Modelling the effects of temperature on the rates of seedling emergence and leaf appearance in legume cover crops

Simple models of the growth and development of legume cover crops in differ ent environments may aid the selection of species and genotypes most suitab le for particular farming systems. The effects of temperature and photoperi od on the daily rate of seedling emergence (1/t(50%)) and the rate of leaf appearance (leaves d(-1)) were quantified for 12 diverse legume cover crop species. Six tropical or subtropical species were grown in 12 combinations of mean temperature (16.8, 21.8 and 26.8 degrees C) and photoperiod (11.5, 12.5, 13.5 and 14.5 h d(-1)). Another six genotypes of temperate species we re grown in nine combinations of mean temperature (16.8, 21.8 and 26.8 degr ees C) and photoperiod (12.5, 13.5 and 14.5 h d(-1)). Durations from sowing to 50% seedling emergence (t(50%)) were recorded and the number of leaves on the main stem counted during early vegetative growth. Photoperiod did no t affect either 1/t(50%) or leaves d(-1). However, the effects of temperatu re, photoperiod response group (short-day or long-day response), genotype o r species and their interactions were all significant. An optimal temperatu re between 21.8 and 26.8 degrees C for leaves d(-1) was identified for Vici a sativa, Vicia dasycarpa and Lupinus mutabilis. Below the optimum temperat ure, 1/t(50%) and leaves d(-1) were positive linear functions of temperatur e for all legume species except Vicia dasycarpa. Estimates of base temperat ure (T-b) and thermal time (theta) for 1/t(50%) and leaves d(-1) were obtai ned for each legume cover crop from these functions. Common values of T-b w ere found within each photoperiod response group: 9 degrees C and 1 degrees C for 1/t(50%), and 10 degrees C and -2 degrees C for leaves d(-1) for sho rt-day and long-day species respectively. Differences among species were re presented by variant estimates of theta. The practical consequences of the combined effects of these values of T-b and theta for selecting legume cove r crops for particular farming systems are discussed.