2/2/2015
Can we live without plants?
Outline:
Introduction, syllabus for part 2
Why plants/plant science?
Reproduction – Alternation of generations
Dr. Uwe Hacke, Associate Professor
251 ESB
[email protected]
Photo: U. Hacke
Plants and plant scientists feed the world.
Can we live without plants?
A major objective of plant
science is to increase food
production; current
estimates indicate that we
need to increase production
by 70% in the next 40 years.
World population, billions
•Plants produce most of the
oxygen we breathe.
•Food
•Wood products
Is this realistic? Why or why
not?
•Coffee, beer, tea, wine, fruit
juice,…
Photo: U. Hacke
Teaching Tools in Plant Biology, The Plant Cell
•Useful chemicals.
Plants and plant scientists feed the world.
What’s needed
What’s grown
Globally, more than one billion people per year are
chronically hungry
World population, billions
That’s more than the total population of the USA, Canada and the EU.
Plant growth is often limited by drought stress
Wheat yield trend from 1951 to 2005 in
Sonora, Mexico.
Teaching Tools in Plant Biology, The Plant Cell
Ortiz et al. Climate change: Can wheat beat the heat?
Agriculture, Ecosystems & Environment (2008) 126: 46 - 58
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Even mild drought stress reduces yields
Mild drought stress reduces the rate of
photosynthesis and growth, whereas
extreme drought stress is lethal.
We need plants to slow down desertification and dust storms
Students planting trees in China
Asian Dust; dust originates in deserts
and is carried by wind over large areas
http://tinyurl.com/ky6vbg5
http://en.wikipedia.org/wiki/Asian_Dust
http://www.reuters.com/article/2012/07/19/uk-usa-drought-mosquitoes-idUSLNE86I00V20120719
Besides the real need for plants/plant science – plants just
make us happy
Plants are cool: tallest organism, biggest, oldest ….
People at work who can see
plants report significantly
greater job satisfaction than
those who can’t.
Photo: U. Hacke
Photo: U. Hacke
Seed: definition, significance
Reproduction – Alternation of generations
Seeds are the dispersal units of seed plants.
Outline:
• Seed: definition, significance
• Alternation of generations (ch. 12)
• Homospory and heterospory
A seed is a mature, fertilized ovule
containing 3 basic parts:
 Embryo
 Seed coat (testa)
 Food source
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Seed: definition, significance
Seed habit has been very successful (250,000
flowering plants and several hundred gymnosperms);
why?
Alternation of generations
All plants characterized by an “alternation of
generations” life cycle
seed coat (testa) → protects embryo
food source → nourishes embryo
Seeds often become widely scattered by wind or
animals
Advantage over free-sporing plants, whose embryos
are at the mercy of the environment.
Alternation of generations
The gametophyte is the haploid gamete-producing
phase in the life cycle.
Gametes are formed in sexual organs: antheridia
(→sperm) and archegonia (→egg).
The sporophyte is the diploid spore-producing
phase in the life cycle.
Alternation of generations
Why trend for favoring the 2n generation?
• 2n may offer more subtle expression of
genetic information.
Def: alternation between a haploid (n)
gametophyte phase and a diploid (2n)
sporophyte phase in the life cycle of a sexually
reproducing plant
Alternation of generations
Gametophyte
dominant
•Bryophytes
(mosses,
hornworts,
liverworts)
Sporophyte
dominant but
gametophyte freeliving and
photosynthesizing
•Ferns
•Club-mosses
•Horsetails
Sporophyte dominant
and gametophyte
greatly reduced
(gametophyte reduced to
few cells, 7-2000)
•Gymnosperms and
angiosperms
Transition from homospory to heterospory
• Homospory: 1 type of spore; basic condition
Lycopodium,”ground pine” (Figures from Stern)
• 2n protects organism from deleterious
mutations.
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Transition from homospory to heterospory
• Heterospory: 2 types of spores
Transition from homospory to heterospory
Selaginella reproduction
Selaginella, a “spike moss”
Transition from homospory to heterospory
 Some homosporous plants evolved heterospory.
Mosses and most ferns are homosporous.
Conifers and angiosperms are heterosporous
 With heterospory, the female gametophyte is
enclosed and protected; this led to evolution of
seeds
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