Seed and Seedling Nursery Characteristics for 10 USDA Citrus Rootstocks

Abstract

<jats:p>Six new hybrid rootstocks, ‘US-1279’, ‘US-1281’, ‘US-1282’, ‘US-1283’, ‘US-1284’, and ‘US-1516’, were released from the U.S. Department of Agriculture (USDA) citrus breeding program to provide improved tree tolerance to huanglongbing (HLB), the most destructive disease facing the citrus industry in the United States and many other parts of the world. Five of these new rootstocks were released based on field performance in trials with the rootstocks propagated by stem cuttings, rather than the traditional propagation using nucellar seedlings. In this study, we evaluated the fruit, seed, and seedling characteristics of these new rootstocks, along with four other USDA rootstocks of commercial importance. The study included a determination of the percentage of true-to-type and off-type seedlings by both plant morphology and simple sequence repeat (SSR) markers. All 10 rootstocks produced an acceptable number of seeds and good seedling emergence from those seeds. The rootstocks ‘Swingle’, ‘US-802’, ‘US-812’, ‘US-1283’, ‘US-1284’, and ‘US-1516’ had a high percentage of true-to-type seedlings and correspondingly good potential to be propagated by seeds. However, no true-to-type plants were observed among seedlings from the rootstocks ‘US-1279’, ‘US-1281’, and ‘US-1282’, indicating that economical seed propagation will be impossible for these cultivars. The 10 SSR marker sets used in this study were observed to easily differentiate the 10 rootstocks studied, and readily distinguished true-to-type and off-type seedlings among progeny from all 10 rootstock clones. This study presents information of significant value for commercial nurseries involved in propagation of citrus rootstocks, and those involved in citrus rootstock breeding and development around the world. We propose the use of these 10 SSR marker sets as readily applicable for accurate identification of most citrus rootstock cultivars and their true-to-type seedlings.</jats:p>

Rootstock Influences on Health and Growth Following Candidatus Liberibacter asiaticus Infection in Young Sweet Orange Trees

“…Eleven citrus rootstocks were used in the study (Table 1), five rootstocks of long-term commercial importance (including two that were released by USDA more than 40 years ago), five popular USDA hybrid rootstocks released in the last 20 years, and one other USDA hybrid rootstock being considered for commercial release [12,16]. All eleven rootstocks are efficiently propagated by uniform nucellar seedlings [21], and for this study were grown from certified seed source trees located at the Whitmore Foundation Farm (Groveland, FL, USA) for the respective rootstocks. The seed were harvested from the source trees in the previous season, treated with 8-hydroxyquinoline sulfate, and stored at 4 • C until use.…”

Horticultural Attributes and Root Architectures of Field-grown ‘Valencia’ Trees Grafted on Different Rootstocks Propagated by Seed, Cuttings, and Tissue Culture

“…The phenomenon of nucellar embryony in citrus allows the true-to-type production of plants from the seeds of many citrus species (Koltunow et al, 1995), including all rootstocks of commercial importance. However, because of the high demand for new HLB-tolerant rootstocks, there is a shortage of seeds for some of the most desired cultivars (Albrecht, et al, 2020). In addition, although nucellar embryony is common, not all cultivars exhibit this trait and some outstanding new rootstock cultivars may be unsuitable for seed propagation (Bisi et al, 2020).…”

“…However, because of the high demand for new HLB-tolerant rootstocks, there is a shortage of seeds for some of the most desired cultivars (Albrecht, et al, 2020). In addition, although nucellar embryony is common, not all cultivars exhibit this trait and some outstanding new rootstock cultivars may be unsuitable for seed propagation (Bisi et al, 2020). For these reasons, it is valuable to also make use of alternative propagation methods such as cuttings and tissue culture to produce genetically identical rootstocks that can be used as liners for grafting (Albrecht et al, 2017a).…”

“…One of the main concerns is the different root system architecture of vegetative propagated rootstocks, particularly the absence of a taproot. During the nursery stage, seed-propagated rootstocks usually have a well-defined single tap root system, whereas cuttings-and tissue culture-propagated rootstocks have an adventitious root system with multiple smallerdiameter roots (Albrecht et al, 2017a(Albrecht et al, , 2020. It is generally assumed that the lack of a tap root system can render trees more susceptible to wind-induced uprooting, which is of concern in Florida where tropical storms and hurricanes are an annual threat (Castle, 1977;Crane et al, 1993).…”

“…It is well documented that different rootstock cultivars exhibit differences in their root architecture (Albrecht et al, 2020;Castle and Youtsey, 1977;Eissenstat, 1991) that contribute to their influence on the aboveground horticultural traits (Bowman et al, 2016a(Bowman et al, , 2016bBowman and Joubert, 2020) and may interact with the propagation method. In our laboratory, we recently investigated root architectures of rootstock liners and grafted trees during the nursery stage (Albrecht et al, 2017a, 2020).…”