To recycle or steal?

Submitted by editor on 5 July 2016.Get the paper!

Have you ever wondered how plants could save energy by doing things more efficiently? Like other organisms, plants can develop strategies that allow them to be more efficient in acquiring and conserving nutrients. To optimize the benefits of nutrient conservation, plants have evolved strategies such as investing in defence against herbivory, storage for future use or controlling leaf senescence by re-absorbing nutrients from leaves prior to leaf fall - a process known as nutrient resorption. Therefore, plants face important trade-offs between the energy spent on acquiring and conserving nutrients, and the energy used for other metabolic purposes. Acquiring nutrients from the soil or through the resorption process have metabolic costs, and the balance between use of soil-derived and resorption-derived nutrients should be set by their relative costs. Usually, nutrients are so limited in the soils that plants heavily invest in nutrient resorption, and high proportions (> 60%, on average) of leaf nitrogen, phosphorus and potassium are resorbed from leaves before they are shed.

 

Plants usually acquire nutrients by investing carbon to build roots, and fostering symbiotic relationships with N-fixing bacteria and/or mycorrhizal fungi. However, not all plants evolved to build root systems to forage for soil nutrients. There is a large group of plants that, instead of relying on the soil, they acquire nutrients from another plant through a “sucking root” named haustorium (Figure 1). These plants are parasitic organisms. Mistletoes are xylem-tapping parasitic plants that tap water and nutrients from their hosts. This process is presumably much “cheaper” in terms of energetic costs compared to accessing nutrients in the soil, because there is no need to invest a considerable proportion of their carbon budget in building and maintaining root systems. Therefore, we expect that mistletoes would show very low proportional nutrient resorption, because the cost of resorption-derived nutrients would surpass the cost of host-derived nutrients. However, this expected balance might not be true if the host is also limited by some soil-derived nutrient.

 

To investigate how mistletoes would deal with this balance between host nutrient limitation and leaf nutrient resorption, we measured green and senescent-leaf nutrient concentration in 18 mistletoe–host species pairs distributed across three sites with notably low-phosphorus levels in soil in Australia and Brazil. We found little or no evidence of nitrogen, calcium or magnesium resorption by mistletoes. However, even though proportional resorptions found for mistletoes were much lower than the reported values for non-parasitic plants, on average ∼30% of phosphorus and ∼20% of potassium were resorbed prior to leaf fall. Moreover, the lower the phosphorus concentration mistletoe’s leaves show, the longer they tend to live, which reinforces the importance of an efficient use of this nutrient.  We can conclude that mistletoes growing in low-P environments and parasitising hosts adapted to deal with very low phosphorus availability actively modulate phosphorus use. These findings are very exciting since nutrient resorption affects many ecosystem processes such as rates of litter decomposition, nutrient-carbon cycling and, hence, ecosystem productivity.

Figure 1. Different types of haustorium from mistletoes in Australia (a and b) and in Brazil (c and d). (a) The mistletoe Muellerina eucalyptoides parasitizing the host Eucalyptus hemastoma; (b) The mistletoe Amyema miquelli parasitizing the host Eucalyptus miniata; (c) The mistletoe Struthanthus polyanthus parasitizing the host Miconia albicans; (d) The mistletoe Psittacanthus robustus parasitizing the host Qualea grandiflora.

Figure 2. Scalon M.C. at the Brazilian Savanna site (Recor/IBGE) with the mistletoe Psittacanthus robustus parasitizing the host Miconia albicans.

Marina C. Scalon

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