Seasonal patterns of root C and N reserves of lucerne crops (Medicago sativa L.) grown in a temperate climate were affected by defoliation regime

Lucerne crops exhibit a seasonal cycle of accumulation and depletion of C and N reserves in perennial organs (crown and taproot). The level of perennial reserves influences shoot growth rate, mainly after defoliation and during early-spring regrowth. The frequency and time of defoliation (grazing or cutting) also influences yield and stand persistence but the impact on the seasonality of perennial reserves is unknown. Patterns of accumulation and depletion of reserves in lucerne crops were examined during two growth seasons in Canterbury, New Zealand. To create contrasting levels of reserves, four defoliation regimes were imposed through a combination of a long (L, 42 days) or a short (S, 28 days) grazing cycle, applied at two times of the growth season (before and/or after mid-summer). Crops grazed consistently every 42 days (LL treatment) yielded 23 t/ha/year of shoot dry matter (DM). Those on a year round 28-day regrowth cycle (SS treatment) produced only 50–60% of this. Crown plus taproot DM cycled from ∼3.0 to 5.5 t/ha for the LL and from 2.2 to 3.5 t/ha for SS treatment. The concentrations of starch in taproots ranged from ∼4 to 30% DM and differed seasonally but were consistently reduced by frequent defoliations. Nitrogen reserves accumulated in autumn to ∼1.8% DM and were depleted in spring to 1.2% DM in the LL and 1.0% in the SS treatment. The levels of soluble sugars declined abruptly from 9 to 4% DM in frequently defoliated crops but recovered to seasonal levels similar to the LL treatment. In the 2003/04 season, the mid-season switch in defoliation frequency from a short to a long regrowth duration (SL treatment) restored C and N in taproots to levels 25–50% greater than the SS treatment. The amount of nitrogen in taproots during winter was the strongest predictor (R2 = 0.76) of spring shoot growth rates. The seasonal patterns of change in crown plus taproot DM and the concentrations of C and N in taproots occurred regardless of defoliation regime. This indicates that environmental signals imposed a strong control over the DM partitioning to lucerne roots.