Aspects of Plant Biology What Makes Plants Tick?

Plant Water Relations (from CH 36) Plants need water for –Nutrient transport –Metabolic solvent –Turgor pressure –Cooling

Plant Water Relations Plants obtain water passively Plants transport water without a systemic pump –Plants transport water through two different, but interconnected, systems

Transport in Plants water travels down a gradient of water potential water potential = osmotic potential + pressure potential  s +  p

Transport in Plants water moves through membranes by osmosis in response to water potential differences water moves through xylem or phloem by bulk flow diffusion vs. unidirectional flow

Transport in Plants Bulk flow in xylem and phloem is driven by different forces –phloem transport is under positive pressure –xylem transport is under tension (negative pressure)

Transport in Plants phloem transport –at the source sugar is pumped into sieve tube by active transport (phloem loading) as sieve tube  s decreases, sieve tube  decreases extracellular water moves into sieve tube by osmosis sieve tube  p (and  ) increases

Transport in Plants phloem transport –at the sink sugar is pumped out of sieve tube (phloem unloading)  s (and  ) in sieve tube increases water moves out of sieve tube by osmosis  p in sieve tube decreases

Phloem transport Figure 36.14

Transport in Plants phloem transport –phloem sap flows from source to sink down a gradient of  dominated by the difference in  p –flow can be from any source to any sink THE PRESSURE-FLOW MODEL

Transport in Plants xylem transport –  is lower high in the transpiration stream water is under increasing tension (negative pressure) up the stem –water is pulled up the transpiration stream –evapotranspiration through the stomata drives the transpiration stream

transpiration stream Figure 36.8

pressure bomb Figure 36.9

evapotranspiration of water from a leaf Figures 35.23, 36.1

 gradient increases with temperature vpd =  gradient

RH drops as temperature rises Leaf & air temperature (˚C) RH of air (%) Vapor Pressure in leaf Vapor Pressure in air Vapor Pressure Gradient

airflow increases transpiration T a = transpiration rate of plant in still air T b = transpiration rate of plant in wind T b /T a : wind/still air

Gentle Breeze steep gradient shallow gradient steep gradient leaf surface stoma

Transport in Plants xylem transport –tension can drive the transpiration stream because water is cohesive water is adhesive to xylem cell walls THE COHESION-ADHESION TENSION THEORY

Table 36.1

Figure 35.2

Developmental Regulation Like animals –Environmental signals provide cues –Receptors receive and transduce signals –Hormones integrate responses Unlike animals –Growth is indeterminate Organs are modular Most tissues contain totipotent cells

Plant Responses to Challenges (from CH 40) Challenge: Pathogenesis Response(s): –cell wall and cuticle repel most pathogens –reinforcement of the cell wall prevents pathogen from spreading –lignin precursors are toxic –toxic phytoalexins are produced rapidly by infected cells and their neighbors –PR proteins mediate a variety of defenses

Hypersensitive response Figure 40.2

Magnesium salicylate R-COO - 2 ·Mg 2+ Salicylic acid Page 767

Gene-for-gene resistance Figure 40.3

composite of cellular responses Figure 40.1

Plant Responses to Challenges (from CH 40) Challenge: Pathogenesis Response(s): –resistant plants employ the hypersensitive response to limit the spread of pathogens –systemic acquired resistance is non-specific immunity mediated by salicylic acid –gene-for-gene resistance triggers defensive mechanisms to specific pathogen strains –siRNAs block viral infection

interference RNA (RNAi) Figure 16.11

Plant Responses to Challenges Challenge: herbivory

Response to herbivory Figure 40.4

toxic amino acid analog page 770

Plant Responses to Challenges Challenge: herbivory Response(s): –Modular development - replace lost parts –Chemical defenses - toxic or repellent secondary metabolites

Table 40.1 > 10,000 characterized compounds in these and other classes

Leaf positioning in a eucalypt

stomatal crypts in Nerium oleander Figure 40.8

Plant Responses to Challenges Challenge: Water problems Response(s): anatomical –Leaf position –Surface hairs –Stomatal position & condition –Leaf abscission - seasonal or more often

preparation for seasonal leaf drop

opportunistic leaf production in ocotillo Figure 40.9

Sequestering salt in the extracellular compartment Figure 40.13

Plant Responses to Challenges Challenge: Water problems Response(s): metabolic –Fermentation –Salt sequestration –Heavy metal detoxification

Grasses resistant to heavy metal waste Figure 40.15

Plant Responses to Challenges Challenge: temperature Response(s): –Heat shock protein synthesis –Cold hardening –Antifreeze protein synthesis

Information Processing in Plants stepplantsanimals responseless elaborateelaborate stereotypedselected asymmetricrapid morphogeniccomplex advantagesenergy savingsmotility