Translocation of some assimilates from the sink to the donor in apple tree

The shoot apex or fruitlets of Jonathan apple trees grafted on M IX rootstock and grown in pots in a greenhouse were exposed to¹⁴CO₂ in an assimilation chamber. The translocation of¹⁴C-labelled assimilates from treated organs to other parts of the plant was studied. It was found that a very small amount of¹⁴C-labelled compounds was translocated from the shoot apex and very young fruitlets to the shoot stem. Preliminary chromatographic studies show that the chemical composition of the labelled substances detected below assimilation chamber differs profoundly from that of those remaining in the supplied leaves. The results support the view that there exists a translocation of some substances, possibly regulators from the sink to the donor.     

Distribution of labelled assimilates within a young apple tree after supplying14CO2to a leaf or shoot

Labelled carbon dioxide was supplied for 22 hrs to a leaf of the leader or to the lateral shoot in two-year-old apple seedlings. The distribution of radioactive assimilates within the plant following this treatment was investigated by using radioautography. The transport of labelled assimilates from the young leaf of the leader was very meagre and affected only parts of the stem and the leaves situated in the close vicinity of the treated leaf. The¹⁴C-labelled assimilates from the mature leaf of the leader were transported in a considerable amount to the apex and to the other leaves of the leader. They were also found in an appreciable amount in the stem and the roots, as well as in some lateral shoots. After supplying¹⁴CO₂ to the lateral shoot remarkable transport of labelled assimilates was observed. Radioactivity was detected in the tip and in the youngest leaves of the leader, as well as in the roots. Their path in the stem was studied by dissecting the plant and examining the cross section from each internode. This method revealed that the assimilates from the treated leaf or shoot were transported downward only on one side of the stem in a helical pattern. The lateral shoots situated on the radioactive side of the stem were also labelled, whereas those situated on the opposite (non-radioactive) side were not labelled.     

Floral stimulus movement in perilla and flower inhibition caused by noninduced leaves

Photoperiodic control of flowering in the short day plant Perilla involves the transmission of a floral stimulus from induced leaves to the shoot apex. We have studied the basipetal movement of this stimulus and of ¹⁴C-labeled assimilates in plants with an induced leaf (donor) grafted into the uppermost internode of a vegetative plant in which the axillary shoots at various nodes along the stem function as receptors.     

Effects of shoot bending of apple trees on accumulation and translocation of14C-labelled assimilates

The translocation of¹⁴C-labelled assimilates from a single leaf in bent and intact apple shoots was studied in varying stages of shoot development. In actively growing shoots¹⁴C-labelled assimilates translocated from the treated leaf and accumulated mainly in the shoot apex. In moderately growing apple shoots radioactive assimilates were translocated from the treated leaf in both directions towards and down the shoot. In apple shoots showing only slight growth activity the¹⁴C-labelled assimilates were transported from the treated leaf mainly to the base of the shoot, stem and roots. Bending shoots changed the pattern of distribution of radioactive assimilates. Bending actively and moderately growing shoots resulted in higher concentration of 14-carbon in the shoot apex than in controls. In slowly growing and non-growing apple shoots bending caused a higher accumulation of radioactive assimilates in the bent section than in an equal section of control shoots.     

The Export and Distribution of 14C-labelled Assimilates from each Leaf on the Shoot of Lolium temulentum during Reproductive and Vegetative Growth

In both reproductive and vegetative plants of Lolium temulentumL., the export of ¹⁴C-labelled assimilates from each healthyleaf on the main shoot to terminal meristem, stem, tillers,and roots was measured each time a new leaf was expanded, fora period of 5 to 6 weeks. Some labelled assimilates moved fromeach leaf on the main shoot to every meristem in the same shoot,as well as to the tops and roots of adjacent organically attachedtillers. The terminal meristem of the reproductive shoot, which includedthe developing inflorescence, received 70–80 per centof the carbon assimilated by the emerged portion of the growingleaf, 15–25 per cent of the carbon assimilated by thetwo youngest expanded leaves, and 5–10 per cent of thatfrom each of the older leaves. A similar pattern of carbon supplyto the terminal meristem was found in vegetative shoots, exceptthat older leaves on young vegetative shoots supplied even lessof their carbon to the terminal meristem. The general conclusionis that developing leaves at the tip of the shoot receive aboutthe same proportion of carbon from each leaf as does a developinginflorescence. Young expanded leaves provided most labelled assimilates forstem growth; during both reproductive and vegetative growth,expanded leaves increased their export of labelled carbon tostem, and exported less of their ¹⁴C to roots and sometimesto tillers. In these reproductive and vegetative shoots, grown in a constantexternal environment, the major changes in the pattern of distributionof labelled assimilates appeared to be the result of increasedmeristematic activity in stem internodes; the development ofan inflorescence had no obvious direct effect on the carboneconomy of shoots.     

Effect of assimilate utilization on photosynthetic rate in wheat

Summary Two weeks after anthesis, when the grain is filling rapidly, the rate of photosynthesis by flag leaves of wheat cv. Gabo was between 20 and 30 mg CO₂ dm^-2 leaf surface hour^-1 under the conditions used. About 45% of flag-leaf assimilates were translocated to the ear, and only about 12% to the roots and young shoots.On removing the ear, net photosynthesis by the flag leaves was reduced by about 50% within 3–15 hours, and there was a marked reduction in the outflow of ¹⁴C-labelled assimilates from the flag leaves.Subsequent darkening of all other leaves on plants without ears led to recovery of flag-leaf photosynthesis, as measured by gas analysis and ¹⁴CO₂ fixation, and to increased translocation of assimilates to the roots and young shoots. Minor changes in the rates of dark respiration accompanied these major, reversible changes in photosynthetic rate.After more than a week in continuous, high-intensity light, the rate of photosynthesis by flag leaves of intact plants had fallen considerably, but could be restored again by a period in darkness, or by inhibiting photosynthesis in the ears by spraying them with DCMU. The inhibition of ear photosynthesis increased translocation of labelled assimilates from the flag leaf to the ears, without affecting leaf sugar levels.The application of TIBA to the culm below the ear inhibited auxin movement throught the culm, but had no influence on flag-leaf photosynthesis.These results suggest that, at least in this system, photosynthesis by the flag leaf is regulated directly by the demand for assimilates from the flag leaf and not indirectly through action in the leaf of auxins produced by the sink organs.     

The role of sink demand in carbon partitioning and photosynthetic reinvigoration following shoot decapitation

Photosynthesis, growth, and carbon partitioning of vigorous coppice shoots were compared with the slower growing intact shoots of Populus maximowiczii × nigra L. MN9 to determine the relationship between carbon partitioning and photosynthetic rate. Relative height growth rate of coppice shoots was 2.2 times that of intact shoots with net photosynthetic rate 1.9 times that of intact shoots. Coppice leaves exported a larger proportion of newly-fixed assimilate (11% compared with 6%) after a 4-h chase. The greater export from coppice leaves was correlated with a greater proportion of [¹⁴C]-labelled photosynthate deposited as starch in stems 4 cm below the point of label application. Coppice leaf assimilate levels were reduced to 15% that of leaves on intact plants, but coppice leaves had twice the concentration of labelled sucrose. Carbohydrates constituted 55% of the water-soluble [¹⁴C]-labelled photosynthate in leaves of coppice shoots compared with 40% in intact shoots. The results suggest that carbon allocation and partitioning in coppice shoots were altered towards production and export of new assimilate, and support the hypothesis that photosynthetic rate is responsive to sink demand for assimilates.     

The Acquisition and Utilization of Carbon in Early Spring by Kiwifruit Shoots

Measurements of net photosynthetic rate (at 1450µ molm^-2s^-1photosynthetically active radiation) of leaves, of leafand stem respiration, and of shoot growth of potentially-fruitinglaterals on kiwifruit (Actinidia deliciosa ) were used to estimateweekly shoot carbon balances over the first 10 weeks of shootgrowth (budburst to anthesis). Consistent differences in therate of shoot elongation, of internode expansion and of increasein basal diameter were found among shoots. Faster-growing (long)shoots acquired carbon by photosynthesis at a faster rate evenin the first few weeks after budburst, but the amount of carbonrequired to sustain this growth resulted in shoot carbon deficitswhich were approx. seven times greater than those of the slower-growing(short) shoots. It was estimated that the transition from shootcarbon deficit to carbon surplus occurred 3–4 weeks afterbudburst, irrespective of shoot growth rate. As a result ofsubsequent rapid increases in shoot photosynthetic rate, longshoots had a shoot carbon surplus of 4.4 g C week^-1in the weekbefore anthesis, approx. three times that of the short shoots.Defoliation (66%) of shoots 1 week after budburst, and subsequentremoval of later-emerging leaves to maintain the level of defoliation,had the effect of slowing shoot growth in the carbon deficitperiod, particularly for the long shoots. However, the durationof shoot expansion in the defoliated shoots was longer, resultingultimately in shoots which were longer than the control shoots.Linkages among early carbon balance dynamics of shoots, shootlength at anthesis, and fruit growth are discussed. Actinidia deliciosa ; kiwifruit; shoot growth; carbon acquisition; respiration; photosynthesis     

Seasonal differences in the pattern of assimilate movement in branches of Coffea arabica L

At Ruiru, Kenya, ¹⁴CO₂ was fed to single leaves at different distances from the apex on branches of mature fruiting and non-fruiting trees of Coffea arabica on six occasions from the Short Rains (November) 1966 to the Long Rains (April-May) 1967. The location of ¹⁴C-labelled assimilates in the treated branches was determined 26 h later by autoradiography. The direction of movement of labelled assimilates indicated large seasonal differences in the relative sink strengths of the shoot tip, fruit and trunk-root systems. On vegetative trees the sink strength of trunk-roots was much smaller, as compared with the shoots, at the beginning of the Long Rains than at the end of the previous Short Rains or in the intervening dry season. Assimilate use by growing fruits did not alter the pattern of distribution of assimilates to the other sinks at the end of the Short Rains, but it did restrict assimilate movement to both shoot tips and trunk-roots at the_ beginning of the Long Rains. In the dry season, virtually all assimilates were utilized by growing fruits when these were present. Vegetative secondary shoots provided assimilates to growing fruits and trunk-roots at the end of the Short Rains and in the dry season. Some practical implications are noted.     

Translocation of 14C-Assimilates in Roses

Movement of ¹⁴C-assimilates from young and mature leaves to young rose shoots (Rosa hybrida cv. Marimba) was examined in two developmental stages. In the first stage after bud breaking the young shoot, especially its tip, depends for its supply of assimilates mainly on the mature foliage. At this stage young leaves are powerful sinks and retain 97% of their own photosynthates. The translocated 3% move mainly to the roots. At a later stage, just after the appearance of the flower bud, most of the leaves on the shoot become a source. The upper leaves supply assimilates to the flower bud and to the upper part of the stem. The ¹⁴C-assimilates from the lower leaves move in two directions, the larger part being directed downward.     

Alterations in source-sink patterns by modifications of source strength

Bean plants, trimmed to a simplified “double source, double sink” translocation system (the paired primary leaves serving as the double source and the paired lateral leaflets of the immature first trifoliate leaf as the double sink) were used to study the magnitude and short-term time course of change in the allocation ratio (partition ratio) of assimilates translocated from the labeled primary leaf to its respective “near” and “far leaflet” sinks in response to an increase or decrease in the source strength of the opposite primary leaf (the “control” leaf). If the rates of net photosynthesis in the two primary leaves were similar, assimilates from the labeled source leaf partitioned to the leaflet sinks in the ratio of 5:1 or higher, the dominant sink being the leaflet “nearer” to the labeled source leaf. If the rate of net photosynthesis in the control leaf was increased substantially above that of the labeled source leaf, the rate of translocation from the labeled source to either the near leaflet sink or far leaflet sink remained unaffected, despite, presumably, a higher translocation rate from the control leaf, and hence a higher phloem pressure gradient (or increased cross-sectional area) in the transport pathway from the control leaf to the leaflet sinks. If the control leaf was excised, thus reducing the source leaf area by about a half, the translocation rate from the remaining source leaf rapidly doubled, the partition ratio becoming equal to unity. If the control leaf was darkened, the partition ratio adjusted to an intermediate value. Although export rates from the labeled source leaf were increased either by excising or darkening the control leaf, the rate of net photosynthesis in the labeled leaf remained constant.     

Role of evergreen foliage in the nitrogen economy during shoot growth ofTernstroemia gymnanthera,a warm-temperate broadleaf tree

Accumulation and redistribution of nitrogen were examined during the shoot growth ofTernstroemia gymnanthera, a warm-temperate evergreen broadleaf tree species. Measurements and analyses were confined to the shoot units comprising 2-year-old, 1-year-old and developing current shoots with the foliage of respective ages. Budbreak occurred in early May and nitrogen was rapidly translocated into curent shoots with the progress of their growth. In all of the old organs of the shoot unit, nitrogen concentrations decreased gradually from the time of budbreak to early July. During this period, those old organs supplied more than 60% of the amount of nitrogen needed for the developing current shoots within the same shoot unit. The rest was supplied from the basal organs outside the shoot units comprising branches older than 2 years, stem and roots, by redistribution and/or by absorption from soil. Old leaves, mainly 1-year-old ones, provided about 72% of the total nitrogen derived from the old organs in the shoot units. It was concluded that the evergreen broadleaves served as a large source of nitrogen for the early shoot growth.     

Magnesium deficiency results in accumulation of carbohydrates and amino acids in source and sink leaves of spinach

Accumulation of assimilates in source leaves of magnesium‐deficient plants is a well‐known feature. We had wished to determine whether metabolite concentrations in sink leaves and roots are affected by magnesium nutrition. Eight‐week‐old spinach plants were supplied either with a complete nutrient solution (control plants) or with one lacking Mg (deficient plants) for 12 days. Shoot and root fresh weights and dry weights were lower in deficient than in control plants. Mg concentrations in deficient plants were 11% of controls in source leaves, 12% in sink leaves and 26% in roots, respectively. As compared with controls, increases were found in starch and amino acids in source leaves and in sucrose, hexoses, starch and amino acids in sink leaves, whereas they were only slightly enhanced in roots. In phloem sap of magnesium‐deficient and control plants no differences in sucrose and amino acid concentrations were found. To prove that sink leaves were the importing organs they were shaded, which did not alter the response to magnesium deficiency as compared with that without shading. Since in the shaded sink leaves the photosynthetic production of metabolites could be excluded, those carbohydrates and amino acids that accumulated in the sink leaves of the deficient plants must have been imported from the source leaves. It is concluded that in magnesium‐deficient spinach plants the growth of sink leaves and roots was not limited by carbohydrate or amino acid supply. It is proposed that the accumulation of assimilates in the source leaves of Mg‐deficient plants results from a lack of utilization of assimilates in the sink leaves.     

Endogenous Regulation of Photosynthetic Rate in Primary Leaves of Bean (Phaseolus vulgaris L.)

The rate of photosynthesis and/or dry matter production wasstudied in fully-expanded primary leaves of bean (Phaseolusvulgaris cv. Bulgarian) plants which had been subjected to varioussurgical and hormonal treatments. Between 30 and 40 per centof the assimilates produced by the primary leaves, over a 4-dayperiod starting with expansion of the first trifoliate leaf,were diverted to the growing shoot above the insertion of theprimary leaves. In detopped plants (i.e. lacking all leaves,stem and buds above insertion of primary leaves), both the rateof net photosynthesis (NP) of the primary leaves 4 days afterdetopping, and the mean net assimilation rate (NAR) over thisinterval, did not differ significantly from those of intactplants. The assimilate normally diverted to the top in intactplants was distributed between the remaining organs of the detoppedplant. When translocation of assimilates from the primary leaveswas stopped by girdling their petioles, both NAR and NP wereas in untreated control plants after a 2-day period. The assimilatesproduced during that period accumulated in the mesophyll chlorenchymain the form of starch granules. Intact plants supplied withGA₃, or IAA, through the primary leaves as well as detoppedplants supplied with IAA through the stump, differed from untreatedcontrol plants in the pattern of distribution of the assimilatesproduced: IAA favoured dry-matter accumulation in the roots,while GA₃ favoured the tops. Nevertheless, neither NP, nor NARdiffered significantly from the corresponding controls.     

A Quantitative Analysis of the Uptake of Carbon and of the Supply of 14C-labelled Assimilates to Areas of Meristematic Growth in Lolium temulentum

A quantitative analysis of the ¹⁴C-labelled assimilate suppliedby leaves on the main shoot to terminal meristem, stem, tillers,and roots was conducted during parallel periods of reproductiveand vegetative development in Lolium temulentum. The initial rate of entry of carbon into the shoot varied withthe area and photosynthetic efficiency of the assimilating leaf.Subsequently, respiratory losses of carbon during translocationand incorporation of assimilate at the site of utilization alsovaried. The combined effect of these differences resulted inthe supply of recently assimilated carbon being twofold greaterin reproductive shoots than in vegetative shoots, while withinshoots the carbon supply of the youngest fully expanded leafranged from four-or five-fold greater than the oldest leaf inyoung shoots, to two-or three-fold greater in older shoots.In both reproductive and vegetative shoots, the two or threeyoungest leaves thus dominated the supply of carbon for meristematicgrowth. Meristematic tissue in expanding leaves and leaf primordia atthe terminal meristem of the vegetative shoot received 18–27per cent of the total shoot carbon. This meristem utilized aboutthe same proportion of shoot carbon when it developed into aninflorescence, indicating no major change in the level of meristematicactivity. The proportion of shoot carbon utilized in stem growthincreased as both reproductive and vegetative shoots aged; thisincreased meristematic activity in stem internodes was accompaniedby reduced export of carbon to roots, which received less than10 per cent of the shoot carbon when the experiments ended.The main shoot translocated 20–30 per cent of its recentlyassimilated carbon to developing and rooted tillers, which assinks for carbon were thus as important as the terminal meristemand stem. This outward flow of carbon continued relatively uncheckedwhen donor and receptor shoots developed inflorescences.     

Seasonal patterns of 13C partitioning between shoots and nodulated roots of N2- or nitrate-fed Pisum sativum L

The effect of nitrogen source (N(2) or nitrate) on carbon assimilation by photosynthesis and on carbon partitioning between shoots and roots was investigated in pea (Pisum sativum L. 'Baccara') plants at different growth stages using (13)C labelling. Plants were grown in the greenhouse on different occasions in 1999 and 2000. Atmospheric [CO(2)] and growth conditions were varied to alter the rate of photosynthesis. Carbon allocation to nodulated roots was unaffected by N source. At the beginning of the vegetative period, nodulated roots had priority for assimilates over shoots; this priority decreased during later stages and became identical to that of the shoot during seed filling. Carbon allocation to nodulated roots was always limited by competition with shoots, and could be predicted for each phenological stage: during vegetative and flowering stages a single, negative exponential relationship was established between sink intensity (percentage of C allocated to the nodulated root per unit biomass) and net photosynthesis. At seed filling, the amount of carbon allocated to the nodulated root was directly related to net photosynthesis. Respiration of nodulated roots accounted for more than 60 % of carbon allocated to them during growth. Only at flowering was respiration affected by N supply: it was significantly higher for strictly N(2)-fixing plants (83 %) than for plants fed with nitrate (71 %). At the vegetative stage, the increase in carbon in nodulated root biomass was probably limited by respiration losses.     

Structural and hydraulic correlates of heterophylly in Ginkgo biloba

This study investigates the functional significance of heterophylly in Ginkgo biloba, where leaves borne on short shoots are ontogenetically distinct from those on long shoots. Short shoots are compact, with minimal internodal elongation; their leaves are supplied with water through mature branches. Long shoots extend the canopy and have significant internodal elongation; their expanding leaves receive water from a shoot that is itself maturing. Morphology, stomatal traits, hydraulic architecture, Huber values, water transport efficiency, in situ gas exchange and laboratory-based steady-state hydraulic conductance were examined for each leaf type. Both structure and physiology differed markedly between the two leaf types. Short-shoot leaves were thinner and had higher vein density, lower stomatal pore index, smaller bundle sheath extensions and lower hydraulic conductance than long-shoot leaves. Long shoots had lower xylem area:leaf area ratios than short shoots during leaf expansion, but this ratio was reversed at shoot maturity. Long-shoot leaves had higher rates of photosynthesis, stomatal conductance and transpiration than short-shoot leaves. We propose that structural differences between the two G. biloba leaf types reflect greater hydraulic limitation of long-shoot leaves during expansion. In turn, differences in physiological performance of short- and long-shoot leaves correspond to their distinct ontogeny and architecture.     

Post-Anthesis Economy of Carbon in a Cultivar of Cowpea

Budgets for transfer of carbon from individual leaves and othersource organs to fruits and nodulated roots were constructedfor stages of the post-flowering development of symbiotically-dependentcowpea (Vigna unguiculata L. Walp. cv. Vita 3-Rhizobium strainCB756). Exportable surpluses of carbon from sources, assessedfrom net exchanges of CO₂ and changes in carbon content, wereallocated to sink organs in proportion to carbon consumption(growth and respiration) and the ability of each sink organto attract assimilates from the sources, as demonstrated by¹⁴C-feeding. The first 10 d after flowering showed high sinkactivity by roots, stem and petioles, low consumption by fruits,with the upper three trifoliate blossom leaves providing thebulk of the required assimilates. The next 10 d showed a sharpdecline in photosynthesis of the leaf subtending the oldestfruit followed by similar declines in leaves at the other fruitingnodes. All leaflets at fruiting nodes abscised during the final10 d period, while the two lower leaves, not subtending fruits,remained green and supplied most of the carbon required by developingfruits and roots. Throughout fruiting all currently-active sourcessupplied all sinks, with only slight evidence of blossom leavesspecializing in nourishing their subtended fruits. Of the carbontranslocated from leaves during fruiting 32% came from the topmostleaf, 28% from the leaf below this, 16% from the next leaf,and the remaining 24% from the lowest three leaves. Some 80%of the fruit's total intake of carbon came from leaves, therest from mobilization of stored carbon (partly sugars and starch)fromother vegetative parts. Key words: Carbon, Translocation, Cowpea     

The impact of two gall-forming arthropods on the photosynthetic rates of their hosts

The impact of herbivores on host plant photosynthetic rates can range from negative to positive. While defoliation by chewing herbivores can result in increases in photosynthesis followed by compensatory growth, other herbivore guilds, such as mesophyll feeders which damage photosynthetic leaf tissues, almost always reduce photosynthetic rates. The impact of galling herbivores on host photosynthesis has rarely been examined, even though the limited tissue disruption and the strong metabolic sinks induced by gall-forming herbivores could potentially stimulate photosynthetic rates. I examined the hypothesis that gall-inducing herbivores could stimulate photosynthesis in neighboring leaves in response to increased sink-demand by the gall. To address this hypothesis, I measured photosynthetic rates of galled leaves or leaflets, neighboring ungalled leaves or leaflets, and ungalled leaves or leaflets on ungalled shoots on naturally growing Prunus serotina (wild cherry) and Rhus glabra (smooth sumac). The leaves of wild cherry were galled by an eriophyid mite, Phytoptus cerasicrumena; the leaves of smooth sumac by an aphid, Melaphis rhois. I found that both species reduced the photosynthetic rates of the leaves or leaflets they galled from 24 to 52% compared to ungalled leaves in ungalled areas of the plants. Contrary to my hypothesis, mite galls on wild cherry reduced photosynthesis of neighboring ungalled leaves within the same shoot by 24% compared to ungalled leaves on gall-free shoots. Aphid galls on sumac leaflets did not significantly alter the photosynthetic rates of neighboring leaflets relative to ungalled leaves on ungalled shoots. Although gall-formers would appear to have the potential to stimulate photosynthesis in the same manner as defoliating herbivores, i.e., by increasing sink demand relative to source supply, I found only negative impacts on photosynthesis. I suggest that sink competition for nutrients between developing leaves and growing gall tissue may account for the negative impacts of sink-inducing gallers on photosynthesis. Received: 17 October 1997 / Accepted: 2 February 1998     

The utilization of recently assimilated carbon in graminaceous plants

Isotopic carbon and infra-red gas analysis techniques were used to measure the following growth attributes in maize, sorghum, winter wheat and perennial ryegrass: the rate of entry of carbon into each main shoot leaf; the rate of translocation of leaf assimilate to meristems; the fraction of leaf and total shoot assimilate respired in one diurnal period; and the distribution of residual assimilate to new leaf, stem, axillary shoots and root. The two tropical plants possessed higher leaf assimilation rates and larger leaves than the temperate species, but their efficiency of translocation was only marginally superior in the experimental conditions. In all species, c. 25% of the assimilate generated in the 8·4h photoperiod was respired in in the same diurnal period. Maize and sorghum partitioned a greater proportion of their total shoot assimilate to new leaf tissue at the main shoot apex and to root than wheat and ryegrass. On the other hand, wheat and ryegrass exported up to 30% of their assimilate to axillary shoots; in sorghum, little assimilate was translocated to axillary shoots, while in maize this activity was completely absent. Plant habit, as exemplified by the contrast between the annual, single-axis maize plant and the perennial, multi-tillering ryegrass plant, appears to be a reflexion of the pattern of assimilate distribution to areas of potential growth. With the exception of superior leaf assimilation rates in maize and sorghum, the four species showed no marked differences in respect of the production, transport and respiratory utilization of assimilates.