Effects of nitrogenous materials on translocation and stump exudation in root systems of tomato

Abstract

An application of urea or of potassium nitrate to potted tomato plants in the 11- to 18-leaf stage of growth increased markedly the quantity of solutes in the stump exudate for the next 48 to 72 hours. This increase of solutes was most pronounced during the maximum part of the diurnal rhythmic cycle. A large proportion of the increased quantity of solutes was composed of asparagine, glutamine, aspartic acid, glutamic acid, nitrate, ammonium, etc. and thus directly related to the metabolism of the applied nitrogen. In addition, however, increased quantities of phosphorus and, at certain times, of potassium were noted.In detopped tomato plants the increased transfer of solutes from the treatment with nitrogenous materials increased the maximum rate of stump exudation from 2 ml/(plant × hour) for water control plants to 8 ml/(plant × hour). Single plants produced 80 ml of stump exudate in 24 hours. The time course effect on rate of exudation by the two forms of nitrogen differed in that potassium nitrate produced its maximum effect in from 6 to 8 hours following an addition at 0830 hours but urea produced its maximum effect in from 28 to 30 hours. By the fourth day the rate of exudation from nitrogen-treated plants was approximately the same as from water control plants.The Q10 for the rate of exudation of plants treated with nitrogenous compounds varied from 1.6 to 5.2 with an average value of 3.0. As the soil temperature was decreased from 18° to 8 °C the osmotic pressure of the stump exudate increased and the difference was statistically significant at the 1% level. Between soil temperatures of 18° and 28 °C, however, the difference in osmotic pressure of the exudates were not statistically significant. A reduced transport of water by physical means is suggested as the cause of the increase in osmotic pressure of the stump exudate at 8 °C.When plants treated with nitrogenous materials were compared to water control plants no support was found for the theory that active mechanisms participated in the transport of the water. In fact the marked increase in the osmotic pressure of the augmented exudates indicated that a lag existed in the movement of the water.It was concluded that the addition of nitrogenous materials to the soil increased by active processes the movement of solutes into the xylem sap of the plants. By osmosis the increased quantity of solutes then increased the quantity of water transported through the roots and collected as stump exudate.