Estimation of hydrothermal time required for emergence of two weed species Hordeum spontaneum Koch. and Lactuca serriola L.

Seed germination and emergence is a key event that determines the success of a weed in an agro ecosystem that is influenced by many environmental factors such as temperature, pH, light, salinity and moisture (Chauhan & Johnson, 2008). Temperature plays a major role in determining the periodicity of seed germination, emergence and the distributions of weed species (Guan et al., 2009). Also, osmotic stress can reduce, delay or prevent germination (Zhou & Deckard, 2005). An understanding of weed development is needed to design effective weed management programs. A better understanding of the biology of seed germination and emergence could contribute to the development of weed management technologies to help counter those undesirable shifts in weed populations. The hydrothermal time models have been widely applied to describe the germination responses of seeds to temperature and water potential. This study was carried out with the aim to predict the time and maximum seedling emergence of two weed species, wild barley (Hordeum spontaneum Koch.) and prickly lettuce (Lactuca serriola L.).

Different temperatures (15, 25 and 35°C) and water potentials (-0.045 and -0.172 MPa) as full factorial experiments applying a randomized experimental design with four replications were investigated in laboratory and planting dates (October and November) for H. spontaneum seeds and March, April and May for L. serriola and three irrigation intervals: every 3 days once (level1), every 5 days once (level2) and every 7 days once (level3) for two species as full factorial experiments applying a completely randomized experimental design with three replications were investigated in field. To express the changes in the emergence process, hydrothermal time model was applied and the base temperature and water potential were calculated. Then, MPa-°C-days required for emergence was calculated with the base temperature and water potential and soil moisture and temperature.

The results of the laboratory experiments showed that the maximum germination percentage of wild barley at 15 and 25°C was 95.9 and 83.7 %, respectively and for prickly lettuce at 15, 25 and 35°C was 96.3, 79.9 and 60.9 %, respectively. The highest and lowest maximum emergence percentage of wild barley was at 15°C, -0.045 MPa and 25°C, -0.172 MPa, respectively that was 92.6 and 66.3 %, respectively and for prickly lettuce was at 15°C, -0.045 MPa and 35°C, -0.172 MPa, respectively that was 95.5 and 20.0 %, respectively. Thermal time required for germination of wild barley at 15 and 25 °C was 43.8 and 65.1 °C-days, respectively and for prickly lettuce at 15, 25 and 35 °C was 44.6, 31.7 and 32.9 °C-days, respectively. The highest and lowest thermal time required for 50% emergence of wild barley was at 25 °C, -0.045 MPa and at 15°C, -0.172 MPa, respectively that was 209.6 and 99.8 °C-days, respectively and for prickly lettuce was at 15°C, -0.172 MPa and at 35°C, -0.172 MPa, respectively that was 70.0 and 49.7 °C-days, respectively. The results of the field experiment showed that the highest emergence percentage of wild barley and prickly lettuce was 97.6 and 56.2% that occurred in November with irrigation interval of every 3 days once and April with irrigation interval of every 5 days once, respectively. The lowest emergence percentage of wild barley and prickly lettuce was 78.2 and 21.3% that occurred in October with irrigation interval of every 3 days once and May with irrigation interval of every 7 days once, respectively. The highest hydrothermal time for 50% emergence of wild barley and prickly lettuce was 384.2 and 179.2 °C-days-MPa in October with irrigation interval of every 7 days once and May with irrigation interval of every 5 days once and the lowest was 84.6 and 43.3 °C-days-MPa in November with irrigation interval of every 3 days once and march with irrigation interval of every 3 days once, respectively.