Radiation use efficiency and biomass partitioning of lucerne (Medicago sativa) in a temperate climate

This research quantifies the influence of seasonal variations in solar radiation, temperature and biomass partitioning on lucerne production in a temperate climate. Above ground biomass (shoot) production of fully irrigated ‘Kaituna’ lucerne was measured in the field over 5 years and 33 regrowth cycles in Canterbury, New Zealand. Shoot production increased linearly (R2 of 0.93 ± 0.07) with intercepted total radiation within each regrowth cycle but radiation use efficiency (RUEshoot, in g DM/MJ total radiation) ranged from 0.29 to 1.09 g DM/MJ. Covariate analysis showed season and temperature both influenced RUEshoot with temperature adjusted RUEshoot(RUEshoot*) decreasing from 1.01 g DM/MJ in September to 0.77 g DM/MJ between October and February, decreasing again to 0.47 g DM/MJ in March and April before increasing back to 0.99 g DM/MJ in May. A second fully irrigated experiment with lucerne plants grown in plastic columns under near field conditions investigated the seasonality of biomass partitioning between shoots and perennial biomass (roots and crowns). The proportion of total biomass partitioned to shoots (Pshoot) was 0.90 in September, ∼0.67 from October to February but only 0.35 in March and could be related to photoperiod. These Pshoot values were closely correlated with RUEshoot* showing seasonal changes in RUEshoot were partly caused by changes in biomass partitioning. Field data for RUEshoot were divided by Pshoot to estimate the RUE for total biomass production (RUEtotal). The RUEtotal increased linearly from 0.60 to 1.60 g DM/MJ as mean air temperatures increased from 6 to 18 °C. These results quantified the effects of solar radiation and mean temperature on total lucerne biomass production and its seasonal partitioning between shoots and perennial biomass. The influence of regrowth duration on this partitioning was also investigated.