Tree Physiology 18, 277--280
© 1998 Heron Publishing----Victoria, Canada
Comparative seed ecophysiology of wild and cultivated Carica papaya
trees from a tropical rain forest region in Mexico
LEONCIO PAZ1 and CARLOS VÁZQUEZ-YANES1,2
1
Instituto de Ecología UNAM, Apartado Postal 70-275, Ciudad Universitaria, 04510 Mexico D.F.
2
Author to whom correspondence should be addressed
Received March 20, 1997
Summary To ascertain the effects of centuries of cultivation
practices on seed behavior and dormancy mechanisms, we
compared seed size and germination characteristics of wild and
cultivated (domesticated) populations of Carica papaya L.
Germination experiments were carried out under various conditions of temperature, light, seed soaking and gibberellic acid
treatments. Wild papaya seeds showed responses to treatment
that are characteristic of seeds of many rain forest pioneer trees.
Seeds were small and light sensitive, whereas cultivated papaya
seeds were 33% larger and their light responses as well as other
physiological traits indicated that cultivation had resulted in a
lessening in the importance of specific environmental conditions for dormancy breaking and germination.
Keywords: dormancy, germination rate, giberellic acid treatment, light sensitivity, seed dormancy, seed size, seed soaking,
temperature alternations.
Introduction
Carica papaya L. (papaya) is a pantropical tree crop cultivated
both for its nutritious sweet fruit and for the commercially
important proteolytic enzyme papain (meat tenderizer). There
are many cultivated varieties of papaya that differ in traits such
as fruit size, color, flavor and tree size (Smith et al. 1992).
Wild papayas still grow spontaneously in many parts of the
area of origin of the species in tropical America, usually
associated with disturbed humid and sub-humid tropical forests. Sometimes wild varieties are also found in home gardens
of ethnic groups like the Mayans from Yucatan, Mexico, who
use wild papaya fruits to make a sweet glaze (Terán and
Rasmussen 1995). Wild papaya trees in the Mexican rain forest
behave like typical fast-growing short-lived pioneer trees; they
become established rapidly and grow only in recent, relatively
large canopy gaps in mature forest as well as in early secondary
forests. Gaps in mature forest can be opened as a result of
human activity or naturally by falling trees. Wild papaya are
relatively abundant in recent large man-made clearings that are
not plowed. Large natural forest gaps of 1--5 years of age
sometimes accommodate several papaya trees.
Wild papayas of Veracruz are considered to belong to the
same taxonomic species as the cultivated varieties (Moreno
1980). Sometimes isolated individuals of papaya carry inter-
mediate morphological characteristics between those of wild
and cultivated plants. This introgression may indicate occasional mating between population types.
Domestication involves separation of a portion of a plant
population and selection under environmental conditions different from those operating in nature. Over time, cultivated
plants become morphologically and physiologically modified
relative to their wild counterparts (Evans 1993). As a consequence, they tend to have larger seeds with less stringent
requirements for germination than seeds of their wild progenitors. The genetic consequences of such manipulations on seed
populations and the way that the new traits are fixed in the
genome have rarely been studied.
The presence of wild and cultivated papaya in the forest
reserve of our university provided the opportunity to assess
effects of domestication on seed morphology and physiology.
In this study we investigated whether the seed dormancy attributes of pioneer trees and shrubs like Cecropia obtusifolia
Bertol. (Vázquez-Yanes and Smith 1982), Piper spp. (OrozcoSegovia and Vázquez-Yanes 1989), Urera caracasana (Jacq.)
Griseb (Orozco-Segovia et al. 1987), and Heliocarpus donnell
smithii Rose (Vázquez-Yanes and Orozco-Segovia 1982) are
found in wild papaya and if they have changed during domestication. The seed characteristics we studied included germination rate, light sensitivity, responses to daily temperature
alternations, and responsiveness in darkness to gibberellic acid
(GA). Several reports indicate that GA treatments and the
washing or removal of the seed sarcotesta increase the rate of
germination of cultivated papaya (Chow and Lin 1991, Andreoli and Khan 1993). Our hypothesis is that, as in other
cultivated plants, domestication of papaya has resulted in an
enlargement of the seed and a reduction in the complexity of
the seed dormancy mechanisms (Evans 1993).
Materials and methods
Plant material
Seeds were collected in the tropical mountain range covered
by rain forest of ‘‘Los Tuxtlas,’’ in the State of Veracruz
(18°10′ N and 94°42′ W), Mexico. The area is partially forested, mean temperature is 26 °C, and mean annual rainfall is
about 4000--5000 mm. The vegetation and environmental
278
PAZ AND VÁZQUEZ-YANES
characteristics of the area have been described in detail by
Bongers et al. (1988).
Wild papaya grow to 5 m in height, start fruiting the first
year, and produce spherical fruits from 5 to 8 cm in diameter
with approximately 50 to 300 seeds each. Cultivated papaya
are widespread on farms and home gardens in the region. The
most common variety is the ‘‘Criollo’’ type bearing fruits that
sometimes reach over 4 kg and contain hundreds of seeds.
Seeds were gathered during late summer from at least
15 cultivated and 15 wild papaya trees; each sample of trees
was considered a provenance. Fruit flesh was discarded and
seeds were washed, cleaned and air-dried at room temperature
before transportation to the laboratory in Mexico City where
they were stored in paper bags at room temperature (± 20 °C)
for 1 month. Papaya seeds are classified as intermediate (i.e.,
between orthodox and recalcitrant types; Ellis et al. 1991),
because they may remain viable for some years when stored
dry at temperatures above 0 °C.
One hundred clean seeds of each wild and cultivated papaya
provenance were weighed and measured individually. The
seeds were taken at random from the seeds available (about
22,000 wild-type seeds and 30,000 cultivated-type seeds).
Seed germination
Within a month of seed collection, the seed samples of both
wild and cultivated papaya were divided into batches of 25
seeds. Each germination experiment included both types of
seeds and four replicates of each per treatment. Germination
tests were carried out in petri dishes containing 15 ml of 10%
pure agar gel in distilled water as the germination substrate.
This germination medium provides stable moisture availability with little microbial contamination over the often long
periods of time that seeds of many wild plants require for
germination (Vázquez-Yanes and Orozco-Segovia 1996a).
Germination was carried out in incubators (Biotronette LabLine Instruments Inc., Melrose Park, IL) that provided a 12-h
photoperiod supplied by white fluorescent light. For dark treatments, light was excluded by wrapping petri dishes with two
layers of aluminum foil before placing them in the incubators.
Control series, which were included in each test, consisted of
untreated seeds in petri dishes containing 15 ml of agar--distilled water medium incubated at 25 °C in either a 12-h photoperiod or continuous darkness.
Seeds for the fluctuating temperature experiments were prepared as described above but the petri dishes were exposed to
a daily temperature alternation of 15 °C for 16 h and 30 °C for
8 h, in either a 12-h photoperiod or continuous darkness. The
treatment was intended to reproduce the effects of the daily
temperature fluctuations that occur on bare soil during winter
in the collection area.
For the GA treatments, the agar--distilled water medium
contained 500 ppm GA. Seeds were subjected to constant or
fluctuating temperatures in either a 12-h photoperiod or continuous darkness.
A soaking treatment consisted of seeds in water for 24 h
followed by washing. The soaked seeds were subjected to
constant or fluctuating temperatures in either a 12-h photoperiod or continuous darkness.
To test whether light with a low red/far red (R/FR) ratio
inhibited germination of wild and cultivated papaya seed,
plastic boxes were assembled with blue and red transparent
Plexiglas (Rhom & Hass, Mexico D.F.) and illuminated with
incandescent lamps. Vázquez-Yanes et al. (1996) showed that
this technique produces light enriched in far red (i.e., with a
R/FR ratio of less than 0.01). Petri dishes containing seeds
were placed inside the Plexiglas box and maintained at 25 °C
in a 12-h photoperiod for 60 days.
Arcsine transformed data were subjected to multifactorial
ANOVA using the Statistica software package (Statistica Version 4.3, 1993; Graphic Software Systems Inc., Rockville,
MD) to determine treatment effects and their interactions.
Results and discussion
Although seeds collected from wild and cultivated papaya trees
were similar in color and shape, seeds from cultivated trees were
about 25% heavier and larger than seeds from wild trees (Figure 1). Germination behavior also differed significantly between wild and cultivated seeds. At the end of 2-month
germination tests in a 12-h photoperiod, both total number of
seeds germinated and rate of germination were significantly
higher for cultivated papaya than for wild papaya (Figure 2A).
Germination in darkness also differed between populations
because wild papaya seeds exhibited a strong light requirement
for germination that was not shown by seeds of cultivated
plants. Many pioneer seeds are photoblastic (i.e., require a high
R/FR ratio for germination; Vázquez Yanes et al. 1996).
Fluctuating temperature had no effect on germination of
cultivated papaya seeds, whereas it partially inhibited germination of wild papaya seeds (Figures 2A and 2A′ ). The inhibition was eliminated by soaking the seeds or by treating them
with GA (Figures 2B′ and 2C′, respectively).
Under constant temperature conditions, however, the soaking pretreatment had no effect on either total germination or
germination rate of wild papaya seeds, whereas it greatly
increased both total germination and germination rate of cultivated papaya even in darkness (Figure 2B).
Figure 1. Mean weight (A) and mean length (B) of the seeds taken
randomly from the mixed seed samples of wild and cultivated papaya.
TREE PHYSIOLOGY VOLUME 18, 1998
ECOPHYSIOLOGY OF WILD AND CULTIVATED PAPAYA SEED
279
Figure 2. Time course of germination in a 12-h photoperiod (graph)
and total final germination in continuous darkness (histogram). Wild
papaya is denoted by solid circles
and bars and cultivated papaya is
denoted by open circles and bars.
Left panels: constant temperature
of 25 °C; right panels: alternating
temperatures of 30 °C for 8 h and
15 °C for 16 h. A, A′: control; B,
B′: 24-h soaking pretreatment; and
C, C′: 500 ppm GA in the germination media. Values are means ± SD.
The GA treatment increased germination rates of cultivated
papaya seeds and promoted germination in darkness of seeds
of wild papaya (Figures 2C and 2C′). Germination of seeds of
both wild and cultivated papaya was strongly inhibited by FR
light (Figure 3). Gibberellic acid reversed the inhibitory effect
of FR light in both seed types (Figure 3). Induction of germination of photoblastic seeds by GA in darkness has long been
known (e.g., Lewak and Khan 1977). The finding that seeds of
cultivated papaya germinated in darkness but not in FR light
may indicate that the ratio between active phytochrome (Pfr)
and total phytochrome (Pt) reached during seed ripening permits imbibed seeds to reach the germination response threshold in darkness. However, in some other species, a low R/FR
ratio reduces the Pfr/Pt ratio resulting in inhibition of germination (Smith and Whitelan 1990). If a similar mechanism functions in papaya seeds, it would imply that the Pfr/Pt ratio in
ripe wild papaya seeds is lower than in cultivated papaya seeds.
Statistical analyses to determine treatment differences and
their interactions are given in Table 1.
Figure 3. Effect of light enriched in far red (720 nm) and GA treatment
on germination of papaya seeds. Seeds were exposed to a 12-h photoperiod × 60 days at 25 °C. GA treatment consisted of 500 ppm of GA
in the germination media. Control: j = cultivated papaya seeds and
d = wild papaya seeds; GA treatment: h = cultivated papaya seeds
and s = wild papaya seeds.
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PAZ AND VÁZQUEZ-YANES
Table 1. Summary of ANOVA of data presented in Figures 2 and 3. Abbreviation: GA = Gibberellic acid.
Effect
Sum of squares
df
Mean square
F-ratio
P-value
Data presented in Figure 2
Seed
Light
Treatments
Seed × Light
Seed × Treatments
Light × Treatments
Seed × Light × Treatments
18975.8
795.3
8274.8
1869.6
1645.8
1406.4
648.5
1
1
2
1
3
3
3
18975.8
795.3
2758.2
1896.6
548.6
468.8
216.2
616.1
25.8
89.5
60.7
17.8
15.2
7.0
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
Data presented in Figure 3
Seed
Light
GA
Seed × Light
Seed × GA
Light × GA
Seed × Light × GA
1241.4
3063.7
8535.0
40.9
488.5
1179.6
91.7
1
1
1
1
1
1
1
73.1
180.5
502.8
2.4
28.8
59.5
5.4
< 0.001
< 0.001
< 0.001
0.133
< 0.001
< 0.001
< 0.001
Wild and cultivated papaya seeds differ in size, rate of
germination, dormancy mechanisms and light sensitivity
(Ladizinski 1987). Wild seeds of papayas can remain dormant
for extended periods of time when buried (Pérez-Nasser and
Vázquez-Yanes 1986, Vázquez-Yanes and Orozco Segovia,
1996a), reflecting their strong light requirement for germination (Vázquez-Yanes and Orozco-Segovia 1993, 1996b). Domestication has eliminated these characteristics in the
cultivated population studied.
Acknowledgments
This research was funded in part with resources provided by The
National Council of Science and Technology of Mexico (CONACyT):
Research Grant Ref. G0011-N9607. We appreciate the assistance and
recommendations from A. Orozco-Segovia, M. Rojas-Arechiga and
M.A. González-Méndez.
References
Andreoli, C. and A.A. Khan. 1993. Improving papaya seedling emergence by matriconditioning and gibberellin treatment. Hortscience
28:708--709.
Bongers, F., J. Popma, J. Meave del Castillo and J. Carabias. 1988.
Structure and floristic composition of the lowland rainforest of Los
Tuxtlas, Mexico. Vegetatio 74:75--80.
Chow, Y.J. and C.H. Lin. 1991. Para-hydroxybenzoic acid as the major
phenolic germination inhibitor of papaya seed. Seed Sci. Technol.
19:167--174.
Ellis, R.H., T.D. Hong and E.H. Roberts. 1991. Effect of storage
temperature and moisture on the germination of papaya seeds. Seed
Sci. Res. 1:69--72.
Evans, L.T. 1993. Crop evolution, adaptation and yield. Cambridge
University Press, Cambridge, 500 p.
Ladizinski, G. 1987. Pulse domestication before cultivation. Econ.
Bot. 41:60--65.
Lewak, S. and A.A. Khan. 1977. Mode of action of gibberellic acid and
light on lettuce seed. Plant Physiol. 60:575--577.
Moreno, N.P. 1980. Caricaceae. Flora de Veracruz. Fascículo 10.
Instituto de Ecología A.C. Xalapa, Veracruz, Mexico, 18 p.
1241.38
3063.7
8535.0
40.9
488.5
1179.6
91.7
Orozco-Segovia, A. and C. Vázquez-Yanes. 1989. Light effect on seed
germination in Piper L. Acta Oecol. Oecol. Plant. 10:123--146.
Orozco-Segovia, A., C. Vázquez-Yanes, R. Coates-Estrada and
N. Pérez-Nasser. 1987. Ecophysiological characteristics of the seed
of the tropical forest pioneer Urera caracasana (Urticaceae). Tree
Physiol. 3:375--386.
Pérez-Nasser, N. and C. Vázquez-Yanes. 1986. Longevity of buried
seeds from some tropical rain forest trees and shrubs from Veracruz.
Malay. For. 94:352--356.
Smith, H. and G.C. Whitelan. 1990. Phytochrome, a family of photoreceptors with multiple physiological roles. Plant Cell Environ.
13:695--707.
Smith, N.J.H., J.T. Williams, D.L. Plucknett and J.P. Talbot. 1992.
Tropical forests and their crops. Cornell University Press, Ithaca,
NY, 568 p.
Terán, S., and C.H. Rasmussen. 1995. Genetic diversity and agricultural strategy in 16th century and present day Yucatecan Milpa
agriculture. Biodivers. Conserv. 4:363--381.
Vázquez-Yanes, C. and A. Orozco-Segovia. 1982. Seed germination of
a tropical rain forest tree Heliocarpus donnell smithii in response to
diurnal fluctuations of temperature. Physiol. Plant. 56:295--298.
Vázquez-Yanes, C. and A. Orozco-Segovia. 1993. Patterns of seed
longevity and germination in the tropical rainforest. Annu. Rev.
Ecol. Syst. 24:371--376.
Vázquez-Yanes, C. and A. Orozco-Segovia. 1996a. Comparative longevity of seeds of five tropical rain forest woody species stored
under different moisture conditions. Can. J. Bot. 74:1635--1639.
Vázquez-Yanes, C. and A. Orozco-Segovia. 1996b. Physiological
ecology of seed dormancy and longevity. In Tropical Forest Plant
Ecophysiology. Eds. S. Mulkey, R. Chazdon and A. Smith. Chapman and Hall, New York, pp 535--558.
Vázquez-Yanes, C. and H. Smith. 1982. Phytochrome control of seed
germination in the tropical rainforest pioneer trees Cecropia obtusifolia and Piper auritum and its ecological significance. New Phytol.
92:477--485.
Vázquez-Yanes, C., M. Rojas-Arechiga, M.E. Sánchez-Coronado and
A. Orozco-Segovia. 1996. Comparison of light regulated seed germination in Ficus spp. and Cecropia obtusifolia: ecological implications. Tree Physiol. 16:871--874.
TREE PHYSIOLOGY VOLUME 18, 1998