Differential response of citrus rootstocks to CuEDTA concentration in sand culture

Abstract: Florida citrus groves that have been under continuous production for many years often have high levels of soil-fraction copper (Cu) from the use of Cu-containing fertilizers and pesticides. On such groves, citrus trees may develop Cu toxicity, a disorder that impacts both plant growth and nutrition. The objectives of this study were to investigate the growth and nutritional response of six citrus rootstock seedling varieties grown in sand in 3.8 L containers to increasing concentrations of Cu-ethylenediaminetetraacetic acid (CuEDTA). Citrus rootstocks included in the study were: 'Swingle' citrumelo [SC (Citrusparadisi x Poncirus trifoliata)], 'Volkamer' lemon [VL (C. volkameriana)], 'Cleopatra' mandarin [CM (Citrus reticulata)], 'Flying Dragon' trifoliate [FD (P trifoliata)], 'US-812' [US812 ('Sunki' mandarin x 'Benecke' trifoliate)], and 'US-897' [US897 (CMx FD)]. Incorporated into a complete nutrient solution, Cu was supplied at 0.05, 0.25, 1.00 and 2.00 mg L^-1. Citrus rootstock but not Cu treatment was significant for root and leaf dry mass with FD and VL having the least and greatest total plant dry mass, respectively. Rootstock and Cu treatment was significant for root and leaf Cu. As a mean of Cu treatments, foliar Cu ranged from 4.05 ug g^-1 (CM) to 7.74 ug g^-1 (US812); and root Cu ranged from 30.18 ug g^-1 (FD) to 61.08 ug g^-1 (VL). Rootstock but not Cu treatment was significant for Ca, K, Mg, P, Fe, Mn and Zn. 'Volkamer' lemon had significantly higher levels of foliar Ca, K, and Mg than the other rootstocks; and along with US812, the highest level of foliar Fe. For all nutrients analyzed except for Mg, accumulation was greater in roots than in leaves. Magnesium, as a mean of rootstocks, accumulated equally in roots and leaves. Subjective visual observations of plants at harvest for nutrient disorder revealed that young terminal-growth leaves of VL and SC in the highest Cu treatment (2.00 mg L^-1) showed few to pronounced symptoms of a micronutrient-type disorder, respectively, that correlated with increasing Cu treatment. Based on visual symptoms in the highest Cu treatment (interveinal chlorosis and leaf/leaflet deformation/cupping), plants segregated as follows from greatest to least expression of the observed micronutrient-like disorder: SC > CM/FD > US812/US897 > VL.