Cuticular penetration of potassium salts:: Effects of humidity, anions, and temperature

Cuticular penetration of potassium salts across astomatous cuticular membranes isolated from pear and Citrus leaves has been studied. Penetration was a first order process and was greatly affected by humidity and hygroscopicity of salts. Temperature did not significantly influence rates of penetration in the range of 10 - 25 degreesC. Penetration required dissolution of the salt and this is determined by the point of deliquescence (POD) and humidity. When humidity is above the POD the salt residue on the cuticle dissolves, while below a solid residue is formed and penetration ceases. The POD for K2CO3 is 44% at 20 degreesC and above this humidity salt penetration was possible. Rate constants of penetration increased from 0.022 to 0.045 h(-1) when humidity increased from 50 to 90%. With KCl (POD of 86%) rates were highest at 90% and lower at 100% while with KNO3 (POD of 95%) and KH2PO4 (POD of 97%) highest rates were measured at 100% humidity and at 90% penetration was extremely slow. With KCl, KNO3 and K2CO3 maximum rate constants of penetration across pear CM were around 0.04 - 0.05 h(-1) which correspond to half times of penetration of 14 - 17 h. Rate constants measured with Citrus CM were up to two times higher but response to humidity was similar. The potassium salts studied did not only differ in POD and sensitivity to humidity but also in potassium content, which is highest with K2CO3 (57%) and KCl (52%) and much lower with KNO3 (38%) and KH2PO4 (29%). Thus, K2CO3 is best suited for foliar applications as rates of penetration were large at 50% humidity and higher, while with the other salts humidity must be 90 - 100% for maximum rates of penetration.