Authors: SOURAV GHOSH1* and DEBARATI DATTA2
1 Ph.D Research Scholar, Indian Agricultural Research Institute, Pusa, New Delhi, India
2 Ph.D Research Scholar, Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand, India
Phalaris minor : Spread, ecology and biology
Phalaris is a native of the Mediterranean region but has been introduced into many other parts of the world. At present, 22 species of Phalaris are recognized in the world.
It probably reached India through the import of Mexican wheat (Lerma rojo and Sonora 64 through PL-480) which was observed to be a problem by the 1970s. The most affected states in India are Punjab and Haryana, however, P. minor infestation is one of the serious causes of concern in Uttar Pradesh, Uttaranchal, Madhya Pradesh and parts of Bihar, and Himachal Pradesh, where it reduces wheat production significantly.
Phalaris minor competes with the wheat crop for all growth factors and may reduce grain yield to the tune of 25-50% and under very severe infestation the losses may go upto 80% or even more. During the initial growth stages, the plants of P. minor are morphologically similar to wheat plants and thus escape hand weeding/hoeing which is done mainly in between the crop rows.
The success of P. minor in the rice-wheat rotation appears to be related to high surface moisture for seedling emergence, high input levels, and a phenology which is ideally suited to the climatic conditions. The weed also tends to be surface rooting, which may partly account for its preference for the rice-wheat system where adequate soil moisture is available for extended periods throughout the growing season.
Optimum temperature for seed germination of P. minor is in the range of 10-200C. Phalaris minor cannot emerge from depths greater than 4-5 cm, therefore, availability of moisture in the surface soil is necessary for seedling emergence and growth of this weed. It prefers acidic pH. Phalaris seeds can remain dormant for 3-4 months after maturity however seeds are capable of tolerating anaerobic conditions by entering into secondary dormancy for upto 12 months, which perhaps is one of the reasons of its better adaptation in rice-wheat cropping systems.
Dhiman et al. (2002) have outlined following possible reasons, which help P. minor to survive in rice-wheat cropping systems:
- It is susceptible to solarization. Presence of water in rice fields lowers the temperature of soil, and thus helps in its survival in rice-wheat system as compared to other cropping systems.
- Puddling helps in deep placement of seed in the soil and hence exposure to relatively lower temperature.
- The increased and prolonged activity of alcohol dehydrogenase in P. minor is known to play a detoxifying role in anaerobic respiration, hence retaining viability
- Its tolerance to anoxia might be due to inherent ability of seed in using NO3 as an alternate electron acceptor in Electron Transport System (ETS) with the help of nitrate reductase activity.
|Phalaris minor Retz.||Wheat (Triticum aestivum L.)|
|1.||At early stage (seedling), there is dark green/bluish green colour of leaves.||At this stage, the leaves are light green in colour.|
|2.||Upto initial 50 days, the lower parts of leaf and sheath have pink colour.||It is yellowish green/ greenish-yellow.|
|3.||High tillering (upto 42 / plant)||Low tillering|
|4.||Three times longer ligules than wheat and no auricle.||Ligules and auricle are small but hairy.|
|5.||Length of internodes is more than wheat, therefore, it has increased height.||Internodes comparatively smaller than Phalaris.|
|6.||There is branching and tillering both, tillering is of rosette type.||Only tillering and no branching and plant grows erect.|
|7.||Pink colour sap exudes from stem, if the plant is removed / broken from near to soil surface.||Water coloured exudation from stem.|
|8.||About 8-9 cm long earhead with compact spikelets.||10-15 cm long earhead, having 18-22 spikelets and not so compact|
|9.||300-450 seeds / earhead.||About 30-50 seeds/earhead.|
|10.||1000-grain weight is 1.5 to 2.0 g .||1000-grain weight varies between 40-45g.|
|11.||Seed is generally black and oval shaped.||Seed is amber to red in colour.|
Up until the early 1990s, Phalaris minor could be effectively controlled by farmers spraying the crop with isoproturon, a substituted urea herbicide first recommended in 1977–78 and widely used since the early 1980s. But continuous use of isoproturon from 1980 as single herbicide for 10-15 years accentuated by poor application rates, spray techniques and timing resulted in evolution of resistance (Malik and Singh 1995). Due to development of resistance in 1992-93, farmers in some parts of Haryana have restored to ploughing up of immature wheat in a population range of 2000 to 3000 plants/m2. On an average, a loss of 25 to 50% in wheat yield was quite common.
The resistance affected area ranged from 0.8 to 1.0 million hectare in N -W India. To achieve 50% growth reduction, resistant biotypes of this weed now require 8 to 11 times more isoproturon than susceptible biotypes. This was the most serious case of herbicide resistance in the world resulting in total crop failure under heavy infestation.
Urea herbicides inhibit photosynthesis by either blocking or interfering with electron flow in the thylakoid membrane of chloroplast by attaching of herbicide molecule to D1 protein. The blocking of electron transport by PS II inhibiting herbicides generates oxidative stress resulting in the destruction of the reaction centre and the photooxidation of lipid and chlorophyll molecules.
The isoproturon-resistance mechanism is metabolic degradation and enhanced detoxification, mediated by membrane-bound NADPH-dependent P-450 monooxygenase enzymes. Few biotypes have shown alteration of target site (D1) through four point mutations in the nucleotide sequence of psbA gene of P. minor.
Cross / Multiple herbicide resistance
Based on large-scale experimentation and field demonstrations from 1995 to 1998, four alternate herbicides clodinafop, fenoxaprop, sulfosulfuron and tralkoxydim were recommended. GR
50 values of fenoxaprop, clodinafop, sulfosulfuron and tralkoxydim have been reported to consistently increase year after year of their use (Yadav and Malik, 2005). During 2000-01, there were large scale failures in respect of fenoxaprop against
P. minor in Haryana, India. Also cases of multiple herbicide resistance development across three modes of action photosynthesis at the photosystem II site A, ACCase, and ALS inhibition arose during 2006. However, these multiple resistant populations were sensitive to triazine (metribuzin and terbutryn), Dinitroaniline (pendimethalin), pinoxaden, mesosulfuron + iodosulfuron, sufosulfuron + metsulfuron herbicides.
1. Dhiman, S. D., Om, H., Kumar, S., Goel, S. K., 2002, Biology and management of Phalaris minor in rice-wheat system. CCS HAU Rice Research Station, Kaul Pub. 102pp.
2. Malik, R. K. and Singh, S., 1995. Littleseed canary grass (Phalaris minor Retz.) resistant to isoproturon in India. Weed Technology, 9: 419-425.
3. Yadav, A. and Malik, R. K., 2005, Herbicide resistant Phalaris minor in wheat: A sustainability issue. Resource Book. Hisar, India: Department of Agronomy and Directorate of Extension Education, C. C. S. Haryana Agricultural University.
About Author / Additional Info:
I am a Ph.D research scholar working at the division of Agronomy, Indian Agricultural Research Institute, New Delhi