Optimizing the Water Relations of Cuttings During Propagation Fred Davies Dept. of Horticultural Sciences Texas A&M University College Station, Texas
So, How Do Cuttings Gain & Lose Water???
Water Water universal solvent brings minerals from the roots for biosynthesis within the leaf. Water needed photosynthesis & growth about 1 to 2% total. Water cools the leaf through evaporative cooling liquid to gas 98% water lost to transpiration. Transpiration is the Engine that pulls (lifts) water up from the roots.
Environmental Factors Affecting Transpiration Light plants transpire more rapidly stimulates stomatal opening, warming of leaf. Temperature transpiration higher because water evaporates more quickly as temperature increases leaf transpires 3X greater at 86F/30C than 68F/20C. Humidity diffusion of water out of leaf more rapid when surrounding air is drier.
Lower Relative Humidity of Unsaturated Surrounding Air, i.e. 70% RH 100 % RH inside leaf H 2 O
Water Potential = ψ Water moves from high to low water potential (ψ).(
Vapor Pressure Deficit (VPD) Vapor pressure determined by temperature & RH. VPD = Gradient measured as difference between water vapor pressure in leaves V leaf and surrounding air (V air ). @ @ 85F Air inside leaf saturated at 100% RH (0.60 psi); 75% RH of air outside leaf (0.45 psi). VPD = 0.60 psi 0.45 psi = 0.15 psi.
Water Relations of Cuttings Balance between Transpirational losses & uptake of water. Water travels: soil (media) roots stems leaves. Cells must maintain adequate turgor for growth and initiation & development of adventitious roots. Roots produce ABA (abscisic acid) to signal soil/media drying and reduce leaf transpiration (stomata).
The Problem Cuttings initially do not have roots lack effective organs to replace transpired water lost. Cuttings take up water POORLY through the base of the stem until adventitious roots are formed. Cutting base & foliage immersed in prop media are main entry points of water until adventitious roots form. Roots increase H 2 O uptake Poor H 2 O Uptake
The Problem (con.) Water absorption through leaves NOT major source/contributor of H 2 O balance. Water uptake in cuttings and tissue RWC declines after initially inserted in prop media. Decline in hydraulic conductivity of cuttings caused by blockage of xylem vessels, collapse of tracheids similar similar to problems observed with cut flowers, i.e. bent neck neck of roses.
Important to maintain hydraulic contact between cutting base and propagation media thus improving water uptake of cuttings. Wounding increases contact area between cutting base and propagation media improving improving water uptake of cuttings.
Control of Water Loss in Cuttings Intermittent Mist: Water droplets > 50 μm; 50 to 100 μm. Condenses Forms film f of water on leaf surface. Water evaporates from leaf surface rather than from internal water in leaf tissue.
Intermittent Mist Decreases V leaf by reducing leaf temperature. Modest increase in V air by increasing RH. Bottom Line: Lowers Leaf to Air VPD (vapor pressure gradient) & slows down transpiration.
Problems with Intermittent Mist Rapidly Leaches cuttings of nutrients N, P, K, Mg Water saturated prop media reduces media aeration anaerobic conditions death death of cuttings. Evaporative cooling can lead to suboptimal propagation media temperatures. Bottom Heat
Fog Systems Fine Water droplets 15 μm. High surface: volume ratio. Remain suspended in air as a vapor (gas) to maximize evaporation. Does not condense avoids over saturation of media & foliar leaching, as occurs with mist.
Fog Systems Maximize V air by increasing RH. Decreases V leaf by decreasing leaf & air temperature. Bottom Line: Lowers Leaf to Air VPD (vapor pressure gradient) & slows down transpiration.
High cost. Problems with Fog Systems Maintenance of system, clogging & wearing out of nozzles. Filtration/deionizing system to remove salts from H 2 O. Salt accumulation
Contact Systems/Nonmisted Enclosures Water loss from foliage reduced; condensation increases RH of air. Simple, inexpensive, cost effective. Minimal condensing avoids over saturation of media & foliar leaching, as occurs with mist. Nonmisted Enclosures Contact System
Contact Systems/Nonmisted Enclosures Maximize V air by preventing escape of water vapor uses humidification since only V air is affected V leaf leaf somewhat affected, leaf temp cooler with condensation in contact poly system Bottom Line: Lowers Leaf to Air VPD (vapor pressure gradient) & slows down transpiration
Problems with Contact Systems/Nonmisted Enclos. Critical to control irradiance (shade system) & temperature. Easily traps heat via light irradiance can reduce RH of air, leaf temp rises, increase leaf to air VPD/gradient.
Static Mist Control Systems Inexpensive, easily installed. Rely on clocks and timers. Unable to respond to daily fluctuations in light irradiance, cloud cover, RH, Temperature or stage of root development. Moderate conditions reduce evaporative demand by reducing VPD Too much mist cloudy days, solar radiation low. VPD air. Too little mist very sunny, windy days when net radiation is high.
Light Sensors Dynamic Mist Control Systems Respond to changes in environment affecting VPD. Evapotranspiration based mist control system (dynamic control) air temp, time interval between misting, calculated VPD air RH Sensor Computerized Environmental Sensors Screen Balance
Ideal Propagation Media. Air filled porosity: 15 40%; 20 25% 25% ideal. Water Holding Capacity (WHC) 20 60% after gravitational drainage.
Stock Plant Maintenance
Maintain Plant s Momentum
12 80 Net photosynthesis (µmol CO 2 m 2 s 1 ) 10 8 6 4 2 Stomatal Conductance Net Photosynthesis 70 60 50 40 30 20 10 Stomatal Conductance (mmol H 2 O m 2 s 1 ) 0 0 0 5 10 15 20 25 Days 100% rooting Root elongation Svenson,Davies (1995) Root primordia
Take Home Message Maintain the Plants Momentum: Season, Minimize Stress. Control Stress Light, Temperature and Humidity (RH) to Reduce VPD, i.e. atmosphere of low evaporative demand=low transpirational losses from cuttings. Don t increase light until cuttings start to form roots. Apply just enough mist to form a thin film of water on leaf surface. Use a loose propagation media for proper aeration. Group cuttings in propagation by species requirement for moisture, i.e. Zelkova & Chinese Elm vs. River Birch tolerance for wetter media.