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Root morphological traits and biomass allocation were studied in 2-year-old ‘Duke 7’ avocado (Persea americana) trees propagated using seedling and clonal techniques. The plants either were or were not grafted with the scion ‘Hass’. Whole tree excavation 1 year after planting revealed that the propagation technique affected the root growth angle of the main roots (third order roots), the root length density (defined as the total length of roots per volume of soil), and the number of first and second order roots present. The root system of clonal trees showed a typical morphology of rooted cuttings, with a crown of roots originating from a relatively short stem, resulting in a shallow root system. Clonal trees, compared with seedlings, produced main framework roots with shallower angles and more fine roots (first and second order roots) that increased the root length density (defined as the total length of roots per volume of soil). Nongrafted seedlings exhibited a main taproot and lateral roots with narrow angles that penetrated deeper into the soil and increased the aboveground biomass but had a lower root-to-shoot ratio than nongrafted clonal trees. The grafting of both clonal and seedling trees resulted in similar root architecture and revealed that grafting significantly decreased the soil volume explored and the shoot and root biomass. Although both root systems were shallow, grafted clonal trees had a higher root-to-shoot ratio than grafted seedlings. In this study, a distinct class of roots with large diameter and unbranched growth was more abundant in the root systems of clonal trees. These types of roots (previously undescribed in avocado trees), called pioneer roots, may enhance soil exploration in clonal trees.

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C. Estay, R. Cautín, A. Neaman, and M. Castro. 2016. Clonal propagation of the avocado: effects of the rooting step on graft union formation and development. Cien. Inv. Agr. 43(2):233-241. The etiolation and layering technique that is used on avocados has two critical steps, namely, the rooting of clonal rootstock and graft union formation between the commercial variety and clonal rootstock. This study analyzed the rooting step of the clonal rootstock regarding its influence on graft union formation and development. The optimum times for grafting the commercial variety onto clonal avocado rootstock were determined under two greenhouse conditions, heating and no heating. The results showed that under greenhouse conditions without heating, there was higher survival (%) in plants that were grafted 60 days after the rooting steps in the clonal scion, and, in the experiments in a heated greenhouse, the highest survival rate (%) was observed in plants that were grafted 15 days after the rooting treatments. For both assays, the lowest survival rate was observed for grafts performed 30 days after the wounding and auxin application process. In histological cross sections of rootstocks at the wounding zone and in auxin treatments, a large number of emerging adventitious roots were developed at 30 days.

Citronella mucronata (C. mucronata), a tree species endemic to Chile, has become threatened in its natural habitat and is currently listed as vulnerable. Tree population parameters have deteriorated due to indiscriminate logging and other anthropogenic activities, warranting research on mass propagation as a means of recovery. This study, unprecedented for this native species, has developed a successful method for its micropropagation. The objective was to establish a protocol for in vitro propagation of C. mucronata to produce large quantities of high-quality seedlings in an accelerated plant acquisition process. The best results were achieved by growing explants on Murashige and Skoog (MS) basal culture medium supplemented with 4.44 μM 6-benzylaminopurine (BAP) and 14.76 μM indole-3-butyric acid (IBA). Explant survival rate was 78 %, the average shoot length reached 3.2 cm, the number of lateral shoots was 3.9, and rooting rate was 60%. Furthermore, stimulation with red and blue light in a 1:2 ratio, supplemented with 14.76 μM IBA, improved the rooting rate to 93%. The survival rate of rooted explants reached 100% in the acclimatization stage when using peat and perlite substrate (1:1 v/v).

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Alstroemeria pelegrina (A. pelegrina), a Chilean endemic, is considered vulnerable as its natural habitat is currently threatened. The decline in the reproductive capacity of the species due to anthropogenic impacts and climate change has made it imperative to address the problem by developing large-scale propagation methods. The objective of this study was to establish protocols for breaking the dormancy and in vitro germination of A. pelegrina seeds to speed up the germination and seedling production processes. The research began with morphological observations of the reproductive process, followed by in vitro sowing. The results showed that the seeds reached full maturity in 51 days, and physiological maturity in 41 days, at which point the seeds could be harvested for in vitro germination. The mechanical scarification pretreatment improves the in vitro germination rate to 96% and the germination time to 7 days, showing that the species is characterized by physical seed dormancy. On the other hand, if the seed coat incisions are deeper than 0.5 mm, 30% of the potential plants are lost due to embryo damage. The study provides scientific evidence for the feasibility of large-scale in vitro propagation of the species and establishes an efficient method of seedling production.