Adjustment of Media pH Improves In Vitro Pollen Germination In Plumeria

Plumerias are important ornamental plants to Hawaii’s horticulture and landscape industries.  However, the majority of Plumeias are affected by Plumeria rust disease.  Although methods for controlling this disease are available, these methods are laborious and costly.  A more feasible solution would be to develop Plumeria hybrids that are rust-resistant.  Establishing a breeding program will help to achieve this goal.  However, a basic understanding of Plumeria pollen is needed prior to breeding for resistance.  As part of basic pollen studies in a breeding program, the ability of pollen to germinate on artificial media is commonly used to assess pollen viability.  This requires an optimum germination medium and environment.  As an environmental component, pH plays an important role in in vitro pollen germination studies and can affect viability results.  Thus, adjusting media pH may improve pollen germination.  Previous Plumeria pollen studies led to the elucidation of media optimized for tube growth.  However, germination rates were low, and pH was not a factor considered.  In this experiment, the response of Plumeria pollen to pH was tested.  Using modified Brewbaker and Kwack medium optimized for each genotype, P. caracasana, ‘Celadine,’ ‘Lei Rainbow,’ ‘Pops,’ and P. pudica pollen grains were subjected to media of six pH levels (no adjustment, 4.5, 5.0, 5.5, 6.0, 6.5).  Analyzed as a 5x6 factorial within a completely randomized design, analyses of pollen germination and pollen tube lengths revealed a genotype-dependent response to different levels of pH.  This indicates that genotypes require different media pH to achieve high rates of pollen germination (>40%) and long pollen tubes (>1000 µm).  In conclusion, genotype-specific protocols can now be developed to properly assess in vitro pollen viability in support of breeding for rust resistance.