Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
ISBN: 9781305389892



Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
ISBN: 9781305389892
Textbook Problem

As Chapter 37 discusses, the plant hormone ABA (abscisic acid) inhibits seed germination, while any of several GAs (gibberellins) stimulate it. For hundreds of plant species, exposure to a substance in wood smoke—one of a family of plant growth regulators called karrikins, or KARs—markedly improves the germination rate. When flames destroy the existing vegetation at a fire scene, more sunlight reaches the ground. In the presence of ample sunlight, KARs apparently promote germination by enhancing the expression of genes that encode GA.

The Figure below summarizes experiments by Australian researchers studying KAR activity in mutant A. thaliana plants (called gal-3), which do not synthesize GA. The graph tracks germination activity after two groups of identical test seeds began imbibing water in a medium that contained the gibberellin GA3. All seeds were exposed to identical levels of a karrikin called KAR1. The red symbols designate seeds of mutant ABA-deficient plants that were supplied with supplementary ABA. The white symbols are ABA-deficient seeds that did not receive the supplement.

Based on this graph:

Did the presence or absence of ABA have a significant effect on the experiment’s results?

Chapter 36, Problem 2ITD, As Chapter 37 discusses, the plant hormone ABA (abscisic acid) inhibits seed germination, while any

FIGURE Response of phytohormone mutants to KAR1: Germination of A. thaliana ga1-3 mutants in the presence of 1 or 10 μM GA3 6 1 μM KAR1.

Source: David C. Nelson, Julie-Anne Riseborough, Gavin R. Flematti, Jason Stevens, Emilio L. Ghisalberti, Kingsley W. Dixon, and Steven M. Smith. 2009. Karrikins discovered in smoke trigger Arabidopsis seed germination by a mechanism requiring gibberellic acid synthesis and light. Plant Physiology 149:863–873. Plant physiology by AMERICAN SOCIETY OF PLANT PHYSIOLOGISTS. Copyright 2009. Reproduced with permission of AMERICAN SOCIETY OF PLANT BIOLOGISTS in the format Textbook via Copyright Clearance Center.

Summary Introduction

To review:

The change observed in the experiment in the presence and absence of abscisic acid (ABA).


The mutant A. thaliana is studied for the activity of karrikin (KAR) growth regulators. The mutants taken are unable to synthesize gibberellins. The seeds are imbibed in water for germination. One set of seeds is imbibed in water with ABA supplement and another set is imbibed in normal water. The results of germination processes are observed in both sets of seeds.


ABA is a hormone involved in regulating plant growth in stressful conditions. It is known for prolonging seed dormancy and inhibiting seed germination. The germination of seeds is observed in ABA-deficient seeds and in seeds with supplementary ABA in the given experiment. It can be clearly stated after studying the graph that the presence of ABA does not have any major effect on the percentage of germination.

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