Effect of low irradiance on the photosynthetic performance and spiking of Phalaenopsis
Effect of low irradiance on the photosynthetic performance and spiking of Phalaenopsis
Y.-C. LIU 0 1
C.-H. LIU 0 1
Y.-C. LIN 0 1
C.-H. LU 0 1
W.-H. CHEN 0 1
H.-L. WANG 0 1
0 Department of Life Sciences
1 , Institute of Biotechnology
Lowering irradiance can delay the flower stalk, i.e., spike development, in order to schedule flowering time of Phalaenopsis; however, the effect on photosynthetic performance and spiking inhibition remains poorly understood. We compared light and shade treatments of Phalaenopsis aphrodite subsp. formosana in order to determine how limiting light affects day-night changes in the photosynthetic capacity of leaves and the carbon pool of leaves and stems resulting in delayed spiking. The low irradiance treatment [20 μmol(photon) m2 s1] for six weeks did not affect potential functions of photosynthetic apparatus estimated by chlorophyll a fluorescence analysis, but it significantly reduced the net CO2 uptake and O2 evolution rates, carbohydrate and organic acid concentrations, and amplitudes of CAM activity in new and fully expanded leaves of Phalaenopsis and delayed the spiking compared with the control kept at 150 μmol(photon) m2 s1. The shortened stem contained a remarkably high sucrose concentration, accounting for more than 80% of total soluble sugars for both treatments throughout the day. Moreover, the sucrose concentration was unaffected by the lowering of irradiance. The relationship between the sucrose content and spiking seemed to be loose; the major factor(s) for spiking in Phalaenopsis remained to be ascertained as the flower stalk bud is attached to the shortened stem. Additional key words: CAM; carbohydrate; chlorophyll fluorescence; flower stalk; gas exchange; oxygen evolution.
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Phalaenopsis orchids are popular and valued potted
ornamental plants worldwide for their beautiful
longlasting flowers with a variety of shapes, sizes, and colors
(Endo and Ikusima 1992, Chugh et al. 2009). In order to
meet market demands, flowering time must be precisely
scheduled. One of key steps in regulating Phalaenopsis
flowering is to control the emergence of the flower stalk or
spiking.
Lowering of irradiance is a known strategy to delay
spiking in Phalaenopsis (Kubota and Yoneda 1993, Wang
1995, Wang 1997, Hisamatsu et al. 2001, Liu et al. 2010,
Wu et al. 2013). For example, Wang (1995) demonstrated
that the spiking of Phalaenopsis cultured in a growth
chamber was inhibited under irradiance of 8 μmol(photon)
m2 s1 during a 12-h photoperiod or complete darkness for
6 weeks. Many studies have mentioned that the decrease
in the sucrose concentration under lower irradiance may be
an inhibitory signal for spiking, but their deduction was
only based on the results from studies where new and fully
expanded leaves were used (Konow and Wang 2001,
Kataoka et al. 2004, Guo and Lee 2006, Tsai et al. 2008,
Wu et al. 2013).
Phalaenopsis is a monopodial orchid with thick leaves
alternating on each side of the plant and the bases
of leaves are connected to a shortened stem. One or two
dormant spike buds emerge from the stem at the base of
the third to fourth leaves that are numbered basipetally
from the aerial portion of the plant (Sakanishi et al. 1980).
Although the leaf is the site of light perception, it is
reasonable to speculate that the signal in the stem of
Phalaenopsis is involved more directly with spike
development than with the leaf, because the spike bud is
connected with the stem. To our knowledge, little is known
about the relationships between carbohydrates,
particularly, sucrose concentrations in the stem, and spiking
inhibition of Phalaenopsis subjected to low irradiance.
A reduction in photosynthesis by lowering of
irradiance is axiomatic. Photosynthetic capacity can be
investigated by noninvasive methods, such as net CO2
assimilation rate and chlorophyll (Chl) a fluorescence, and
the invasive method, e.g., O2 evolution rate. Phalaenopsis
is an obligate CAM plant with a day-night fluctuation of
Materials and methods
Plant material and growth conditions: Mature
P. aphrodite subsp. formosana plants were purchased
from the Wusulin Farm of the Taiwan Sugar Corp., Tainan
County, southern Taiwan (23°34'N, 120°38'E) and
transplanted in transparent plastic pots (10.5 cm) filled
with sphagnum moss in an environment-controlled
greenhouse at 28.0 1.2°C for four months in order to inhibit
spiking. The plants with six leaves were then kept for six
weeks in a growth chamber under a photoperiod with 14 h
of light (06:0020:00) and 10 h of dark (20:0006:00) with
150 and 20 μmol(photon) m2 s1 of PAR supplied with
cool-white fluorescence tubes (F20T12, GE Inc.,
Schenectady, NY, USA) at the tops of the plants for
control (CK) or shade (SH) treatment, respectively. The
day and night temperature for both treatments were 28.5
0.4 and 20.5 0.4°C, respectively. The plants were
irrigated once a week, alternating between water and
Peters fertilizer (1.0 g L1) (Hyponex Corp., Marys (...truncated)