The Horticulture Journal 91 (4): 448–452. 2022.
doi: 10.2503/hortj.QH-006

The Japanese Society for
JSHS

Horticultural Science

http://www.jshs.jp/

Effects of Drought Stress on Flower Number in ‘Summer Queen’ Passion Fruit
Tomohiro Kondo1* and Hana Morizono2
1

Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
Faculty of Regional Innovation, University of Miyazaki, Miyazaki 889-2192, Japan2

To determine the effects of drought stress, especially light drought stress, on flower number in passion fruit,
one-year-old passion fruit plants grown in 7.5 L plastic pots were subjected to different soil water content
treatments, namely wetness, light drought, and heavy drought for two months. Average, maximum, andminimum soil water contents (v/v) were 44, 47, and 41% in the wetness treatment, 23, 40, and 11% in thelight drought treatment and 11, 33, and 6% in the heavy drought treatment. Flower number decreased as
the strength of drought stress increased, although the number of nodes and flower buds did not. Flowering
periods were from June 27 to July 19 in the wetness treatment and June 26 to July 16 in the light drought
treatment with three peaks around July 1, 6, and 13. In the heavy drought treatment, the flowering period wasfrom July 11 to 18 with one peak. The flower bud number was not affected by drought stress. Light drought
stress did not suppress vegetative growth, such as vine length, leaf number, leaf length, or photosynthetic rate,
although heavy drought stress did. Stomatal conductance was suppressed by light drought only at 12:00PMand by heavy drought throughout the day. Leaf water potential was decreased by heavy drought at 3:00PM,
but not by light drought. In the wetness and light drought treatments, visible wilting was not observed, andin the heavy drought treatment the plants wilted before irrigation, although they recovered about 15 min after
irrigation. In conclusion, even light drought stress, which did not suppress vegetative growth, reduced theflower number in passion fruit. Drought stress suppressed flower bud development but not differentiation.
Key Words: flower bud, leaf water potential, Passiflora edulis, photosynthetic rate, soil water content.

Introduction
Passion fruit (Passiflora edulis Sims) is a subtropicalfruit which is native to Latin America, and is now wide‐ly cultivated in tropical and subtropical regions(Morton, 1987). In Japan, the cultivation area and the
production has been increasing mainly for fresh con‐sumption (Kumamoto et al., 2017). High quality fruitwith low acidity and good appearance for fresh con‐sumption are mainly cultivated in greenhouses. In
greenhouses, irrigation systems, heaters, and artificial
lights make it possible to produce high quality fruit
year-round, but the cost is generally high. Therefore,
cultivation methods to increase yield are now required.
Using artificial pollination, the fruit set rate wasmore than 90% (Akamine and Girolami, 1959; Ishihata,1993), and so an increase in flower number may lead toincreased yield. Therefore, effects of cultivation condi‐
Received; March 31, 2022. Accepted; June 22, 2022.
First Published Online in J-STAGE on August 20, 2022.
Special Issue ‘Reproductive biology of fruit and nut tree crops’.
* Corresponding author (E-mail: kondo.tomohiro.0z@kyoto-u.ac.jp).

tions on flower number were determined recently;
strong light (Menzel and Simpson, 1988, 1989;
Shimada et al., 2019), air temperature lower than 30°C
(Matsuda et al., 2005), and NaCl application (Kondo
and Sato, 2022) all increased the flower number. How‐ever, these findings are not so useful to establish newcultivation methods; increasing light intensity artificial‐ly and lowering air temperature below 30°C in summer
are costly and NaCl application in greenhouses carries arisk of salinity. On the other hand, to maintain the soil
water content is relatively inexpensive and easy to prac‐
tice because irrigation systems installed to produce highquality passion fruit are often used in greenhouses.
Therefore, the effects of soil water content on flower
number in passion fruit would be useful information forfarmers. Effects of drought stress on flower number
varied with the intensity of the stress and crop; lightdrought stress without photosynthetic reduction reduced
the flower number in blueberry (Guo et al., 2021), 12
days of drought stress induced flowering in pummelo(Phadung et al., 2011), while severe drought stress with
−3.94 MPa of leaf water potential did not induce floral
morphogenesis, but did induce earlier floral budbreak,© 2022 The Japanese Society for Horticultural Science (JSHS), All rights reserved.

Hort. J. 91 (4): 448–452. 2022.

in mango (Nunez-Elisea and Davenport, 1994). There‐
fore, for each crop the effects of drought stress on flow‐
er number should be determined, paying attention to theintensity of the stress.
In passion fruit, heavy drought stress, which sup‐pressed vegetative growth significantly reduced theflower bud and flower number (Menzel et al., 1986;
Shimada et al., 2018; Staveley and Wolstenholme,1990). Findings regarding the detrimental effects ofheavy drought stress are not useful to establish new cul‐
tivation methods in greenhouses with irrigation systems
because such heavy drought stress seldom occurs. On
the other hand, a high soil water content reduced theflower number drastically (Iida, 2019) as well as vege‐
tative growth due to root rot (Shimada et al., 2018).
Therefore, farmers often reduce the irrigation amountand frequency because they are afraid of those risks,and so light drought stress may sometimes occur.
Therefore, in this study the effects of drought stress,
especially light drought stress, on flower number were
determined.

Materials and Methods
One-year-old hybrid passion fruit (Passiflora edulis
Sims × P. edulis f. flavicarpa Deg. ‘Summer Queen’)plants were grown in a greenhouse at Kyoto University(35.0°N, 135.8°E). Fifteen plants were propagated by
cutting in October 2020 and transplanted to 7.5 L plas‐
tic pots in December. Decomposed granite soil and barkcompost were mixed at a ratio of 4:3 by volume for the
pot soil. A pumice stone was placed at the bottom of thepots at a depth of 5 cm to improve drainage. The pots
were placed in three rows with five pots per row. The
pots were spaced in rows at 90 cm between pots and90 cm between rows. CaCO3 and MgCO3 were applied
at 2.5 g each to prevent a pH decline and Ca and Mg
nutrients were added on the transplanting day and in
April 2021. Chemical fertilizer at 15 g per plant con‐taining N:P2O5:K2O = 10:10:10 was applied weekly. In
the greenhouse, the minimum temperature was kept at
10°C during the winter and the maximum temperature
was not regulated. The plants were hedge trained, as
described by Kondo and Higuchi (2011), with a heightof 140 cm and four fruit-bearing vines per plant.
The 15 plants were divided into three groups of fiveplants. Each group was subjected to a different soil
water content treatment, namely wetness, light drought,
and heavy drought from May 31 to August 4. For thewetness treatment, a bottom-irrigation tray of 45 cm
diameter and 14.5 cm height was placed the bottom of
the pot, and the water level was maintained at a depthof around 8 cm to keep the soil water content high(Fig. 1). Water in the bottom-irrigation tray was
replaced every three days. Fifteen 1 cm diameter drain
holes per pot were made at 10 cm above the bottom of
the pot. To supply oxygen, a rectangular airing stone
10 cm long and 2 cm wide (Best Bio AIR 100; GEX.

Fig. 1.

449

Diagram of the experimental pot in the wetness treatment.

Co., Ltd., Osaka, Japan) was placed the bottom of the
pot with an air pump (e-AIR 6000WB; GEX. Co., Ltd.).
In addition to bottom-irrigation, from the top of the soil
surface 700 mL of water was supplied once a day fromMay 31 to June 9 and three times a day from June 10 toJuly 1. After that, 1,400 mL of water was supplied threetimes a day. On rainy days, water was not supplied. For
the light and heavy drought treatments, water was sup‐plied only from the top of soil surface without bottomirrigation. The amount and frequency of irrigation fromthe top pot soil in the light drought treatment were thesame as in the wetness treatment. For the heavy drought
treatment, water was supplied every three days fromMay 31 to June 9 and after that once a day, keeping theamount the same. Soil water content (v/v) at a 6–12 cm
depth and air temperature were measured once every
15 min during the experiment using a soil moisture sen‐sor (EC-5; METER Group, Inc., WA, USA) and a ther‐mometer (VP-4; METER Group, Inc.). These data wererecorded on a data logger (EM50; METER Group,Inc.). After the cultivation experiment, water content(v/v) and water potential of the soil used for the experi‐
ment were measured by the soil moisture sensor and asoil water potential sensor (TERSO-21; METERGroup, Inc.) to determine the relationship between thetwo variables. Then, the soil water potential during theexperiment was calculated. Soil water content (v/v) ismore informative for evaluating soil wetness than water
potential, while water potential is an indicator of soil
dryness regardless of soil type. Thus, both water con‐tent (v/v) and water potential of the soil are described inpresent study.
Flower buds 2–3 mm long and bloomed flowers werecounted. After flower counting, artificial pollination
was conducted. One week after pollination, we regard‐ed enlarged ovules as fruiting and counted the fruit
number. Leaf number, length of fruit-bearing vines, andlength of the largest leaf were measured weekly. Plant
appearance was observed before and after irrigation.
Stomatal conductance of the largest leaf on fruitbearing vines was measured at 6:00AM, 9:00AM,

450

T. Kondo and H. Morizono

Results
Average, maximum, and minimum soil water con‐tents were 44, 47, and 41% in the wetness treatment,
23, 40, and 11% in the light drought treatment and 11,33, and 6% in the heavy drought treatment (Fig. 2). A
water content higher than 18% represented a higher soil
water content than −0.1 kPa, while 11 and 6% watercontents represented −39.3 and −298.8 kPa, respective‐ly (data not presented). The flower number decreased as
the drought stress increased, although the number ofnodes and flower buds did not (Table 1). The fruit num‐
ber decreased only under heavy drought stress. Flower‐
ing periods were from June 27 to July 19 in the wetness

treatment and June 26 to July 16 in the light drought
treatment with three peaks around July 1, 6, and 13(Fig. 3). In the heavy drought treatment, the flowering
period was from July 11 to 18 with one peak. The fruitbearing vine length in the light drought treatment was
longer than that in the wetness treatment after June 28(Fig. 4). Heavy drought suppressed vine elongation.
Leaf number was not affected by light drought, except
for July 5 and the leaf number in the light drought treat‐ment was higher than that in the wetness treatment(Fig. 5). The increase in leaf number was suppressed byheavy drought, although at the end of the experiment
there was no statistical difference. Leaf length was not

8
Wetness

Flower number / plant

12:00PM, 3:00PM, and 6:00PM on July 19 using a leaf
porometer (SC-1; METER Group, Inc.). The photosyn‐
thetic rate of the largest leaf, well exposed to sunlight,
on fruit-bearing vines was measured at 10:00–11:00AM
and 1:30–2:30PM on July 19 using a photosynthetic
rate measurement device (MIC-100; Masa International
Co., Ltd., Kyoto, Japan). The measurement conditions
were as follows: photosynthetically active radiation(PAR) 1200 μmol·m−2·s−1, stabilization time 3 s, mea‐
surement start CO2 concentration 400 ppm, and mea‐
surement CO2 span 10 ppm. Leaf water potential was
also measured at 6:00AM, 9:00AM, 12:00PM, 3:00PM,
and 6:00PM on July 19 using a pressure chamber(Model670; PMS Instrument Co., OR, USA).
The data were analyzed by analysis of variance, andthe statistical differences among treatments were sub‐jected to further analysis using Tukey’s test.

Light drought

6

Heavy drought

420June 26

July 1

July 7
Date

July 13

July 19

Fig. 3. Effect of drought stress on the flower number in ‘Summer
Queen’ passion fruit. Bars indicate SE (n = 5).

150

a

a

a

a

a

Vine length (cm)Soil water content (%)

50

40

30

100

a

b

b

c

b

b

b

b

b

b

b

c

a
a

50

Wetness
Light droughtc

Heavy drought

NS

20

b

0

June 11 June 17

10

June 28 July 5 July 11

July 20 July 26

Aug. 4Date
0June 5

Fig. 4. Effect of drought stress on vine length in ‘Summer Queen’passion fruit. Bars indicate SE (n = 20). Different letters within
Datethe same day indicate statistical differences by Tukey’s test at
Fig. 2. Changes in soil water content (v/v) during the experiment.
P < 0.05.
Table 1. Effects of drought stress on the number of nodes, flower buds, flowers, fruit per vine and photosynthetic rate in ‘Summer Queen’ passion
fruit.
Table 1.

June 20

July 5

July 20

Aug. 4

Effects of drought stress on the number of nodes, flower buds, flowers, fruit per vine, and photosynthetic rate in ‘Summer Queen’ pas‐sion fruit.

Wetness
Light drought
Heavy drought

Node number
per vine

Flower bud number
per vine

Flower number
per vine

Fruit number
per vine

Photosynthetic rate
(μmol·m−2·s−1)

15.6 a15.8 a16.5 a14.4 a14.3 a14.5 a

8.1 a
5.5 b
2.7 c

4.7 a
4.0 a
2.6 b

20.6 a21.9 a12.5 b

Different letters within columns indicate statistical differences by Tukey’s test at P < 0.05 (n = 20).

451

20

a

Leaf number / vine

a

15

a

a

b

b

a

10

a
a

5

0

NS

a

ab

a
a

NS

b

b
c

Wetness
Light droughtb

Heavy droughtbJune 11 June 17

June 28 July 5 July 11

July 20 July 26

Aug. 4

Stomatal conductance (mmol·m-2·s-1)

Hort. J. 91 (4): 448–452. 2022.

25

800
600

20
a

15

0

a

a

a

a

a

a

a

a

a

a
a

a
ab

a

b

b
b

200

a

b

6:00AMb

9:00AM

12:00PM

3:00PMabb

6:00PM

Measuring time

Fig. 7. Effect of drought stress on stomatal conductance in‘Summer Queen’ passion fruit. Bars indicate SE (n = 5). Differ‐
ent letters within the same measuring time indicate statisticaldifferences by Tukey’s test at P < 0.05.

Measuring time0

6:00AM

9:00AM

12:00PM

3:00PM

6:00PM

a

10
5

a

a

a
a

400

0

b

b

b

b

b

Wetnessb

June 11 June 17b

Light drought
Heavy drought
June 28 July 5 July 11

July 20 July 26

Aug. 4Date

Fig. 6. Effect of drought stress on leaf length in ‘Summer Queen’passion fruit. Bars indicate SE (n = 20). Different letters within
the same day indicate statistical differences by Tukey’s test at
P < 0.05.

affected by light drought, but was reduced by heavy
drought throughout the experiment (Fig. 6). In the wet‐ness and light drought treatments, visible wilting was
not observed, and in the heavy drought treatment theplants wilted before irrigation but recovered about
15 min after irrigation. Stomatal conductance was sup‐pressed by light drought only at 12:00PM and by heavy
drought throughout the day (Fig. 7). Light drought did
not suppress the photosynthetic rate, but heavy droughtdid (Table 1). Leaf water potential was reduced byheavy drought at 3:00PM, but not by light drought(Fig. 8).

Discussion
The flower number in the light drought treatment was
lower than that in the wetness treatment, although vege‐
tative growth, such as vine length, leaf length, and thenumber of nodes and leaves, did not decrease and novisible wilting was observed. There were no differencesin leaf water potential and photosynthetic rate, althoughstomatal conductance was slightly lower in the lightdrought treatment. In conclusion, even light drought

Leaf water potential (MPa)

Leaf length (cm)aLight drought
Heavy drought

Date

Fig. 5. Effect of drought stress on the leaf number in ‘Summer
Queen’ passion fruit. Bars indicate SE (n = 20). Different
letters within the same day indicate statistical differences by
Tukey’s test at P < 0.05.

Wetness

1000

NS

a

−0.5
a

NS

−1.0
WetnessNS

NS

Light drought
Heavy drought

−1.5

b

Fig. 8. Effect of drought stress on leaf water potential in ‘Summer
Queen’ passion fruit. Bars indicate SE (n = 5). Different letterswithin the same measuring time indicate statistical differences
by Tukey’s test at P < 0.05.

stress that did not suppress vegetative growth, reduced
the flower number in the present study, even though adecrease in flower and flower bud number due to strong
drought stress that suppressed vegetative growth has
been reported (Menzel et al., 1986; Shimada et al.,2018; Staveley and Wolstenholme, 1990). On the other
hand, maintaining a high soil water content and highleaf water potential resulted in a drastic reduction inflower number to about 10% (Iida, 2019). Iida (2019)concluded that a temporary decrease in leaf water
potential was necessary to promote flowering. In thepresent study, leaf water potential decreased to−0.8 MPa in the wetness treatment possibly because ofrestriction of the root zone due to the pot, and so theflower number may not have been reduced.
The flower bud number was not affected by light or
heavy drought stresses. Therefore, drought stress might
not affect flower bud differentiation, but caused flower
bud drop and reduced the number of flowers. Flower
bud development may be more sensitive to drought

452

T. Kondo and H. Morizono

stress than flower bud differentiation. In contrast,
strong drought stress reduced the number of flower
buds, so strong drought stress may suppress flower bud
differentiation (Menzel et al., 1986; Shimada et al.,2019). Vegetative growth and photosynthetic rate were
not suppressed by light drought stress. Thus, the flower
bud drop in the light drought treatment was not caused
by a reduction in vegetative growth or photosynthetic
rate induced by the stress. The flower bud drop may
have been caused by changes in the synthesis of plant
hormones, such as ethylene, induced by the stress.
In addition to the detrimental effect of drought stressin the light drought treatment, in the heavy drought
treatment the reduction and delay of vegetative growthcaused by a decrease of photosynthetic rate may also
have dropped the flower buds. The photosynthetic rate
was suppressed by heavy drought stress, and vegetative
growth was delayed. As a result, the flowering period in
the heavy drought treatment was also delayed because
flowering began in all treatments when the fruit-bearing
vine length reached about 100 cm. Passion fruit flower‐
ing was suppressed by air temperatures higher than
30°C (Chang and Cheng, 1992). In the present study,
the flowering period ended in all treatments when the
maximum temperature exceeded 35°C after July 19. In
the heavy drought treatment, the delayed flowering
period due to delayed vegetative growth may haveresulted in a shortened flowering period because themaximum temperature exceeded 35°C immediately
after the flowering period began.
The fruit number was not decreased by light drought
stress, although the flower number was decreased. In
passion fruit, the fruit set rate decreased as the cropload increased (Kondo and Higuchi, 2011, 2021). In thepresent study, 4 to 5 fruits per fruit-bearing vine would
have represented full capacity, beyond which the plants
could not set additional fruit because relatively small,
140 cm tall plants were used. As a result, there may
have been no difference in the fruit number between thewetness and light drought treatments. Bigger plants
than those used in the present study are generally culti‐
vated in commercial orchards, so a reduction in theflower number induced by light drought stress may
result in a decrease in yield.
In passion fruit, the flower number was reduced bylight drought stress that was not detected visibly. There‐
fore, to increase the flower number, a high soil watercontent close to the field water capacity should bemaintained. On the other hand, a temporary decrease in
the leaf water potential was reported to be necessary forflower formation (Iida, 2019). Therefore, further studies
on the relationships among soil water content, leaf
water potential, and flower number are needed.

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