POTENTIAL EFFECTS OF UV-B ON PHOTOSYNTHES IS AND PHOTOSYNTHETIC PRODUCTIVITY OF HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 134

Table 1 . Analysis of the responses of chlorophyll fluorescence, carbon assimilation to intercellular
CO 2 concentration and stomatal conductance parameters in pea plants suddenly exposed for 14h or
developed during 24d to high UV-B doses in a greenhouse. Parameters estimated were as follows:
maximum (F v /F m ) and relative (I PSII ) quantum efficiency of PSII photochemistry in the dark
measured at Photosynthethically active Photon Flux Density (PPFD) = 450 Pmol m -2 s -1 ; light
saturated CO 2 assimilation rate ( A sat ), maximum carboxylation velocity of Rubisco ( V c,max ), maximum
potential rate of electron transport contributing to RuBP regeneration ( J max ) and stomatal limitation ( l )
to A sat measured at PPFD = 1200 Pmol m -2 s -1 , leaf temperature = 25 ± 0.5 o C and VPD = 1.5 kPa; and
adaxial, abaxial and total (adaxial plus abaxial) g s measured at PPFD = 450 Pmol m -2 s -1 . Means of at
least three replicates ± 1 SE. Data modified from Nogués et al. 14-15

Parameter

Non-UV-B Suddenly exposed

for 14h

Developed
during 24d

F v /F m

0.81r0.01 0.80r0.01 0.79r0.02

I PSII

0.54r0.03 0.45r0.02 0.47r0.03

A sat ( P mol m -2 s -1 )

23.3r1.0 7.7r1.4 18.0r2.1

V c,max ( P mol m -2 s -1 )

99.5r7.1 42.2r11.1 91.3r15.2

J max ( P mol m -2 s -1 )

238r11 99r28 332.9r50.9

Stomatal limitation (%)

11.1r1.4 19.8r1.9 34.0r1.6

Ada. g s (mol m -2 s -1 )

0.42r0.05 0.24r0.07 0.13r0.06

Aba. g s (mol m -2 s -1 )

0.50r0.07 0.20r0.03 0.44r0.11

Total g s (mol m -2 s -1 )

0.92r0.08 0.44r0.07 0.57r0.12

Calvin cycle

Decreases in photosynthesis on exposure of plants grown in the absence of UV-B
to high UV-B doses have been attributed to deactivation or a loss of Rubisco activity 19 .
Wilson et al. 20 reported the appearance of a 66 kDa protein during UV-B exposure in
oilseed rape, pea, tomato and tobacco which they attributed to a photomodified form of
the large subunit of Rubisco. Levels of mRNA coding for both the large (LSU, 55 kDa)
and small (SSU, 14.5 kDa) submits of Rubisco have been also reported to decline 2 .
Furthermore, decline in the maximum rate of electron transport contributing to RuBP
regeneration also occurred in UV-B-irradiated leaves (Table I). Since PSII was
excluded as a primary factor limiting CO 2 assimilation during UV-B exposure, other
Calvin cycle enzymes must be involved in the photosynthetic decreases by UV-B 21 .
Allen et al. 1 were the first to show large reductions in the content of sedoheptulose-1,7-
bisphosphatase (SBPase), a key regulatory enzyme in the Calvin cycle.
The process by which UV-B radiation induces the loss of some Calvin cycle
enzymes is unknown. The selective loss of specific enzymes suggest that this
mechanism is not a non-specific oxidation of leaf proteins resulting from UV-B
absorption by these proteins, or a UV-B-induced production of reactive oxygen species,
unless these particular proteins are not easily repaired or are relatively unprotected by