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

摘要

This study compared physiological responses of fully-irrigated seedling and regrowth lucernecrops (Medicago sativa L.) grown under similar environmental field conditions.Measurements occurred for 2–4 years after sowing on 24 Oct, 15 Nov, 05 Dec and 27 Dec2000 at Lincoln, Canterbury, New Zealand. Irrespective of the date of sowing, on averagelucerne accumulated less shoot dry matter (DM) in the seedling year (11±0.44 t.ha⁻¹) thanduring the regrowth year (18±0.76 t.ha⁻¹). Slower shoot-growth rates in seedlings wereexplained 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 areaexpansion rate (0.009 m² m⁻² °Cd⁻¹) than regrowth crops (35±1.8°Cd leaf⁻¹ and 0.016 m² m⁻² °Cd⁻¹, respectively). There were no differences in canopy architecture with a commonextinction 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 ) inseedlings and 1.9±0.24 g DM MJ⁻¹ PARᵢ in regrowth crops. Reproductive development wasslower in seedling than regrowth crops due to an apparent juvenile period ranging from 240to 530°Cd in seedlings. For both seedling and regrowth phases, the thermal timeaccumulation to reach 50% buds visible (Tt₀-bv) and 50% open flowers (Tt₀-fl) increased asphotoperiod shortened in autumn. The minimum Tt₀-bv, or the thermal-time duration of thebasic vegetative period (TtBVP), was estimated at 270±48°Cd at photoperiods >14 h forregrowth crops. The theoretical threshold below which reproductive development is projectedto cease, or the base photoperiod (Ppbase), was estimated at a common 6.9 h for seedling andregrowth crops. The transition from buds visible to open flowers (Ttbv-fl) was mainlycontrolled by air temperature and ranged from 161°Cd for seedlings to 274°Cd for regrowthcrops. These results can be used as guidelines to develop differential management strategiesfor seedling and regrowth crops and improve the parameterization of lucerne simulationmodels.