Growth and phenological development patterns differ between seedling and regrowth lucerne crops (Medicago sativa L.)

This study compared physiological responses of fully irrigated seedling and regrowth lucerne crops (Medicago sativa L.) grown under similar environmental field conditions. Measurements occurred for 2–4 years after sowing on 24 October, 15 November, 05 December and 27 December 2000 at Lincoln, Canterbury, New Zealand. Irrespective of the date of sowing, on average lucerne accumulated less shoot dry matter (DM) in the seedling year (11 ± 0.44 t ha−1) than during the regrowth year (18 ± 0.76 t ha−1). Slower shoot-growth rates in seedlings were explained by less intercepted light and reduced efficiency in conversion of light to biomass. Specifically, seedlings had a longer phyllochron (47 ± 2.3 °Cd leaf−1) and slower leaf area expansion rate (0.009 m2 m−2 °Cd−1) than regrowth crops (35 ± 1.8 °Cd leaf−1 and 0.016 m −2 m −2 °Cd, respectively). There were no differences in canopy architecture with a common extinction coefficient of 0.93. The radiation use efficiency (RUE) for shoot production (RUEshoot) was 1.2 ± 0.16 g DM MJ−1 of intercepted photosynthetically active radiation (PARi) in seedlings and 1.9 ± 0.24 g DM MJ−1 PARi in regrowth crops. Reproductive development was slower in seedling than regrowth crops due to an apparent juvenile period ranging from 240 to 530 °Cd in seedlings. For both seedling and regrowth phases, the thermal time accumulation to reach 50% buds visible (Tt0-bv) and 50% open flowers (Tt0-fl) increased as photoperiod shortened in autumn. The minimum Tt0-bv, or the thermal-time duration of the basic vegetative period (TtBVP), was estimated at 270 ± 48 °Cd at photoperiods >14 h for regrowth crops. The theoretical threshold below which reproductive development is projected to cease, or the base photoperiod (Ppbase), was estimated at a common 6.9 h for seedling and regrowth crops. The transition from buds visible to open flowers (Ttbv-fl) was mainly controlled by air temperature and ranged from 161 °Cd for seedlings to 274 °Cd for regrowth crops. These results can be used as guidelines to develop differential management strategies for seedling and regrowth crops and improve the parameterization of lucerne simulation models.

► Growth and development rates are slower in lucerne seedling crops than regrowth crops. ► Lucerne seedlings have reduced leaf appearance, leaf expansion and RUE. ► At short photoperiods, time to flowering increases in seedling and regrowth lucerne crops. ► Parameterization of lucerne simulation models should differ for seedling and regrowth crops.