Plant Growth Regulation :Plant Growth Regulation Growth: Cell division and enlargement Irreversible Hydration and vacuolation Differentiation Dependent upon an interaction of genotype and environment DNA codes the sequencing of amino acids into specific proteins and enzymes, establishing a genetic potential for growth, development, and complete morphogenesis


Plant Growth Regulation (cont.) :Plant Growth Regulation (cont.) Plant production is dependent upon maximizing growth rates and yield through genetic and environmental manipulation Dry weight accumulation is commonly used as a parameter to describe growth (height, volume, and leaf area can also be used)


Plant Growth Regulation (cont.) :Plant Growth Regulation (cont.) External Factors 1. Climatic: Light, temperature, water, daylength, wind, gases (including pollutants) 2. Edaphic (soil): Texture, structure, organic matter, cation exchange capacity (CEC), pH, base saturation, and nutrient availability 3. Biological: Weeds, insects, disease organisms, various types of herbivores, and soil microorganisms including N2 fixing and denitrifying bacteria, and mycorrhiza


Plant Growth Regulation (cont.) :Plant Growth Regulation (cont.) Internal Factors 1. Resistance to climatic, edaphic, and biological stresses 2. Photosynthetic rate 3. Respiration 4. Partitioning of assimilate and N 5. Chlorophyll, carotene, and other pigment contents 6. Type and location of meristems 7. Capacity to store food reserves 8. Enzymatic activity 9. Direct gene effects (e.g., heterosis, epistasis, flowering) 10. Differentiation


Plant Growth Regulation (cont.) :Plant Growth Regulation (cont.) Liebig Law of Minimum “A deficiency or absence of one necessary constituent, all others being present renders the soil barren of crops....” Mitscherlich Law of Deminishing Returns “The increase in any crop produced by a unit of increment of a deficient factor is proportional to the decrement of that factor from the maximum”


Plant Growth Regulation (cont.) :Plant Growth Regulation (cont.) Limiting Factor Qualifications 1. Biological reactions are complex and may proceed by more than one pathway. 2. Factors substitute for other factors 3. Factors modify or affect other factors 4. Plants affect other non-plant factors (e.g., environmental factors) 5. More than one factor may be acting simultaneously


Plant Growth Regulation (cont.) :Plant Growth Regulation (cont.) Considerations: 1. Allometry Growth rates of an individual organ 2. Root:Shoot Ratio Influence of drought stress Fertigation (Irrigation + nutrients) 3. Apical and Lateral Growth 4. Growth and Differentiation Dependent upon: Available assimilate in excess of most metabolic processes Favourable temperatures Proper enzyme system to mediate the differentiation response


Plant Growth Regulation (cont.) :Plant Growth Regulation (cont.) Growth Dynamics 1. Leaf area index 2. Leaf area duration 3. Net assimilation and partitioning rate 4. Economic biomass


Physiology of Plants Under Stress :Physiology of Plants Under Stress Any external constraint that reduces the ability of a plant to develop to its genetically predetermined level Plant species are highly variable in their optimum environments and tolerance to extreme conditions (e.g., water potential, salinity, temperature, etc.)


Physiology of Plants Under Stress (cont.) :Physiology of Plants Under Stress (cont.) Principal environmental stresses can include the following High temperatures (heat) Low temperatures (chilling, freezing) Excess water (anoxia, flooding) Water deficit (drought, low water potential) Salinity Radiation (PAR, ultraviolet) Chemical (pesticides, heavy metals, air pollutants) Biotic (pathogens, competition)


Plant Responses Under Stress :Plant Responses Under Stress Some plants are injured by stress in which they exhibit one or more metabolic disfunctions If moderate and short term: injury may be temporary and the plant can recover when the stress is removed If severe: flowering and seed formation may be impaired or the survival of the plant Stress escapers include ephemeral plants (short-lived desert plants) which germinate, grow, and flower very quickly following seasonal rains Complete their life cycle during a period of adequate soil moisture


Plant Responses Under Stress (cont.) :Plant Responses Under Stress (cont.) Stress Avoidance Mechanisms reduce the impact of a stress Deep root systems Thick cuticles Fleshy leaves


Plant Responses Under Stress (cont.) :Plant Responses Under Stress (cont.) Stress Tolerance Requires the plant to come to thermodynamic equilibrium with the stress Internal conditions are in equilibrium with conditions outside of the plant E.g., Plant can survive the desiccation of the protoplasm without injury. It also has the ability to rehydrate the protoplasm without injury and retaining the ability to resume normal growth upon rehydration of the protoplasm


Plant Responses Under Stress (cont.) :Plant Responses Under Stress (cont.) Adaption Heritable modifications in structure or function which increase the fitness of the plant in a stressful environment E.g., the morphological and physiological adaptations of CAM plants


Plant Responses Under Stress (cont.) :Plant Responses Under Stress (cont.) Acclimation Non-inheritable adaptations that occur over the lifespan of a plant Modifications are induced upon gradual exposure to the stress such as chilling temperatures or drought stress Enable the plant to adapt, live, and reproduce in stressful environments Capacity to acclimate is a genetic trait, but the specific changes brought about by the stress are not necessarily passed on to the next generation


Plant Responses Under Stress (cont.) :Plant Responses Under Stress (cont.) Strategy A genetically programmed sequence of responses that a plant will use to survive in aparticular environment


Types of Stress :Types of Stress 1. Water Stress Caused by an excess or deficit of water Osmotic stress


Water Stress (cont.) :Water Stress (cont.) Membranes and Water Stress Detrimental effects of desiccation on the protoplasm Increase in solute concentration Protoplast volume may shrink and this may have serious metabolic and structural consequences Integrity and structure of membranes are also influenced by desiccation (membranes become exceptionally porous) Loss of membrane integrity, loss of protein structure, and high concentration of cellular electrolytes all contributes to a disruption of metabolism in the cell


Water Stress (cont.) :Water Stress (cont.) Photosynthesis and Water Stress Affected by water stress in 2 ways (i) Stomatal limitation (closure of stomata) (ii) Mesophyll limitation (low cellular water potential)


Water Stress (cont.) :Water Stress (cont.) Stomatal Responses to water Deficit Importance of vapour pressure gradient? Hydropassive closure Guard cells are not protected by a thick cuticle Closure of stomata as a result of direct evaporation of water from the guard cell Hydroactive closure Metabolically dependent event Reversal of the ion fluxes that cause stomatal opening Involves ABA and other plant growth regulators


Drought Stress :Drought Stress Osmotic Adjustment Net increase in solute concentration within a cell as a result of metabolic processes triggered by drought stress Relatively slow process Maintains cell turgor Allows Pn to occur under conditions of moderate stress Also assists the plant in quickly retaining turgor upon rehydration occurs


Osmotic Adjustment and Drought Stress (cont.) :Osmotic Adjustment and Drought Stress (cont.) Most chemicals associated with osmotic adjustment do not normally interfere with normal metabolic processes Examples: Proline (amino acid) Sorbitol (sugar alcohol)


Drought Stress (cont.) :Drought Stress (cont.) Shoot and Root Growth Leaf Area Adjustment Loss of turgor Abscission of older leaves (cotton)


Temperature Stress :Temperature Stress Chilling Stress Many plants, especially those native to warm habitats are injured when exposed to low nonfreezing temperatures Examples: maize, cucumber, cotton, tomato, etc. Young seedlings typically show signs of reduced leaf expansion, wilting, and chlorosis


Chilling Stress (cont.) :Chilling Stress (cont.) Influences a wide array of activities including impaired cytoplasmic streaming, reduced respiration, photosynthesis, and protein synthesis Causes reversible changes in the physical state of the membranes Chilling-sensitive plants tend to have a higher proportion of saturated fatty acids


Temperature Stress (cont.) :Temperature Stress (cont.) (ii) Freezing Stress Acclimation to lower temperatures is critical to many perennial and biennial crops It is ice formation (not low temperatures) that causes damage to membranes Rapid freezing reduces the size of ice crystals formed (e.g., cryogenic storage)


Temperature Stress (cont.) :Temperature Stress (cont.) Thermal analysis of freezing Upon gradual freezing, a sudden rise in tissue temperature occurs as a result of ice formation in the apoplastic space (extracellular freezing of xylem tracheary elements and intercellular spaces in cortex, bark, and phloem) Water first freezes in apoplastic space as a result of relatively low solute content of water in apoplast


Thermal analysis of freezing (cont.) :Thermal analysis of freezing (cont.) Water migrates from the symplast to the apoplast as a result of a large water potential gradient (i.e., water potential of ice is very low) Migration of water from the cytoplasm to the apoplastic space results in the creation of a second exotherm at a lower temperature Deep supercooling (i.e., a lack of freezing until very low temperatures are attained) occurs as a result of tissue water lacks nucleating agents, and relatively small volumes of water which minimizes ice crystal size Effective freezing mechanism avoidance (e.g., apple buds are not affected by temperatures >-40 C)


High Temperature Stress :High Temperature Stress Combination of high temperature and irradiance stresses Few plants can tolerate leaf temperatures >50 to 55 °C (exception: cacti, etc.) Effects on membranes and metabolism Irreversible denaturization of membranes Reactions in thylakoid membranes are most sensitive to heat stress Light reaction (especially photosystem II) is particularly sensitive to injury


High Temperature Stress (cont.) :High Temperature Stress (cont.) Heat shock proteins (HSPs) Low molecular mass proteins Can be synthesized very rapidly upon an abrupt increase in temperature Function: protect membrane integrity?