Attempts to use artificially lit cabinets to grow plants identical to those growing in sunlight have provided compelling evidence of the importance of light quality for plant growth. Changing the balance of red (R) to far-red (FR) radiation, but with a fixed photosynthetic input can shift the phytochrome photoequilibrium in a plant and generate large differences in plant growth. With FR enrichment the plants elongate, and may produce more leaf area and dry matter. Similar morphogenic responses are also obtained when light quality is altered only briefly (15-30 min) at the end-of-the-day. Conversely, for plants grown in natural conditions the response of plant form to selective spectral filtering has again shown that red and far-red wavebands are important as found by Kasperbauer and coworkers. Also, where photosynthetic photon flux densities (PPFD) of sunlight have been held constant, the removal of far-red alone alters plant growth. With FR depletion plants grown in sunlight are small, more branched and darker green. Here we examine the implications for plant growth and flowering when the far-red composition of incident radiation in plant growth chambers is manipulated

Bagnall, D. J.

King, R. W.

English

NASA Technical Reports Server

PHYTOCHROME, PLANT GROWTH AND FLOWERING
R.W. King and D.J. Bagnall
CSIRO Division of Plant Industry, GPO Box 1600 Canberra, ACT, 2601
:? : ;;",--L_.,
N96- 18134
Australia
INTRODUCTION
Attempts to use artificially lit cabinets to grow plants identical to those growing in sunlight
have provided compelling evidence of the importance of light quality for plant growth.
Changing the balance of red (R) to far-red (FR) radiation, but with a fixed photosynthetic
input can shift the phytochrome photoequilibrium in a plant and generate large differences in
plant growth. With FR enrichment the plants elongate, and may produce more leaf area and
dry matter (see Smith, 1994 these proceedings). Similar morphogenic responses are also
obtained when light quality is altered only briefly (15-30 min) at the end-of-the-day (Batlare
1994; these proceedings). Conversely, for plants grown in natural conditions the response of
plant form to selective spectral filtering has again shown that red and far-red wavebands are
important as found by Kasperbauer and coworkers (Kasperbauer, 1992). Also, where
photosynthetic photon flux densities (PPFD) of sunlight have been held constant, the removal
of far-red alone alters plant growth (Mortensen and Stromme 1987; McMahon et al., 1991).
As shown in Table 1 for chrysanthemum, with FR depletion plants grown in sunlight are
small, more branched and darker green. Here we examine the implications for plant growth
and flowering when the far-red composition of incident radiation in plant growth chambers is
manipulated.
TABLE 1. Influence of filtered sunlight on growth and flowering of chrysanthemum cv Yellow
Mandalay in long days of summer. Adapted from McMahon et al. (1991).
Filter R:FR Pfr:Ptot Plant Leaf Chlorophyll Leaf Visible
Height (g cm -2) Number Flowering (days)
(cm)
Red 1.16 .71 30.3** 36.7 22 - 52
Blue 0.99 .66 29.3 39.5 21.6 - 52
Far-red 3.3 .79 16.9"* 55.4** 17.1"* 46-49
Control 1.16 .71 28.6 39.8 21.5 - 52
Significant differences; * p = 0.05 or ** p -- 0.01 vs control
R:FR ratio 655-665 nm vs 725-735 nm
Pfr:Ptot calculated over 350 to 850 nm
103
https://ntrs.nasa.gov/search.jsp?R=19960011698 2020-06-16T05:54:09+00:00Z
FAR-RED ENRICHMENT AND PLANT GROWTH IN ARTIFICIALLY LIT CHAMBERS;
As with spectral filtering of sunlight, differences in the R:FR ratio of light in growth chambe:-s
can lead to large effects on growth (Figs 1, 2). Here PPFD was held constant across the two
chambers (560 mols m-as -1) and there were no major thermal differences, leaf temperatures
across the chambers being within 1 to 2°C. Thus, the greater stem elongation with mixed
metal halide/quartz halogen versus fluorescent lamps (Fig. 1) can be most simply explained a:;
a phytochrome mediated response to FR enrichment. This FR-induced change was not driver
by photosynthesis as stem elongation increased rapidly especially in sunflower (< 1 week,
Fig. 1) and preceded by 1-2 weeks any increase in dry matter accumulation and leaf area
production. To reiterate, there was firstly a change in plant form (Fig. 1), then later a change
in dry weight indicating an initial photomorphogenically-driven increase in leaf area with
subsequent photosynthetic increase, a conclusion suggested by Smith and coworkers from their
studies over the last decade (see Smith 1994). On the other hand, photosynthetic capacity
and/or leaf assimilate export could respond directly to FR enrichment. Chow, Anderson and
coworkers have reported many FR effects on photosynthetic light harvesting pigment
components particularly of young pea seedlings and these responses can result in slightly
increased CO2 exchange rates per unit leaf area (Chow et al., 1990).
Surprisingly large effects on dry matter allocation to roots were observed as a consequence of
FR enrichment (see Figure 2). Root growth has not always been measured in these types of
experiment (e.g. Tibbitts et al., 1983) but there could also be a trivial explanation for the data
summarized in Figure 2. Greater dry matter allocation to the roots was evident only at the last
( week 4) harvest. For tomato, for example, the root:shoot ratio doubled to 0.53 over the last
week of growth in FR-enriched conditions whereas in the R-rich cabinet it remained at ca 0.3.
However, this dry matter reallocation occurred when total dry matter was also increasing
exponentially. Thus, the rapid increase in leaf assimilate supply may have temporarily
exceeded stem demand leading to a shunting of assimilate to the roots and a transient shift in
the root:shoot dry weight balance in FR-enriched conditions. Further studies are needed of
responses of roots to FR-enriched conditions especially since our findings with tomato are the
opposite of those noted earlier by Kasperbauer (1992) where only end-of-day light quality was
altered.
Although sunflower and tomato grew optimally in FR-enriched conditions (Fig. 2) wheat was
rather insensitive (Figs. 1,2) as also found for wheat by Tibbitts et al. (1983). The slower
growth of the eucalypt (Fig. 1) may have masked positive responses. However, there was a
significant reversal of response compared to other species in that FR-enrichment led to the
formation of fewer leaves (61.23.5 vs 91.7_10.0) and branches (7.8_0.8 vs 13.2 1.5). This
data for eucalypt requires confirmation but large differences in sensitivity to FR between
species have been reported previously (Tibbitts et al., 1983 and see references therein).
104
80 t Sunflower/
°'_ 60 h
o 40
°l
-I-_ 20
0 i I ; I i
Tomato Wheat
I 1 1 1
Eucalypt
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1600 P-
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_1200 _
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• /
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//
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i
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0 2 4
Weeks of growth
I • +FR I
IoF
2 4
Fig. l. Growth in height, plant dry
weight and leaf area of 4 plant species
over 4 weeks of exposure to FR-rich (0)
or R-rich (D) lamps. Irradiance 560 mol
m-2s ' of photosynthetically active radiation
(PAR). The ratios of R:FR (660:730 nm)
were FR= 1.53 R=5.08. Daylength was
12-h with a day:night temperature of
21:16°C.
O
FR R
Sunflower
©
O
O,
rO
0
0
0
O
O
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dd
/
0
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Tomato
ooo
7 I'
FR R
Wheal
C C
0 O0
o o8
0 0
0
0 O
to
°,
I
FR R
Eucalyptus
Fig. 2. Relative proportion of dryweight
(R value set at 1) in root (V) stem (length
of uprights) and leaf (circles x 2) after 4
weeks growth in a FR- or R-rich cabinet.
Conditions as in Fig. 1.
105
FLOWERING AND FAR-RED ENRICHMENT
Potential for change in time to flower with FR enrichment is one morphogenic response that
has received little attention in the designing of lamp types for plant growth chambers. For
short-day plants, red rather than far-red rich conditions favour flowering (see Salisbury, 1965)
but in most published reports there have generally been confounding effects of daylength
change and photosynthetic input. With equalization of photosynthetic inputs as in the study of
McMahon et al. (1991) with the short-day plant chrysanthemum, there was actually a slight
enhancement of flowering time for plants grown in long days with removal of far-red (see
Table 1). However, the response to the presence or absence of far-red may also vary with the
time of the day. For the short-day plant Pharbitis nil a FR interruption of as little as 90 min
during continuous light can promote or inhibit flowering depending on its timing (Heide et al..
1986). This FR response cycles with a period of about 12-h of a semidian rhythm (see Table
1).
By contrast with short-day plants, promotion of flowering by FR enrichment can be expected
in long-day plants (see Vince-Prue, 1975; Deitzer, 1984). However, there are also very few
comparisons of effects of luminaries on flowering of long-day plants when photosynthetic
input has been fixed. In the studies of Tibbitts et al. (1983) mustard in 16-h long days reached
anthesis about 2 to 3 d earlier (in a total of 25 to 29 d) when it was exposed to FR-rich lamps.
Wheat was unaffected reaching anthesis at 57 d. However, an almost halving of days to
flower (40 to 24 d) and of leaf number at flowering was found by Bagnall (1993) for thefca
mutant ofArabidopsis (Landsberg strain) exposed to long days and ratios of R:FR ranging
from 5.8 to 1.0.
The importance of FR-mediated effects of phytochrome on flowering in long-day plants is
shown clearly by recent studies of Bagnall and coworkers (1994). They found that transgenic
Arabidopsis plants constitutively overexpressing the far-red sensing phytochrome A gene
flowered very early relative to the isogenic wild type (28 vs 64 d). Conversely, a mutant
lacking phytochrome A is late to flower in FR-enriched conditions (Johnson et al., 1994)
involving low PPFD (10 mol m-2s 1) FR-rich tungsten daylength extensions.
A PHOTOSYNTHETIC ROLE IN FLOWERING
Although FR plays a central role in determining flowering time, photosynthetic input also
influences expression of the long day response. Lolium temulentum, for example, flowers in
response to a single long day and shows enhanced flowering the greater the photosynthetic
irradiance. However, Lolium remains vegetative in short days independent of the irradiance of
sunlight up to 1200 mol m2s" (King and Evans, 1991). Photosynthesis alone is insufficient
for flowering whereas a single non-photosynthetic long day given as a 16-h incandescent low-
irradiance daylength extension is sufficient (Fig. 3) and less effective is an extension using a
106
fluorescent,FR-deficient lamp. For either lamp, in associationwith a long day, increasing
their photosyntheticcontribution givesparallel andlinear increasesin flowering response(Fig.
3). There is noevidencehereof interactionbetweenlight quality - thephytochrome-mediated,
response- andthe photosyntheticresponseto this long day. Thus, photosynthesisin Lolium
must be considered as beneficial but not sufficient for flowering. On the other hand, a more
direct photosynthetic effect is evident in another long day plant, Sinapis alba. Its requirement
for a single FR-rich long daylength extension can be bypassed by increasing photosynthesis for
more than three short days although this effect involves four times the photosynthetic
irradiance applied during a single photoinductive long day (Bodson et al., 1977).
Since phytochrome and the photosynthetic pigments can act in concert to promote flowering of
long-day plants, then, with increasing irradiance the photosynthetic contribution to flowering
will range from nothing to apparently over-riding control by photosynthesis. As a
consequence, action spectra could range from dominance by red wavelengths to a balance in
the contribution by red and far-red and to the classic dominance by far-red wavelengths. The
literature contains illustrations of all these combinations of wavelength and response of
flowering to red and far-red. (Deitzer, 1984; Carr-Smith et al. 1989) and, clearly, some
detailed reexamination of wavelength and irradiance interactions is required.
2.0
E T8
E
r-- 16N
e-
-- 14
x
O.
m 1.2
o
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i
08 a
t i i i
I_ 2_ 3_ 4_
E
t-
O
o
£
(/1
x
o.
<
b
c A L I I
_00 200 300 400
Irradiance (pmol PAR m -2 s-I)
20
O)
_ 15
0,) u_
10
t'N _
O s
.,J
o
/
/
/° c
I I I i
0 200 400 600 800
Fig. 3. Dependence on irradiance from fluorescent lamps during a single 16-h daylength
extension of (a) flowering response in terms of shoot apex length after 3 weeks and (b) apex
sucrose content at the end of the 16-h extension. Short day (I) and a low PPFD incandescent
16-h extension (o) are also included. Leaf CO2 exchange (c) was determined during the main
photoperiod. From King and Evans (1991).
107
OVERVIEW
As a broad generalization, far-red rich lamps are beneficial and sometimes essential, for plant
growth and flowering in artificially lit chambers. Thus fluorescent and sodium lamps, being
FR deficient may cause stunting and poor flowering. Brief end-of-day FR exposure may
alleviate some of the stunting of growth but will probably have complex effects on flowering.
A more beneficial approach appears to be continued FR enrichment over the whole
photoperiod. A further complexity is that the need for FR input may vary cyclically over the
day.
REFERENCES
Bagnall, D.J. 1993. Light quality and vernalization interact in controlling late flowering in
Arabidopsis ecotypes and mutants. Annal. Bot. 71: 5-83.
Ballare, C.L. and A. L. Scopel. 1994. Plant Photomorphogenesis and Canopy Growth.
(This proceedings)
Bodson, M., R.W. King, L.T. Evans and G. Bernier. 1977. The role of photosynthesis in
flowering of the long-day plant Sinapis aIba.
Carr-Smith, H.D., C.B. Johnson and B. Thomas. 1989. Action spectrum for the effect of
day-extensions on flowering and apex elongation in green, light-grown wheat (Triticum
aestivum L.) Planta 179:428-432.
Chow-, W.S., D.J. Goodchild, C., Miller and J.M., Anderson. 1990. The influence of high
levels of brief or prolonged supplementary far-red illumination during growth on the
photosynthetic characteristics, composition and morphology of Pisum sativum
chloroplasts. Plant, Cell, Environ. 13:135-145.
Deitzer, G.F. 1984. Photoperiodic induction in long-day plants, p.51-68. In: D. Vince-Pruz,
B. Thomas and K.E. Cockshull (eds.). Light and the flowering process. Academic Pre_s,
London.
Heide, O.M., R.W. King and L.T. Evans. 1986. A semidian rhythm in the flowering
response of Pharbitis nil to far-red light I. Phasing in relation to the light-off signal.
Physiol. 80:1020-1024.
Phmt
lO8
Johnson,E., N.P. HarberdandG.C. Whitelam. 1994. Photoresponsesof light grownPHYA
mutants of Arabidopsis. Phytochrome A is required for the perception of daylength
extensions. Plant Physiol. In Press.
Kasperbauer, M.J. 1992. Phytochrome regulation of morphogenesis in green plants: from the
Beltsville spectrograph to coloured mulch in the field. Photochem. and Photobiol.
56:823-832.
King, R.W. and L.T. Evans. 1991. Shoot apex sugars in relation to long-day reduction of
flowering in Lolium temulentum. Aust. J. Plant Physiol. 18:121-135.
McMahon, M.J., J.W., Kelly, D.R., Decotean, R.E., Young and R.K. Pollock. 1991.
Growth of Dendranthemum x grandiflora (Ramat.) Kitamura under various spectral
filters. J. Amer. Soc. Hort. Sci. 116:950-954.
Mortensen, L.M. and E. Stromme. 1987. Effects of light quality on some greenhouse crops.
Scientia Hortic. 33:27-36.
Smith, H. 1994. Phytochrome-mediated responses: implications for controlled environment
research facilities. (This proceedings)
Tibbitts, T.W., D.C. Morgan and I.J. Warrington. 1983. Growth of lettuce, spinach, mustard
and wheat plants under four combinations of high-pressure sodium, metal halide, and
tungsten halogen lamps at equal PPFD. J. Amer. Soc. Hort. Sci. 108:622-630.
Vince-Prue, D. 1975. Photoperiodism in plants. McGraw Hill, London, N.Y.
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PLANT REQUIREMENTS
NON-PHOTOSYNTHETIC (BLUE AND ULTRAVIOLET)
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112


Phytochrome, plant growth and flowering

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