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162 Plant Soil (2008) 304:157–168Dry matter (g.plant -1 )7654321K+ rootsK0 rootsK+ shootsK0 shoots0-50 0 50 100 150 200 250Thermal time since starvation (°C days)Fig. 2 Roots and shoots dry matter (in g per plant) as afunction of thermal time after starvation for treatments K0 andK+ (means±standard deviation)Shoots and roots were affected similarly, so that theroot biomass/plant biomass ratio was not stronglyaltered. It was however slightly higher for K0 plantsjust after K deprivation (between the 5th and 7thvisible leaf stages) and became slightly lower later on(Fig. 3). If compared day by day, the root biomass/R / (R+S)0.450.400.350.300.250.200.154 5 6 7 8 9 10Plant stage (number of visible leaves)Fig. 3 Ratio between the root dry biomass (R) and the totalplant dry biomass (R+S), as a function of the plant stage(number of visible leaves) for treatments K0 and K+ (means±standard deviation)K-K+plant biomass ratio was significantly higher for the K-deprived plants at the third sampling date (97.4°Cdays) and significantly lower at the sixth sampling date(196.8°C days). Although no statistical tests could beperformed, the calculated values of RUE were verysimilar for K0 and K+ treatments for the twocalculation periods just after K starvation, whereasthey differed strongly later on (Fig. 4).Leaf elongation ratesWhen expressed on a daily basis (in mm per day) theleaf elongation rates were linearly related to airtemperatures (R 2 >0.8, data not shown). This resultconfirms that leaf elongation is strongly controlled bytemperature and emphasises the value of expressingleaf elongation rates on a thermal time basis (Ben-Haj-Salah and Tardieu 1995). Significant correlationswere also observed between LER expressed on athermal time basis and the incident PAR accumulatedduring the 3 days preceding the leaf elongationmeasurement (0.34