Estimation of Potassium Recirculation in Tomato Plants by Comparison of the Rates of Potassium and Calcium Accumulation in the Tops with Their Fluxes in the Xylem Stream

Abstract

A procedure to estimate the extent of K recirculation in plants is proposed. This is based on the ratio of the upward fluxes of K to Ca in the xylem sap from root to shoot with the ratio of K to Ca accumulation in plant tops.In a preliminary investigation the factors influencing the K to Ca ratio in the xylem sap were considered. Tomato plants were grown at three levels of K nutrition and harvested at different times during the 24-hour day period. It was shown that the K to Ca ratio in xylem sap changed dramatically depending on the time of sap colection after decapitation, the values falling from over 2 to less than unity over the 4-hour period of colection. Diurnal effects on exudation were less marked but also of significance. The level of K nutrition was oflittie importance. It is suggested that a representative xylem sap from tomato plants can best be obtained from samples taken between 15 and 60 minutes after decapitation.In a second experiment K recirculation was estimated. At nine harvesting stages over a 24-hour period the K to Ca ratio in the xylem sap was invariably higher than the K to Ca ratio of accumulation in the tops over the same period. From this information it was calculated that about 20% of the upward flux of K in the xylem stream resulted from recirculated K.There is current interest in the possible occurrence of K recirculation in plants, followng the work of Dijkshoom (7) and Ben Zioni et al. (2). The scheme these authors have proposed is as follows: K+ and N03 as the major ions taken up by the root are translocated to the shoot in the xylem. The reduction of N03-in the upper plant parts induces OH-production NO3-+ 8H+ + 8e -* NH3 + 2H20 + OHwhich in tum stimulates malate synthesis and accumulation from P-enolpyruvate (12,20). Some of the K+ is then redirected via the phloem to the roots in association with malate ions. In the roots malate is decarboxylated to pyruvate and a negative charge is excreted from the plant as HCO3 in exchange for the uptake of a further N03-ion. This is consistent with the general observation that intact plants supplied with N03N almost always take up an excess of anions over cations (6,14,15 (3,5,10). Once Ca is translocated to the tops via the xylem pathway it is not redirected. Indeed, Ca is known to be the most immobile macronutrient (1 1). A comparison of the K/Ca ratio of accumulation in plant tops with that in the xylem sap should thus provide a means of estimating the extent to which K is recirculated within the plant. If for a given time period there is evidence of a lower K/Ca ratio of accumulation in the tops than in the upward flux, this is indicative of K recirculation in the plant. In other words if K is recirculating, the upward flux of K comprises two components: one directly from absorption from the nutrient medium, and the other from recirculation of K translocated from the tops via the roots. Using this concept we have estimated the extent of K recirculation in tomato plants.The difficulty in dealing experimentally with the above app...

Effects of NaCl on Flows of N and Mineral Ions and on NO3- Reduction Rate within Whole Plants of Salt-Sensitive Bean and Salt-Tolerant Cotton

“…1A and 2A) was less than the 80% reported for pea (Duarte and Larsson, 1993 the 68% for soybean (Touraine et al, 1988), or the 33% for lupin , but was in the range of the 20% for tomato (Armstrong and Kirkby, 1979). The presence of 50 mM NaCl in the culture solution similarly reduced K+ uptake (by 41 and 12% in bean and cotton, respectively) and xylem transport from roots to shoot (by 49 and 14% in bean and cotton, respectively); however, NaCl reduced phloem translocation of K+ (by 74 and 61% in bean and cotton, respectively) to a greater extent than uptake or xylem transport (Figs.…”

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

“…The extent to which the movement of organic anions to the roots, with metabolism and OH" excretion, occurs in plants reducing NO3" in their shoots is variable (e.g. Armstrong & Kirkby, 1979;Van Beusichem et al, 1985;Allen & Raven, 1987;Van Beusichem, Kirkby & Baas, 1988;Touraine, Grignon & Grignon, 1988).…”

Ion distribution in relation to leaf age inLeptochloa fusca*(L.) Kunth (Kallar grass)

“…Ricinus; Jeschke & Pate, 1991) and corresponds with a continued Ca uptake via xylem and low phloem mobility of Ca (MacRobbie, 1971;Armstrong & Kirkby, 1979). Interestingly, Mg concentrations also increased with leaf age (Figs 2 and 5), indicating that xylem import exceeded phloem export even though Mg is easily phloem-mobile (MacRobbie, 1971) and is readily exported from leaves (Jeschke, Pate & Atkins, 1987;Jeschke & Pate, 1991).…”

“…If Mg ions were not re-exported from the leaf, then the ratios of increase in Ca and Mg concentrations should resemble the ratios in the xylem sap. Secondly, knowing the Ca/Mg ratio in the xylem sap and assuming that Ca export via phloem was negligible, phloem export of Mg can be estimated from the increase in Ca and Mg content in the leaf (Armstrong & Kirkby, 1979). On this basis it was estimated that 40 or 47 °o of xylem-imported Mg was re-exported from the laminae of mature leaves of Leptochloa fusca grown in the presence of 10 or 100 mM NaCl.…”