2019 ASHS Annual Conference

In the epidemic of the devastating citrus disease huanglongbing (HLB), also known as citrus greening, Florida growers are seeking solutions to boost net returns on production. High-density planting is one promising strategy to fetch profitable returns during the early years of production before trees start to decline from HLB. Use of rootstocks that induce small tree size or dwarfing is key to the successful implementation of high-density planting in a commercial setting. In several fruit tree crops, tree vigor was shown to be associated with the xylem vessel anatomy, specifically the diameter of the vessel lumen. Other root anatomical and architectural traits such as root diameter, root hairs, and specific root length (SRL) may also play a role in the influence of rootstock on tree vigor. Root anatomical studies in citrus showed that root xylem vessel characteristics were related to the hydraulic properties of trees. The objective of this study was to decipher the xylem anatomy, specifically xylem vessel number, size, and arrangement, and other rootstock-specific traits that may be associated with tree vigor and productivity. We compared two commercial rootstock varieties, ‘US-897’ (Citrus reticulata Blanco x Poncirus trifoliata L.Raf) and ‘US-802’ (C. paradisi Macfad x P. trifoliata) which are known for their different vigor-inducing properties. ‘US-897’ induces small tree-size and production of high-quality fruit. In contrast, ‘US-802’ produces large trees with lower quality fruit. Our study showed that SRL (the ratio of root length to biomass of fine roots) was significantly higher in ‘US-897’compared with US-802, suggesting a negative correlation of rootstock vigor and SRL. In addition, microscopical analysis of roots from two-year-old plants showed differences in vessel element (VE) arrangement. VEs in ‘US 802’ were mostly present in a pore chain arrangement where multiple pores (VEs) are arranged radially, while in ‘US-897’ more solitary pores and few pore chains were observed. There were no quantitative differences in the diameter of the VE lumen and vessel density between ‘US-802’ and ‘US-897’ in roots with different diameters. This study further investigated the importance of plant age, root order, and root diameter on the ability to predict rootstock influence on tree vigor. Characterization of the root system and root xylem architecture at a young age will be valuable for phenotyping citrus rootstocks in a breeding program and to predict the vigor control capacity of new citrus rootstock varieties.