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 137

determine total leaf area of plants, leaf size and number. Gonzalez et al. 24 and Nogués et
al. 14 demonstrated that UV-B decreases leaf cell number and cell division. Repair of
UV-B damage to DNA before replication and direct UV-B-induced oxidation of tubulin,
delaying microtubule formation 25 have been suggested as mechanisms for direct slowing
of cell division. UV-B may also affect the key stages of cell division through
transcriptional repression of the genes encoding for a mitotic cyclin and a p34 cdc2 protein
kinase 26 . Reductions in leaf area and cell division in all measured cell types were
observed at all stages of leaf development with the effects becoming more pronounced
approaching full expansion 14 . This indicates it is unlikely that UV-B acts directly on the
dividing cells because leaves at early stages were still enclosed by the folded bracts and
so would have experienced a very low UV-B exposure.
UV-B also inhibited cell expansion of the exposed surface of leaves. UV-B could
reduce cell expansion by changing turgor pressure or cell wall extensibility, and Tevini
and Iwanzik 27 suggested that direct oxidation of indole-acetic acid by UV-B reduces cell
wall expansion. Therefore, decreases in the ability of the pea plants to capture
photosynthetically active radiation are also a major factor in determining the reduction
in the photosynthetic productivity of these plants.
4. Field studies
Glasshouse experiments may overestimate the response of photosynthesis to
UV-B radiation. This is because such experiments frequently compare plants grown
under very high levels of UV-B against those at non-UV-B radiation, and/or are
conducted under low levels of UV-A and photosynthethically active photon flux density
(PPFD) compared to field studies 1 . Such conditions can be expected to increase plant
sensitivity to damage by UV-B.
To overcome these problems research effort was moved towards applying
enhanced UV-B in the field. In most instances supplementary UV-B was applied as a
fixed levels for a fixed number of hours each day (“square wave” systems). However,
this approach takes no account of variation in cloud cover, and enhanced levels were
frequently based on the predicted consequences of ozone depletion under clear sky
conditions at the summer 28 . A few recent studies have utilised filters to compare near
ambient UV-B with below ambient UV-B irradiances (“UV-B exclusion experiments”)
to identify whether current UV-B levels are detrimental to plants. However, exclusion
experiments do not enable predictions to be made about the impact of future UV-B
increases, in response to continued O 3 depletion. These problems can be overcome by
the use of modulated systems, which provide a proportional UV-B supplement by
modulating UV-B lamp output in proportional to ambient UV-B levels (“modulated
supplementation systems” 23,29 ).
When pea plants were grown in the field and subjected to a modulated 30%
increase in ambient summer UV-B radiation, no significant effects on photosynthesis
were found (Figure 1 23,29 ). Moreover, UV-B exposure had no significant effects on plant
biomass, leaf area or partitioning assimilates from shoots to roots (Table II 23 ). The only
UV-B effect in the field was an increase in leaf flavonoid content. This increase will
give protection against the deleterious effect of UV-B irradiation.