Influence of Citrus Rootstocks and Propagation Method on Root Structure and Growth in Nursery and Field Environment

Rootstock is important for production of many tree crops, especially citrus. In Florida, demand for new rootstocks has increased tremendously due to the negative impact of huanglongbing (HLB, a.k.a. citrus greening). As most commercial scion varieties are highly susceptible to HLB, tolerant rootstock varieties are used to mitigate the negative impacts of the disease. Because the current demand for new HLB-tolerant rootstocks exceeds the available supply, nurseries are often making use of rootstock plants propagated by tissue culture or by cuttings. This study examines root traits of different commercially important rootstock varieties propagated by seed, cuttings, and tissue culture, both at the nursery stage and under field conditions. During the first months in the nursery, significant differences were observed for several root parameters, such as root mass fraction, number of roots and specific root length (SRL) among rootstocks as well as among propagation methods. Root number and SRL were significantly correlated at this growth stage and were highest in rootstocks generated from cuttings and lowest in seedlings. Rootstock-scion interactions were observed. The impact of propagation method was less evident in the grafted field-ready tree, but root architectural traits remained consistent within rootstock variety. The highest SRL was measured for the lowest vigor-inducing rootstock, whereas the lowest SRL was found for the highest vigor-inducing rootstock. Plants from the same experiment were planted in the field in November 2017 in a split-plot design with rootstock as the main factor and propagation as the subplot factor. Root growth was measured monthly using a minirhizotron root imaging system. Tissue culture appeared to positively influence root growth in the early months after planting. The application of ground penetrating radar technology for non-invasive imaging of the rooting system is being investigated.