Introduction

Stratospheric ozone decreases have heightened the concern over the ecological implications of increasing solar ultraviolet-B (UV-B; 280 nm - 320 nm) radiation on agricultural production (Caldwell et al. 1998). Biologically effective UV (UV-Bbe) varies seasonally, geographically, and with changes in cloud cover. While long periods of low stratospheric ozone and corresponding high UV-B are unlikely, short-term events of 70 kJ m-2 - 80 kJ m-2 are likely to occur during the summer. This extremely high UV-B intensity, consisting of 2 to 4 days of sequential high exposure (Grant and Slusser, 2003), may affect large portions of cropland (Grant and Slusser, 2002).

The effects of solar UV-B on plants involve both acute and chronic exposures to moderate to high levels of UV-B radiation (Grant and Slusser, 2003). Depending on species, experimental conditions, and duration of UV-B exposure (Gwynn-Jones et al., 1999), there appears to be a variety of mechanisms through which UV-B affects growth and physiology of plants. Long-term exposure of plants to UV-B is known to inhibit photosynthesis (Nogues et al., 1998) through the inhibition of reaction centers in photosystem II which results in decreased plant productivity. Ultraviolet-B is also reported to inhibit ribulose 1,5-bisphosphate carboxylase/ oxygenase (rubisco) activity, ATP hydrolysis, and reduce thylakoid membrane proteins (Bornman, 1989; Jordan et al, 1991; Jordan et al., 1992). In a few cases, UV-B increased stomatal conductance in some Ericaceae (Musil and Wand, 1993), but not in others (Teramura et al., 1983).

Different plant species and crop cultivars vary in their tolerance to UV-B and appear to respond to UV-B stress in a variety of ways (Murali et al., 1988; Kankani et al., 2003). Some plants tolerate UV-B through the production of photosynthetic and protective pigments and through changes in electron transport capability (Teramura, 1983; He et al., 1994; Teramura and Sullivan, 1994). Some long-term UV-B effects on plants are known; however, little work has been done to examine the transient UV-B effects on plants, in particular on soybeans. Studying the immediate responses of plants during short-term UV-B exposure may further our understanding of the mechanisms of UV-B damage and tolerance in plants. Soybean cultivars responded differently to prolonged UV-B exposure, with Essex being the most sensitive to UV-B while Williams 82 was the least affected (Murali et al., 1988).

Pigments are integrally related to the physiological function of leaves. Recently, leaf reflectance has been regarded as a good measure of pigment contents (Gamon and Surfus, 1999; Sims and Gamon, 2002). Changes in leaf reflectance due to stress factors are likely due to metabolic disturbance on chlorophyll concentrations (Knipling, 1970; Gamon and Surfus, 1999). Leaf reflectance measurements have been used in several UV-B studies (Bornman and Vogelmann, 1991; Visser et al., 1997; Qi et al., 2003; Kakani et al., 2004). However, little is known about how these different levels of UV-B and duration of exposure affect leaf spectral reflectance in soybeans.

Most plant species are thought to have photoreactivation or photorepair mechanisms to deal with elevated UV-B intensity (Beggs et al., 1986). These mechanisms include production of UV-B absorbing compounds (Murali and Teramura, 1986) and photosynthetic pigments (Bornman and Vogelman, 1991) that are correlated with UV-B tolerance. The UV-B absorbing compounds are known to reduce the level of damaging effects of UV-B radiation reaching the sensitive organelles in mesophyll tissues (Tevini et al., 1991; Teramura and Sullivan, 1994). The production of UV- protective compounds in plants is dependent on the availability of carbohydrates from photosynthesis and storage (Matsuki, 1996). Therefore, provision of resources may be crucial to maintain protection from, and repair following, UV-B exposure.

It has been reported that the effects of UV-B on soybeans are more injurious during early stages of seedling growth (Murali and Teramura, 1986). Additionally, the synthesis of UV-B absorbing compounds and alterations in photosynthetic pigments are one of the immediate responses to UV-B exposure (Jordan et al., 1994; Battaglia and Brennan, 2000). Essex and Williams 82 cultivars were chosen in the present study because they have shown contrasting UV-B sensitivities in many studies (Tevini, 1993). We hypothesized that UV-B tolerance in Williams 82 is conferred by its ability to accumulate UV-B absorbing compounds and photosynthetic pigments.

The present study examined the short-term effects of UV exposure on stomatal conductance (gs), concentrations of photosynthetic pigments and UV-B absorbing compounds, photosynthetic carbon fixation (A), transpiration (E), and photosystem II quantum efficiency (0pSn) in William 82 and Essex cultivars (Table 16.1) growing under greenhouse and field conditions. This study is one of the few documenting short-term effects of UV radiation on the physiology of soybean cultivars. Our objective was to build on the work of Bawhey et al. (2003) regarding the mechanisms of injury and tolerance of soybeans to UV-B exposures.

Table 16.1 Experimental conditions and physiological measurements

Experiment

Mean exposure rates at plant leaf height

Exposure duration

Cultivar(s) and phonological stage*

Physiological measurements made

Greenhouse, Spring 2004

UV exposure UV-Bbe: 7.8 kJ m 2 d 1 UV-A: 4.1 MJ m 2 d 1 PAR: 18.7 mol m 2d 1 Control

UV-A: 0.45 MJm 2 d 1 PAR: 25.3 mol m 2d 1

6h/d for 3

days

W82 and Essex at V2

gs, Pigments and

UV-absorbing compounds

(Continued)

Experiment

Mean exposure rates at Exposure plant leaf height duration

Cultivar(s) and phonological stage*

Physiological measurements made

Greenhouse, UV exposure Winter 2007 UV-Bbe: 12 kJ m 2 d 1 UV-A: 6.3 MJ m 2 d 1 PAR: 17.4 mol m 2 d 1 Control

UV-A: 0.54 MJ m 2 d 1 PAR: 22.7 mol m 2 d 1 Greenhouse, UV exposure Winter 2008 UV-Bbe: 12 kJ m 2 d 1 UV-A: 6.3 MJ m 2 d 1 PAR: 15.9 mol m 2 d 1 Control

UV-A: 0.57 MJ m 2 d 1 PAR: 19.7 mol m 2 d 1 Field, UV exposure

Summer 2004 UV-Bbe: 4.6 kJ m 2 d 1 UV-A: 2.5 MJ m 2 d 1 PAR: 62 mol m 2 d 1 Control

8 h/d for 6 days ambient gs, E, A, PSII efficiency

Essex at VC, V1, gs, E, A, PSII V2, V3, V4 efficiency

Essex, W82

gs, E, A, Pigments and UV-absorbing compounds

* The following growth and development stages are described in detail in the measurement section.

Was this article helpful?

0 0

Post a comment