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166 Plant Soil (2008) 304:157–168Table 4 Relative (K0/K+) growth (leaves, axile and lateral roots) and physiological (RUE, water content, sugar content) responses toK deficiency, towards thermal time, stage and K concentration in starved plantsThermal Visible [K] intime afterstarvationleafstagePlantDM K0[K] in plant waterLeafElongationRateRoot Elongation Rate RUE Water contentSoluble sugars˚C days g.g -1 1 0.9 0.7 0.5 0.3 0.1 1 0.9 0.8 0.7 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 1 0.9 0.8 0.7 1 0.99 0.98 0.97 0.96 0.95 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.730 5,0 0.04 + + + +60 5.9 0.034 + + + +90 6.8 0.018 + + . + +120 7.8 0.015 + + . + +150 8.7 0.011 + + . + +180 9.6 0.009 + + . +leaves+ axile roots. lateral roots+ leaves + axile rootsMain gross tendanciesvalues of K0/values of K+) for most of thesevariables. On the whole, the plant response patternduring the initial period (moderate K deficiency) maybe interpreted as the consequence of an initial effectof K deficiency on the ability of leaf growing zone toexpand correctly. Conversely, photosynthetic processesdon't seem to be affected. According to Lockart(1965), cell growth is controlled by turgor pressureand cell wall extensibility. Potassium is involved inthe control of both processes, so that the observedreduced leaf elongation may be the consequence ofaltered cell-wall properties, osmotic and turgor pressures(Triboulot et al. 1997). These unfavourablebiophysical conditions are not likely to affect the rootcompartment, especially in our hydroponic conditions(Sharp et al. 1988, 1990), whereas roots are likely toget more carbohydrates from shoots since theirutilisation in aerial parts is limited by the reducedleaf growth. This latter interpretation is supported byfour observations (1) root elongation was stimulatedbetween 45 and 75 degree-days, whereas leaf elongationwas reduced, (2) this stimulation was associatedwith an increased soluble sugar content. Thishigher sugar content may contribute to osmoticpotential and provide carbon for growth, (3) in ourexperimental conditions (low incident PAR) rootgrowth was likely to be limited by C, which isconfirmed by the existence of correlations betweenRER and incident PAR, (4) the RUE was not affectedby K treatments during the first part of the experiment,which suggests that net carbon assimilation perunit leaf area was not reduced, but its allocationbetween shoots and roots was altered, as confirmedby the root biomass/plant biomass ratio. This “unfavourablebiophysical conditions” hypothesis and itsassociated effects on carbon allocation, which seemssufficient to account for the plant response during thefirst part of the experiment, however failed to explainwhat happened during the second part of theexperiment. The root elongation was reduced, bothon axile roots and to a greater extent on first-orderlateral roots, and the RUE was severely reduced,which suggests that net carbon assimilation per unitleaf area was affected. Other physiological processesare likely to be affected, possibly due to thebiochemical functions of K.Theoritical K concentration in cell compartments (mM)120100806040200[K]vacuole[K]cytoplasmRelative DM0 10 20 30 40 50 601.21.00.80.60.40.20.0K content in dry matter (mg . kg -1 )Fig. 8 Relative dry matter yield (DM K0/DM K+) andcalculated concentrations of K in the cytoplasm and vacuoleas a function of K content in the dry matter (mg kg −1 ). Figureadapted from Leigh and Wyn Jones 1984, with the assumptionsthat [K] cyt =101 mM, [K] min vac =19 mM and that the proportionof intracellular volume occupied by the cytoplasm is 10%Dry matter K0 / K+