been used to determine gibberellin activity in plant extracts. These basic responses have generally been
used in various modifications with different plant materials. These assays are convenient, specific, easy
to use, and detect nanogram levels of gibberellins. Nishijima and Katsura [65] have improved the dwarf
rice bioassay to detect picogram quantities of gibberellins. Among the commonly used bioassays, the bar-
ley endosperm assay [66] has generally been preferred. In this bioassay, sterile de-embryoed barley (cv.
Himalaya) half-seed (endosperm) is incubated in GA 3 solutions for 24 to 48 hr. The presence of GA 3 stim-
ulates-amylase synthesis in the aleurone layers (two to four layers of live but undividing cells), which
breaks up the starch and builds up the reducing sugars. The amount of reducing sugar, analyzed colori-
metrically, is dependent on the GA 3 concentration in the test solution.
- Growth Promotion
The isolation of gibberellins from plants, combined with their physiological responses to applied GAs,
suggests that they do play a role in the regulation of various phases of their development. At the same
time, if a process fails to respond to a certain GA, it could not be used as evidence that GA is not required.
It may be that a different GA is required to elicit the response. In Silene, GA 3 fails while GA 7 induces
flowering under noninductive conditions, suggesting the involvement of gibberellin in this process. Sev-
eral species of conifers show little or no elongation to GA 3 treatment, but they do respond to a mixture of
GA 4 and GA 7 [67].
During the vegetative growth phase, mitotic activity in subapical meristem is regulated by gib-
berellins. A reduction in its level causes a severe imbalance between internode and leaf growth, resulting
in a form of growth called a rosette, first noted in Hyoscymus nigerand later in many other plants. In
plants such as cabbage (Brassica oleracea capitata), leaf development is profuse and internode growth is
retarded during the vegetative phase. But before the start of reproductive growth, a marked elongation of
the internode, called bolting, takes place. When treated with GA 3 during their rosette phase, such plants
bolt and flower, whereas nontreated plants remain rosetted. There is evidence that endogenous gibberellin
levels are higher in the bolted plants than in the rosetted plants. In addition, higher concentrations have
been found in the bolted long-day Rudbeckia speciosaand cold-requiring Chrysanthimum morifoliumcv.
Shuokan than in their nonbolted forms [63]. Thus it appears that the influence of gibberellin in such a re-
sponse includes the stimulation of cell division as well as cell elongation.
For many crop species there are genetic dwarf mutants that are deficient in gibberellin. Dwarfs of
rice, maize, and peas phenotypically attained the height of normal varieties when treated with gibberellin.
These mutants have been used successfully for gibberellin bioassay and in breeding programs for in-
creasing crop productivity. Dwarf rice responded to as little as 4 pg of GA 3 per plant [64]. Five different
gibberellin-synthesis mutants are known which are underproducing dwarf mutants. Each mutant has a
mutation on a different gene, and each gene controls a different enzyme needed for gibberellin synthesis.
The work of MacMillan and Phinney [68] suggests that only GA 1 controls elongation in maize, and all
five dwarf mutants lack the enzyme(s) that can convert other gibberellins to GA 1. Other evidence also in-
dicates that GA 1 is the main gibberellin needed by dwarf rice, rape (Brassica napus), peas, sweetpeas,
tomato (Lycopersicon esculentum), and some wheat (Triticum aestivum) cultivars for stem elongation.
Mutants are not only lacking GA 1 , but GA 1 -overproducing mutants with abnormally long internodes have
been reported in Brassica rapa(syn.campestris) [69].
- Dormancy of Buds and Seeds
Buds and seeds of many plants show the ability to retain viability while having limited metabolic activ-
ity and no observable growth during an unfavorable season. This physiological condition is commonly
known as dormancy, and plants that grow in regions with a pronounced season usually adopt this strategy
in late summer or early fall. Buds in dormant conditions are relatively more cold and drought tolerant than
are actively growing buds. Similarly, seeds of many noncrop plant species remain dormant when they ma-
ture and will not germinate even if favorable conditions are provided. Dormancy of buds and seeds must
be broken at a time when conditions are suitable for their growth and germination, respectively, during
the spring. Long-day or brief red-light exposures have been found to break seasonal dormancy in many
species. Gibberellins have also been found effective in overcoming both kinds of dormancy in buds as
well as in seeds. Treatment with gibberellins has been observed to substitute effectively for long-day,
low-temperature, or red-light exposure requirements. Due to ease in handling, much more is known about
seed dormancy, but it is likely that much of this information may also be applicable to buds.
510 NAQVI