Impact of a controlled infestation of pea aphids on spatial distributions of growth, water and dry matter deposition in elongating alfalfa stems.

Stem elongation is one of the main processes contributing to shoot growth and yield in alfalfa (Medicago sativa L.). Stem elongation rate (SER) is very sensitive to aphid infestation. In alfalfa stem, the elongation zone (EZ) is a small part of the entire stem and even within the EZ, the growth activity is very heterogeneous. To analyse aphid effect on growth, it is thus important to locate the EZ and to quantify the extent and spatial. distribution of the growth reduction under aphid infestation. In this study, a growth kinematical analysis of the effect of a controlled aphid infestation (with Acyrthosiphon pisum Harris) on the spatial distribution of i) growth and ii) water and dry matter (DM) net deposition rates is reported. Aphid infestation (200 apterous adults in pre-reproductive phase, located on the elongating zone during 24h) caused a reduction of the SER of 70%. This involved shortening the EZ by 42% as well as reducing the maximum of relative elemental growth rate (REGR (h(-1))) by 60%. The spatial patterns of water net deposition rates were similar in both treatments (control and infested) and were related to REGR distribution. But absolute rates of water deposition rates were lower in infested stems than in control throughout the EZ. The spatial patterns of DM net deposition rates were also similar in both treatments but differed from REGR distribution. Absolute rates of DM deposition were lower in infested stems than in control stems along and beyond the EZ. From the results, it is concluded that the effect of a controlled aphid infestation on alfalfa SER resulted in a severe reduction of water and DM fluxes entering the EZ, supporting a trophic hypothesis for the impact of aphids.