Defoliation frequency and season affected radiation use efficiency and dry matter partitioning to roots of lucerne (Medicago sativa L.) crops

Radiation use efficiency (RUE), and subsequent partitioning between shoots and roots were investigated for ‘Grasslands Kaituna’ lucerne crops grown in the cool temperate climate of Canterbury, New Zealand. Crops were grazed by sheep every 28 or 42 day and yielded 12 and 23 t DM/ha year, respectively. The RUE for above ground shoots (RUEshoot) was 1.7–2.0 g DM/MJ of intercepted photosynthetically active radiation (PARi) in spring but decreased systematically to ≤1.0 g DM/MJ PARi in autumn. The RUE for total biomass (RUEtotal) ranged from 1.3 to 3.1 g DM/MJ PARi in response to air temperature and defoliation treatment. The lowest RUEtotal in mid-summer for the treatment defoliated every 28 days was related to a 20% decline in the leaf photosynthetic capacity measured at 1000 μmol photons/m2 s (Pn1000) and at saturating light (Pmax). In turn, the reduction in Pn1000 was related to differences in specific leaf nitrogen (SLN), through changes in specific leaf weight (SLW) rather than the leaf N concentration of 4–6% DM.The fractional partitioning of DM to roots (proot) increased from near zero in winter/early-spring to >0.45 in autumn, which explained the observed seasonality of RUEshoot. For the treatment defoliated each 42 days, proot increased linearly from ∼0.05 to >0.45 as Pp increased from 10.5 to 16.5 h. In decreasing photoperiods proot averaged 0.45. Agronomic treatments that result in suboptimal N reserves post-grazing can be expected to produce conservative canopy characteristics but reduced photosynthetic capacity of the first five main stem leaves. Beyond this development stage, canopy expansion may be reduced with more conservative leaf N.