The components of lucerne (Medicago sativa) leaf area index respond to temperature and photoperiod in a temperate environment

Irrigated crops of 'Grasslands Kaituna' lucerne were grown for 5 years in a temperate climate at Lincoln University, Canterbury, New Zealand (43°38′S, 172°28′E). From these the response of the components of leaf area index (LAI) to environmental factors was determined. A broken stick temperature threshold with a base temperature (Tb) of 1 °C at air temperatures (Ta) <15 °C and a Tb = 5 °C for Ta ≥ 15 was required to accumulate thermal time (Tt). Using this, the appearance of nodes on the main-stem (phyllochron) was constant in Tt within a re-growth cycle (30-42 days). The phyllochron was 37 ± 7 °Cd but declined from 60 to 37 °Cd as photoperiod decreased from 15.7 to 11.4 h. Branching began at the appearance of the fifth main-stem node with 2.5 secondary nodes produced per main-stem node in spring re-growth cycles but only 1.7 produced in summer. Leaf senescence increased from 0.3 to 1.08 leaves per main-stem node after the appearance of the ninth node. Spring re-growth cycles had a mean individual leaf area of 170 mm2 compared with 400 mm2 for summer re-growth cycles. These results demonstrate systematic variation in LAI components and suggest they need to be considered separately in response to environmental factors to provide a quantitative framework for crop simulation analyses of lucerne canopy development.