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

Irrigated crops of 'Grasslands Kaituna' lucerne were grown for 5 years in a temperate climate at Lincoln University, Canterbury, New Zealand (43 degrees WS, 172 degrees 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 (T-b) of 1 degrees C at air temperatures (T-a) <15 degrees C and a T-b = 5 degrees C for T-a >= 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 degrees Cd but declined from 60 to 37 degrees Cd as photoperiod decreased from 15.7 to 11.4h. 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 mm(2) compared with 400 mm(2) 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. (c) 2005 Elsevier B.V. All rights reserved.