Handbook of Vanilla Science and Technology

Vanilla seeds are difficult to germinate, and this is one of the factors limiting development of new Vanilla cultivars. Vanilla seeds have been found germinating on rotten wood (Childers et al. 1959), which suggests that fungi may be required to break the seed coat and allow water to reach the embryo. Symbiotic germination techniques may be useful in overcoming this barrier, both by stimulating germination and by reducing contamination.

16.5.2 Seedling growth and survival

Seeds and mericlones of Vanilla and other orchids are usually propagated in sterile culture under laboratory conditions. When plantlets are removed from culture flasks and moved to the greenhouse they are suddenly exposed to environmental conditions and potential pathogens such as bacteria and fungi. During this transition, plants become very susceptible to infections and desiccation, and many plantlets die or lose vigor in the process.

We tested whether inoculation with mycorrhizal fungi aided growth and survival of sterile Vanilla plantlets (Porras-Alfaro and Bayman 2007). Mericlones of V. planifolia × V. pompona were bought from Twyford Laboratorio de Plantas SA, Costa Rica Division, Alajuela, CR. Flasks were inoculated with the same fungi used for seed germination experiments. Twenty plants were used for each treatment, and thirty for uninoculated controls. Plant height and number of leaves was measured once a month for 5 months. Some control Vanilla mericlones were naturally colonized by Ceratobasidium after 5 months in the greenhouse, which suggests that airborne inoculum is common.

Significant differences were found among treatments after 1 month’s growth (Figure 16.6). Plants inoculated with a Ceratobasidium isolate from Vanilla and three isolates from the orchid Ionopsis utricularioides grew as much or more than control plants without fungi, and produced as many or more new leaves than control plants. However, plants inoculated with four isolates of Ceratobasidium and three isolates of Tulasnella grew significantly less than uninoculated control plants and produced significantly fewer new leaves. Moreover, plants inoculated with two Ceratobasidium isolates and one Tulasnella isolate showed a significant loss of leaves (Porras-Alfaro and Bayman 2007).

Fig. 16.6 Effects of mycorrhizal fungi on growth of mericloned Vanilla plants one month after inoculation. (A) change in plant height; (B) change in number of leaves. Error bars = s.e., N = 20 plants/treatment, and N = 30 for uninoculated controls. Bar shading indicates the fungi used: Tulasnella, Ceratobasidium isolated from Vanilla, Ceratobasidium isolated from I. utricularioides, and control seeds without fungi. From Porras 2004, Porras-Alfaro and Bayman 2007.

Closely related fungi tended to have similar effects on growth of Vanilla plantlets (Figure 16.7; Porras-Alfaro 2004). The two Ceratobasidium isolates that reduced both seed germination and plant growth were very closely related to each other. The three isolates from Ionopsis, which stimulated both germination and growth, were also very closely related to each other. A similar finding was reported for Ceratobasidium isolates from epiphytic orchids in Puerto Rico (Otero et al. 2005). This association between phylogeny and function is useful, because it suggests a strategy for finding new useful isolates: choose isolates that are closely related to known, successful isolates.

Fig. 16.7 Fungal phylogeny predicts effects of mycorrhizal fungi on seed germination and plant growth. Relationships between fungal isolates are shown in phylogenetic tree below, based on Figure 16.4. Top: effects on numberof leaves of mericloned Dendrobium Jaquelyn Thomas hybrid plants in flasks. Flaskswith approximately 300 Dendrobium plants were inoculated with fungi, six flasks per treatment. After 2 weeks plantsweretaken outoftheflasks, sown in sterile peatand keptinanoutdoorgreenhouse under natural light. Survival after 2 weeks and the number of leaves after 5 months are shown. Only four isolates were used. Middle: germination of Vanilla planifolia seeds and growth of V. planifolia × pompona hybrids, as seen in Figures 16.5 and 16.6. Bar shading indicates the fungi used: Tulasnella, Ceratobasidium isolated from Vanilla, Ceratobasidium isolated from I. utricularioides, and control seeds without fungi. From Porras-Alfaro 2004, Porras-Alfaro and Bayman 2007.