Foliar nutrient allocation patterns in Banksia attenuata and B. sessilis

Phosphorus (P) and nitrogen (N) are essential nutrients for plant growth, and a lack of these nutrients can limit growth. Plants react to phosphorus and nitrogen deficiencies in different ways. Understanding how plants adapt to poorer soils could help develop more robust crops. To see how plants cope with low phosphorus, Han and colleagues looked at two species of Banksia, Banksia attenuata and Banksia sessilis. These are two trees that grow in the most P-impoverished soils in southwestern Australia. They’ve had to adapt their photosynthesis to use phosphorus efficiently, but how?

Han and colleagues hypothesised that as soil phosphorus decreased, so would the phosphorus concentration in leaves. They also expected Banksia sessilis to have a higher ratio of nitrogen to phosphorus in its leaves than B. attenuata when grown on the same substrate. They also expected B. sessilis to invest more phosphorus in nucleic acids. The reason is due to how the two plants cope with fire.

B. attenuata is a slow-growing species. When fire strikes, it resprouts from buds or tubers. A fire doesn’t mean the end of its life cycle. B. sessilis, in contrast, grows from seed after a fire. That means it has to get things done fast and invest in a seed bank if the species is to survive.

The botanists found they were right about phosphorus concentration dropping in leaves in poorer soils, but not about differences in foliar phosphorus. The leaves of both species had similar concentrations. The team found that the phosphorus was more likely to be in nucleic acids in B. sessilis than B. attenuata. This, the authors say, “is likely to be needed for greater protein turnover associated with rapid growth rates.”

Writing in a commentary on the paper, John Raven adds: “This intriguing finding focuses attention on the important ecological role of nucleic acid investment, which has often been overlooked because the cost of nucleic acids is relatively low in plant tissues. However, because the study focused on P allocation in mature leaves, the underlying mechanism becomes a concern… Thus, for the moment it is not possible to determine the extent to which the RNA content of mature leaves can be explained in terms of the requirement for protein turnover.”