Extension growth of Impatiens glandulifera at low irradiance: importance of nitrate and potassium accumulation.

BACKGROUND AND AIMS The summer annual Impatiens glandulifera can reach 3 m in height within deciduous woodland. The primary objective was to determine if NO(3)(-) accumulation, and hence its osmotic effect, is an important physiological mechanism allowing Impatiens to achieve substantial height under low irradiance. METHODS Stem extension, concentrations of K(+) and NO(3)(-) in leaves and concentrations of K(+), NO(3)(-) and other inorganic anions, malate, sugars, total N and total osmoticum in stem were measured in I. glandulifera sampled at different irradiance levels in deciduous woodland and in a glasshouse. Also, the energetic costs, as absorbed photons, of generating osmolarity in stem cell vacuoles with KNO(3), K(2)malate or hexose sugar were determined. KEY RESULTS Results were similar in the woodland and glasshouse. At 50-100 % relative irradiance (Ir; open ground PAR = 100 % Ir) and 2-10 % Ir, plant height increased from 7-14 cm to 130-154 cm in 64-67 d. Leaf and stem NO(3)(-) concentrations were negligible at 50-100 % Ir while K(+), malate(2-) and sugars, respectively, accounted for 33.2-50.1 %, 19.3-20.8 % and 2.0-2.6 % of total osmoticum in stems. At 2-10 % Ir, NO(3)(-) concentrations were four to eight times greater in stems than leaves. Here, NO(3)(-) constituted 26.7-34.3 % of the total osmotic concentration in the stem and NO(3)(-)-N constituted 69-81 % of total N in stem tissue. Also at 2-10 % Ir, K(+) comprised 44.9-45.9 % and malate plus sugars 2.2-3.1 % of total osmotic concentration. The energy cost of osmoticum as KNO(3) was calculated as less than half that of malate and less than one-seventh that for hexose. Further calculations suggest that use of KNO(3), K(2)malate or glucose as osmoticum at low irradiance would, respectively, cost approx. 7 %, 16 % and 50 % of the total construction cost of the stem. CONCLUSIONS It is concluded that accumulation of NO(3)(-) in place of organic molecules in stems is an important mechanism allowing I. glandulifera to achieve substantial height at low irradiance.