R e c e i v e d for Publication November 1, 1 9 8 1 INTRODUCTION
Sucrose concentration of s u g a r b e e t s (Beta vulgaris L.) grown in the U.S. v a r i e s over a wide r a n g e of 10 to 20 p e r c e n t . Within a climatic zone such as southern I d a h o , sucrose concen- t r a t i o n v a r i e s over a narrower but still wide r a n g e of 14 to 20 p e r c e n t . This v a r i a t i o n in sucrose concentration is due to many factors t h a t include v a r i e t y (19, 24, 26), nitrogen (N) level (18, 23), growth p a t t e r n s of the crop (3, 16, 25, 29), climatic conditions ( 1 , 22, 28), and other factors t h a t are not fully understood. Refined sucrose production is based on the product of root yield and e x t r a c t a b l e sucrose c o n c e n t r a t i o n . Therefore, it is of prime importance to have p r a c t i c e s and c o n d i - tions t h a t provide a d e q u a t e root growth while m a i n t a i n i n g suf- ficiently high sucrose percentages and p u r i t y for profitable s u - crose extraction and y i e l d .
Sugarbeet q u a l i t y , mainly due to sucrose c o n c e n t r a t i o n , h a s been s t e a d i l y decreasing in southern Idaho as well as other sugarbeet growing a r e a s of the U.S. since the e a r l y 1950's ( 2 ) . This decrease in beet root q u a l i t y h a s accompanied an i n c r e a s e and in some c a s e s , excessive use of N fertilizer for the growth of t h i s crop (14, 15). However, within seasons between a d j a - cent fields where the total a v a i l a b l e soil and fertilizer N were s i m i l a r , l a r g e differences have been measured in sucrose concen- t r a t i o n and root q u a l i t y . These differences in the q u a l i t y f a c - tors may be due to differences in time and amount of N u p t a k e , i r r i g a t i o n l e v e l s , c u l t u r a l p r a c t i c e s or other unknown r e a s o n s . A better u n d e r s t a n d i n g of the reason for t h i s decrease in s u c -
* Contribution from the U.S. Department of Agriculture, Agricultural Research Service; in cooperation with the University of Idaho Col- lege of Agriculture Research and Extension Center, Kimberly, Idaho.
The author is Soil S c i e n t i s t , Snake Fiver Conservation Research Center, Kimberly, ID 83341.
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rose concentration and q u a l i t y is needed so t h a t a r e v e r s a l of this trend can be achieved with increased production efficiency t h a t will economically benefit the consumer, producer, and manu- f a c t u r e r .
The objective of this study was, by the use of d a t a col- lected at several locations in southern Idaho since 1966, to identify and e v a l u a t e the effect of factors and conditions t h a t significantly affect sucrose concentration and root q u a l i t y such as N a p p l i c a t i o n , N u p t a k e , i r r i g a t i o n level, location, y e a r , and growth p a t t e r n s during the growing season.
MATERIALS AND METHODS
Experiments on s u g a r b e e t s have been conducted since 1966 by scientists located at Kimberly, with experimental plots at several locations in southern I d a h o . The procedures used in these experiments have been published in numerous a r t i c l e s since the initiation of these s t u d i e s . The specific procedures used for each of these experiments can be found in the a r t i c l e for the y e a r : 1966 (6), 1967 (6), 1968 (7, 8 ) , 1969 (7), 1971 (15), 1972 (14), 1976 (12), 1977 (11), 1978 (9), 1979 (13), and 1980 ( 5 ) . These experiments were conducted on Portneuf silt loam soil (Durixerollic Calciorthids; c o a r s e - s i l t y , mixed, mesic) with the exception of some of the plot a r e a s in the 1971 and 1972 s t u d i e s .
Most of the agronomic p r a c t i c e s such as v a r i e t y , p l a n t i n g d a t e , and i r r i g a t i o n level were r a t h e r uniform between y e a r s . However, v a r i a t i o n in these p r a c t i c e s t h a t caused significant changes in the sugarbeet growth and yield components are given in the t a b l e s , figures, or the discussion of t h i s information.
In most c a s e s , the s u g a r b e e t s (Amalgamated AH10) were planted in e a r l y to mid-April in either 56 or 61 cm rows and were thinned to a 23 to 30 cm spacing in e a r l y June. P r e p l a n t and mid-June N applications were applied as a broadcast and sidedress a p p l i c a t i o n , r e s p e c t i v e l y , as ammonium n i t r a t e . Later a p p l i c a t i o n s of N were b r o a d c a s t as urea and moved into the soil with s p r i n k l e r i r r i g a t i o n .
Alternate furrow (every other furrow and a l t e r n a t i n g
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furrows at each i r r i g a t i o n ) or s p r i n k l e r i r r i g a t i o n were u s e d . Previous experiments (.10) h a v e i n d i c a t e d t h a t the i r r i g a t i o n method (furrow or s p r i n k l e r ) h a d l i t t l e effect on root a n d s u c - rose y i e l d s . Experimental a r e a s were a d e q u a t e l y i r r i g a t e d (20) except where deficit i r r i g a t i o n was i n t e n t i o n a l l y imposed.
The s u g a r b e e t s were h a r v e s t e d d u r i n g the season and in October by t a k i n g 3-m row l e n g t h s or by mechanically h a r v e s - ting l a r g e r a r e a s of each plot at final h a r v e s t in October. All beet roots were a d e q u a t e l y crowned before d u p l i c a t e or t r i p l i c a t e root samples (16 to 18 roots per sample) were taken for p u r i t y and sucrose a n a l y s e s . The sucrose concentration in the beet roots was determined by the Amalgamated Sugar Company ( 9 ) . The beet t o p , root, and crown samples were dried at 65 C and t h e i r d r y weights determined. The dried samples were ground to p a s s through a 40-mesh sieve and t o t a l N was d e t e r - mined by the macro- or semimicro- Kjeldahl procedure ( 4 ) , both modified to include n i t r a t e . Nitrogen u p t a k e was estimated by assuming t h a t the N concentration was the same in both the fibrous and storage roots and the weight of the u n h a r v e s t e d fibrous roots was e q u a l to 25% of the total h a r v e s t e d s t o r a g e root weight (21).
RESULTS AND DISCUSSION
Total N uptake by the s u g a r b e e t crop at h a r v e s t was l i n e a r l y r e l a t e d to the total a v a i l a b l e N (NT) t h a t was v a r i e d by p r e p l a n t and seasonal N fertilizer addition at one location in 1977 (Figure 1), and in 1976. This r e l a t i o n s h i p also a p p l i e d when NT was v a r i e d by p a s t N fertilizer management (14, 15).
I n c r e a s i n g the N a v a i l a b l e to the s u g a r b e e t p l a n t from r e s i d u a l sources or by N addition at any s t a g e of p l a n t growth i n c r e a s e d the p l a n t p a r t N content and the amount of N u p t a k e . Mid-June and mid-July a p p l i c a t i o n s g e n e r a l l y i n c r e a s e d the efficiency and amount of N u p t a k e by the p l a n t as compared with similar amounts a p p l i e d p r e p l a n t . This was p r o b a b l y a r e s u l t of m i n i - mizing the time between N a p p l i c a t i o n and N u p t a k e by s u g a r - beets which allowed less o p p o r t u n i t y for N to be leached out of the root zone, d e n i t r i f i e d , or incorporated into the soil micro- organisms a n d t h e i r b y - p r o d u c t s . Mid-August N a d d i t i o n at
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F i g u r e 1. E f f e c t of t o t a l , a v a i l a b l e N (NT) on t o t a l N u p t a k e ( r = 0 . 9 9 , 0 , 9 8 , 0 . 9 9 , 0 . 9 6 f o r p r e p l a n t , m i d - J u n e , m i d - J u l y , mid-August N a p p l i c a t i o n s , r e s p e c t i v e l y ) . Arrow i n d i - c a t e s maximum s u c r o s e y i e l d f o r p r e p l a n t N .
higher N r a t e s decreased the N fertilizer efficiency (Ef) and N uptake below those of other treatments which was probably- due to the short time period between N application and cessation of active p l a n t growth.
The wet root (Figure 2A) and dry root (Figure 2B) sucrose concentrations and root percent dry matter (Figure 2C) at h a r - vest were l i n e a r l y decreased with increased total N uptake by the p l a n t s that was v a r i e d by p r e p l a n t and seasonal N a d d i - t i o n s . Applying N during the growing season and subsequent late season N uptake reduced sucrose percentage more than did the N preplant a p p l i c a t i o n . This was p a r t i c u l a r l y noticeable from the mid-August N a p p l i c a t i o n . The assumption can be made t h a t the decrease in sucrose concentration in the wet roots with N addition and N uptake by the p l a n t was caused p r i m a r i l y by a decrease in the dry matter concentration of the roots and by a lesser but still important decrease in the sucrose concen- t r a t i o n in the dry matter. Therefore, t h i s relationship between N uptake level and dry matter concentration was associated with a strong positive r e l a t i o n s h i p between percent dry matter and sucrose concentration in the beet roots (Figure 2D).
Sucrose concentration of the dry roots (Figure 3A, B) a n d wet roots (Figure 3C, D) during the season was dependent upon the N fertility level and time the N was taken up by the s u g a r - beet. I n c r e a s i n g the N uptake by N fertilizer addition e i t h e r p r e p l a n t or during the season, g e n e r a l l y decreased both the s u c -
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F i g u r e 2 . E f f e c t of: A ) T o t a l N u p t a k e o n s u c r o s e p e r c e n t a g e i n t h e wet r o o t ( r = 0 . 9 4 , 0 . 9 1 , 0 . 9 5 , 0 . 9 2 ) + , B ) T o t a l N u p t a k e o n s u c r o s e p e r c e n t a t e i n t h e d r y r o o t ( r = 0 . 7 7 , 0 . 6 6 , 0 . 6 4 , 0 . 6 4 ) + , C ) T o t a l N u p t a k e o n p e r c e n t r o o t d r y m a t t e r ( r = 0 . 8 7 , 0 . 8 5 , 0 . 8 8 , 0 . 8 8 ) + , a n d D ) P e r c e n t r o o t d r y m a t t e r o n s u c r o s e p e r c e n t a g e i n t h e w e t r o o t ( r = 0 . 9 2 , 0 . 8 8 , 0 . 8 4 , 0 . 9 0 ) + . + r v a l u e f o r p r e p l a n t , m i d - J u n e , m i d - J u l y , and m i d - A u g u s t N a p - p l i c a t i o n s , r e s p e c t i v e l y . Arrows i n d i c a t e maximum s u c - r o s e y i e l d f o r p r e p l a n t N .
rose concentration in the wet and dry roots from the time of a p p l i c a t i o n until h a r v e s t . When 112 kg N/ha was a p p l i e d p r e - p l a n t , which was the r a t e for maximum sucrose y i e l d , the s u c - rose concentration of the wet and dry roots was at l e a s t e q u a l to t h a t of the check by the end of the season. However, s u c - rose concentration g e n e r a l l y was decreased by each i n c r e a s e in the N a p p l i c a t i o n r a t e and by each delay in its a p p l i c a t i o n time.
Sucrose concentration of the wet and d r y roots i n c r e a s e d most r a p i d l y d u r i n g June a n d July for the check and for a l l p r e p l a n t N t r e a t m e n t s . From l a t e July u n t i l h a r v e s t , the r a t e of i n c r e a s e in sucrose concentration of the wet roots was r a t h e r uniform when no N was a p p l i e d d u r i n g t h i s p e r i o d . The r a t e of i n c r e a s e in sucrose concentration of the beets fertilized at mid-
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( C , D ) a s a f f e c t e d b y t i m e o f s a m p l i n g , N f e r t i l i z e r l e v e l , a n d t i m e o f N a p p l i c a t i o n i n 1 9 7 7 .
season was g r e a t e r during the l a t t e r p a r t of the growing season than those fertilized e a r l i e r . This would indicate t h a t an e x - tended season through good weather conditions for plant growth during September and October in the intermountain area or delaying h a r v e s t as long as weather conditions permit should improve both sucrose yield and concentrations r e g a r d l e s s of the N n u t r i t i o n a l s t a t u s of the c r o p .
The decrease in the sucrose concentration with N addition p r e p l a n t or during the season was due to a decrease in the p e r - cent sucrose of the dry matter and to a decrease in dry matter concentration in the wet roots at a l l plant growth stages at one location in 1976, 1977, and 1978 (Table 1). A change in s u c - rose concentration between locations during the same season or between seasons was also due to a change in these two compon- ents in the beet root at h a r v e s t . Although sucrose concentration of the dry matter influenced the sucrose concentration in the wet roots, the major changes due to location-to-location, y e a r - to-year v a r i a t i o n or N uptake by the plant occurred because of a change in the dry matter concentration in the beet roots.
High sucrose concentration in the wet roots occurred when both
Table 1. Effect of N fertilizer level and time of sampling on dry matter and sucrose concentrations in sugarbeet roots in 1976, 1977, and 1978.
N Year Level
1976 Max. Suc. Yd.+
High N
Max. Suc. Yd.
1977 High N
Max. Suc. Yd.
1978 High N
+ Maximum sucrose yield.
Component
Dry Matter Sucrose, Dry‡
Sucrose, Wet§
Dry Matter Sucrose, Dry Sucrose, Wet Dry Matter Sucrose, Dry Sucrose, Wet Dry Matter Sucrose, Dry Sucrose, Wet Dry Matter Sucrose, Dry Sucrose, Wet Dry Matter Sucrose, Dry sucrose, Wet
‡ sucrose, %dry 5 July
16.9 59.3 10.0 15.3 54.1 8.3 14.7 54.1 7.9 14.0 49.1 6.9 14.2 54.2 7.7 13.3 53.9 7.2 wt. §
19 July
17.0 66.1 11.2 14.1 64.8 9.1 16.6 62.2 10.3 15.1 60.3 9.1 17.5 64.6 11.3 16.1 61.8 10.0 sucrose, %
2 Aug.
16.7 71.1 11.9 15.5 66.8 10.3 20.2 65.9 13.3 17.8 65.3 11.6 20.4 70.7 14.5 18.9 71.0 13.4 wet wt.
Date Sampl 16 Aug.
17.8 73.5 13.1 16.8 71.3 12.0 19.5 76.3 14.8 17.8 70.6 12.5 22.7 74.6 16.9 21.8 74.1 16.2
.ed (± 1 30 Aug
18.6 75.0 13.9 17.8 72.4 12.9 20.8 72.8 15.1 18.7 70.4 13.2 22.5 74.7 16.8 21.6 73.3 15.9
day) 13 Sept.
19.8 75.4 1.4.9 17.7 73.3 13.0 22.4 78.1 17.5 20.1 71.7 14.4 21.9 77.0 16.9 20.8 75.8 15.8
27 Sept.
22.2 71.8 15.9 19.3 72.8 14.1 23.5 78.3 1.8.4 21.9 72.4 15.9 22.6 77.3 17.5 21.6 77.0 16.6
18 Oct
22.4 76.6 17.1 20.8 74.8 15.6 25.2 75.6 19.0 23.5 71.5 16.8 24.3 77.6 18.8 23.0 75.2 17.3
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dry matter concentration and percent sucrose of the dry matter were high and low levels of sucrose were obtained when both of these components were low. This may be due to y e a r - t o - y e a r v a r i a t i o n or to treatment within any p a r t i c u l a r y e a r . Excessive or late additions of N fertilizer and uptake by the plants may make l a r g e decreases in sucrose concentration of the wet root because there is g e n e r a l l y a major reduction in both these com- ponents in the roots. Therefore, any treatment or agronomic practice that will maintain a high dry matter concentration and sucrose level of the dry matter will assure a high q u a l i t y beet root for processing.
Sucrose concentration of the wet roots was d r a m a t i c a l l y influenced by moisture s t r e s s of the p l a n t during the growing season and at h a r v e s t (Figure 4C, D). When i r r i g a t i o n water was a d e q u a t e , sucrose concentration increased most r a p i d l y during June and July and progressed at a r a t h e r constant d e - creased r a t e from late July u n t i l h a r v e s t . If an irrigation was delayed or stopped, sucrose concentration of the wet roots s t a r t e d to increase significantly above the control about 2 weeks after the l a s t i r r i g a t i o n when the surface soil became dry and the sugarbeet leaves showed signs of water s t r e s s . The r a t e of increase in sucrose concentration was generally higher during this i n i t i a l period of p l a n t s t r e s s . Following this i n i t i - al l a r g e increase in sucrose concentration due to water s t r e s s , the r a t e of increase was similiar to t h a t of the control. The increase in sucrose concentration above t h a t in the control was not evident when sucrose concentration was calculated on a d r y weight b a s i s (Figure 4A, B). This indicated t h a t the increase in sucrose concentration as determined on a wet weight b a s i s was l a r g e l y due to dehydration of the roots. This was further shown in 1978 on all treatments by the decrease in sucrose con- centration after the water application by i r r i g a t i o n or r a i n f a l l to stressed p l a n t s . This demonstrated t h a t i r r i g a t i o n level can influence the dry matter and sucrose concentration in beet roots at h a r v e s t . However, root q u a l i t y may be improved by i n - creased sucrose concentration, but sucrose yield will not be b e n e - fitted by this p r a c t i c e .
Total N uptake by the sugarbeets at h a r v e s t was l i n e a r l y
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F i g u r e 4. S u c r o s e p e r c e n t a g e of d r y roof. (A, B) and wet r o o t (C, D) as a f f e c t e d by t i m e of s a m p l i n g and i r r i g a t i o n w a t e r t r e a t m e n t in 1977 and 1978 (M - f a r m l e v e l i r r i g a t i o n , M =1 August w a t e r c u t o f f and l i g h t i r r i g a t i o n 1 S e p t e m - b e r , M =1 August w a t e r c u t o f f , M4=15 J u l y w a t e r c u t o f f . ) r e l a t e d to NT at each of e i g h t sites in 1972 with the amount and r a t e of N u p t a k e v a r y i n g with site and treatment (Figure 5A). The amount or r a t e of t o t a l N u p t a k e with N fertilizer (Nf) addition at each site h a d l i t t l e r e l a t i o n s h i p with the s t a r t - ing NT v a l u e s . Sucrose concentration of the roots was also l i n e - a r l y r e l a t e d to NT at each of the eight sites with the r a t e of decrease with i n c r e a s e d NT v a r y i n g with the site (Figure 5B).
If the slopes of the r e g r e s s i o n l i n e s for i n c r e a s e d N u p t a k e a n d decreased sucrose concentration with i n c r e a s e d NT a r e used in a r e g r e s s i o n a n a l y s i s , the r a t e of decrease in sucrose (S) con- c e n t r a t i o n depended upon the r a t e of i n c r e a s e in t o t a l p l a n t N u p t a k e (N ) with fertilizer a d d i t i o n [ y ( D S/D NT) = 0.00055 0.0141 (DN / D Nf) , r = 0.89 or y ( D S / D NT) = 0.00083 - 0.0094 (DNup/DNf at 65% N fertilizer efficiency ( Ef) , r = 0 . 8 9 ] . There was v e r y l i t t l e r e l a t i o n s h i p between the NT v a l u e s and the N u p t a k e at the v a r i o u s sites and the s t a r t i n g (check) sucrose*
concentration v a l u e s in 1972 [y(%S) = 17.39 - 0.0018 N r = up
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F i g u r e 5 . E f f e c t o f t o t a l a v a i l a b l e N (NT) o n t o t a l N u p t a k e (A) and s u c r o s e p e r c e n t a g e (B) a t d i f f e r e n t l o c a t i o n s i n s o u t h e r n I d a h o i n 1 9 7 2 . R e g r e s s i o n l i n e i s NT r a n g e f o r e a c h s i t e . Arrows i n d i c a t e maximum s u c r o s e y i e l d NT l e v e l f o r each s i t e .
0 . 2 2 ] . However, a better r e l a t i o n s h i p was obtained between these components at the different sites in 1971 [y(%S) = 19.66- 0.0105 Nup , r = 0 . 7 3 ] . This would indicate t h a t between sites and location, there was a factor or factors other than total N uptake t h a t was influencing the components t h a t determine s u - crose concentration in beet roots.
When the v a r i o u s parameters t h a t influence sucrose concen- tration in beet roots were compared at maximum sucrose y i e l d , an excellent correlation g e n e r a l l y existed between these com- ponents if the d a t a were obtained at one location during any one year (Table 2 ) . If these same parameters were compared using data from different locations during the same year or at the same location during different y e a r s , correlations were smaller. However, the better correlations using d a t a at a l l l o - cations and y e a r s existed between the total N uptake and p e r - c e n t dry matter or sucrose concentration of the wet roots and
between dry matter and sucrose concentration of the wet roots.
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This r e l a t i o n s h i p between N u p t a k e , percent dry matter and sucrose concentration would indicate t h a t the amount of N uptake necessary to achieve maximum sucrose yield and its effect on dry matter concentration was a major contributing factor to the v a r i a t i o n in sucrose concentration in the wet roots at the v a r i - ous locations and between different y e a r s . However, other factors were contributing to these differences in sucrose concen- t r a t i o n .
One of the closest correlations at the different locations and different y e a r s was the effect of percent dry matter on s u - crose concentration. The effect of these factors was compared in Figure 6. The slopes of the regression lines were essentially the same in all but one treatment (check, 1972) when percent dry matter was compared to sucrose concentration during differ- ent y e a r s and locations (Figures 6B, C, D). The slopes of these regression lines were the same as those obtained at one location and year by plant water stress and dehydration of the roots (Figure 6A). The slope of the regression line changed because of N fertilizer (Nf); therefore, Nf seemed to be the major contributing factor to the decrease in sucrose concentra- tion due to the decrease in percent dry matter (Figure 6A).
This was probably caused by major decreases in both percent dry matter and percent sucrose of the dry matter with N a d d i - tion. Whereas, between locations and y e a r s at maximum sucrose yield, the difference in sucrose concentration can be a t t r i b u t e d more to the change in percent dry matter than to the change in percent sucrose of the dry matter. However, because the slopes of the regression lines were uniform and the slopes of these lines were less than those for N differences alone, a factor or factors other t h a n N level are probably major c o n t r i - buting factors to the differences in dry matter and sucrose con- centration at the different locations and y e a r s . Some of these factors could be i r r i g a t i o n level, time of N uptake due to either application or soil location of the a v a i l a b l e N, growth p a t t e r n s of the crop, climatic conditions d u r i n g the season, or insect damage to the roots e a r l y in the season. Ail of these c o n d i - tions could either increase or decrease the dry matter in the
roots or delay the N uptake by the p l a n t s to a period in p l a n t
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growth where it could have i n c r e a s i n g effect on sucrose concen- t r a t i o n .
F i g u r e 6 . E f f e c t o f p e r c e n t d r y m a t t e r o n s u c r o s e p e r c e n t a g e i n t h e wet r o o t i n : A) Same y e a r and same l o c a t i o n w i t h v a r y i n g N and m o i s t u r e l e v e l s i n 1 9 7 7 , B ) D i f f e r e n t y e a r s and same l o c a t i o n ( 1 9 6 6 - 1 9 7 9 ) , C) Same y e a r and d i f f e r e n t l o c a t i o n s (Check and maximum s u c r o s e y i e l d in 1 9 7 1 ) , D) Same y e a r and d i f f e r e n t l o c a t i o n s (Check and maximum s u c r o s e y i e l d i n 1 9 7 2 ) .
Beta v a r i e t i e s , v a r y i n g widely in t h e i r root yield p o t e n t i a l and sucrose c o n c e n t r a t i o n , were grown at one location at two N levels in 1980 (5) and showed an important r e l a t i o n s h i p b e - tween dry matter and sucrose concentration (Table 3 ) . There was an i n v e r s e l i n e a r r e l a t i o n s h i p between root yield and d r y matter or sucrose concentration in s u g a r b e e t roots grown at farm level i r r i g a t i o n (M1) and at mid- to l a t e - s e a s o n moisture s t r e s s (M3) within each N l e v e l . The sucrose concentration in the roots was p r i m a r i l y dependent upon the d r y matter concentration within the roots with a lesser but s i l l important sucrose concen- t r a t i o n within the dry matter. Increased N l e v e l , as previously shown, reduced the sucrose concentration in the wet roots by reducing the percent dry matter and the sucrose concentration of the d r y m a t t e r . Therefore, sucrose yield was p r i m a r i l y d e - pendent upon the dry matter yield at the different i r r i g a t i o n
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Effect of root yield on percent root dry m a t t e r and p e r - cent s u c r o s e , percent root dry m a t t e r on percent s u c r o s e , and dry m a t t e r yield of roots on sucrose yield as a f - fected by N fertilizer l e v e l , m i d - to late-season m o i s - ture s t r e s s , and B e t a g e n o t y p e . t
water and N l e v e l s . This emphasizes the point that the v a r i e t y of sugarbeets grown within any climatic zone and N level can have an influence on the sucrose concentration and yields o b - t a i n e d .
Within any climatic zone, season, and v a r i e t y , the major contributing factors t h a t affect sucrose concentration is the a- mount of N uptake by the p l a n t , the p l a n t growth period of the N u p t a k e , and the water s t a t u s of the p l a n t . Petiole NO3-N is an excellent indicator of the N s t a t u s of the crop at any time during the growing season (27). The level and the r a t e of change in petiole NO3-N reflects the net uptake and N a s s i m i l a - tion r a t e s . Therefore, the accumulative effect of both time and N uptake r a t e is taken into consideration when petiole NO^-N is used to determine the effect of N on sucrose concentration and growth p a t t e r n s of s u g a r b e e t s . The N s t a t u s late in the s e a - son can be predicted from soil and petiole samples (17) or
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petiole samples (6) t a k e n e a r l i e r in the season. Sucrose concen- t r a t i o n at h a r v e s t at one location d u r i n g the same y e a r (1968) was shown to be inversely r e l a t e d to the amount of N fertilizer t h a t was applied p r e p l a n t (r = 0 . 9 9 ) , to the NO3,-N concentration on 21 August (r = 0.96), and to the a v e r a g e (r = 0.99) or i n t e - g r a t e d a v e r a g e (r = 0.99) petiole NO3-N from 8 July to 21 August ( 8 ) . In the work d e s c r i b e d , petiole NO3-N was used to determine the N s t a t u s throughout the season at maximum s u - crose yield for s u g a r b e e t s grown with a d e q u a t e water at differ- ent locations d u r i n g the same y e a r (Figure 7B) and at the same location (Kimberly) d u r i n g different y e a r s (Figure 7A). High sucrose concentration, in every c a s e , depended upon an e a r l y N (peak NO3--N concentration) and upon a decline to a low NO3- N level d u r i n g the l a t t e r p a r t of the growing season. The peak concentration, N , was a t t r i b u t e d to high a v a i l a b l e soil and fertilizer N and a low r a t e of N use by the p l a n t which u s u a l l y occurred d u r i n g the e a r l y growth s t a g e s . If t h i s peak concen- t r a t i o n is delayed due to known or unknown problems in p l a n t growth, low sucrose concentration in the roots may occur. The level of NO„-N in the petioles at N seems to have l i t t l e effect 3 r o on the sucrose concentration provided t h a t the r a t e of decline in petiole NO3-N is high enough so t h a t low levels a r e obtained d u r i n g the l a t t e r p a r t of the growing season. The c r i t i c a l low r a n g e for NO3-N h a s been e s t a b l i s h e d at 1000 ppm (27) and experience in t h i s a r e a i n d i c a t e d t h a t petiole N03-N should be n e a r 1000 ppm by 20 August to maximize y i e l d s , sucrose concen- t r a t i o n , and p u r i t y . A b e t t e r method of determining the N s t a t u s for high sucrose concentration may be the use of the a v e r - age or i n t e g r a t e d a v e r a g e petiole NO3-N d u r i n g August when the maximum sucrose is being p a r t i t i o n e d to the roots for storage (11). The i n t e g r a t e d a v e r a g e is preferred because it can be predicted at mid-season using the given equation (Figure 7 ) . Generally, the petiole N03-N concentration d u r i n g August was associated with the level of sucrose concentration at h a r v e s t at a n y one location. However, there was v a r i a t i o n caused by site location.
Maximum sucrose yield was obtained at r a t h e r high petiole N03-N levels late in the season at some locations where added
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F i g u r e 7. E f f e c t of y e a r (A) and l o c a t i o n (B) on t h e NO„-N c o n c e n - t r a t i o n i n s u g a r b e e t p e t i o l e s and i t s e f f e c t o n s u c r o s e c o n c e n t r a t i o n in t h e wet r o o t s . t N is t h e NO3-N c o n c e n - t r a t i o n a t t i m e t , N0 i s t h e c o n c e n t r a t i o n a t t h e f i r s t s a m p l i n g d a t e a f t e r t h e peak o c c u r s , t i s any t i m e a f t e r t h e f i r s t s a m p l i n g d a t e , and C i s a c o n s t a n t f o r any g i v e n t r e a t m e n t o r b e e t f i e l d ( 6 )
N was necessary to achieve yield benefits. This was probably caused by high levels of N being a v a i l a b l e from r e s i d u a l sources in the lower soil profiles caused by past management practices t h a t resulted in N accumulation. This resulted in low sucrose concentration on a l l treatments including the zero-N treatment.
Yield benefits were achieved by fertilizer application to a N deficit surface soil by its effect on e a r l y p l a n t growth. In every c a s e , where the field was p r e p a r e d for sugarbeets by growing a g r a i n crop without fertilizer (1968b, 1971, 1977,
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1978), maximum sucrose yield was obtained at low levels of p e - tiole NO3-N d u r i n g August r e s u l t i n g in high sucrose c o n c e n t r a - tion at h a r v e s t .
The r e s u l t s of these experiments at s e v e r a l different locations and y e a r s showed t h a t sucrose concentration in the s u g a r b e e t roots was the r e s u l t of the level of dry matter concentrations and the sucrose concentration within the dry matter of the roots. Within any climatic zone, these factors a r e normally con- trolled by the effect of N level on p l a n t growth and the i r r i g a - tion water level imposed on the p l a n t s . Optimum N levels applied e a r l y in the growing season will cause e a r l y N uptake and p l a n t growth. Early p l a n t growth will optimize leaf a r e a e a r l y in the season when solar energy is h i g h e s t . Under these conditions, photosynthate production will be maximized for the location r e s u l t i n g in high sucrose concentration and y i e l d s . Addition of excess N or N u p t a k e l a t e in the season when sucrose storage is h i g h e s t , will r e s u l t in the energy from solar r a d i a - tion being used for top growth r a t h e r t h a n for sucrose storage c a u s i n g lower or low sucrose concentration within the r o o t s . These effects of N on sucrose concentration d i r e c t l y affects the root and sucrose y i e l d s . However, increased sucrose c o n c e n t r a - tion within the roots by moisture s t r e s s placed on the roots by limiting i r r i g a t i o n s , is b a s i c a l l y caused by d e h y d r a t i o n of the roots and h a s l i t t l e , if a n y , effect on sucrose production at h a r v e s t .
In the production of high q u a l i t y roots, it is extremely im- p o r t a n t t h a t fields be selected for the growth of t h i s crop t h a t have low levels of r e s i d u a l N at all soil depths t h a t a r e within the root zone of s u g a r b e e t s . Fields where p a s t management h a s applied too much N for the crop grown or h a s leached the N to lower depths within the profile, should be avoided in s u g a r b e e t production. Soil t e s t i n g at a l l depths within the root zone of s u g a r b e e t s would be a d v a n t a g e o u s and would locate fields favor- able for q u a l i t y beet root p r o d u c t i o n . However, soil t e s t i n g by u n i v e r s i t i e s commercial c o n s u l t a n t s , and fertilizer companies do not normally sample or recommend sampling below 60 cm.
Past cropping and N fertilizer management provides an a l t e r n a t e