Nitrogenase-dependent acetylene reduction, leaf, herbage, and root growth, and total nonstructural carbohydrate accumulation of alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.) were compared to learn how nitrogen fixation capacity and vegetative growth respond to partial (75-85%) or total shoot and leaf removal, and floral debudding. Treatments were imposed on greenhouse-grown plants during two successive harvest cycles.Both species displayed an initial decline in total nitrogenase activity within 2 days of harvest and a subsequent recovery of activity after 10 to 21 days. Rate of recovery varied with the amount of leaf area removed. Periodic flower removal did not significantly alter total nitrogenase activity of either species compared with the unharvested controls. In the first harvest cycle, partial leaf area removal did not affect nitrogenase activity of alfalfa, but activity of trefoil was reduced 56%. In the second harvest cycle, partial leaf area removal reduced total nitrogenase activity of alfalfa 46% and that of trefoil 69%. Complete leaf and shoot removal reduced total nitrogenase activity of alfalfa 78% after the first harvest and 86% after the second harvest.Recovery of nitrogenase activity after harvest paralleled leaf area expansion in both species. After the initial decline following the first partial harvest, total nitrogenase activity and leaf area of alfalfa increased 170 and 500%, respectively. After the initial decline following the second partial harvest, nitrogenase activity and leaf area of alfalfa increased 280 and 800%, respectively. Partly harvested trefoil and completely harvested alfalfa showed similar response patterns. Release of bud dormancy and leaf area expansion after flowering of nonharvested alfalfa apparently caused an increase in nitrogenase activity, but patterns of acetylene reduction and leaf area were not otherwise closely correlated in controls of either species. Decline and accumulation of total nonstructural carbohydrates in both species varied with defoliation treatment. Patterns of nonstructural carbohydrates in root tissue were not closely related to the changes in total nitrogenase activity caused by shoot removal. associated with photosynthate supply and partitioning in a number of species. Supplemental light (10), grafting of a second shoot (21), and CO2 enrichment (6) enhanced TNA of soybeans (Glycine max L. Merr.). Higher PAR and LA accompanied increased TNA of several forage legumes (3, 4). Floral debudding and the associated promotion of vegetative growth stimulated nodule growth (16) and TNA (11) of pea (Pisum sativum L.). The TNA of soybeans declined after CO2 deprivation (9), shading (10), defoliation (10), stem girdling (9), shoot excision (12), and prolonged darkness (12). Similarly, TNA decreased after exposure of subterranean clover to low light intensity (4), removal of pea shoots (13), and continuous darkening of pea and alder (23). The depression of TNA by such competing sinks as developing fruits (1, 7, 10) fur...

Differences in source-sink relationships between short-and longseason cotton (Gossypium hirsutum L.) cultivars, the contribution of main stem leaves to boll growth, and the distribution of nonstructural carbohydrates late in the season are not well understood. This study compared the cultivars Tamcot CAMD-E (a short-season type) and Stoneville 213 (a long-season type) for two seasons in the field. To study photosynthate partitioning, the uppermost fully expanded leaf on the main stem subtending a reproductive axillary branch was pulsed with 14CO~ at the onset of reproductive growth (ORG) and at early reproductive growth (ERG), and sampled 48 h later. To study nonstructural carbohydrate partitioning, 14C-starch was injected into the base of the main stem just above the soil surface durifig late reproductive growth (LRG). At ORG in 2 yr and ERG in the year with poor growing conditions, there were no cultivar differences in partitioning of accumulated dry matter throughout the plant or recent ~4C-photosynthate. However, at ERG in a year with good growing conditions, Tamcot CAMD-E distributed a greater percentage (45%) of its dry matter to reproductive growth than did Stoneviile-213 (30%). Tamcot CAMD-E also partitioned 85% of its ~4C-photosynthate to the reproductive structures compared with only 60% in Stoneville-213. Main stem leaves were an important source of photosynthate to the developing bolls in both cultivars and years. The total nonstructural carbohydrates of the main stem and root contained from 20 to 90% of the 14C-photosynthate from the main stem leaves partitioned to the main stem or root at ORG and ERG. Both cultivars partitioned more than one-half of the injected 14C-starch to the main stem and its vegetative branches in both years at LRG. At this later stage of reproductive growth, cotton production could benefit from a different pattern of partitioning that favored the distribution of stored carbohydrates to reproductive structures.

A greenhouse experiment used a replacement series design to compare the vegetative growth 6 wk after emergence in pure cultures and mixtures of winter wheat and Italian ryegrass, with phosphorus (P) levels recommended by soil testing. The planting proportions of wheat and Italian ryegrass were 100 and 0%, 75 and 25%, 50 and 50%, 25 and 75%, and 0 and 100%, respectively. There was no alleopathic interaction between the species. Both species in all pure and mixed cultures had substantially less growth in the low-P than in the recommended P treatment. However, the relative performance of the two species differed between P treatments. In the recommended P treatment in pure culture, Italian ryegrass had more tillers and greater root weight and length than wheat. Pure culture wheat in the low-P treatment exceeded pure culture Italian ryegrass in leaf area, weights of leaves, stems, and roots, and root length. Thus, the growth of wheat was inhibited less by P deficiency than the growth of Italian ryegrass in pure culture. In the 50:50 mixture of the recommended P treatment, wheat had greater leaf, stem, and root weights than Italian ryegrass. In the 50:50 mixture of the low-P treatment, the two species were very similar in growth, except that Italian ryegrass had about three times more tillers than did wheat. Whereas P deficiency limited the growth of wheat less than Italian ryegrass in pure culture, P deficiency did not affect the relative competitiveness of Italian ryegrass as much as wheat in mixed cultures. The ability of Italian ryegrass to compete with wheat when P was limiting may result from a difference in root growth. Italian ryegrass had a greater fresh root length to fresh root weight ratio than did wheat in the low-P treatment in pure culture and in the 50:50 mixture. The greater surface area of Italian ryegrass roots likely enhanced the competitiveness of Italian ryegrass relative to wheat under P-deficit conditions. Thus, the use of the recommended P nutrition from soil testing may be a key component to diminish Italian ryegrass competition in wheat fields.

Greenhouse experiments in central Texas assessed the relative importance of above- and belowground interactions of semidwarf Mit wheat and Marshall ryegrass during vegetative growth. One experiment used partitions to compare the effect of no (controls), aboveground only, belowground only, and full interaction for 75 d after planting (DAP) one wheat and nine ryegrass plants in soil volumes of 90, 950, and 3,800 ml. The results with the different soil volumes were similar. Wheat growth in the aboveground interaction only did not differ from controls. However, the full or belowground only interaction of wheat with ryegrass reduced wheat height, leaf number, tillering, leaf area, percent total nonstructural carbohydrates in shoot, and dry weights of leaves, stems, and roots 45 and 75 DAP compared to controls. Wheat in full and belowground interaction only did not differ from one another in growth. A replacement series experiment of 56 d also showed that the competitive advantage of ryegrass was relatively greater in root than in shoot growth. No allelopathic response of wheat to ryegrass occurred. While the tallness of the semidwarf wheat minimized aboveground interference by ryegrass, the root growth of the thinner and more fibrous roots of ryegrass greatly enhanced its belowground competitiveness.

The net partitioning of current photosynthate among vegetative organs of nodulated alfalfa (Medicago sativa L.) was investigated by determining radiolabel distribution from upper and lower source leaves, at different times of day, and during successively longer chase periods. Photosynthate was exported sooner and more completely by a fully expanded lower than by a fully expanded upper source leaf. The radiolabel pulse from the lower source leaf peaked in the main stem within 1.5 hours, in the crown and nodules after 3 hours, in the unexpanded leaves and apex of the main stem after 6 hours, and in the shoots growing from leaf axils on the main stem after 24 hours. The results suggest that the crown, apex, axillary shoots, and nodules both imported and mobilized photosynthate originating at a lower source leaf, while roots and shoots growing from the crown showed net accumulation. The pulse from the upper source leaf was initially rapidly exported by the main stem and imported by the root between 1.5 and 3 hours, but there was no net change of label content of these organs during the ensuing 21 hours. Rapidly growing organs had the highest concentrations, and the largest organs had the highest content of radiolabel. These results provide new information about the accumulation and circulation of photosynthate within the alfalfa plant. recently begun to receive attention (1), whereas much more is known of photosynthate usage by nodules of annual legumes (e.g. 14). Studies in a number of species, chiefly monocots, showed that the partitioning of photosynthate from source to sink was affected by relative sink size (3), source-sink distance (3, 12, 17, 18), growth stage (1), intersink competition (3), and time of day (9, 16). However, there is a paucity of comparable information on photosynthate partitioning among organs in nodulated perennial legumes.Our objective was to investigate the net partitioning of current photosynthate among vegetative organs of nodulated alfalfa during a growth stage often harvested for forage. We measured the net allocation of 14C-photosynthate to organs from two source leaf positions, at different times of the day, and during a succession of chase periods to develop an initial understanding of the dynamic patterns ofphotosynthate partitioning in this important perennial legume.MATERIALS AND METHODS Plant Culture. Seeds of 'Saranac' alfalfa (Medicago sativa L.) treated with commercial inoculum (Nitragin,3 Milwaukee, WI) were sown and cultured in 10-cm clay pots filled with nil-nitrate sand. Plants were grown in a naturally lit glasshouse at 25/20C day/night with photoperiod extension to 16 h with cool white fluorescent tubes that provided a supplemental photosynthetic photon flux density of 300 jmol m-2 s-' on the upper leaves.Plants were grown for either two or three harvest-regrowth cycles, each of 30-d duration, and fertilized after each herbage harvest with a total of 0.3 g P and 0.5 g K during the experimental period. Inter-and intrarow pot spacing was 20 cm.Plant Selection for...