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Abstract
In recent years, the pressure for replanting and resetting huanglongbing (HLB or citrus greening) affected citrus groves has led to an inadequate seed supply for the most popular rootstock cultivars in the State of Florida, United States. Early fruit harvesting of citrus rootstock source trees might reduce fruit losses and enhance seed availability, especially in HLB-endemic and hurricane susceptible areas, if the physiological quality of the seeds is adequate. The effects of fruit maturity on seed quality and seedling performance of US-802, US-897, and US-942 citrus rootstocks were investigated for two consecutive growing seasons. The study included the evaluation of seed germination and nursery performance of the citrus rootstock seedlings. The germination test was performed in vitro, where seeds were hand-peeled, surface-sterilized and placed in culture tubes containing basal Murashige and Skoog medium. For the emergence test, seeds were sown in seedling trays containing sterilized growing substrate in a greenhouse with controlled-environment conditions. Rootstock fruits from all three varieties harvested in August and September had seeds with higher germination potential, as more than 90% of the seeds generated seedlings. US-942 had more % of emergence than US-802 and US-897, resulting in faster seed germination; in contrast, US-802 had the faster shoot growth rate. Assays on fruit abscission response showed that by August, fruit from all three varieties were responsive to ethylene and abscised, although response varied and was higher in US-942, suggesting the seeds were mature enough. Taken together, our findings indicate that fruits these three rootstocks can be harvested as early as August in contrast to the current procedures without losing germination potential. This will result in an increase in available seeds for nurseries in Florida.
Highlights
Florida’s citriculture faces an unprecedented crisis due to the widespread presence of citrus greening or Huanglongbing (HLB), a disease associated with the phloem-limited bacterium Candidatus Liberibacter asiaticus that has become endemic in the State
We show that sensitivity to abscission induction is a good marker for seed viability, allowing advancing fruit harvesting significantly without loss of seed vigor, and resulting in better yield of seeds of highly demanded citrus rootstocks in Florida
We found that all three rootstocks started to respond to ACC in advancing abscission as early as July, but response was maximized in August; Fruit from US-942 incubated with water (Figure 9 upper central panel) reached 100% abscission by day 6 of incubation, whereas after ACC treatment this time was reduced to 4 days; in the case of US-897, the abscission response after ACC treatment was advanced from 9 to 4 days in fruit harvested in August, whereas in fruit from US-802 abscission was advanced from 10 to 7 days, showing a milder response to the ethylene precursor This set of data further supports the notion that seeds from all three varieties may be ready to be harvested as early as August (Figure 9)
Summary
Florida’s citriculture faces an unprecedented crisis due to the widespread presence of citrus greening or Huanglongbing (HLB), a disease associated with the phloem-limited bacterium Candidatus Liberibacter asiaticus that has become endemic in the State. Citrus trees affected by HLB are characterized by root loss, branch die-back, yellow shoots, blotchy mottle chlorotic patterns on Frontiers in Plant Science | www.frontiersin.org de Carvalho et al. New Citrus Rootstock Seeds Performance leaves, and off-tasting and malformed fruit. HLB infection inhibits root growth, reduces nutrient uptake, and promotes leaf and fruit drop, and whole tree decline that is often lethal (Pustika et al, 2008; Etxeberria et al, 2009). HLB plays a major role in several aspects of citrus production, including the choice of the citrus rootstock. Replanting HLB-affected trees with disease-free nursery trees may require rootstocks resistant to soil diseases as Phytophthora gummosis, in order to guarantee the reestablishment of the grove (Graham et al, 2013)
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