JAMES V L A M I S AND A L B E R T U L R I C H1
Received for publication March 2, 1973
The extensive work of Eaton (4)
2established sugar beet plants as having a high requirement for boron and a high tolerance for boron excess. His work and that of others indicates that plant tissues in the deficiency range tend to run about 10 to 35 parts per million boron (1, 7, 12).
In the toxic range Eaton showed that the tops and laminae of sugar beets could contain hundreds of ppm B without affecting yield very markedly.
This work presents the results of experiments designed to study the growth of sugar beets in solutions of high concentrations of B and to correlate growth with the B content of selected tissues. A previous report covered the B nutrition of sugar beets in the deficiency range (12).
Materials and Methods Plant culture
Sugar beet seeds were planted in vermiculite and irrigated with a nutrient solution containing 0.25 ppm B (Table 1). The seeds used (F58-554H1-MS of NB1 X NB4) were previously treated with a fun- gicide, Phygon XL, at a rate of 1 gram per 100 grams of seed.
Eighteen days later the seedlings were each wrapped with Dacron fiber and transferred to cork rings. Three seedlings were placed in
Table 1.—Chemical composition of nutrient solution added at two intervals.
Salts added, meq/1 Microelements, mg/1
Ca(N03)2 5.0 Mn, as M n S 04 • 4 H2O 0.25
KNO3 2.5 Zn, as ZnSO* • 7 H 2 O 0.025
M g S 04 2.0 Cu, as C u S 0 4 • 5 HzO 0.010
KH2PO4 1.0 Mo, as M0O3 0.005
K2SO4 0.5 Fe, as E D T A 2.5
N a C l 0.5 B o r o n (see T a b l e 2)
1Plant Physiologists, D e p a r t m e n t of Soils a n d Plant Nutrition, University of California, Berkeley.
2N u m b e r s in p a r e n t h e s e s refer to literature cited.
V O L . 17, N o . 3, APRIL 1973 2 8 1 p e r f o r a t e d m a s o n i t e c o v e r s m o u n t e d o n 5-gallon c a n s p a i n t e d with a l u m i n u m p a i n t o n t h e o u t s i d e a n d A m e r c o a t N o . 3 3 o n t h e i n s i d e . T h e m a s o n i t e lids w e r e p a i n t e d w i t h a l u m i n u m o n t h e u p p e r s u r f a c e a n d V a l s p a r u n d e r n e a t h . T h e c o n t a i n e r s w e r e filled with n u t r i e n t s o l u t i o n s with c o m p o s i t i o n s as g i v e n in T a b l e 1.
B o r o n w a s a d d e d t o t h e s o l u t i o n s s o a s t o give t h e following r a n g e of B in p p m : 0 . 5 , 2, 4, 8, 16, 3 2 , 6 4 , a n d 128 ( T a b l e 2). In two of t h e e x p e r i m e n t s a 1 p p m B s o l u t i o n was i n c l u d e d . T h r e e r e p l i c a t e s of e a c h r a t e w e r e set u p . T h e e x p e r i m e n t a l p e r i o d for t h e m a i n e x p e r i m e n t e x t e n d e d f r o m A p r i l 2 6 t o J u n e 8 . M i d w a y i n this p e r i o d a n o t h e r d o s e o f n u t r i e n t s was a d d e d t o e a c h p o t , b u t w i t h o u t a f u r t h e r a d d i t i o n o f B .
T h e test was c o n d u c t e d i n a s m o g - f r e e c l e a r glass g r e e n h o u s e . T h e n u t r i e n t s o l u t i o n s w e r e k e p t a t a p H o f 5.0 t o 6 . 5 , a n d distilled w a t e r w a s a d d e d a s n e e d e d .
Harvesting
T h e p l a n t s w e r e h a r v e s t e d 6 w e e k s a f t e r t r a n s p l a n t i n g . F r e s h w e i g h t s w e r e t a k e n o f t h e s h o o t s a n d r o o t s s e p a r a t e l y . T h e leaves w e r e d i v i d e d i n t o t h r e e a g e g r o u p s — y o u n g , m a t u r e , a n d o l d . L e a v e s w h i c h h a d n o t r e a c h e d full d e v e l o p m e n t w e r e classified y o u n g , a n d t h o s e s h o w i n g signs o f s e n e s c e n c e w e r e p l a c e d i n t h e old c a t e g o r y . T h e i n t e r m e d i a t e leaves w e r e called m a t u r e . T h e leaves w e r e t h e n s e p a - r a t e d i n t o b l a d e s a n d p e t i o l e s for b o r o n analysis. T h e r e s i d u e c o n s i s t e d o f d e a d o r wilted leaves. T h e f i b r o u s r o o t s w e r e r e m o v e d f r o m t h e s t o r a g e r o o t , c e n t r i f u g e d t o r e m o v e f r e e w a t e r , a n d fresh w e i g h t s t a k e n .
Preparation of samples and chemical analysis
T h e p l a n t p a r t s w e r e p l a c e d i n p a p e r bags a n d d r i e d a t 70°C i n a f o r c e d - d r a f t o v e n f o r 7 2 h o u r s . T h e d r i e d m a t e r i a l was w e i g h e d , g r o u n d in a Wiley mill w i t h a 4 0 - m e s h s c r e e n , a n d s t o r e d in plastic vials.
T h e s a m p l e s w e r e a n a l y z e d for b o r o n a t r o o m t e m p e r a t u r e b y t h e c u r c u m i n m e t h o d , u s i n g a n a c e t a t e b u f f e r (6). All o t h e r a n a l y s e s w e r e m a d e b y e s t a b l i s h e d m e t h o d s (2, 8).
R e s u l t s Boron excess and yield
T h e d a t a o b t a i n e d a r e s h o w n i n T a b l e 2 a n d f i g u r e 1 . T h e g r o w t h c u r v e f o r t h e t o p s s h o w s t h a t m a x i m u m yields w e r e o b t a i n e d a t 0 . 5 , 2 , 4, a n d 8 p p m B in t h e e x t e r n a l s o l u t i o n . T h i s is followed by a s t e e p d r o p a t 1 6 p p m a n d t h e n a m o r e g r a d u a l d e c l i n e i n yield for t h e r e m a i n d e r o f t h e c u r v e . T h e b e e t c u r v e follows t h e o n e for t h e t o p s e x c e p t for a small shift t o t h e left. T h u s , t h e first significant d r o p i n b e e t yield o c c u r s a t 8 p p m a s o p p o s e d t o 1 6 p p m f o r t h e t o p s . T h e f i b r o u s r o o t r e s p o n s e r e s e m b l e s v e r y closely t h a t o f t h e t o p s .
Table 2.—Growth of tops and roots of sugar beet plants.
Boron Hoagland
rate 1 4 8 16 32 64 128 256
supply Boron,
PPm
0.5 2.0 4.0 8.0 16.0 32.0 64.0 128.0
_ Symptoms
none none none trace slight severe severe acute
Tops dry weight,
g 37.8 38.0 38.2 37.6 26.1 17.0 7.1 0.9
Fibrous roots dry weight,
g 4.37 4.29 3.83 4.25 2.83 1.71 0.74 0.30
Beet roots Fresh weight,
g 155 143 141 128 80 36 11
—
% Sucrose
8.5 8.2 7.9 8.1
- -
— —
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V O L . 17, N o . 3, A P R I L 1973 2 8 3
SOLUTION BORON PPM
Figure 1.—Relative yields of tops and roots of sugarbeets as a function of parts per million boron in nutrient solution. The vertical scale for fibrous roots may be gauged by the arrow indicating 50% yield.
Toxicity symptoms
T h e r e w e r e n o o b s e r v a b l e s y m p t o m s o f toxicity i n t h e f o u r lowest B r a t e s . T h e f i r s t i n d i c a t i o n o f toxicity a p p e a r e d i n t h e s u g a r b e e t s g r o w n i n 1 6 p p m B o r h i g h e r a b o u t 1 0 d a y s after t r a n s p l a n t i n g . A t this c o n c e n t r a t i o n t h e e d g e s o f t h e o l d leaves c u r l e d i n w a r d a n d a mosaic p a t t e r n o f c h l o r o s i s a p p e a r e d o n t h e leaf b l a d e s . T h e y o u n g a n d m a t u r e l e a v e s w e r e n o r m a l . A t 3 2 p p m B t h e s y m p t o m s b e c a m e m o r e p r o n o u n c e d a n d a f f e c t e d s o m e o f t h e m a t u r e leaves a s well a s t h e old o n e s . I n a d d i t i o n t o t h e s e s y m p t o m s , b r o w n n e c r o t i c spots a p p e a r e d o n t h e tips o f t h e o l d leaves. 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 o f B all leaves s h o w e d s e v e r e c u r l i n g a n d m o s a i c c h l o r o s i s , a n d b r o w n s p o t t i n g h a d s p r e a d o v e r a g o o d p a r t o f t h e o l d leaves, w o r k i n g d o w n w a r d f r o m t h e tips a n d m a r g i n s t o w a r d t h e b a s e s o f t h e leaves (Fig. 2).
T h e r o o t s s h o w e d a d e c r e a s e i n size b u t w e r e o t h e r w i s e n o r m a l . T h e y s e e m e d h e a l t h y a n d s h o w e d n o sign o f n e c r o s i s , d i s c o l o r a t i o n , o r m o r p h o l o g i c a l d i s t o r t i o n s .
Boron content of tissues
T h e r e s u l t s o f t h e B analysis a r e p r e s e n t e d i n T a b l e 3 a n d F i g u r e 3 for t h e y o u n g , m a t u r e , a n d o l d b l a d e s . I n all t r e a t m e n t s t h e old blades
2 8 4 JOURNAL OF THE A. S. S. B. T.
Figure 2.—On the left, leaf series of one plant grown in 64 ppm B showing severe symptoms on old leaves. On the right, closeup of second oldest leaf with severe symptoms of B toxicity.
PPM BORON IN BLADES
Figure 3.—Relation of top dry weights to boron content of young, ma- ture, and old blades.
VOL. 17, No. 3, APRIL 1973
Table 3.—Boron content in ppm of petioles, blades, and roots of sugar beets.
Boron solution
0.5 2.0 4.0 8.0 16.0 32.0 64.0 128.0
*Blades with distinct Tops dry weight, _
g 37.8 38.0 38.2 37.6 26.1 17.0 7.1 0.9 toxicity symptoms.
Young 55 77 129 256 333 631 965*
-
Blades Mature 57 97 173 422 551 1002*
1810*
-
Old 66 138 288 655 853*
1633*
2570*
-
Young 33 26 34 47 65 101 145
—
Petioles Mature 33 37 44 55 71 102 181
-
Old 30 35 47 56 72 107 201
-
Roots Fibrous
40 46 64 80 77 101 187
-
Storage 46 47 57 64
—
—
—
-
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have t h e highest B c o n t e n t a n d y o u n g blades h a v e t h e lowest. At t h e inflection of t h e c u r v e , or w h e r e injury has b e c o m e first manifest (at a b o u t a 10% r e d u c t i o n in t o p g r o w t h ) , t h e old blades have a B c o n t e n t o f over 6 0 0 p p m , t h e m a t u r e blades a b o u t 4 5 0 p p m , a n d t h e y o u n g blades 3 0 0 p p m .
In T a b l e 3 a n d F i g u r e 4 a r e t h e c o m p a r a b l e results for t h e petioles.
T h e B c o n t e n t s o f t h e s e tissues a r e u n i f o r m l y m u c h lower t h a n t h e blades. T h e d a t a for fibrous roots in T a b l e 3 r e s e m b l e t h e values o b t a i n e d for t h e petioles. T h e s a m e is t r u e of t h e beets.
Figure 4.—Relation of top dry weights to boron content of young, ma- ture, and old petioles.
Beet sugar content
T h e sucrose c o n t e n t of t h e beets was n o t affected as l o n g as t h e B in solution was u n d e r 8 p p m ( T a b l e 2). T h e s u g a r c o n t e n t is generally at a low level because t h e plants w e r e h a r v e s t e d at a relatively early stage a n d t h e solutions w e r e still s u p p l y i n g n i t r a t e to t h e plants.
D i s c u s s i o n
T h e results o f these e x p e r i m e n t s show t h a t s u g a r beets can toler- ate c o n c e n t r a t i o n s of B as h i g h as 8 p p m in c u l t u r e solutions w i t h o u t a serious r e d u c t i o n in yield. By i n t e r p o l a t i o n we find t h a t t h e 5 0 % r e d u c t i o n in yield of tops a n d fibrous r o o t s is r e a c h e d w h e n t h e e x t e r - nal B c o n c e n t r a t i o n is a b o u t 28 p p m . F o r t h e beet r o o t s this value is
VOL. 17, No. 3, APRIL 1973 287
closer to 16 ppm. This puts the sugar beet plant in the category of plants tolerant of excess B in comparison with many cultivated plants (11). In the extensive work by Eaton (4) several varieties of sugar beet were all found to be fairly tolerant of high B in the external solution. A recent study by El-Sheikh et al., (5) showed that cucumber, squash, melon, and corn had a 50% reduction in growth with 6, 12, 12, and 16 ppm B in solution, respectively.
In the case of sugar beets the focus has to be on the beet root more than on the shoot. T h e first suggestion of a significant decrease in beet yield is at 8 ppm B. T h e tissue analysis, however, shows that there was very little B accumulation in either the beet or the fibrous roots.
Therefore, the reason the beet yield dropped at a lower B concentra- tion than the tops must be an indirect one. T h e photosynthetic rate must be adversely affected before the yield of the shoots is decreased so that there is enough sugar manufactured for the growth of the tops and fibrous roots but not enough for the storage root.
T h e boron analysis of the tissues indicates that the yield and the appearance of the leaves are not affected until the B content of the old leaves reaches 600 ppm. At the same point the mature leaves have 450 and the young leaves 300 ppm, both without symptoms. Brandenburg showed that B content increases with the physiological age of the tissues (1). In this instance, since the injury first affects the old leaves, it is preferable to use the B content of these tissues as an indication of the status of sugar beet plants from the toxicity point of view.
It is of economic interest that sugar beets tolerate high B since there are many areas where high boron occurs in soils or irrigation waters. These areas occur primarily in the western parts of this country (3,9, 10) where sugar beets should be given preference over crops with a greater sensitivity to high B situations.
Summary