AG 2470 Crop Water Use, Drought Stress, Soil Health & Irrigation pt 2

What is the primary physiological reason crop yield is reduced under drought stress?

Stomatal closure limits the inward diffusion of CO2 into leaf cells, restricting photosynthesis.

Approximately how many liters of water are required by a plant to produce 1 kg of grain?

500 L of water.

Why is cellular water depletion for the biochemical reaction of photosynthesis not the main driver of drought stress?

Cells contain roughly 55 mol/L of water; if water levels drop enough to stall the reaction, proteins and cell structures have already denatured.

What maximum percentage of total plant water requirement is accounted for by turgor pressure?

At most 5% of total water needs.

Why do plants lose vast amounts of water through transpiration?

Water loss is an unintended consequence of stomata opening to allow inward diffusion of atmospheric CO2

Correct the misconception: 'Stomata close only after a plant wilts.'

Stomata are actively regulated: when roots sense dry soil, chemical signals are sent to leaves to trigger stomatal closure before wilting occurs.

What physical driving force moves water in its liquid phase through a plant?

A pressure difference moving water down a negative thermodynamic gradient from wetter regions (soil) to drier regions (leaves).

How does water move through the gas phase inside leaf tissue?

Via diffusion driven by high vapor concentration gradients within intercellular air spaces out through stomata.

List four environmental factors that determine the rate of crop water use.

Temperature, solar radiation/light availability, humidity, and wind speed.

In what unit is crop water use measured?

Millimeters (depth), which represents volume per unit surface area.

How does a crop's daily water consumption change as it develops through its growth stages?

Water consumption increases in proportion to leaf area expansion and the total solar radiation intercepted.

What is reference evapotranspiration ETo

An estimate of expected water loss via transpiration and evaporation from an idealized, well-watered, uniform vegetative surface under current weather conditions.

What primary meteorological factor causes large day-to-day fluctuations in reference evapotranspiration ETo?

Solar radiation levels (cloud cover variations).

Why is breeding crops solely for larger root systems not a complete solution for drought stress?

Allocating metabolic resources toward excessive root growth diverts energy away from harvestable yield.

What are aerenchyma tissues, and how do they benefit marsh and wetland plants?

Air spaces inside plant stem tissue that conduct air down to roots, allowing nutrient uptake without excessive metabolic cost.

What hypothetical epidermal modification could eliminate the trade-off between transpiration and photosynthesis?

A semi-permeable membrane replacing stomata that freely allows inward CO2 diffusion while restricting outward water vapor movement.

What wavelength range corresponds to photosynthetically active radiation (PAR) and visible light?

400 to 700 nm

What plant physiological processes are influenced by near-infrared light in the 700-800 nm range?

Seed germination, phototropism, and flowering.

Which man-made greenhouse gas has a global warming potential 23,000 times greater than CO2

Sulfur hexafluoride SF6

How does paddy rice production contribute significantly to atmospheric methane (CH4) emissions?

Anaerobic conditions in flooded soil promote methanogen activity, and rice plant stems act as a 'straw' conduit pumping methane into the air.

By what biochemical process is nitrous oxide (N2O) produced in agricultural soils?

Denitrification of nitrate in wet soils and wetlands.

What defines the field capacity of a soil?

The maximum amount of water a soil profile can hold against the downward pull of gravity.

Distinguish between soil infiltration and soil percolation.

Infiltration is the ease with which water enters the soil surface; percolation is how easily water moves down through the soil profile.

What is the permanent wilting point of soil?

The soil moisture level at which water is held so tightly by matrix forces that plants can no longer extract it.

What is the volumetric ratio of pore space to solid space in an ideal soil?

50% pore space and 50% solid space.

What are the specific volumetric proportions of mineral, organic, water, and air components in an ideal soil at field capacity?

45% mineral, 5% organic matter, 25% water, and 25% air.

How do sandy soils and clay soils compare regarding water holding capacity and availability?

Sandy soils hold less total water, but water is easily available to plants; clay soils hold a large volume of water but retain much of it tightly.

What are hydathodes, and what substance leaves the plant through them?

Pores located at leaf tips or margins through which liquid water droplets containing high concentrations of sugars exit (guttation).

Contrast cohesive forces and adhesive forces in xylem water transport.

Cohesive forces bind water molecules to one another; adhesive forces pull water molecules to xylem cell surfaces.

What are three key physiological functions of transpiration in plants?

Maintaining cell turgidity, pulling nutrients up through xylem against gravity, and evaporative cooling.

What short-term morphological and structural changes do plants exhibit when experiencing drought stress?

Increased root length/density, stomatal closure, cuticle thickening, and production of antioxidant enzymes.

What cellular metabolic impairment occurs during extended, severe drought stress?

ATP synthesis is obstructed, leading to drastic reductions in photosynthesis and crop yield.

What is the difference between drought resistance and drought tolerance in crop plants?

Drought resistance prevents stress impact (e.g., thick cuticles); drought tolerance allows the plant to function despite stress exposure.

Why is maintaining optimal soil Potassium (K+) levels crucial for managing drought stress?

Potassium plays an essential regulatory role in turgor changes of guard cells responsible for opening and closing stomata.

Why should excess readily available Nitrogen (N) be avoided under drought conditions?

Excess N stimulates shoot/vegetative growth, increasing transpiration area and placing heavy demand on root systems.

How does furrow surface irrigation operate?

Water is gravity-fed through small, shallow trenches carved between rows of crops.

What is the purpose of a soil mound (bund) in basin irrigation for fruit trees?

It isolates localized water away from direct contact with the tree trunk to prevent fungal rot.

What are two major disadvantages of surface irrigation systems?

Low water use efficiency (high evaporative loss) and heightened disease/weed risks from standing water.

Why is sprinkler irrigation generally unsuitable for tree fruit orchards?

Tree trunks and canopies block the overhead water stream, resulting in uneven water distribution.

Why is microirrigation (drip irrigation) considered the most efficient irrigation method?

It applies water directly to the root zone, minimizing evaporation losses and foliage wetting.

What is fertigation?

The practice of applying dissolved liquid fertilizers directly through a drip or microirrigation system.

Why is nighttime irrigation strictly discouraged for turfgrass maintenance?

Prolonged leaf wetness overnight creates an ideal moist environment for fungal pathogens to thrive.

What toxic gases accumulate in waterlogged soils, and how do they impact crop roots?

CO2, ethylene, and methane accumulate due to anaerobic microbial activity, suffocating roots and causing root death.

What is the primary objective of Controlled Traffic Farming (CTF)?

To confine machinery compaction to precise, permanent wheel tracks ('soil roads'), preserving uncompacted soil across the rest of the field.

How do reduced-till or no-till farming practices improve soil water retention?

By increasing soil organic matter, conserving natural soil pore structure, and enhancing water infiltration.

How does a raised bed crop system protect plant roots from excess water?

It elevates the root zone above standing water, allowing excess water to drain away into furrows.

How is 'soil quality' defined in agricultural science?

The capacity of soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote health.

How does the concept of 'soil health' differ from 'soil quality'?

Soil health explicitly emphasizes the continuous capacity of soil as a vital living ecosystem.

List three chemical indicators used to assess soil health.

Soil pH, Cation Exchange Capacity (CEC), and Electrical Conductivity (EC).

List four physical indicators evaluated during soil assessment.

Bulk density, texture, aggregate stability, and porosity/infiltration rate.

List three biological indicators of soil health.

Earthworm populations, microbial respiration rate, and enzyme activity.

What soil properties are influenced by soil texture?

Drainage, porosity, water-holding capacity, Cation Exchange Capacity (CEC), bulk density, and nutrient availability.

How does high soil organic matter enhance soil structure?

It stimulates root exudates and microbial binding agents, increasing macroaggregate formation and stability.

What is soil bulk density?

The dry weight of soil per unit volume.

Why is elevated bulk density harmful to crop development?

It restricts root penetration physically and reduces soil pore space, restricting oxygen and aerobic microbial activity.

What is Cation Exchange Capacity (CEC) in soil science?

The total capacity of soil particles (negatively charged sites on clay and organic matter) to hold and exchange positively charged ions (cations).

How does specific surface area affect a soil's Cation Exchange Capacity (CEC)?

Greater surface area provides more negatively charged binding sites for cations, resulting in higher CEC.

What parameter does Electrical Conductivity (EC) measure in a soil test?

Soil salinity (the concentration of soluble salts carrying an electrical current).

How do earthworm populations improve physical soil quality?

By forming stable soil aggregates, creating macro-channels for water/air movement, and decomposing residues.

Which structural plant compound is particularly resistant to decay and requires specialized soil microorganisms for decomposition?

Lignin.

What is the rhizosphere?

The narrow region of soil directly surrounding and chemically influenced by plant root excretions and root growth.

Which microbial group is present in the absolute highest numerical abundance in healthy soils?

Non-filamentous bacteria.

What is Induced Systemic Resistance (ISR) as triggered by Plant Growth-Promoting Rhizobacteria (PGPR)?

A state of heightened physical and chemical defense triggered systemically throughout the plant by beneficial root-colonizing bacteria.

What structural and functional features characterize actinomycetes in soil?

They are filamentous bacteria that resemble fungal hyphae, grow slowly, and produce natural antibiotics (e.g., Streptomycin).

How do fungi compare to bacteria regarding population numbers versus total biomass in soil?

Fungi have lower total population numbers than bacteria, but they represent the largest proportion of total microbial biomass.

Why is excessive or non-targeted fungicide application detrimental to soil health?

It disrupts beneficial fungal networks that form the dominant living biological biomass essential for organic decomposition.

What is biofumigation in crop management?

Incorporating volatile-releasing plant tissue (like brassicas) into soil to suppress soil-borne pests and pathogens naturally.

What is aflatoxin, and what produces it?

A potent natural carcinogenic mycotoxin produced by fungi infecting crops such as corn and peanuts.

Why does soil lacking plant or vegetative ground cover ('bulk soil') show low microbial activity?

Without live roots exudating organic carbon compounds, microbes lack the continuous food supply present in the rhizosphere.

How does animal manure addition benefit soil physical quality beyond nutrient supply?

It improves soil structure and aggregate stability while fueling biological microbial activity.

What advantages do municipal biosolids (treated sewage sludge) offer as a soil amendment?

They supply organic matter along with a broad array of essential macro- and micronutrients.

What is biochar, and how does it enhance long-term soil water and nutrient storage?

Recalcitrant carbon produced via biomass pyrolysis; its high porosity and surface area boost water retention and nutrient binding sites.

Adding simple sugars like molasses to soil causes a rapid increase in which specific microbial population?

Simple, fast-growing bacteria.

What is the consequence of low microbial species richness in an agricultural soil ecosystem?

Reduced overall microbial activity, lowered functional redundancy, and diminished resistance to soil pathogen outbreaks.

Why is soil tillage described as being best managed at a 'moderate rate'?

Moderate tillage alleviates severe compaction without over-destroying natural soil aggregates and fungal hyphal networks.

How does strip tillage maintain water conservation advantages relative to full conventional tillage?

It disturbs soil only in narrow seedbed strips, leaving undisturbed residue cover between rows to slow evaporation.

Why does irrigation with high-salinity water degrade soil physical structure over time?

Salt accumulation causes clay dispersion, breaking down stable soil aggregates and reducing surface permeability.

What modern technological advances allow agronomists to quantify specific, unculturable soil microbes?

Molecular techniques including whole-genome sequencing and quantitative Polymerase Chain Reaction (qPCR).