Planta (Berl.) 95, 351--354 (1970) 9 by Springer-Verlag 1970
Short Communication
The Proportion of Sieve Elements in the Phloem of Some Tropical Trees* J. 1~. S. LAWTO• ~ a n d M. J. CANNY Botany Department, Monash University, Clayton, Victoria, Australia Received August 5, 1970
Summary. Measurements have been made of the proportion of the area of sieve elements in the cross-sectional area of the secondary phloem of trees of two tropical genera in which the presence of storied sieve plates makes the recognition of sieve elements particularly easy. This proportion, often accepted as one fifth in the literature on phloem and translocation, rises as high as three quarters in one of the trees measured. I n considering t h e m a g n i t u d e of t h e m o v e m e n t s of organic m a t e r i a l s b y t h e p h l o e m t r a n s l o c a t i o n s y s t e m a convenient m e a s u r e is t h e m a s s of d r y weight m o v e d p e r u n i t t i m e per u n i t cross-sectional area of t h e p a t h . The p u b l i s h e d m e a s u r e m e n t s l e a d to similar values of this m e a s u r e of t r a n s l o c a t i o n which has been called t h e specific mass t r a n s f e r b y one of us (Canny, 1960). I n t h a t review i t was shown t h a t for a wide range of t r a n s l o c a t i n g p l a n t p a r t s t h e values of t h e specific mass t r a n s f e r were close to 4 g r a m s p e r hour per square c e n t i m e t r e of phloem. Since t h a t t i m e i t has become increasingly a c c e p t e d t h a t a m o n g t h e cells of t h e p h l o e m i t is t h e sieve t u b e s t h a t are c a r r y i n g t h e organic stream, a n d t h a t a m o r e correct a n d useful m e a s u r e of specific m a s s transfer w o u l d be g r a m s p e r hour p e r square c e n t i m e t r e of sieve tube. I t is i m p o r t a n t , then, to k n o w w h a t f r a c t i o n of t h e cross section of t h e t o t a l p h l o e m is occupied b y sieve elements. A figure w i d e l y a c c e p t e d a n d used for physiological calculations is t h a t of one fifth, based on some m e a s u r e m e n t s of Crafts (1931) on p o t a t o t u b e r stalks a n d Cucurbita stems. I t is t h e p u r p o s e of this p a p e r to show t h a t this factor is n o t u n i v e r s a l l y applicable, a n d t h a t in some p h l o e m t h e p r o p o r t i o n of sieve elements m a y be v e r y m u c h higher. Two other groups of workers, a n t e d a t i n g Crafts, h a d come i n d e p e n d e n t l y to t h e conclusion t h a t t h e sieve elements m a d e u p a larger fraction of t h e phloem. Mason a n d Maskell (1928a, b) h a d referred to t h e "sievet u b e g r o u p s " of s e c o n d a r y c o t t o n p h l o e m a n d used their area as t h e d e n o m i n a t o r in a n expression of t r a n s l o c a t i o n essentially t h e same as t h e * I t is with regret that we record the death of Dr. Lawton in the closing stages of this investigation.
352
J . R . S . Lawtont and M. J. Canny: Sieve Elements
specific mass transfer. T h e y i m p l y t h a t t h e groups were composed largely of sieve elements. M/inch (1930, p. 83), in considering t r a n s l o c a t i o n in t h e p h l o e m of t e m p e r a t e trees considered t h a t two t h i r d s of t h e p h l o e m should be r e c k o n e d as sieve elements. W e p r e s e n t some d a t a on t r o p i c a l trees where t h e d e p t h of functioning p h l o e m has been shown ( L a w t o n a n d L a w t o n , in press) to be v e r y m u c h greater t h a n in M/inch's t e m p e r a t e trees. Samples of phloem were collected from forest-grown dominants of Sterculia tragacantha Lindl. and Bombax buonoloozense Beauv. which had been growing in isolation for the past 20 years. The details of this collection and of the sectioning and staining have been described elsewhere (Lawton and Lawton, in press). Briefly, sledge-microtome sections of bark fixed in FAA were stained with aniline blue and photographed by UV light with an orange filter in the ocular. Prints like those in Figs. 1 and 2 were enlarged considerably and the areas of phloem delimited from medullary rays, fibres, and parenchyma around the bands of fibres. The cross-sectional areas of the phloem and sieve elements were computed by cutting out their images from the prints and comparing the weights of the paper with that of a measured area of the same print. For Sterculia, individual sieve elements were cut out. In addition to their area, their individual radii were measured and computed in micrometres from the known magnification. For Bombax the sieve elements could not easily be separated with certainty: their cross-areas were computed in groups, but their radii were not determined. T h e storied c a m b i u m in these trees produces sieve plates in t h e s e c o n d a r y p h l o e m which lie closely a t t h e same level a n d are n o t m o v e d from this level b y s u b s e q u e n t elongation of t h e sieve elements, so t h a t t h e y all a p p e a r in one or two sections from t h e sledge m i c r o t o m e over a wide area of t h e phloem. The fluorescing s t a i n e d eallose on t h e sieve plates is seen in t h e U V microscope occupying a large p r o p o r t i o n of t h e phloem. W h e r e t h e sieve p l a t e is n o t seen, t h e rich deposits of callose in pits on t h e walls of t h e element are u s u a l l y plain. I n one section a t t h e right level it is v e r y easy to recognise all t h e sieve elements a n d to see t h a t t h e p r o p o r t i o n of t h e t o t a l p h l o e m cross section t h e y o c c u p y is high (Fig. 1). The n a t i v e fluorescence of t h e fibres, which a p p e a r s similar in a b l a c k - a n d - w h i t e p h o t o g r a p h , is clearly distinguishable in t h e microscope. M e a s u r e m e n t s of the t o t a l - p h l o e m a n d sieve-element cross sections from t h e enlarged p r i n t s gave t h e p r o p o r t i o n s shown in t h e Table which also gives t h e a v e r a g e radius of t h e Sterculia sieve elements. Thus, in this material, where it is easy to recognise u n e q u i v o c a l l y t h e ceils t h a t are sieve elements, a n d to distinguish t h e m from phloem p a r e n c h y m a w i t h o u t t h e need for following t h e m t h r o u g h serial sections, t h e sieve elements o c c u p y well over half t h e a r e a of t h e phloem. I t is n o t e a s y to see how Crafts, using fresh h a n d sections a n d no stain e x c e p t (occasionally) acid violet, a n d w i t h o u t serial sections, could be certain t h a t all t h e cells he classed as phloem p a r e n c h y m a or c o m p a n i o n cells were indeed so. I n our experience of p h l o e m sections of herbaceous material, m a n y cells in a n y one section p r e p a r e d b y t h e b e s t m o d e r n
Figs. 1 and 2. Callose/aniline blue fluorescence of pitted sieve-tube walls and sieve plates in transverse sections of the secondary phloem of (1) Bombax buonopozense (• and (2) Sterculia tragacantha (• 120). The groups of sieve elements are separated b y bands of fibres t h a t are also fluorescing. 8t sieve tubes, /fibres, c cambial zone, x xylem
354
J . R . S . Lawton t and M. J. Canny: Sieve Elements
Table. Area percent and radius o/sieve elements in Sterculia tragacantha and Bombax buonopozense Species
Area percent of sieve elements in the phloem cross section
Mean radius of sieve elements Qxm)
Sterculia tragacantha
72
27.5:~3
Bombax buonopozense :
tree 1 tree 2
54 74
fixations a n d sectioned a t 1 ~m in glycol m e t h a c r y l a t e c a n n o t w i t h c e r t a i n t y be classed as sieve elements or s o m e t h i n g else b y light microseopy. Only b y using fluorescent cailose stains a n d following t h e cells t h r o u g h serial sections does it become clear, b y t h e presence of a sieve plate, t h a t a certain file of cells is in fact a sieve tube. W i t h o u t going into t h e details of this work here, we suggest t h a t e s t i m a t e s of t h e p r o p o r t i o n of sieve e l e m e n t s b a s e d on single sections should be a c c e p t e d with reserve, a n d as furnishing only lower limits to t h e p r o p o r t i o n . Geiger et al. (1969) used b e t t e r f i x a t i o n t h a n Crafts, a n d t h i n n e r sections (6 ~m) to p r o d u c e a p r o p o r t i o n of 29% in Beta petioles, b u t t h e i r Fig. 9 has a n u m b e r of cells m a r k e d as p a r e n e h y m a cells which m i g h t be sieve elements if followed in serial sections. F o r physiological calculations, t h e r e seem to be grounds for suspecting t h a t a larger p r o p o r t i o n should be used t h a n one fifth, a n d if a figure is to be selected, t h e t w o - t h i r d s factor of Miinch seems on p r e s e n t evidence to be m o r e reasonable. References
Canny, 5I. J. : The rate of translocation. Biol. Rev. 35, 507-532 (1960). Crafts, A. S. : Movement of organic materials in plants. Plant Physiol. 6, 141 (1931). Geiger, D. R., Saunders, M. A., Cataldo, D. A. : Translocation and accumulation of translocate in the sugar beet petiole. Plant Physiol. 44, 1657-1665 (1969). Lawton, June R., Lawton, J. R. S. : Seasonal variations in the secondary phloem of some forest trees from Nigeria. I. General monthly variations in the depth of the phloem. New Phytologist 70 (1971) in press. Mason, T. G., Maskell, E. J. : Studies on the transport of carbohydrates in the cotton plant. I. A study of diurnal variation in the carbohydrates of leaf, bark and wood, and of the effects of ringing. Ann. Bot. 42, 189-253 (1928a). -- - II. The factors determining the rate and the direction of movement of sugars. Ann. Bot. 42, 571-636 (1928b). Miinch, E. : Die Stoffbewegungcn in der Pflanze. Jena: Fischer 1930. Dr. M. J. Canny Botany Department Monash University Clayton, Vic. 3168, Australia
Short Communication
The Proportion of Sieve Elements in the Phloem of Some Tropical Trees* J. 1~. S. LAWTO• ~ a n d M. J. CANNY Botany Department, Monash University, Clayton, Victoria, Australia Received August 5, 1970
Summary. Measurements have been made of the proportion of the area of sieve elements in the cross-sectional area of the secondary phloem of trees of two tropical genera in which the presence of storied sieve plates makes the recognition of sieve elements particularly easy. This proportion, often accepted as one fifth in the literature on phloem and translocation, rises as high as three quarters in one of the trees measured. I n considering t h e m a g n i t u d e of t h e m o v e m e n t s of organic m a t e r i a l s b y t h e p h l o e m t r a n s l o c a t i o n s y s t e m a convenient m e a s u r e is t h e m a s s of d r y weight m o v e d p e r u n i t t i m e per u n i t cross-sectional area of t h e p a t h . The p u b l i s h e d m e a s u r e m e n t s l e a d to similar values of this m e a s u r e of t r a n s l o c a t i o n which has been called t h e specific mass t r a n s f e r b y one of us (Canny, 1960). I n t h a t review i t was shown t h a t for a wide range of t r a n s l o c a t i n g p l a n t p a r t s t h e values of t h e specific mass t r a n s f e r were close to 4 g r a m s p e r hour per square c e n t i m e t r e of phloem. Since t h a t t i m e i t has become increasingly a c c e p t e d t h a t a m o n g t h e cells of t h e p h l o e m i t is t h e sieve t u b e s t h a t are c a r r y i n g t h e organic stream, a n d t h a t a m o r e correct a n d useful m e a s u r e of specific m a s s transfer w o u l d be g r a m s p e r hour p e r square c e n t i m e t r e of sieve tube. I t is i m p o r t a n t , then, to k n o w w h a t f r a c t i o n of t h e cross section of t h e t o t a l p h l o e m is occupied b y sieve elements. A figure w i d e l y a c c e p t e d a n d used for physiological calculations is t h a t of one fifth, based on some m e a s u r e m e n t s of Crafts (1931) on p o t a t o t u b e r stalks a n d Cucurbita stems. I t is t h e p u r p o s e of this p a p e r to show t h a t this factor is n o t u n i v e r s a l l y applicable, a n d t h a t in some p h l o e m t h e p r o p o r t i o n of sieve elements m a y be v e r y m u c h higher. Two other groups of workers, a n t e d a t i n g Crafts, h a d come i n d e p e n d e n t l y to t h e conclusion t h a t t h e sieve elements m a d e u p a larger fraction of t h e phloem. Mason a n d Maskell (1928a, b) h a d referred to t h e "sievet u b e g r o u p s " of s e c o n d a r y c o t t o n p h l o e m a n d used their area as t h e d e n o m i n a t o r in a n expression of t r a n s l o c a t i o n essentially t h e same as t h e * I t is with regret that we record the death of Dr. Lawton in the closing stages of this investigation.
352
J . R . S . Lawtont and M. J. Canny: Sieve Elements
specific mass transfer. T h e y i m p l y t h a t t h e groups were composed largely of sieve elements. M/inch (1930, p. 83), in considering t r a n s l o c a t i o n in t h e p h l o e m of t e m p e r a t e trees considered t h a t two t h i r d s of t h e p h l o e m should be r e c k o n e d as sieve elements. W e p r e s e n t some d a t a on t r o p i c a l trees where t h e d e p t h of functioning p h l o e m has been shown ( L a w t o n a n d L a w t o n , in press) to be v e r y m u c h greater t h a n in M/inch's t e m p e r a t e trees. Samples of phloem were collected from forest-grown dominants of Sterculia tragacantha Lindl. and Bombax buonoloozense Beauv. which had been growing in isolation for the past 20 years. The details of this collection and of the sectioning and staining have been described elsewhere (Lawton and Lawton, in press). Briefly, sledge-microtome sections of bark fixed in FAA were stained with aniline blue and photographed by UV light with an orange filter in the ocular. Prints like those in Figs. 1 and 2 were enlarged considerably and the areas of phloem delimited from medullary rays, fibres, and parenchyma around the bands of fibres. The cross-sectional areas of the phloem and sieve elements were computed by cutting out their images from the prints and comparing the weights of the paper with that of a measured area of the same print. For Sterculia, individual sieve elements were cut out. In addition to their area, their individual radii were measured and computed in micrometres from the known magnification. For Bombax the sieve elements could not easily be separated with certainty: their cross-areas were computed in groups, but their radii were not determined. T h e storied c a m b i u m in these trees produces sieve plates in t h e s e c o n d a r y p h l o e m which lie closely a t t h e same level a n d are n o t m o v e d from this level b y s u b s e q u e n t elongation of t h e sieve elements, so t h a t t h e y all a p p e a r in one or two sections from t h e sledge m i c r o t o m e over a wide area of t h e phloem. The fluorescing s t a i n e d eallose on t h e sieve plates is seen in t h e U V microscope occupying a large p r o p o r t i o n of t h e phloem. W h e r e t h e sieve p l a t e is n o t seen, t h e rich deposits of callose in pits on t h e walls of t h e element are u s u a l l y plain. I n one section a t t h e right level it is v e r y easy to recognise all t h e sieve elements a n d to see t h a t t h e p r o p o r t i o n of t h e t o t a l p h l o e m cross section t h e y o c c u p y is high (Fig. 1). The n a t i v e fluorescence of t h e fibres, which a p p e a r s similar in a b l a c k - a n d - w h i t e p h o t o g r a p h , is clearly distinguishable in t h e microscope. M e a s u r e m e n t s of the t o t a l - p h l o e m a n d sieve-element cross sections from t h e enlarged p r i n t s gave t h e p r o p o r t i o n s shown in t h e Table which also gives t h e a v e r a g e radius of t h e Sterculia sieve elements. Thus, in this material, where it is easy to recognise u n e q u i v o c a l l y t h e ceils t h a t are sieve elements, a n d to distinguish t h e m from phloem p a r e n c h y m a w i t h o u t t h e need for following t h e m t h r o u g h serial sections, t h e sieve elements o c c u p y well over half t h e a r e a of t h e phloem. I t is n o t e a s y to see how Crafts, using fresh h a n d sections a n d no stain e x c e p t (occasionally) acid violet, a n d w i t h o u t serial sections, could be certain t h a t all t h e cells he classed as phloem p a r e n c h y m a or c o m p a n i o n cells were indeed so. I n our experience of p h l o e m sections of herbaceous material, m a n y cells in a n y one section p r e p a r e d b y t h e b e s t m o d e r n
Figs. 1 and 2. Callose/aniline blue fluorescence of pitted sieve-tube walls and sieve plates in transverse sections of the secondary phloem of (1) Bombax buonopozense (• and (2) Sterculia tragacantha (• 120). The groups of sieve elements are separated b y bands of fibres t h a t are also fluorescing. 8t sieve tubes, /fibres, c cambial zone, x xylem
354
J . R . S . Lawton t and M. J. Canny: Sieve Elements
Table. Area percent and radius o/sieve elements in Sterculia tragacantha and Bombax buonopozense Species
Area percent of sieve elements in the phloem cross section
Mean radius of sieve elements Qxm)
Sterculia tragacantha
72
27.5:~3
Bombax buonopozense :
tree 1 tree 2
54 74
fixations a n d sectioned a t 1 ~m in glycol m e t h a c r y l a t e c a n n o t w i t h c e r t a i n t y be classed as sieve elements or s o m e t h i n g else b y light microseopy. Only b y using fluorescent cailose stains a n d following t h e cells t h r o u g h serial sections does it become clear, b y t h e presence of a sieve plate, t h a t a certain file of cells is in fact a sieve tube. W i t h o u t going into t h e details of this work here, we suggest t h a t e s t i m a t e s of t h e p r o p o r t i o n of sieve e l e m e n t s b a s e d on single sections should be a c c e p t e d with reserve, a n d as furnishing only lower limits to t h e p r o p o r t i o n . Geiger et al. (1969) used b e t t e r f i x a t i o n t h a n Crafts, a n d t h i n n e r sections (6 ~m) to p r o d u c e a p r o p o r t i o n of 29% in Beta petioles, b u t t h e i r Fig. 9 has a n u m b e r of cells m a r k e d as p a r e n e h y m a cells which m i g h t be sieve elements if followed in serial sections. F o r physiological calculations, t h e r e seem to be grounds for suspecting t h a t a larger p r o p o r t i o n should be used t h a n one fifth, a n d if a figure is to be selected, t h e t w o - t h i r d s factor of Miinch seems on p r e s e n t evidence to be m o r e reasonable. References
Canny, 5I. J. : The rate of translocation. Biol. Rev. 35, 507-532 (1960). Crafts, A. S. : Movement of organic materials in plants. Plant Physiol. 6, 141 (1931). Geiger, D. R., Saunders, M. A., Cataldo, D. A. : Translocation and accumulation of translocate in the sugar beet petiole. Plant Physiol. 44, 1657-1665 (1969). Lawton, June R., Lawton, J. R. S. : Seasonal variations in the secondary phloem of some forest trees from Nigeria. I. General monthly variations in the depth of the phloem. New Phytologist 70 (1971) in press. Mason, T. G., Maskell, E. J. : Studies on the transport of carbohydrates in the cotton plant. I. A study of diurnal variation in the carbohydrates of leaf, bark and wood, and of the effects of ringing. Ann. Bot. 42, 189-253 (1928a). -- - II. The factors determining the rate and the direction of movement of sugars. Ann. Bot. 42, 571-636 (1928b). Miinch, E. : Die Stoffbewegungcn in der Pflanze. Jena: Fischer 1930. Dr. M. J. Canny Botany Department Monash University Clayton, Vic. 3168, Australia
