Tasawer Abbas

Glyphosate hormesis in broad-leaved weeds: a challenge for weed management

ABSTRACT Little is known of glyphosate-induced hormesis in weeds and how this might influence weed management. To test the hormetic effect of low doses of glyphosate on broad-leaved weeds, two experiments were conducted, in the laboratory and the screenhouse. The hormetic effects of glyphosate solution in growth media (0, 65, 130, 250, and 500 g acid equivalent (a.e) ha−1) and foliar spray (0, 4, 8, 16, 32, and 64 g a.e. ha−1) were tested on four broad-leaved weeds (Coronopus didymus, Chenopodium album, Rumex dentatus, and Lathyrus aphaca). Glyphosate solution in the range 65–250 g a.e. ha−1 stimulated the germination and seedling growth of all tested weeds. However, at 500 g a.e. ha−1 inhibition of germination and growth was observed. Foliarly applied glyphosate in the range 4–32 g a.e. ha−1 increased root and shoot length, dry biomass, and seed production ability of all four weeds species; however, the stimulatory response was species dependent. These results indicate that glyphosate hormesis could play a significant role in altering crop/weed competition and might influence weed management.

Weeds Cause Losses in Field Crops through Allelopathy

A large number of weeds are known to be associated with crops and causing economic losses. Weeds interfere with crops through competition and allelopathy. They produce secondary metabolites known as allelochemicals, which belong to numerous chemical classes such as phenolics, alkaloids, fatty acids, indoles, terpens etc. However, phenolics are the predominant class of allelochemicals. The allelochemicals release from weed plants takes place through leaf leachates, decomposition of plant residues, volatilization and root exudates. Weeds leave huge quantities of their residues in field and affect the associated, as well as succeeding crops, in various cropping systems. Liberation of allelochemicals from weeds affects the germination, stand establishment, growth, yield and physiology of crop plants. They cause substantial reduction in germination and growth of the crop plants by altering various physiological processes such as enzyme activity, protein synthesis, photosynthesis, respiration, cell division and enlargement, which ultimately leads to a significant reduction in crop yield. In crux, allelopathic weeds represent a potential threat for crop plants and cause economic losses.

Evaluation and management of acetyl-CoA carboxylase inhibitor resistant littleseed canarygrass (Phalaris minor) in Pakistan

ABSTRACT A field survey was conducted for the sampling of Acetyl-CoA carboxylase (ACCase) inhibitor resistance littleseed canarygrass, a major weed of wheat, from Punjab, Pakistan in 2014 for confirmation of resistance. The surveyed regions encompassed four different cropping systems including rice–wheat, maize–wheat, cotton–wheat and mixed cropping. Dose–response assay was conducted for confirmation of resistance. Efficacy of herbicide mixtures including clodinafop–propargyl, metribuzin, pinoxaden and sulfosulfuron at a range of doses was investigated to manage littleseed canarygrass. Results revealed that all populations were resistant to fenoxaprop except PM-BWL-2. The higher level resistance (6.5) was found in populations collected from rice–wheat cropping system. The tested herbicide mixtures at 75% and 100% of the recommended dose of each mixture component provided the effective control of resistant littleseed canarygrass. Mixtures at 50% provided more than 80% control and reduced growth and seed production potential of surviving plants. The confirmation of ACCase inhibitor resistance as the first case of herbicide resistance in Pakistan, leads us to discourage use of ACCase inhibitor herbicides alone. However, herbicide mixtures at 75% and 100% of the recommended dose are suggested to manage this weed for sustainable wheat production in the surveyed cropping systems.

ALLELOPATHIC EFFECT OF LEGUMINOUS WEEDS ON RATE, SYNCHRONIZATION AND TIME OF GERMINATION, AND BIOMASS PARTITIONING IN RICE

The present study was aimed at determining the allelopathic influence of water extracts at varying concentrations (2.5% and 5% w/v) and residues with varying decomposition periods (0, 15 and 30 days) at 2% (w/w) concentration of four weeds (Trigonella polycerata, Vicia sativa, Lathyrus aphaca, Medicago polymorpha) against rate, synchronization and time of germination, and biomass partitioning of rice through plant and soil bioassays. Germination and biomass partitioning in rice plants were inhibited by extracts and residues. The maximum decrease in energy of germination (88.6%) and increase in time for 50% germination (210.4%) were caused by the L. aphaca extract at 5% concentration as compared to control. Root and shoot, fresh weight (13.6 and 28.6%, respectively) and dry weight (20.4 and 41.7%, respectively) were reduced by the M. polymorpha extract at 5% concentration. The Vicia sativa extract at both concentrations caused a great increase in the root/shoot ratio (125-145.8%). Trigonella polycerata and V. sativa residues decomposed for 30 days caused maximum inhibition of energy of emergence (71.4%) and their decomposition for zero and 15 days, respectively, inhibited coefficient of uniformity of emergence (78.9%). Medicago polymorpha and L. aphaca residues decomposed for 30 days exaggerated time for 50% emergence (47.1 and 50.3%, respectively) when compared with control. Medicago polymorpha residues decomposed for 30 days exerted the greatest inhibitory effect on shoot fresh weight (46.1%) and root dry weight (59.9%). Vicia sativa with 30 days and T. polycerata residues without decomposition produced the least root fresh weight (65.3%) and shoot dry weight (52.6%), respectively. The root/shoot ratio (56.8%) was increased by T. polycerata residues without decomposition. Phenolics from leguminous weeds caused inhibition of germination/emergence and biomass partitioning in rice. Therefore, these weeds must be eradicated from the field at their initial growth stages.

Boron application in clay-loam soil for improved growth, yield and protein contents of mungbean in water-stresses

Boron is considered important to improve the drought resistance, yield and protein contents of pulses. Two years of field experiment was conducted to evaluate the effect of boron application and water stress given at vegetative and flowering stages on growth, yield and protein contents of mungbean during spring 2014 and 2015. The experiment was laid out in randomized complete block design with split-plot arrangement giving more emphasis to boron. The experiment comprised three water stress levels (normal irrigation, water stress at vegetative stage and water stress at reproductive phase) and four boron levels (0, 2, 4 and 6 kg ha-1). Final seed yield was significantly increased by different levels of boron application both under normal and water stressed conditions. The increase in yield was mainly due to greater plant height, number of pods bearing branches, number of pods per plant, number of seeds per pod and 1000-grain weight. Boron application at 4 kg ha-1 caused 17%, 10% and 4% increase in grain yield under normal irrigation, stress at vegetative stage and water stress at reproductive phase, respectively. Protein contents were also increased (9-16%) at same boron treatment. Most parameters showed a marked decrease at higher dose (6 kg ha-1) of boron. In conclusion, the boron application at rate of 4 kg ha-1 in clay-loam soil performed the best to enhance mungbean growth, yield and seed protein both under normal and water stressed conditions.

Estimation of economic threshold level of alligator weed (Alternanthera philoxeroides (Mart.) Griseb.) to Tackle Grain quality and yield Losses in Rice

ABSTRACT Alligator weed (Alternanthera philoxeroides) is the most troublesome invasive weed in transplanted rice ecosystems worldwide. A two-year field study was conducted to estimate economic threshold levels and the season long competitive effect of six alligator weed densities (0, 2, 4, 8, 16, and 32 plants m−2) on yield and quality traits of transplanted rice. A gradual linear incline in dry weight of alligator weed was observed with an increase in its density from 2 to 32 plants m−2. Maximum N (15.93–18.26 kg ha−1), P (15.10–16.46 kg ha−1) and K (16.34–17.81 kg ha−1) uptake by alligator weed was recorded at its density of 32 plants m−2. More micronutrient loss to the tune of 6.53, 47.92, 19.99, and 38.42 mg kg−1 for Cu, Fe, Zn, and Mn, respectively was observed at the same density. Increasing density of alligator weed caused more losses in paddy yield (up to 21.37–23.78%), amylose and rice grain protein contents. According to a nonlinear hyperbolic regression model, maximum paddy yield loss of rice at asymptotic value of alligator weed density was 38.8% during 2011 and 42.4% during 2012. Economic threshold value of alligator weed was estimated 1.5 and 1.3 plants per m2 during 2011 and 2012, respectively.