Deficiency of potassium but not phosphorus enhances root respiration

Root respiration of kohlrabi (Brassica oleracea var. gongylodes) was measured non-destructively in vivo by infrared gas analysis of complete root systems, using potted plants in sand culture and nutrient solutions, for six weeks under (a) nutrient sufficiency, (b) deficiency of all mineral nutrients, (c) potassium deficiency or (d) phosphorus deficiency. This was to study the adaptation to nutrient stress in terms of changes in root growth, root respiration, assimilate allocation and energy requirement for nutrient uptake. Both deficiencies of phosphorus and potassium increased the root:shoot-ratio. This was attributed to the plants transferring a larger relative proportion of assimilates to the roots than to the shoots relative to nutrient-sufficient plants. Roots of nutrient-sufficient kohlrabi respired 1.7 or 7.7 mg CO2 h(-1) per g fresh or dry matter, respectively. However, potassium deficiency enhanced root respiration to 2.4 mg CO2 h(-1) or 12.2 mg CO2 h(-1) on a per g fresh or dry weight basis respectively. This originated from an additional 2.6 mg glucose g(-1) dry matter h(-1) allocated to the roots and provided 50 Joule additional energy (150 versus 100 Joule g(-1) dry matter h(-1)) which may become available for the proposed K+:H+ symporter for potassium uptake.