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Initial tests s h o w e d t h a t t h e f i v e s u g a r b e e t cultivars w e r e n o t m a r k e d visibly b y a 2 - h r o z o n e e x p o s u r e a t 2 5 p p h m ( p a r t s p e r h u n - d r e d million) b y v o l u m e . A c c o r d i n g l y , w e u s e d c o n c e n t r a t i o n s o f 4 0 , 5 5 , a n d 7 0 p p h m t o inflict o z o n e s y m p t o m s . Five p l a n t s e a c h o f U S H 6 , U S H 9 B , U S 4 0 1 , a n d SP6822-0-(P) w e r e e x p o s e d t o 4 0 p p h m o z o n e i n tests r e p e a t e d t h r e e t i m e s . T h e s a m e cultivars w e r e f u m i g a t e d a t 5 5 p p h m i n f o u r e x p e r i m e n t s t h a t t o t a l e d 1 8 p l a n t s o f e a c h . U S H 2 0 was a d d e d to t h e list in a t h i r d g r o u p of tests c o n s i s t i n g of 14 p l a n t s o f e a c h c u l t i v a r e x p o s e d a t 7 0 p p h m . Plants u s e d i n e a c h test h a d n o t b e e n e x p o s e d p r e v i o u s l y t o o z o n e . E x p o s u r e s w e r e d o n e for 2 h r a t 28°C, 6 0 t o 7 0 % relative h u m i d i t y , a n d 22.2 K-lux l i g h t i n g f r o m V H O f l u o r e s c e n t l a m p s . E s t i m a t e s o f o z o n e injury w e r e m a d e b y critical e x a m i n a t i o n of b o t h s u r f a c e s of all leaves. Scores f r o m 0 to 10, b a s e d o n t h e relative severity o f s y m p t o m s , w e r e given t o e a c h leaf, a n d t h e d a t a w e r e t o t a l e d for a p e r c e n t - i n j u r y s c o r e for e a c h p l a n t .
R e s u l t s
O z o n e injury to all of t h e s u g a r b e e t cultivars i n c r e a s e d as t h e o z o n e d o s a g e i n c r e a s e d f r o m 4 0 t o 7 0 p p h m ( T a b l e 1). U S H 6 a n d U S H 9 B s h o w e d less injury t h a n U S 4 0 1 a n d S P 6 8 2 2 - 0 - ( P ) . C u l t i v a r s d i f f e r e d significantly ( 1 % level) a t t h e 4 0 - p p h m r a t e , b u t n o t a t 5 5 a n d 7 0 p p h m . T h e s y n t h e t i c SP6822-0-(P) was t h e m o s t o z o n e - s u s c e p t i b l e cultivar a t t h e lowest a n d h i g h e s t o z o n e r a t e . H y b r i d s U S I I 6 a n d U S H 9 B t e n d e d t o b e m o r e t o l e r a n t t o o z o n e .
Table 1.—Estimated ozone injury (%) to sugarbeet cultivars fumigated for 2 hr at various ozone concentrations.
1Mean injury to 15 plants.
2Mean injury to 14 plants.
3Mean injury to 18 plants.
T h e r e l a t i o n s h i p b e t w e e n o z o n e s y m p t o m s a n d leaf m a t u r i t y s h o w e d t h a t m i d - p l a n t leaves w e r e m o r e sensitive t o o z o n e w h e n p l a n t s w e r e e x p o s e d a t 4 0 p p h m (Fig. 1). A t 5 5 p p h m o z o n e , t h e 3 o l d e s t leaves of SP6822-0-(P) d i s p l a y e d a p r o n o u n c e d i n c r e a s e in n e c r o t i c tissue a s c o m p a r e d t o t h e s a m e t h r e e leaves o f t h e o t h e r varieties. T h i s d i s p a r i t y d i m i n i s h e d a t 7 0 p p h m , b u t t h e y o u n g e s t leaves o f all cultivars t e n d e d t o r e m a i n m o r e o z o n e - t o l e r a n t . T h e illus- t r a t i o n o f injury i n r e l a t i o n t o l e a f a g e clearly s h o w s t h e b r e a k d o w n o f
Figure 1.—Relationship between ozone concentration and injury to sugarbeet leaves at various stages of development.
o z o n e t o l e r a n c e o f t h e two h y b r i d s a n d U S 4 0 1 a t 5 5 a n d 7 0 p p h m . S y m p t o m s o n U S 4 0 1 w e r e m o r e s e v e r e t h a n t h o s e s h o w n b y U S H 6 a n d U S H 9 B a t t h e s e r a t e s .
O z o n e s y m p t o m s o n t h e oldest leaves consisted o f b l e a c h e d p u n c - t a t e flecks s c a t t e r e d o v e r t h e u p p e r s u r f a c e s (Fig. 2). M i d - p l a n t leaves d i s p l a y e d a g l a z i n g or silvering of t h e l o w e r s u r f a c e , w h i c h led to a c h l o r o t i c , m o t t l e d a p p e a r a n c e o n t h e u p p e r s u r f a c e (Fig. 2). N o n e c r o - sis o f t h e u p p e r s u r f a c e was e v i d e n t , unless s y m p t o m s w e r e very s e v e r e . S y m p t o m s o n y o u n g e s t leaves a p p e a r e d a s flecks o n t h e u p p e r s u r f a c e n e a r t h e leaf tips. A waxy, shiny a p p e a r a n c e o f t e n d e v e l o p e d o n leaves t h a t a d a y l a t e r b e c a m e f l e c k e d o r glazed o r s o m e t i m e s r e m a i n e d visibly u n m a r k e d . S e v e r e , bifacial necrosis was p r e c e d e d by a very
V O L . 18, No. 1, APRIL 1974 83
Figure 2.—Ozone flecking on upper surface of young, expanding sugar- beet leaf (left), glazing of lower surface (left, center), and chlorosis or mottling (right, center) transmitted to the upper surface of mid-plant sugarbeet leaves. Lower surface of healthy leaf shown at right.
m a r k e d w i t h e r i n g a n d dessication o f leaves. T h e s e s y m p t o m s usually w e r e o b s e r v e d a t t h e c o n c l u s i o n o f o z o n e tests, especially t h o s e a t t h e h i g h e s t c o n c e n t r a t i o n . T h e s y n d r o m e o f injury d i d n o t differ a m o n g t h e f i v e cultivars.
O p e n s t o m a t a a r e essential for t h e u p t a k e o f o z o n e . Silicone r u b - b e r i m p r i n t s o f u p p e r a n d l o w e r leaf s u r f a c e s o f f o u r cultivars t a k e n a t t h e b e g i n n i n g o f o n e e x p e r i m e n t r e v e a l e d t h a t t h e s t o m a t a w e r e o p e n . S u b s e q u e n t o b s e r v a t i o n s s h o w e d t h a t s t o m a t a h a d closed a t t h e e n d o f t h e 2 - h r e x p o s u r e i n t e r v a l o f 5 5 p p h m o z o n e .
D i s c u s s i o n
T h i s i s t h e first r e c o r d e d a c c o u n t o f t h e effects o f c o n t r o l l e d o z o n e d o s a g e s o n d i v e r s e s u g a r b e e t cultivars. T h e lack o f i n f o r m a t i o n a b o u t t h e r e s p o n s e o f s u g a r b e e t s t o p h o t o c h e m i c a l s m o g s e e m s t o reflect t h e g e n e r a l view t h a t o x i d a n t s , i n c l u d i n g o z o n e , h a v e n o a d v e r s e effect o n s u g a r b e e t p r o d u c t i o n . P e r s o n a l c o m m u n i c a t i o n s f r o m J . S . M c F a r - l a n e , A . O . P a u l u s , a n d O . C . T a y l o r n o t e d o c c a s i o n a l u n p u b l i s h e d a c c o u n t s o f p h o t o c h e m i c a l s m o g d a m a g e t o s u g a r b e e t s i n t h e S o u t h C o a s t Basin o f S o u t h e r n C a l i f o r n i a . P a u l u s felt t h a t P A N o r o z o n e h a d
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VOL. 18, No. 1, APRIL 1974 85
done no damage economically to sugarbeets, and that most of the crop was grown outside the air-pollution areas of the state. Taylor's many field observations led to his conclusion that sugarbeets were quite susceptible to PAN, but he was never sure that ozone caused much leaf injury. Anatomical differences and light dependency are reasons cited for the distinction between plant tissues shown by PAN and ozone (7).
Our tests show that the varietal responses of sugarbeets to various ozone concentrations are similar to the varietal characteristics of several other crop species tested for ozone resistance. Ozone usually is more injurious to physiologically mature leaves than to developing, expanding leaves. Ting and Mukerji (8) attributed this sensitivity of ozone to the existence of relatively low levels of soluble constituents in mature leaves. Reducing substances such as ascorbic acid, glutathi- one, and sulfhydryl groups, in sufficient amounts, presumably counter the oxidizing action of ozone. Photosynthetically active tissues usually contain more of these substances. Occasional ozone-pollution episodes with concentrations as high as 50 pphm for 2 hr may temporarily slow the growth processes of sensitive sugarbeet varieties by killing large numbers of cells or by inducing momentary closure of stomata. Hill and Littlefield (4) observed a temporary closure of sugarbeet stomata and a reduced rate of photosynthesis of plants exposed to 65 pphm ozone.
T h e hybrids USH6 and USH9B originated in California several years ago. Their more tolerant ozone response in our tests probably is a coincidence, although some selection toward ozone-tolerance may have been practiced because of the presence of photochemical air pollution in many areas of the state. The "synthetic" cultivar SP-6822- 0-(P), developed by G. E. Coe at Beltsville, was bred for resistance to leaf spot (Cercospora beticola Sacc.) and black root (Aphanomyces cochli-
oides Drechsi). Tolerance to smog was not a factor in the breeding ofSP6822-0-(P), because we have witnessed only one event toxic to sugarbeets at Beltsville in the past 15 years. T h e hybrid USH20 con- tains the synthetic SP6322-0, a close relative of SP6822-0-(P), as one of its progenitors. This sibling relationship may partially explain why USH20 was not one of the more ozone-tolerant hybrids.
Ozone generally injures the upper surfaces of leaves, while PAN
and ozonated-hydrocarbons (HC) damage lower surfaces. T h e princi-
pal action sites for PAN are found in young, developing leaves (7), but
ozonated-HC, like ozone, injures the mature and somewhat older
leaves (3). We observed ozone injury on lower surfaces of mature
leaves and typical flecking on other leaves. T h e field diagnosis of smog
injury to sugarbeets perhaps would be easier if PAN were the only
pollutant, but to distinguish between ozone and ozonated-HC injury
is not possible, because both toxicants cause similar syndromes.
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S u m m a r y
Five s u g a r b e e t cultivars e x p o s e d t o c o n t r o l l e d o z o n e rates o f 4 0 , 5 5 , a n d 7 0 p p h m s h o w e d v a r i o u s d e g r e e s o f leaf injury. T w o - h o u r f u m i g a t i o n s disclosed t h a t t h e s y n t h e t i c variety ' S P 6 8 2 2 - 0 - ( P ) ' was m o r e o z o n e - s u s c e p t i b l e a t 4 0 p p h m t h a n t h e m o n o g e r m h y b r i d s ' U S H 6 , ' ' U S H 9 B , ' a n d m u l t i g e r m , o p e n p o l l i n a t e d ' U S 4 0 1 ' ; a n d a t 7 0 p p h m , SP6822-0-(P) also s h o w e d t h e m o s t e x t e n s i v e o z o n e injury.
H y b r i d s U S H 6 a n d U S H 9 B w e r e a little m o r e o z o n e - t o l e r a n t t h a n t h e o t h e r cultivars. T h e a v e r a g e injury t o all c u l t i v a r s i n c r e a s e d f r o m a b o u t 10% a t 4 0 p p h m t o 5 5 % a t 7 0 p p h m o z o n e . Basal a n d m i d - p l a n t leaves w e r e m u c h m o r e sensitive t o o z o n e t h a n y o u n g , d e v e l o p i n g leaves. O z o n e c a u s e d flecking o n t h e y o u n g e s t leaves, a n d g l a z i n g o r silvering o f t h e l o w e r s u r f a c e s o f m i d - p l a n t leaves. T h e silver-leaf a p p e a r a n c e r e s e m b l e d t h e injury s y n d r o m e associated w i t h o z o n a t e d - h y d r o c a r b o n s a n d p e r o x y a c e t y l n i t r a t e ( P A N ) .
A c k n o w l e d g m e n t
I t h a n k D r . G. E. C o e for his t e c h n i c a l assistance a n d advice in c o n d u c t i n g this s t u d y , a n d D r . J . S . M c F a r l a n e for his criticisms a n d s u g g e s t i o n s i n p r e p a r i n g t h e m a n u s c r i p t .
Literature Cited
(1) BREWER, R. F., F. B. GUILLEMET, a n d R. K. CREVELING. 1961. Influence of N-P-K fertilization on incidence and severity of oxidant injury to mangels and spinach. Soil Sci. 92:298-301.
(2) HAAGEN-SMIT, A. J., E. F. DARLEY, M. ZAITLIN, H. HULL., and VV. NOBLE.
1952. Investigation of injury to plants from air pollution in the Los Angeles area. Plant Physiol. 27:18-34.
(3) HEGGESTAD, H. E., F. R. BURLESON, J. T. MIDDLETON, and E. F. DARLEY.
1964. Leaf injury on tobacco varieties resulting from ozone, ozonated hexene-1, and ambient air of metropolitan areas. Intern. J. Air Water Poll. 3:1-10.
(4) H I L L , A. C. and N. LITTLEFIELD. 1969. Ozone. Effect on apparent photo- synthesis, rate of transpiration, and stomatal closure in plants. F.nv. Sci.
and Tech. 3:52-56.
(5) MENSER, H. A., and H. E. HEGGESTAD. 1964. A facility for ozone fumiga- tion of plant materials. C r o p Sci. 4:103-105.
(6) MENSER, H . A., G. H. HODGES, and C. G. M C K E E . 1973. Effects of air pollution on Maryland (Type 32) tobacco. J. Environ. Qual. 2:253-258.
(7) TAYLOR, O. C. 1969. Importance of peroxyacetyl nitrate (PAN) as a phytotoxic air pollutant. J. Air Poll. Contr. Assoc. 19:347-351.
(8) T I N G , I. P., and S. K. MUKERJI. 1971. Leaf ontogeny as a factor in suscep- tibility to ozone : amino acid and carbohydrate changes d u r i n g expan- sion. Amer. J. Bot. 58:497-504.