Root properties of sugarcane (Saccharum officinarum) in relation to nutrient uptake

Abstract

An investigation was undertaken to select the appropriate methods and sampling times for measuring root cation exchange capacity (C.E.C.) and anion exchange capacity (A.E.C.) and these together with root weight and root surface area index (g saturated calcium nitrate solution covering the root surface) were related to the total uptake of N, P, K, Ca, Mg, Si and S by four sugarcane varieties (HSO-7209, HS7-5174, NCo 310, and HS6-5840) grown in solution and soil culture. The three methods selected for measuring root C.E.C. in decreasing order of differentiation between varieties, were equilibration of Ca45 with chilled fresh roots, exchange of Ca40 with chilled fresh roots and a titration method using ground dry roots. Displacing P from P saturated fresh roots with NaOH solution was appropriate in differentiating reproducibly between varieties, for measuring root A.E.C. The root C.E.C. and A.E.C. results were expressed as weight C.E.C. or A.E.C. (m.e./100 g dry roots), surface area index C.E.C. or A.E.C. (m.e./100 g saturated calcium nitrate solution) and C.E.C. or A.E.C./pot (m.e./ pot). Root type (sett or shoot) had no effect on root C.E.C. values. The C.E.C. for sett roots increased with increasing age whereas that of shoot roots decreased with increasing age. The range of C.E.C. and A.E.C. values for shoot roots was narrow for the varieties examined. Better differentiation between varieties was obtained when C.E.C. was measured by equilibration of Ca45.Varietal rankings were consistently reproduced in the several experiments when C.E.C., as measured by the three methods, was expressed on a surface area index or total pot bases. The root C.E.C. and A.E.C. values increased with increasing root weight when greater amounts of potassium were applied in both culture media. The root C.E.C. decreased in soil with better physical conditions. The cane varieties grown in solution and soil were markedly different in their growth and nutrient uptake. In the solution culture the growth and uptake were in the decreasing order of HS7-5l74, HS6-5840, HSO-7209 and NCo 310. In the soil the array was HS7-5l74, NCo 310 and HSO-7209. The C.E.C. and A.E.C. were essentially unrelated to the amounts of cations or anions sorbed by the plant, probably because total ion uptake is primarily controlled by diffusion and active absorption rather than exchange. In solution culture the root weight, root surface area index and exchange capacity values/pot were significantly correlated with shoot weight and, except for P and Si, all nutrients analyzed for. The greatest relationships were obtained with root surface area index. In the soil the root weight was significantly correlated with shoot weight and the uptake of N, Mg and S. The root weight, root surface area index and exchange capacity values/pot measured in the solution culture were related to the root weight, shoot weight and ion uptake in the soil. Possibly the measurement of root weight, root surface area index and exchange capacity values/pot by the solution culture method may aid in making a preliminary assessment of varietal differences in nutrient uptake in the field.