Handbook of Plant and Crop Physiology

(Steven Felgate) #1

abuses [107]. Although visual evaluation is rapid, it is also subjective. Measurement of pollution-induced
ethylene surge before any visual symptom appears points to its sensitivity and superiority for physico-
chemical determinations. However, the ethylene production surge is not long-lasting; rather, it is a short-
lived phenomenon. Therefore, it has been suggested by Craker [108] that it acts as a trigger mechanism
that initiates the biochemical change(s) expressing the response.
Auxin at supraoptimal concentrations (10^5 to 10^3 M) acts as a natural factor in enhancing ethy-
lene production, and depending on species and severity of stress, its biosynthesis starts after a lag of 30
to 60 min, can continue up to 48 hr, and then declines to normal level. The mechanism of IAA-induced
ethylene production has been studied in pea and mungbean seedlings. The detailed studies have shown
that IAA stimulates ethylene production by enhancing conversion of SAM (S-adenosylmethionine) to
ACC (1-aminocyclopropane-1-carboxylic acid) through its effect on the enzyme ACC synthase [109].
Thus at supraoptimal concentrations, auxin per se does not cause growth inhibition, but it is the induction
of enhanced ethylene production that inhibits growth [91].
A number of developmental processes have been listed by Abeles [110], where auxin-induced ethy-
lene synthesis is considered to mediate auxin action. Therefore, it seems reasonable to assume that like
other stresses, auxin acts similarly in accelerating ethylene production.



  1. Senescence


It can be considered that programmed changes in the metabolic processes may ultimately lead to the death
of a tissue, organ, or the whole plant. In nature, we experience three categories of senescence: sequential,
where the oldest leaves senesce first; synchronous, where all the leaves senesce simultaneously (as in de-
ciduous trees); and senescence of the whole plant after the completion of seed production (as in mono-
carpic crops). Simons [111] believes it is likely that the various types of senescence may result from dif-
ferent control mechanisms in the leaves. Studies have commonly been conducted with detached leaves,
where experimental conditions may influence the result. Thus, it is very difficult to relate results from a
single experimental system to the system operating in situ. Two major biochemical events, extensive pro-
teolysis and chlorophyll loss, have been observed consistently at the beginning of the process. Leaf senes-
cence may be induced or accelerated by a number of environmental factors, including competition for
space, light, and nutrients; pollution; biotic or abiotic stresses; or it may be genetically programmed. At
the cellular level it does seem to be controlled by endogenous growth regulators. It is now known that in
most cases, auxin, cytokinin, ethylene, and ABA play a role in the regulation of senescence in plants and
cytokinins, and auxin can delay senescence in a number of plant species. Thus, according to their actions,
they have been classified as senescence promoters and retardants [112]. Ethylene plays an important role
in accelerating leaf, petals, and fruit senescence, and auxin and cytokinins act as retardants. It is now well
accepted that a balance between auxin and ethylene is a crucial factor in the retention or nonretention of
leaves and/or fruits. Premature fruit drop is common in a number of important fruit trees, such as apple
and mango (Mangifera indica). In mango, fruit drop occurs at all stages of its development but is exten-
sive (90%) during the first 2 to 4 weeks after fertilization [113]. This stage coincides with the maximum
ethylene production by the fruitlet pericarp [114]. Therefore, treatments with Ag, Co^2 , or synthetic
auxin NAA (naphthaleneacetic acid), which regulates ethylene balance, have been observed to enhance
the number of harvested fruits significantly [113,115]. In some studies, salicylic acid (an inhibitor of ethy-
lene biosynthesis) has also been observed to reduce mango fruit drop (S. S. M. Naqvi, unpublished
results).


VI. ABSCISIC ACID


The pioneering work pointing to the possibility that plant growth and development are regulated by lev-
els of both promoter (auxin) and inhibitor is generally credited to Hemberg. Using Avenabioassay, he ob-
served that potato peels contained high levels of growth inhibitors [116]. In the same year, he demon-
strated further the presence of a similar inhibitor that could be correlated with the levels and degree of ash
(Fraxinus excelsior) bud dormancy [117].
Employing paper chromatography to analyze plant growth substances from plant extracts, Bennet-
Clark et al. [118] observed growth inhibitory activity at Rf0.6 to 0.7. This was later shown to be present
in a number of plant species and the levels responded to the changes in environmental conditions. As
pointed out by Hemberg [116,117], these results supplemented the physiological importance of the
growth inhibitor, which was named “inhibitor” [119,120].


518 NAQVI
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