294
L. C. LUCKWILL:
[J.L.s.B. LVI
Factors controlling the growth and form of fruits. By L. C. LUCKWILL
Long Ashton Research Station, University of Bristol
(With 4 Text-figuree)
b!FRODUU'I!ION
The scienti& exploration of a morphological problem, as indeed of any problem in
biology, normally proceeds in three stages: first, the observation of the facts; secondly,
the formulation of a working hypothesis which might explain and correlate the facts;
and thirdly, the carrying out of experiments to test the hypothesis; and it is in this
sequence that I propose to deal with the problem of growth and form in fruits.
Because many are of economic importance, fruits have probably been studied more
intensively than most other organs of the plant. A great deal of information exists on the
way in which fruits grow, on their chemical constitution, their gas exchange, and on the
metabolic changes accompanyinggrowthand ripening, but our knowledgeof the hormonal
factors which control these processes is still very fragmentary and presents a fascinating challenge to the experimental morphologist.
The fruit, unlike other organs of the plant, is not a clearly d e h e d morphological unit,
and for the purposes of this paper I shall follow Nitsch (1952) in describing it simply as
'the structural entity resulting from the (post-floral)development of the tissues which
support the ovules'.
INCREASE
IN SIZE
The most obvious thing about the growth of a fruit is that it involves very large increases
in volume. This is particularly true of cultivated fruits such as the black currant which,
in a period of 10 weeks undergoes a 100-foldincrease, or the apple which may increase its
volume 6000 times during a growing period of only twenty weeks. By plotting the volume
or weight of the fruit against time we obtain a growth curve which falls into one of two
distinot types. I n some plants, e.g. cucumber, tomato, apple (Fig. 3), the growth of the
fruit during the post-fertilization period can be represented by a smooth sigmoid curve,
such as characterizes the growth of many plant organs. In others, e.g. Prunw, spp.,
the fig (Fig. 4), blueberry, fruit growth proceeds in three distinct phases, two periods of
rapid growth being separated by an intermediate period of varying length when little or
no increase in volume occurs. The growth curves of such fruits may be interpreted a8
made up of two successive sigmoid-type curves.
The increases in volume associated with the growth of fruits are due partly to increases
in the number of cells and partly to increase in cell size, though in some fruits (e.g.
apple) expansion of the intercellular spaces may also contribute to the growth of the
fruit during the later stages. I n general, growth by cell division predominates in the
early stages and growth by cell expansion rn the later stages, though there are many
variations on this generaltheme. In Lycopersicon eeculentum Mill. cell multiplication ceases
a t anthesis, the whole of the post-floraldevelopment being attributable to cell expansion,
whereas in the closely related L.p ~ m p ~ n e ~ Mill.
l ~ ~some
o ~ ~cell
~ division
m
continues right
up to maturity (Houghtaling, 1935). I n the apple, cell division ceases 3-4 weeks after
bloom (Bain & Robertson, 1950), and in Cucurbita mschuta Duch. when the fruit reaches
9 cm. in diameter (Kano, Fujimura, Hirose & Tsukamoto 1957). The change from cell
division to cell extension growth may thus occur at different times in the development of
different fruits. The change, *hen it does occur, is frequently quite abrupt, as is shown by
the work of Kano et al. (1957),who have applied the differential growth formula of Huxley
(1932) to the study of the relationship between cell size and fruit size in certain members
of the Solanaceaeand Cucurbitaceae (Fig. 1). In some fruits more complicated patterns of
J.L.S.B. LVI]
FACTORS CONTR0LI;”Q THE GROWTH AND FORM OF FRUITS
296
development are found in which cell division may cease at different times in different
parts of the fruit. Thus, in the strawberry, cell division in the cortex ceases just before
rtnthesis, whereas in the pith it continues right up to maturity (Havis, 1943). In the
Deglet Noor date general cell division throughout the mesocarp terminates 6 weeks after
flowering, but a localized region of meristematic activity at the base of the fruit persists
for a further 6 weeks (Long, 1943).
Ovary diameter (mm.)
Fig. 1. The relation of cell diameter to fruit diameter in the middle w d of the Shirokikuza
cushaw (Curcubita m o a c h t a Duoh.). From Kano et al. (1967).
SHAPEDETERMINATION
Broadly speaking, two main types of shape determination are found in fruits. In the
simplest type, exemplified by the cucumber and tomato, the shape at maturity is already
pre-determined in the ovary at the time of flowering. This means that during its postfloral development the fruit grows at an equal rate in all directions. More commonly
shape differences arise after flowering as the result of unequal growth rates in different
dimensions, so that the shape index, or lengthlbreadth ratio, is changing continuously
during growth, as in the egg plant (Kano et al. 1957) and Capsicum (Kaiser, 1935), the
elongated fruits of which arise from ovaries which are approximately spherical in shape.
Although the size and shape of fruit typical of a particular species is determined
genetically, both these characters are liable to modification within rather wide limits
within the genotype by diverse external and internal factors. Of the external factors
modifying size, nutrition is of obvious importance, and in some fruits (e.g. apple) shape
may be modified by unilateral light or gravitational forces (Schander, 1955a). Of the
internal factors, by far the most important is the number and distribution of seeds within
the fruit.
INFLUENCE OX SEEDS ON FRUIT GROWTH
The dependence of fruit size on seed number is a rather general phenomenon though the
relationship may be masked in some fruits (e.g. certain varieties of Citrus)in which there
is an inherent tendency to parthenocarpy. The correlation is very marked in grapes
(Miiller-Thurgau,1898) and other few-seeded fruits, though, even in the many-seeded
black currant, correlation coefficients ranging from 0.86 to 0.98 were found between
seed number and the diameter of the mature berry (Teaotia t Luckwill, 1956). In apples
the relation between fruit size and seed number is more complex: in some varieties the
relationship is masked by parthenocarpic tendencies, whilst in others, negative as well
as positive correlations have been reported. Usually, where the seed content is low, a
stimulating effect of seeds on flesh development can be demonstrated, but high seed
numbers may depress fruit growth through competition for available carbohydrates
(Schander, 1956).
296
L. 0.LUUKWILL:
[J.L.s.B. LVI
The influence of seeds on the shape of fruits is particularly well shown in certain pears
in which, for genetical reasons, many of the seeds may abort before reaching maturity.
Schander (1955 E ) has found that the earlier in their development the seeds abort, the
more elongated is the fruit in relation to its breadth (Fig. 2). Occasionally, as a result of
frost at blossom time, the seeds are killed at a very early stage of growth and the fruits
which subsequently develop are sausage-shaped rather than pyriform. The influence of
seeds on the growth of the flesh is often strictly local in its effect, so that an uneven distribution of seeds may result in asymmetrical growth of the fruit. I n Pmnus spp. for
instance, failure of one of the two ovules to develop results in unequal growth of the carpel wall on opposite sides (Tukey, 1936), and in the apple asymmetry is attribut$ble
principally to the failure of seed development in one or more of the five carpels. The local
effect of the seeds is very clearly seen in strawberries in which, owing to insuEcient
$
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I
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,
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From Schander (1966~).
pollination, only a small number of fertile achenes have developed. In such fruits the
receptacle tissue expands only in the immediate vicinity of these fertile achenes, giving
rise to extremely diatorted fruits. Many other instances could be quoted of the influence
of seeds, not only on the size and shape of fruits, but also on the chemical composition of
the flesh and the time of ripening.
AUXIXS AND BaUIT GROWTH
The work which I have so far described constitutes the observational phase of our
problem. Having surveyed the main facts of fruit growth we are now ready to proceed
to the second phase-the formulation of an hypothesis which might explain and coordinate these facts. We have seen that fruits grow largely by a process of cell expansion
supplemented by cell division in the early stages, and that the direction and amount of
growth are strongly influenced by the seeds. This suggests that the seeds are the source of
a hormone-like substance capable of stimulating the growth of the adjacent tissues of
the fruit. The principal hormones known to control cell extension in plants are the auxins.
Auxins also inhence cell division, for example in the cambium of woody stems and in
the initiation of adventitious root primordia. I n the stem and the root they have their
origin in the terminal meristems. In the fruit we have no terminal meristem, but we do
have two very active internal meristems, the growing embryo and the endosperm, both
of which grow by cell division. A reasonable hypothesis therefore would seem to be that
J.L.S.B. LVI]
FACTORS CONTROLLING THE GROWTH AND FORM OF FRUITS
297
these meristematic centres in the growing seeds produce auxins and that these auxins.
pass out of the seeds either along the vascular system or by diffusion through the testa,
and directly stimulate the growth of the surrounding tissues of the fruit.
We may note in passing that there exists a considerable body of indirect experimental
evidence in support of the idea that auxins are limiting factors in fruit growth. Thus, in
some fruits, though not in all, fruit development in the absence of fertilization may be
stimulated by the application of various synthetic auxins. The application of auxin
sprays to partially grown fruits at certain stages of development may in some species
result in an increase in the growth rate and in the final size attained, a technique which
has found commercial application for certain varieties of oranges, apricots, figs and grapes.
In other fruits, such as blackberry and strawberry, auxin applications have proved of
value in increasing fruit size where, owing to partial sterility or inadequate pollination, the
seed content and growth rate of the fruit is below normal.
Evidence of this type, based on the effects of synthetic auxins, most of which differ
chemically from their endogenous counterparts, whilst not providing proof of the auxin
hypothesis of fruit growth outlined above, does at least make it appear that the hypothesis is a reasonable one. More direct evidence comes from the results of experiments
in which measurements have been made of auxin production in the seeds, and especially
from those in which such measurements have been accompanied by simultaneousobservations on fruit growth. One of the earliest workers in this field was Gustafson (1939) who
showed that in immature fruits of the tomato the concentration of auxin in the seeds was
very much higher than in the placenta, and that the concentration in the placenta,
central axis and partitions of the fruit, was in turn higher than that in the carpel wall.
Such centrifugal gradients of auxin concentration within the fruit are consistent with the
idea that auxin is being synthesized in the seeds and is moving outwards along a concentration gradient to the other parts of the fruit.
Table 1. Audn distribution in immature tomato fruits in pg. equivalents I A A
per kg. fresh wt.
(Gustafson, 1939)
seeds
Locular jelly
Placentae
Central axis and partitions
Carpel wall
Fruit 1
15.33
6.57
244
2.27
1.27
Fruit 2
30.03
0.45
2.47
2.69
0.63
In my own experiments with apples (Luckwill, 1948, 1953) measurements of auxin
concentration in the seed were made at weekly intervals throughout the growing season
and simultaneous measurements were made of embryo and endosperm development,
fruit growth and the rate of fruit abscission. The results of these experiments, made
over a period of 4 years on a number of different apple varieties, may be summarized as
follows:
(1) The auxin content of the seeds during the first 3-4 weeks after fertilization is very
low. It then rises to a sharp peak and falls again, rising to a second peak 3-5 weeks later
(Fig. 3).
(2) The amount of auxin found in the seed at any given time is approximately proportional to the amount of cellular endosperm tissue present. The auxin concentration
in the endosperm is considerably higher than in the embryo (Table 2).
(3) The sharp fluctuations in seed auxin are not reflected in any corresponding changes
in the fruit itself, the growth of which may be represented by a smooth sigmoid curve
(Fig. 3).
298
L. C. LUCKWILL:
[J.L.s.B. LVI
(4) The changes in auxin level in the seeds are negatively correlated with the rate of
fruit drop, in so far as the periods of maximal auxin production invariably correspond
with the periods of minimal fruit drop.
More recent studies of the same type with the fruit of the Calimyrna fig (Crane,
Bradley & Luckwill 1969)have led to essentially the same conclusions. Here again very
low auxin concentrations were found during the early stages of fruit growth, followed
by a sharp increase at the time of endosperm formation, after which the auxin level
again fell (Fig. 4). As in the apple, no correlation appeared to exist between the growth
rate of the fruit and the auxin concentration, and no evidence was found that lack of
auxin was a limiting factor during the second stage of development of the fruit, when little
or no increase in size occurs.
Fig. 3. Changes in auxin oonoentration in the seeds (full line) and fruit diameter (broken line) in
apple var. Crawley Beeuty, in relation to days after petal-fd. From Luokwill (1963).
Table 2. Auxin content of the embryo and endosperm in partidy developed
seeds of the apple, ua pg, equivalents 2-NOA
(Luokwill. 1948)
Total auxin
Variety and date
Miller’s Seedling,
7 August
Crawley Beauty,
12 August
Part of seed
Embryo
Endosperm
Embryo
Endosperm
Per 100 see&
lb.2
66.4
0.8
2.4
Per g. dry wt.
16.2
277-0
0.9
7.1
A very similar situation exists in the strawberry (Nitsoh, 1950, 1952) where a very
sharp rise in the auxin content of the achenes at the time of endosperm formation appears
to be quite unrelated to the general growth pattern of the fruit (Fig. 4).
The results of experiments with these three diverse types of fruit, therefore, are in
J.L.S.B. LVI]
FACTORS CONTROLLING THE GROWTH AND FORM OF FRUITS
299
agreement with the concept of the seed as a centre of auxin production in the fruit. They
also suggest that the endosperm is by far the most important, and perhaps the only,
auxin producing tissue within the seed. On the other hand, they clearly do not support
the second part of our hypothesis that auxin is the chief factor concerned in the control
of fruit growth, though in the deciduous apple there is good evidence that it does control
fruit abscission. What interpretation should we put upon these results?
First, it is important to realize that the ‘auxin’ we extract from plant tissue by water
or organic solvents is not a simple chemical substance but a rather complex mixture of
several different compounds having auxin-like properties, which can be separated by
chromatographic techniques. Indole-3-acetic acid (IAA), at one time thought to be the
dominant, and perhaps the only naturally occurring auxin, is now known to be only one
of a number of hormones with similar biological properties which occur in plant extracts.
In some species even IAA appears to be absent (Luckwill BE Powell, 1956). Secondly,
not all the auxins which we extract from plants necessarily function as such in the species
which produce them. A good example of this is indoleacetonitrilewhich may be extracted
in relatively large amounts from cabbage and detected by its ability to induce the elongation of oat coleoptiles, but which is nevertheless inactive when tested on the tissue of the
Strawberry
var. Marshall
\I
,
,
.
,
,
.
,
~
30
60
90
0
10
20
30
Days
Fig. 4. Chmges in auxin concentration (full line) and fruit diameter (brokenline) in the Calimyrna
fig (Crane, Bradley & Luckwill, 1969), and the Marshall strawberry (Nitsch, 1960).
cabbage itself. The reason for this apparent &nomalyis that the activity of indoleacetonitrile depends on its enzymatic conversion to IAA, and that only certain tissues are
able to effect this conversion. In fruits, therefore, it is possible that some of the endogenous auxins we extract from seeds are inactive on fruit tissue, and that fruit growth
may be correlated with only certain components of the auxin complex, rather than with
total auxin. This is suggested by the work of Wright (1956) on the development of the
black currant berry, in which three principal auxins occur. Of these three auxins, two
show a possible relationship with the growth curve of the fruit whilst the third, which at
some stages of development accounts for the greater part of the total auxin, is quite
unrelated. A similar chromatographicanalysis of the auxins of the fig, on the other hand,
300
L. C . LUCKWILL:
[J.L.s.B. LVI
failed to reveal a relationship between any particular auxin and the growth of the fruit :
there was, in fact, no essential quantitative or qualitative difference between the auxin
pattern during stage 2, when growth is minimal, and that during stage 3 when very active
expansion is taking place (Crane et al. 1969).
This then is the present position: in spite of the large body of supporting evidence,
derived mostly from experiments with synthetic auxins, studies of endogenous auxins in
fruits and seeds do not suggest that, in general, these substances are normally limiting
factors in the growth of fruits, though they are undoubtedly active in controlling fruit
abscission. Particularly difficult to explain on the ‘auxin hypothesis’ is the essentially
similar auxin pattern found in fruits such as the apple and the fig, the growth curves of
which are quite dissimilar. It now appears that we may have to modify our original
hypothesis and to inquire whether there are other types of growth factors besides auxins
which occur in seeds and which might be active, either alone or in combination with
auxin, in stimulating fruit growth.
OTHERGROWTH FAOTORS
Besides auxins two other types of growth factors are now known to ocour in the seeds
of at least certain species of plants. The liquid endosperm of the coconut contains a
complex of factors which are active in stimulating cell division in certain types of tissue
culture (Steward & Caplin, 1954) and which exhibit marked synergism with synthetic
auxins (Shantz, Steward, Smith & Wain, 1955). These coconut milk factors appear to be
essential for the in vitro culture of immature plant embryos and it seems likely that their
natural function in the plant may also be concerned with embryo development. Although
they have been shown to occur in the free nuclear endosperms of a number of species
besides the coconut (Steward & Caplin, 1954) it has not yet been demonstrated that
they have any activity in stimulating the growth of fruit tissue. Reoent attempts t o
induce parthenocarpic development of fruits such as apples and pears by means of coconut milk extracts, alone and in combination with synthetic auxins, have proved unsuccessful.
The gibberellins, on the other hand, are undoubtedly active in stimulating fruit
growth. Gihberellin A, (gibberellic acid) is effective in stimulating parthenocarpy in
tomato (Wittwer & Bukovac, 1958) and, under certain conditions, it shows synergism
with IAA: it will stimulate parthenocarpy in the fig (Crane, 1968)and increase the growth
rate and final size of certain seedless varieties of vinifera grapes (Weaver, 1958): it has
also been reported as active in increasing fruit-set in pears (Wittwer & Bukovac, 1958)
and in several citrus fruits (Hield, Qoggins & Garber, 1968; Soost, 1958). After many
unsuccessful attempts to stimulate parthenocarpy in apples with auxins I have recently
had some success with gibberellic wid. In these experiments, which were of an exploratory nature only, pollination was prevented by cutting off the styles before the flowers
opened. A few days later the whole tree received a spray application of 100 parts/million
GA. The parthenooarpic fruits that developed were noticeably more elongated than the
pollinated controls, though they developed a t the same rate and matured a t about the
game time.
Not only are gibberellins active in stimulating fruit growth, but evidence is accumulating for the rather widespread occurrenoe of substances with gibberellin-like properties in seeds. So far, such substances have been found in the seeds of Echinocystis,
(West & Phinney 1956), Phaswlus (Radley, 1958),peas (Radley, 1958), wheat (Radley
1958),and coconut (Radley & Dear, 1958),though in none of these plants has the oocurrence of the growth factor been studied in relation to seed and fruit development. Until
this has been done any speculations as to the possible role of naturally occurring gibberellins in the control of fruit growth must, of necessity, be tentative. However, in
conclusion, I think we can say that these recent investigationsof this new type of growth-
J.L.S.B. LVI]
FACTORS CONTaOfrLINO THE GROWTH AXD FORM OF FRUITS
301
regulating compound have upened up an entirely new and promising experimental
approach to this fasbinating and economically important problem of the control of
growth and form in fruits.
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