The rate of net CO2 exchange and the distribution of 14C-assimilates was studied in single plants of L. multi/lorum. Defoliation of tillers, while leaving the main shoot intact, diverted some of the 14C assimilated by leaf 10 of the main shoot to the regrowing tillers.

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Photosynthesis and Assimilate Distribution in Lolium Multiflorum Lam. Following Differential Tiller Defoliation

Black oak (Quercus velutina Lam.) and gray birch (Betula populifolia Marsh.) trees were defoliated in 0, 1, 2, or 3 successive years. Concentrations of 8 minerals, 4 sugars, and 25 amino acids in the foliage of these trees were measured when gypsy moth, Lymantria dispar (L.), reared on them were in instars I, III, IV, and V. These concentrations were tested for changes among years, and changes due to previous-and current-year defoliations. Most foliar constituents varied in concentration from year to year, though relatively few were affected by current or previous defoliations. In black oak, concentration of total free sugar measured during the fifth instar was reduced by current defoliation and correlated with gypsy moth pupal weight. In gray birch no decrease in sugar concentration due to defoliation was apparent, but pupal weights of gypsy moths reared on these trees were correlated with the ratio of total free sugar to calcium in the foliage measured during the fifth instar. Some implications of these apparent relations for gypsy moth larval growth and population dynamics are discussed.

Nutritional changes in host foliage during and after defoliation, and their relation to the weight of gypsy moth pupae

The effect of drought on the fixation and translocation of labelled carbon in the potato (S. tuberosum) was examined. Low leaf water potentials resulted in a decrease in translocation which was proportional to the decline in net photosynthesis, irrespective of whether tubers were absent or present. The ratio of 14C entering polysaccharides to that in ethanol-soluble compounds markedly decreased in droughted leaves. These data suggest that the decrease in growth of tubers under conditions of water deficit can be ascribed to direct effects on tuber growth and photosynthesis but not to interference with vein loading or velocity of transport of photosynthate.

Effect of Water Deficit on Translocation of Carbohydrate in Solanum tuberosum

&p.1: Green pruning was used to induce source limitation in 1-year-old Eucalyptus nitens (Deane and Maiden) Maiden plants in an investigation of regulation of net CO2 assimilation (A). The pruning regimes involved removal of 0, 50% or 70% of the length of green crown, equivalent to 0, 78% or 95% of leaf area respectively. Gas exchange measurements were made on fully-expanded leaves at 80% of crown height prior to pruning, and at weekly intervals over the 8 weeks following pruning. An increase in A was observed in response to pruning a week after treatment. In 50%-pruned plants this increase in A was observed for 6 weeks, but in 70%-pruned plants it was still evident at the end of the experiment. While leaf conductance (g) increased considerably following pruning, stomatal limitation was unaffected, indicating that the most important changes in conductance of CO 2 were in the mesophyll. Both carboxylation efficiency (C e ) and RuBP regeneration capacity (V j ) increased following pruning. The magnitude and duration of changes increased with pruning severity. There was evidence that A was primarily limited by V j , although in 70%-pruned plants Ce may have been the main limitation early in the experiment. The response of A to low p(O 2 ) indicated that A was not limited by rates of triose phosphate utilisation (TPU), and that, at more severe pruning levels, rates of TPU may have increased in the short term. Apparent quantim yield was unaffected by pruning, suggesting no change in the production or consumption of ATP or NADPH 2 . There was a negative linear relationship between A and the ratio of leaf area to aboveground dry mass, and a positive linear relationship between A and specific leaf area. Both suggested a link between rates of assimilation of the level of source limitation. The capacity of E. nitens to regulate photosynthesis in response to pruning increases the potential for maintaining a balance between assimilation and utilisation of carbon, thereby mimimising the impact of pruning on growth. However there will be a level of source limitation at which photosynthesis is limited by the rate of one or more biochemical reactions, and above which complete compensation is not possible. At such levels growth will usually be affected. In this experiment diameter growth was slightly reduced by 50%-pruning, but both height and diameter increment were substantially reduced by 70%-pruning.

Regulation of photosynthesis in Eucalyptus nitens (Deane and Maiden) Maiden following green pruning

In Dupontia fischeri R. Br. the pattern of translocation of photosynthetically incorporated 4C was established within six hr after treatment in situ at Barrow, Alaska. More radiocarbon went to younger rather than older tillers. The most effective sinks were root, lateral root, and rhizome primordia and nonemergent tillers which were actively growing. Substantial amounts of radiocarbon appeared in the stem base and rhizome of the treated tiller. Although relatively little appeared in other plant parts, except for the sinks mentioned above, interdependence of tillers appeared to be maintained until flowering occurred. This pattern of distribution was established early in the season once the soil thawed, and growth began and was maintained until mid-August. Some translocation to active growth centers occurred even while the soil was thawing. Redistribution of radiocarbon to new root and leaf tissues produced after the time of treatment was observed. There was no autoradiographic evidence during the season to indicate storage of large amounts of mobilizable photosynthate in the stem base or rhizome of a tiller at the time these field studies were carried out.

https://www.jstor.org/stable/pdfplus/2441890.pdf

H. T. Oo

Papers

An Investigation into the Rate and Control of Assimilate Movement from Leaves in Pisum sativum