Study of Biological Aspect of Germination and Emergence in Morning Glory (Ipomoea purpurea L.)

Weeds are among the worst limiting factors of the crop yield potential. I. purpurea belongs to Convolvulaceae family and is an annual broadleaf vine and a problem weed in many annual and perennial crops. This weed is becoming an increasing problem in soybean fields of Mazandaran province, Iran. Seed germination is one of the most important processes for weed success in agricultural ecosystems; because the first step is for a weed to compete in an ecological nich. Understanding the biology and germination pattern and emergence of common weeds in crops is an undeniable necessity in effective weed management systems. This process is regulated by several environmental factors such as light, salinity, pH and soil moisture. Among the factors affecting germination, temperature and light are the most important regulatory environmental factors. Temperature has significant effects on germination characteristics such as onset, rate and rate of germination, therefore it is the most critical factor that determines the success and failure of plant establishment. Salinity and drought are one of the most important environmental stresses that affect germination. Weed phenology is determined by the interaction between plant internal factors and external environmental signals such as temperature, day length and drought; phenological stages and their characteristics are very important for the design and implementation of weed management methods. A better understanding of the germination biology of I. purpurea would facilitate the development of better control strategies for this weed. Also, predict the weed phenological developments are useful research tools for advancing our knowledge of population dynamics or crop and weed competition; in this way, the study of phenology allows accurate estimation of weed competition time and its effect on crop yield. The aim of this study was to investigate the effect of temperature, salinity, and drought and to investigate the phenology of this weed based on the growing degree-day.

 These experiments were conducted in the weed science laboratory of Sari Agricultural Sciences and Natural Resources University. The treatments were carried out in a completely randomized design with four replications. The treatments including constant temperature with 8 levels: 5, 10, 15, 20, 25, 30, 35, and 40 °C; salinity stress in 9 levels of control, 0.92, 1.84, 3.68, 7.36, 14.75, 29.5, 35 and 40 (dS m-1 chloride sodium), drought stress in 6 levels of control, -0.2, -0.4, -0.6, -0.8 and -1 MPa. To study the phenology of I. purpurea, a field experiment was accomplished in a randomized complete block design.

The results showed that I. purpurea seeds started to germinate at 10 °C and the germination percentage increased with increasing temperature. No germination occurred at 40 oC. To determine the cardinal temperatures beta, segmented and dent-like models were used. The dent-like model was the best model to predict the germination rate. Based on the dent-like model outputs, base, lower limit optimum, upper limit optimum and maximum temperatures of I. purpurea seed germination were 7.39, 23.54, 29.54, and 39.54 °C, respectively. Also, the salinity level of 31.33 dS m-1 causes a 50% reduction in germination percentage. In drought stress, the highest germination percentage (100%) was observed in the control treatment and the lowest germination percentage (97.5%) was observed in -0.4 MPa. In general, six phenological stages were recorded, including emergence, stem elongation, budding, flowering, fruiting, and seed maturation. The results showed that the shortest and longest growth stages of this weed in terms of time are emergence and stem elongation, respectively. It was observed that the I. purpurea is a weed that completes its phenological stages in 168 days by achieving 2378.01 growing degree-day. These results revealed that the best time to control of this weed is before the flowering stage.

Seed germination of this weed is in the temperature range of 10 to 35 °C and it seems that this weed germinates better at warmer temperatures. Chemical methods are among the most common methods in controlling weeds and other invasive species in the country, so the spread of these species in addition to direct effects can increase the use of chemical pesticides on farms and double the negative consequences of these species. Therefore, with full knowledge of the different stages of I. purpurea phenology, it is possible to plan for proper management. From a managerial point of view, the best time to use the herbicide is in the 3-4 leaf stage; however, in case of negligence at this stage, they should be checked before bud production so that its seeds do not enter the seed bank; because the seeds of this plant are considered as causes of pollution in the field.