The response of field‐grown lucerne to a manipulated range of insect‐induced nitrogen stresses

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Abstract
G. W. BOURDOT*SUMMARY: The growth of a 1‐yr‐old lucerne (Medicago sativa) stand subjected to nitrogen stress was analysed using a simple model that related growth to intercepted photosynthetically active radiation (PAR). Nitrogen stress was systematically induced by establishing a range of populations of Sitona discoideus (Coleoptera: Curculionidae) larvae in the stand. The direct influence of their feeding on the plants' nitrogen‐fixing root rhizobial nodules was confirmed by the strong linear relationship found between larval density and herbage nitrogen concentration (N%). The negative effect of nitrogen stress on herbage production was found to become abruptly apparent at an N% level of 4–7%. Eighty‐seven percent of the loss in dry matter production in the nitrogen‐stressed plants was associated with a decline in the ability of the canopy to use PAR, rather than a reduction in the amount of PAR intercepted.From the results obtained it was concluded that the damage effects of S. discoideus larvae were less drought‐related than initially thought. It is speculated that the abrupt onset of damage occurred when the re‐establishing rhizobial root nodule population demanded more nitrogen than it fixed.
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