The effect of potassium deficiency on growth and N₂‐fixation in Trifolium repens

Abstract

The effects of potassium (K) deficiency on growth, N2‐fixation and photosynthesis in white clover (Trifolium repens L.) were investigated using natural occurring gas fluxes on the nodules in real time of plants under three contrasting relative addition rates of K causing mild K deficiency, or following abrupt withdrawal of the K supply causing strong K deficiency of less than 0.65% in dry matter. A steady‐state below‐optimum K supply rate led to an increase in CO2‐fixation per unit leaf surface area as well as per plant leaf surface. However, nitrogenase activity per unit root weight and per unit nodule weight was maintained, as was the efficiency with which electrons were allocated to the reduction of N2 in the nodules. Abrupt K removals stimulated nodule growth strongly without delay, but as K concentrations decreased in the plant tissue a significant decline in nitrogenase activity per unit root weight as well as per unit nodule mass occurred. Further, the rate of photosynthesis per unit leaf area was unaffected, while the CO2 acquisition for the plant as a whole increased due to an expansion of total leaf area whereas the leaf area per unit leaf weight was unaffected. The ratio between CO2‐fixation and N2‐fixation increased, although not statistically significant, under short‐term K deprivation as well as under long‐term low K supply indicating a downregulation of nodule activity following morphological and growth adjustments. This downregulation took place despite a partly substitution of the K by Na. It is concluded that N2‐fixation does not limit the growth of K‐deprived clover plants. K deprivation induces changes in the relative growth of roots, nodules, and shoots rather than changes in N and/or carbon uptake rates per unit mass or area of these organs.

Functional plasticity of Trifolium repens L. in response to sulphur and nitrogen availability

“…Our study confirms the high white clover plasticity, as already established for other stresses (Frankow-Lindberg 1999;Gautier et al 2001;Hogh-Jensen 2003). This plasticity should allow white clover to develop across a large range of sulphur and nitrogen availability in soils.…”

Soil amendment with biochar increases the competitive ability of legumes via increased potassium availability

“…This led to a significant increase in red clover biomass in the 3-species mixed culture in those treatments where K was applied (either as biochar or as fertilizer) and where no N fertilizer was applied (Table 3). The root system of red clover is shorter, thicker, and less branched than grasses, potentially putting it at a disadvantage in terms of nutrient uptake (Evans, 1977;Høgh-Jensen, 2003). Therefore, under low K conditions, the proportion of legumes in a mixed culture may be expected to decrease as the legumes are outcompeted for K acquisition (Mengel and Kirkby, 2001).…”

Interactions between nitrogen, phosphorus and potassium determine growth and N2-fixation in white clover and ryegrass leys

“…Growing a leguminous crop in mixture with grasses ensures some interspecific complementarity in resource use as the white clover is actively fixing atmospheric N 2 , while the grasses rely on inorganic soil N. The activities of nitrogenase activity in the nodules of legumes are regulated on a whole plant level under P as well as K deficiency (Feigenbaum and Mengel 1979;Høgh-Jensen et al 2002;Høgh-Jensen 2003). Thus, nutrient limitation in mixtures was manifested in yield variations (Table 1).…”

“…Increased access to inorganic nitrogen (N) has been shown to stimulate the growth of grasses relative to that of clovers in mixed leys, thus affecting species diversity and herbage quality (Ledgard and Steele 1992;Høgh-Jensen andSchjoerring 1994, 1997). Also the phosphorus (P) and potassium (K) supplies can change the botanical composition in grasslands (Høgh-Jensen et al 2001;Iversen 1943;Lowe 1967;Wildin 1979).…”