Author: Dr. David Harry, Phd Forest Genetics
TerViva’s aim is to increase the value of under-utilized agricultural land by growing the leguminous tree pongamia (Pongamia pinnata) for its valuable oilseed crop. Pongamia was introduced into the US decades ago, and subsequently planted primarily as a landscape tree. In urban landscapes, pongamia is valued for its deep shade and ability to tolerate the tough urban environments. One of pongamia’s drawbacks in such environments is that it is considered a “messy” tree, brought about by shedding of its profuse flower petals and pods. Ironically, these same traits considered undesirable in an urban setting are exactly what are considered valuable in an agricultural setting. In order to better understand pongamia’s reproductive biology (timing of floral and fruit development, pollinators, fruit drop, etc.), we approached a few urban organizations about examining the reproductive biology of mature pongamia trees in their care. Unfortunately, our overtures were not well received (“Let me get this straight, you want to do what?”). Needless to say, we were thrilled to observe flowering on some of our 2-year-old trees in Texas. Finally we had an opportunity to make some first-hand observations.
The connection between flowers, fruit, and income is likely obvious to many agriculturists, but this significance may be less obvious to others. Simply put, pongamia’s oilseed yield is linked directly to its yield of pods (fruit), which are in turn linked to all of the factors influencing pongamia’s reproductive biology, including: overall amount and timing of flower production, pollinators (mostly bees and insects), pollination efficiency, fruit set, fruit retention, pod and seed development, fruit maturation, fruit drop, and finally, harvest efficiency. In order to optimize yield, we must understand the basics behind each of these important factors, and moreover, how they interact. Detailed observations in field conditions play a key role.
So far our observations of young Texas trees have demonstrated several key points, some of which confirm opportunistic observations of mature landscape trees from elsewhere.
1. The timing of flowering and flower development varies considerably among seedling offspring. Pongamia trees do not all flower at the same time—this is something we have observed repeatedly on trees growing around the world. This is significant because several key studies suggest that pongamia’s ability to set fruit and produce seeds is greatly enhanced when pollinated with pollen from neighbor trees. Effective pollen exchange can only take place among trees with overlapping windows of flower production. Hence pongamia orchards (aka groves) established using clonal varieties must be planned to optimize the individual flowering windows in the varietal mix. We recognize that the timing of flowering among young trees may change as they mature, so we plan to continue monitoring flowering in the years ahead.
2. Local pollinators in Texas visit pongamia’s flowers. We were pleased to observe numerous bees visiting flowers (which bodes well for future crop years. Continued growth and development of some pods suggests that at least some flowers were pollinated.
3. Of the many individual flowers within each of pongamia’s floral clusters, relatively few develop into mature pods. Based on observations of many other trees in Australia, India, Florida, and Hawaii, we know this developmental pattern is simply pongamia’s normal MO. Nevertheless, we had been unable to deduce when this lost reproductive potential happens. We are now fairly convinced that the majority of this loss is due to poor fruit set, but we still don’t know how or why this happens (Figure 2).
4. Very young pods suffer attrition, but as yet our sampling scheme is too sparse to offer meaningful generalizations.
5. Remaining pods rapidly increase in both length and width within about 6 weeks after fruit set. During this same time, immature seeds within
pods appear to increase in size at a much slower relative rate.
What have we learned?
A key to improving yield (and cash flow) for pongamia will be to better understand how to optimize fruit set and pod retention. Unless these critical steps take place, overall oilseed yields can be much reduced. Much of what we’ve recently observed coincides with our earlier observations of mature trees. But now we have a better sense of what to expect in Texas. Futher observations, certain to come in the years ahead, will continue to guide our evaluation and selection of elite pongamia varieties. Increasingly, we will integrate reproductive biology to help us develop specific varietal combinations to maximize oilseed yield and minimize lost reproductive potential.
Dr. David Harry is chief of R+D at Terviva Inc.