Differences in potassium utilization by grape rootstocks: evaluation by relative growth rate, efficiency ratio and utilization efficiency.

Seven 2-years old grapevine rootstocks were grown in an open greenhouse in 15 L pots with different treatments to provide three levels of K fertilization with N and P (OmMK, 60mMK, 120mMK) and a control treatment with no added fertilizer. The effects on growth and total nutrient concentration were evaluated in order to examine for differences in K-utilization efficiency. Growth analyses were carried out to determine K-content, dry weights, K efficiency rate (KER) and K utilization efficiency (KUE); relative growth rate (RGR) was calculated in the 75–105 day period. Pots were daily watered and treatments repeated three times. K concentrations in plant dry matter were different among the seven rootstocks: the K-efficient ones absorbed K in proportion to K availability, whereas the inefficient ones (Rup. du Lot) did not greatly modify tissue K concentration in response to K availability. The K efficiency ratio (KER) values were much greater for the control and the lowest dose (OmMK) than with higher K concentrations. The efficiency decrease was highly significant for K efficient rootstocks (Riparia G.G., Berl. Ress.1, SO4). The potassium utilization efficiency (KUE) had the highest values at control and at OmMK dose for the K-efficient rootstock SO4; Riparia G.G. reached the highest KUE value at 60mMK dose, as a confirmation of its K utilization efficiency. Relative growth rate from 75–105 days decreased greatly irrespective of tissue K content and therefore significant K effects were not detectable. High value of RGR occurred for control vines which in that time had yet a considerable growth rate