wild-type plants by a similar margin (4.8?6.8 mg g?1 DW) irrespective of RAR. Maximum nitrogen productivity (Pn) was 3.9 g DW g?1 N day?1 (Nmin = 7.1 mg g?1 DW) in wild-type plants, and 5.1 g DW g?1 N day?1 (Nmin = 10.7 mg g?1 DW) in stay-green plants. The higher N productivity of stay-green plants indicated these plants used a smaller pool of metabolically available N more efficiently in biosynthesis compared with wild-type plants. The retention of N, absorbed prior to RAR treatments, in senescent laminae was significantly higher in stay-green plants at RAR of 0.03 day?1 after day 21 (i.e. 20% versus 15% of the total N recovered). However, in terms of the whole N economy of the plant the margin represented only 1.7% of the total N content on day 35.

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