Adaptation of the C4 grass Panicum maximum to defoliation is related to plasticity of N uptake, mobilisation and allocation patterns

Abstract

Dry mass production and persistence of Panicum maximum pastures depends on nitrogen supply. Defoliation influences N uptake and allocation patterns yet its effects on plasticity of N dynamics in P. maximum have not been investigated. Stable isotopes of N ( 15 N) were used in order to test the hypothesis that defoliation in terms of proportion of the leaf area removed effects N mobilisation, uptake and allocation patterns in P. maximum. The plants were initially cut weekly to a height of either 0.15 m or 0.30 m for seven weeks. Eight weeks after the first defoliation, all plants were defoliated for a final time to remove 0, 25, 50, 75 or 100 % of the area of each individual leaf blade of the main tiller. Root N uptake was reduced when all leaf area was removed, but more lenient defoliation improved N uptake due to a positive effect on specific N uptake. Young leaves, side tillers and roots were the main sinks for N from root uptake. Roots of P. maximum became a net source of N for mobilisation immediately after severe defoliation. Root uptake was the main source of N for new growth in P. maximum plants. Allocation pattern of mobilised N was different from that of N derived from root uptake. It was concluded that adaptation of P. maximum to defoliation is related to plasticity of N uptake, mobilisation and allocation, but changes in N dynamics did not offset negative impacts of complete defoliation of the plants.

Nitrogen use of Panicum and Brachiaria cultivars vary with nitrogen supply: I. Differences in plant growth

“…In contrast, Panicum maximum cv. Tanzania continued to allocate N primarily towards the growth of new leaves and tillers, but over time, the roots required the largest amount of N. Santos et al (2012) confirmed that the majority of N in the Tanzania cultivar is used for the growth of new leaves and lateral tillers. Martuscello et al (2009) concluded that for Brachiaria brizantha cv.…”

“…The forage grass productivity is related to continuous leaves and tillers growth, which are responsible for LA renewal and increase in canopy photosynthetic capacity after mechanical harvesting or animals grazing (Lemaire and Chapman, 1996). However, under limited N supply, N-efficient cultivars show tillering, leaf growth behavior, and partitioning of accumulated N for biomass production characteristics that may contribute to production system longevity (Brégarda et al, 2000;Martuscello et al, 2009;Anwar et al, 2012;Santos et al, 2012). According to Brégarda et al (2000), biomass partitioning is the key to increase the nitrogen use efficiency (NUE) in grasses.…”

Natural reactive phosphate and poultry litter association on growth of forage grasses in an Amazonian Oxisol

“…The high supply of P due to the poultry litter + Arad phosphate association, indicated by high concentrations of P in diagnostic leaves, caused higher numbers of tillers, which explains the higher dry-matter production of P. maximum shoots at first harvest. Studies show that this result may be linked to P. maximum's higher responsiveness to nutrient supplies when compared to U. brizantha (Santos et al, 2002(Santos et al, , 2012Machado & Assis, 2010). Therefore, U. brizantha at the second harvest was capable of accumulating more dry mass in relation to P. maximum because U. brizantha, under certain conditions, is better adapted to lower supplies of nutrients (Machado & Assis, 2010).…”

Nitrogen use of Panicum and Brachiaria cultivars vary with nitrogen supply: II Nitrogen use efficiency rankings and partition

“…At this N rate, the Panicum spp., Mombaça, Tanzania and Massai cultivars had the highest total N content but the lowest nitrate content in the root system. This finding may be related to the rapid transport of absorbed nitrate to the shoots of these cultivars via the xylem (Marschner, 1995;Santos et al, 2002;Santos et al, 2012). Marandu, Xaraés and Piatã grasses apparently have the ability to accumulate greater amounts of N in their roots in the form of ammonium than do the other grasses; this feature may be related to their ability to easily assimilate ammonium (Table 4).…”

“…Concentrations and contents of N in the plant shoots may be related to the proportion of structural and storage organs containing N, which, in turn, modifies the production of plant dry matter (Greenwood et al, 1990;Primavesi et al, 2004;Corrêa et al, 2007;Yuan et al, 2007). Santos et al (2002) and Santos et al (2012) observed that N taken up by the roots was the main source of N for new lateral tillers and leaves production of Panicum maximum cv. Tanzania and low N rates resulted in the mobilization of accumulated N from the shoots to the root system.…”