plant responses

plant responses

1. tropisms

   • tropisms are a growth response to a directional stimulus, they can be towards the stimulus (positive) or away from the stimulus (negative)
   • phototropism
     • response to light
     • ensures plant get as much light as possible, maximising photosynthesis
     • abiotic
   • geotropism
     • response to gravity
     • abiotic
     • shoots show negative geotropism (they grow against gravity) whilst roots show positive geotropism (they grow with gravity)
     • ensures shoots and roots of seeds grow in the right directions regardless of seed orientation
   • chemotropism
     • response to chemicals
     • abiotic or biotic
     • some plants show a tropic response to certain chemicals (e.g. pollen tubes grow down the flowers stigma towards the ovules due to chemotropism
   • thigmotropism
     • abiotic or biotic
     • response to touch
       • a process similar to thigmotropism occurs in Mimosa Pudica, the leaflets of the touch sensitive plant fold in rapidly when touched, however as this is not a growth response nor is it related to the direction of the stimulus, it is not considered a tropism
     • important to climbing plants (which have adapted to climb to give them access to more light for photosynthesis) as they can detect support and curl around it
   • hydrotropism
     • abiotic
     • response to moisture
     • allows root tips to grow towards damper areas of soil, maximising water uptake

2. nastic movements

   • there is a similar response to thigmotropism that occurs is Mimosa pudica, in which the leaflets of the touch sensitive plant fold in rapidly when touched, however, as this is not related to the direction of the stimulus, it is not considered a tropism
     • this response is caused by rapid water uptake (and therefore inflation) of cells at the base of each leaflet and rapid water los (and therefore collapse) of adjacent cells.
     • this movement occurs very quickly and is likely caused by local bioelectrical signals (as plant hormones cannot respond so quickly
     • it is thought that this movement may be an adaptation to protect the leaflets from herbivorous insects, or to reduce transpiration when the leaves are no longer synthesising (the leaflets also fold in at night and reopen at dawn)
   • non-directional responses
   • the highly specialised leaves that form the "traps" of a venus fly-trap fold together rapidly when stimulated by the touch of an insect or other small organism, which is then trapped and digested to provide certain minerals (eg. nitrogen) to the plant

3. responses to herbivory

   • the consumption of plants by herbivores
   • many plants produce herbivore repellant chemicals (sometimes poisonous), sometimes these are produced in response to damage or stress, but some plants produce these all the time
     • examples
       • tannins
         • water-soluble carbon compounds in the chemical group known as flavonoids
         • stored in the vacuoles of plant cells and can be fatal when to herbivorous insects (toxic chemicals are produced during the breakdown of tannins in the insect gut)
         • they taste bitter to put of herbivores
         • examples
           • gallic acid - found in rhubarb
           • flavan-3-ols (catechins): found in tea and cocoa plants
       • alkaloids
         • nitrogenous compounds derived from amino acids
         • like tannins, they taste bitter and can be toxic, particularly to herbivorous insects and sometimes fungi
         • examples
           • caffeine - found in tea, cocoa, and coffee plants
           • nicotine - found in tobacco plants, tomatoes, potatoes, aubergines and green peppers, a potent neurotoxin that can be fatal to herbivorous insects
       • pheromones
         • chemicals released by one member of a species that affect the physiology or behaviour of another member of the same species
         • not well understood but may be directly toxic to herbivorous insects or cause chemical defences in some plants
         • examples
           • in some plants, oxides of ethene (the chemical that causes the ripening of fruit in nearby plants) appear to switch on the genes for producing chemicals that deter herbivorous insects (acting like a hormone)

4. responses to abiotic stress

   • abiotic stress for plants can occur in many different forms, including: freezing, drought, increased soil water salinity, the presence of heavy metals (eg. lead, copper, zinc, mercury)
     • plants can respond to drought by shutting their stomata and therefore decreasing water loss through transpiration or by dropping their leaves
     • some plants respond to temperatures below freezing by producing and anti-freeze chemical in their cells, that decreases he formation of ice crystals that can be detrimental to cells is allowed to form within them