(a) What is greenhouse effect? Write in detail about various greenhouse gases and their impact on crop production. (20 marks) (b)…

(a) Greenhouse effect, greenhouse gases and crop production

The greenhouse effect is the warming of the Earth's surface because atmospheric gases transparent to incoming short-wave solar radiation absorb outgoing long-wave infrared radiation and re-radiate part of it back, keeping the planet about 33°C warmer than it would otherwise be. Human emissions have intensified it.

Major gases: carbon dioxide (GWP 1), from fossil fuels and land-use change; methane (GWP about 28-30), from flooded rice, ruminants and manure; nitrous oxide (GWP about 273), from nitrogen fertilisers and soils; tropospheric ozone, formed from NOx and volatile organic compounds; and CFCs (GWP in the thousands), from refrigerants and aerosols.

Impacts on crops: elevated CO2 fertilises C3 crops such as wheat and rice, but accompanying warming raises respiration and shortens crop duration, cutting grain filling. Terminal heat causes pollen sterility and poor grain set, as the March 2022 heatwave reduced wheat yields in northwestern India. Rising temperature and erratic rainfall reduce water availability and increase pest and disease pressure. Surface ozone causes leaf chlorosis and lipid peroxidation and lowers yield in wheat and rice. Higher temperatures also deplete soil organic carbon. C4 crops such as maize and millets respond less to CO2 but tolerate heat better.

Agriculture itself contributes methane and nitrous oxide, so mitigation includes alternate wetting and drying in rice, balanced fertilisation and soil carbon sequestration.

(b) Technology and market policy in crop diversification

Technology enables diversification on the supply side. Precision farming and micro-irrigation free crop choice from groundwater abundance, polyhouses allow shifts to high-value flowers and vegetables, and marker-assisted breeding has produced short-duration, climate-tolerant pulses and oilseeds.

Market policy shapes adoption. Assured MSP and procurement locked Punjab and Haryana into paddy-wheat monoculture, with falling water tables. Realignment includes MSP and procurement support for pulses and oilseeds; the Crop Diversification Programme (2013) to move area out of paddy in Punjab, Haryana and western Uttar Pradesh; Haryana's Mera Pani Meri Virasat (2020), which pays farmers to shift from paddy; PM-AASHA price support (2018); millet promotion under the National Food Security Mission and the International Year of Millets (2023); and aggregation through FPOs linked to e-NAM, with contract farming and post-harvest and cold-chain support reducing marketing risk. PM Fasal Bima Yojana covers yield risk for alternative crops.

Diversification works only when price signals, assured markets and risk cover come together; MSP alone does not shift acreage.

(c) Activity and selectivity of herbicides

Activity describes how a herbicide acts. Contact herbicides, such as paraquat, kill only the tissue they touch by generating reactive oxygen and rupturing membranes. Systemic herbicides, such as glyphosate (an EPSPS inhibitor) and 2,4-D (an auxin mimic), move through phloem or xylem to roots and meristems. Pre-emergence herbicides such as pendimethalin act on germinating seedlings, post-emergence ones on emerged weeds.

Selectivity is differential toxicity to crop and weed. It arises from morphology (narrow upright cereal leaves retain less broadleaf spray); physiology (differential absorption and translocation); biochemistry (the crop rapidly detoxifies the chemical, as maize does atrazine through glutathione conjugation); and placement, as shallow-acting pendimethalin spares deep-rooted crops. Safeners and herbicide-tolerant traits can also confer selectivity. Glyphosate and paraquat are non-selective.