assessing some of growth indices in safflower using regression analysis

introduction the study of growth parameters such as net assimilation rate (nar), leaf area ratio (lar), relative growth rate (rgr) and leaf area duration (lad) are important for analyzing yield constraints. the growth parameters can be used to understand the crop response to environmental variables during the growing season. the aims of the present study were to 1) consider the growth indices using regression method and 2) identify relationships between growth parameters and grain yield in different safflower cultivars and sowing dates. materials and methods a factorial experiment based on randomized complete block design with four replicates was conducted in 2012 in research farm of vali-e-asr university of rafsanjan. the factors were included sowing dates (4 april, 25 april and 16 may) and safflower cultivars (411, sina, local esfahan and sofeh). leaf area, leaf dry weight and stem dry weight were measured in 7–14 days intervals during the growth season. grain yield was determined at the end of the growing season. the trend of variation for nar, lar, rgr and lad during the season growth was determined by the nonlinear regression method using sas software. finally, the correlation between grain yield and growth parameters was identified by the proc corr procedure. results and discussion in all sowing dates, nar increased to 43-53 days after sowing and then it showed a declining trend. results showed that there was no significant difference among cultivars in terms of the time to maximum nar among all sowing dates and among cultivars. as the growing season progressed, gradual increases in leaf area improved which resulted in increasing solar radiation interception and nar. the nar declining trend related to increasing leaves shading. in the first and third sowing dates, there was no significant difference in time to achieve maximum lai among cultivars. the maximum lar was obtained in early growth season and its value decreased with slope of 0.033 to 0.051 cm2 g-1 d-1 among the cultivars and sowing dates. the rate of decreasing in lar during growing season was the same among all sowing dates and cultivars. in the early growing season, the value of lar in the first and second sowing dates was higher than third sowing date. it was probably due to high temperature effect on crop growth duration. as the growing season progressed, lar started to decrease due to decrease in partitioning of dry matter to leaves. in the early growing season, the allocated dry matter to leaves were great which is resulted the higher ratio of leaf dry matter to total dry matter (lar). the high lar in early growing season is due to more interception of solar radiation by crop. the trend of rgr during growing season was the same among all sowing dates and cultivars. the estimated maximum rgr varied between 0.149 to 0.169 g g-1 d-1 for the first sowing date, 0.087 to 0.211 g g-1 d-1 for the second sowing date and 0.192 to 0.251 g g-1 d-1 for the third sowing date. for each sowing date, the estimated maximum rgr had no significant difference among cultivars except for the second dates. however, the value of rgr was decreasing with progress towards the end of the growing season. the high rgr in the early growing season was due to the rapid crop growth, the more young tissues in plant and the low initial weight. the value of lad had the increasing trend in the early growth season and its value started to decrease between 50 and 62 days after sowing. in the second and third sowing dates, the maximum value of lad obtained earlier than the third sowing date however, the value of lad was lower than the first sowing date. it is probably due to decreasing the leaf area index and increasing the senesce rate in higher temperature in the last sowing date. it is resulted that the highest grain yield obtained for the 411 and local esfahan cultivars in the first sowing date and the significant and positive correlation between grain yield and lad (r = 0.88, pr ≤ 0.01) observed whereas the value of grain yield increased 12.2 g m-2 per unit increase of lad. increasing the lad increase solar radiation interception by crops and consequently the grain yield would increase. generally, it is concluded that the first sowing date with the highest lad resulted the highest yield.