The effect of planting date and density on leaf Production and Senescence rate in Safflower

 Predicting leaf area is an important part of the crop simulation models. The ability to predict changes in leaf area index is important to estimate the solar radiation received during the growing season and dry matter production. The knowledge about the leaf area development of safflower is incomplete. Therefore the aim of this study was to quantify leaf production and senescence of safflower 'Local Esfahan'.

A factorial experiment was conducted based on completely randomized block design with four replicates in Research Farm of Vali-e-Asr University in 2012. Factors were included sowing date (5 and 25 April) and plant density (15, 40, 65 and 90 plant/m2). Leaf area was measured from the beginning of the emergence stage to the end of the growing season at an interval 10 days. Furthermore, the number of green and yellow leaves on main stem, the number of node on main stem and the number of total yellow leaf on plant were counted from the beginning of the growing season to the end of the flowering at an interval 3-7 days.

The results showed LAImax in 15plant/m2 were 3.91 and 3.37 in the studied sowing dates, respectively, and its value was decreased with increase in density. The leaf production rate (leaf/°Cd-1) was not significantly different among densities of each sowing date and varied from 1.73 to 1.93 leaf/°Cd-1. Leaf production terminated after 1349 to 1395 °Cd-1 and 1300 to 1384 °Cd-1 for studied sowing dates, respectively. The results showed that with increase in leaf number during growing season, leaf area increased as power. The slope of increase in leaf area decreased with increase in density and delay in sowing. The results showed that leaf senescence rate occurred with a steep slope for the lower densities of each sowing date and for the late sowings. Leaf senescence rate based on yellow leaf number on plant for different densities ranged from 0.0025 to 0.0045 leaf/°Cd-1 for the first sowing date and 0.0018 to 0.0037 leaf/°Cd-1 for the second sowing date.

The results showed that leaf area index decreased with increasing plant density. Plant leaf area decreased with increasing plant density due to increasing inter- and intra-plant competition and reducing the number of branches per plant. However, an increase in the number of plants could not compensate the decrease in leaf area index. This is probably due to decreased number of leaves per plant in higher density. The delay in planting resulted in increasing leaf emergence rate and reducing the effective period of leaf production and consequently the final number of leaves was reduced. The node production rate during this period is primarily determined by temperature and then by assimilates availability for leaf growth. With the increase in density, leaf senescence rate was increased. The increase of senescence rate in low planting densities can be attributed to greater inter-plant competition and shading.