Quantifying Leaf Development of Different Wheat Cultivars: I- Production and Senescence of Main Stem Leaves in the Field

A major component in a crop growth model is leaf area development, which has crucial influence on photosynthesis and transpiration. Leaf area development involves the appearance of new leaves, expansion of the newly emerged leaves and senescence of old leaves. Modeling of the leaf growth has been extensively studied in many crops including cereals. Methods of predicting leaf area development are diverge from those dealing with the individual component processes of leaf growth viz. leaf appearance, leaf expansion and leaf death to the models predicting leaf growth at the whole plant or whole crop levels. Leaf appearance and expansion are most sensitive growth processes to environmental conditions and their dependence on temperature revealed in a range of cereals including wheat and barley and legumes including cowpea and soybean. Effects of temperature on leaf appearance rates are usually quantified using some form of thermal time. Air temperature above the canopy has most frequently been used to calculate thermal time. Genetic differences in senescence have also been reported among crop genotypes. Quantitative information regarding leaf area development in wheat especially in environmental conditions with high temperatures for the purpose of crop modeling is scarce. Furthermore, genotypic variations have not been evaluated. Therefore, the goal of this research was to determine parameters related to leaf production and senescence in wheat cultivars in warm environmental conditions.
The aim of this study was to quantify leaf production and senescence of 15 different wheat cultivars. Two field experiments with 15 wheat cultivars (Atrak, Bayat, Chamran, Chenab, Dez, Ineia, Kavir, Marvdasht, Shiraz, S78-18, Yavaroos and shova-Mald) were conducted at the research farm of the Islamic Azad University of Ramhormoz Branch, south-western of Iran in 2005-6 and 2006-7 using a randomized complete block design with four replications. The relationship between main stem leaf numbers (HS) versus degree-days was described using bi-linear regression model.
The results indicated that leaf production on main stem started with receiving 108.5 degree-days after sowing. The leaf appearance rate and along with it phyllochron had no significant across all data. Mean leaf appearance rate ranged from 0.0047 to 0.0082 leaf/˚Cd. At 13 out of 15 cultivars. The cession time (degree-days) of effective leaf production on main stem was not significantly different and ranged from 831.0 to 852.0 ˚C d. Leaf senescence on the main stem started when the main stem had about 4-6 leaves and proceeded at a rate of 0.065 % per each unit increase in degree-days. The relationships found in this study can be used in simulation models of wheat.
Based on results, there was no significant difference between wheat cultivars in terms of parameters related to leaf production and senescence on main stem except the time of cessation of the linear increase in leaf number on main stem. The relationships presented in this study describe leaf production and senescence under well-watered condition and reflect the effects of carbon and nitrogen availability and remobilization under these conditions. However, they do not account for the effects of shortage of carbon, nitrogen or water on leaf development. Other relationships are required to predict these effects.