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Growth analysis was applied to grain sorghum (cv. RS610) grown at low, medium and high population densities, i.e. 14,352, 143,520 and 645,836 plants ha-1 respectively. The medium densities had two arrangements of plants, square (S) and rectangular (R). Crop growth rates, inflorescence growth rates, leaf area indices, net assimilation rates and leaf growth rates were calculated from growth functions of plant dry matter and leaf area over time. Differences in crop growth rate between populations in the early stages were attributed to leaf area development-speci6cally to the initial leaf area (dependent on seedling number) and not to differences in leaf growth rates. Peak crop growth rates were 15.0, 27.5, 26.0 and 45.8 g m-2 day-1 for the low, medium (S), medium (R) and high populations respectively. The large difference between the growth rates of the medium (S) and the high populations was not explained by differences in the amount of radiation intercepted. Although leaf area indices were 4.6 and 10.2 respectively for the two populations, both canopies intercepted almost all of the noon radiation. Light extinction coefficients were 0.45 and 0.29 respectively. The relationship between net assimilation rate and leaf area index was such that for comparable leaf area indices above 2, plants at higher densities showed greater improvement in yield per unit increment in leaf area index. A maximum grain yield of 14,250 kg ha-1 was obtained at the high population density as a result of higher dry matter production, but a similar harvest index to that of the crops grown at the other densities. Inflorescence growth rate (g m-2 day-l) slightly exceeded crop growth rate in the latter part of grain filling, which indicated that there was some retranslocation to the grain of previously assimilated material. The maximum grain yield represents an efficiency of utilization of short-wave solar radiation during crop life of 2.5 x g cal-1.

Sorghum plants (cv. RS610) grown in field stands at two population densities were manipulated to increase the supply of assimilates (by removing neighbouring plants) at one of three developmental stages-10-15 days after floral initiation, 1 week prior to three-quarters anthesis, and 1 week after three-quarters anthesis. Post-initiation exposure increased the number of grains per inflorescence 1.8-fold and 3.5-fold in medium and high density populations respectively, but had relatively less effect on grain size. Higher grain number resulted largely from more grains per secondary branch in the lower part of the inflorescence. Neither of the post-heading exposure treatments influenced grain number, but the higher supply of assimilates resulted in larger grains at both densities. Differences at one density only between yield characteristics of plants exposed at the two times provide evidence of inter-plant competition for assimilates to the extent that the potential size of the grain may be affected. Shading (10% light transmission) of plants grown in a glasshouse, whether for 1 week at anthesis or during grain filling, reduced grain yield at maturity by the same amount as the immediate reduction at the end of the shading period. The experiment was unable to demonstrate changes in the potential size of grains resulting from the loss of assimilates at anthesis. There was substantial compensation for the loss by translocation from other plant parts.

In field and glasshouse experiments with grain sorghum (cv. RS610), the assimilate supply was varied by increasing or decreasing radiation and carbon dioxide supply; the potential grain storage capacity was altered by spikelet removal; and the transport system was reduced by incision of the culm. Plants grown at four population densities in the field were manipulated to increase (by removing neighbouring plants) or decrease (by shading) the supply of photosynthates during grain filling. These treatments affected grain size and thus yield. Removal of some of the spikelets at three-quarter anthesis resulted in a significant increase in the size of those grains remaining at maturity. From anthesis onward, a reduction in the capacity of the transport system in the culm had no significant effect on grain yield. These results are interpreted as evidence that grain yield is not limited by the storage capacity of the grain, or by the transport system involved in moving material from the stem to the grain. Treatments which altered the demand for assimilates by the grain, relative to the supply, did not affect net photosynthesis. Dry matter produced in excess of grain requirements accumulated in other plant parts, including the root. Potential grain size was influenced by interspikelet competition operating within 1 week after three-quarter anthesis.

Two grasses and two legumes were grown at three illuminances: one grass and one legume in a growth cabinet (100, 50 and 34 % relative illuminance) and one of each in a glasshouse (100, 30 and 10% relative illuminance). The response to shading was investigated by using classical growth analysis techniques, and the photosynthesis-respiration balance obtained by Watson and Hayashi's method. Shading caused a reduction in the proportion of root and a corresponding increase in the proportion of leaf, and an increase in the shoot/root ratio in all species except green panic. The relative growth rates of grasses (which were only clearly superior at the highest illuminance) were affected more by shading than were those of legumes, and the greater shading effect resulted from a greater decrease in net assimilation rate, and to a lesser extent, from a smaller compensatory increase in leaf area ratio in the grasses than in the legumes. The greater sensitivity of net assimilation rate of grasses to shading arose from a stronger dependence of the photosynthetic rate on illuminance. The significance of these findings for pasture production is discussed. Both tiller and runner production were dependent upon the total amount of radiant energy received by the plant, and in the growth cabinet, at least, were independent of its intensity, duration and sequence.

The growth analysis technique for the separation of photosynthetic and respiratory components of net assimilation rate used in previous controlled environment studies of early vegetative growth was extended to natural lighting conditions. The use of short-wave radiation rather than days in the light as the quantification of photosynthetic opportunity in the regression analysis improved the accuracy of estimation of photosynthesis and respiration. In one experiment, concurrently with the growth analysis procedure, carbon dioxide exchange measurements were made on one grass and one legume. There was good agreement between the two methods in the estimates obtained. The data support conclusions from previous experiments concerning the superior growth rate of grasses. Species rankings were similar in both sets of experiments.

Effects of photoperiod on growth and development of cassava were studied in controlled environments. Plants of cultivars M Aus 7 and M Aus 10 were grown under short (10-h) and long (16-h) photoperiods and harvested at approximately weekly intervals for a period of 13 weeks. Long photoperiods resulted in large increases in leaf area through increases in both number (both cultivars) and size of leaves (M Aus 7 only). The timing, extent and anatomical characteristics of storage root initiation were similar for both cultivars under 10-h and 16-h photoperiods but the subsequent balance between shoot and storage root growth strongly favoured shoot growth in long days. Dry matter partitioning followed a simple allometric pattern with the proportionality between the relative growth rate of shoots and the relative growth rate of storage roots remaining constant with time but being altered by photoperiod. Distribution to storage roots was quantitatively reduced in another experiment using M Aus 10 with each additional 1-h increment from 10-h to 16-h photoperiods. A treatment consisting of a short day (10 h) plus a 1-h night break in the middle of the dark period produced growth similar to long day treatment. Differences in cultivar response (Ceiba, CMC 84 and M Aus 7) to a range of photoperiods (11, 12, 13 and 14 h) were detected in a third experiment. The possible mechanisms and significance of these responses are discussed.

Four cultivars of French beans (Phaseolus vulgaris L.) were grown at a range of plant population densities and rectangularities in two seasons in south-eastern Queensland. Plant population densities two to three times those used commercially increased yields by 25-30%. This increase resulted from a greater number of pods per unit area with only a small reduction in pod size. Rectangularity in the range 1-8 had no effect on yield. These responses are discussed in terms of time courses of leaf area index development, light interception by canopies, crop growth rates and dry matter distribution between vegetative and reproductive growth after flowering.

The effect of water deficits on the relative photosynthetic rates in heads and leaves of sorghum was measured. When water was withheld, photosynthesis virtually ceased in the leaves but continued in the heads at the same rate. The photosynthetic activity of heads increased from about 12 per cent of the total in a well watered plant to about 88 per cent for a plant at high moisture deficit. Relative transpiration of heads also increased as water status declined. In watered plants, loss from heads was, like photosynthesis, about 12 per cent of the total but increased with water shortage to about 35 per cent.

Experimental data are presented to explain some of the varying effectiveness of phenyl mercuric acetate (PMA) as a stomatal closing antitranspirant. In a comparison of sorghum, maize, cotton, peanuts, and tomatoes, stomatal apertures were affected to different extents by PMA sprays and environmental conditions. In addition, the relationship between aperture and stomatal resistance differed between cotton and sorghum. Thus the effectiveness of PMA depends on the balance between the degree to which stomata may be closed by prevailing conditions, the extent to which PMA can bring about further closure, and what this closure means in terms of diffusion resistance. In pot experiments with these species, PLMA sprays conserved soil water, but the higher transpiration by unsprayed plants led to water deficits which soon caused stomatal closure. The balance between soil water storage, evaporative conditions, and the particular objectives of transpiration reduction would determine the value of application. Trials with sorghum, in both the field and glasshouse, showed that the effect of a single spray of PMA may be short-lived, and that successive applications may be necessary for a persistent effect. The resulting toxicity may be unacceptable.

The relationships between illuminance, leaf resistance, and net photosynthesis were studied in sorghum. Some comparisons were made between varieties chosen on the basis of possible differing environmental adaptation, and with cotton whose photosynthetic response is known to differ from that of sorghum. The after-effects of varying periods and degrees of shading were examined for sorghum. Net photosynthesis of sorghum was not light saturated at 11,000f.c. and increases above intermediate illuminance arises almost entirely from increasing stomatal opening. At lower intensities other factors become important, but leaf resistance still exerts the major control. In contrast, leaf resistance in cotton was much lower at all illuminances, and was relatively little affected by changing illuminance. Light saturation of cotton at 5000 f.c. was associated with constant leaf resistance, while at lower intensities, declining net photosynthesis must depend mainly on factors other than stomatal closure. Variation between four sorghum varieties in response to illumination was attributed solely to differences in leaf resistance. Stomata opened slowly after shading at below 3000 f.c., and at a rate independent of the duration of shading. Shading in the afternoon had greater and more persistent effects. Net photosynthesis was affected according to stomatal aperture.

Pot experiments with nodulated tropical pasture legumes on a soil low in calcium and with a pH of 5.3 showed wide variation between species in response to lime applications. Further experiments with Lotononis bainesii which showed a small response to lime, and Glycine wightii which showed a large response, allowed the lime effects to be resolved into calcium and pH effects. This was done by supplying calcium in a number of forms, and by altering the soil pH with a variety of compounds. The yield of Lotononis was affected only by pH, in spite of large variation in the calcium content of the plant resulting from varying the calcium supply. Glycine responded to both calcium and pH. The pH effect did not involve molybdenum supply or the alleviation of manganese and aluminium toxicities. Yield responses to low levels of calcium nitrate could not be attributed solely to pH effects, and suggest that a nodular dependence on favourable pH may have been circumvented. A calcium content of only 0.2% in the shoot is apparently sufficient for Lotononis, and the soil used was able to supply this.

Inclined point quadrat analysis was applied to pure swards of two pasture species, Desmodium intortum and Setaria sphacelata. Canopy structures are described in terms of the denseness and inclination of foliage and stems. Light penetration patterns throughout the day were calculated. The two species differed greatly in these characteristics. Desmodium leaf was oriented at a uniform mean angle to the horizontal throughout the height of the canopy, but leaf occurred predominantly in the upper layers. In contrast, leaf angles in Setaria varied down the profile, with most of the leaf in the middle layers. There were diurnal variations between and within the species in the amounts and distribution of sunlit leaf area index. Diurnal patterns of canopy photosynthesis were estimated from calculated leaf and light profiles, photosynthesis-illuminance data, and records of incident radiation. There was an overall 1.8 : 1.0 advantage in sward photosynthesis by Setaria, arising from a greater than two-fold superiority at high angles of the sun, falling to a 1.6 : 1 .0 advantage at low angles. This trend follows the diurnal patterns of light penetration into the two canopies. From calculated sward photosynthesis, concurrently measured dry matter production, and other data it was concluded that the relative magnitude of productivity corresponded very closely to the relative photosynthetic capacities of the leaves. Therefore, canopy structures were comparatively unimportant as determinants of yield differences.

The relative contributions of different photosynthetic sites to the filling of the grain in grain sorghum (Sorghum vulgare cv. Brolga) were estimated by measuring the 14C in the grain after exposing various leaves and the head to radioactive carbon dioxide. Methods for preventing photosynthesis were also used. Of the grain yield, 93% was due to assimilation by the head and upper four leaves. The head contribution of 18 % was due equally to direct assimilation of atmospheric carbon dioxide and to reassimilation of carbon dioxide released within the grain by respiration of material translocated from the leaves. The remaining 75 % was equally assimilated by the upper four leaves, the flag leaf being the most efficient contributor per unit area and the third uppermost leaf the least efficient. The percentage contributions to the grain by the flag leaf and fourth leaf, estimated from the decrease in grain yield when they were shaded, agreed closely with the estimates obtained by using 14CO2.

Sorghum plants were allowed to assimilate known relative amounts of 14C in the two periods: (i) from full expansion of the third uppermost leaf to anthesis; and (ii) from anthesis to grain maturity. By comparing relative 14C activity in the mature grain, the relative partitioning of photosynthate between grain production and other uses in the two periods was calculated. Plant and grain dry weight data from another experiment showed the amounts of net photosynthesis in the two periods. It was then possible to calculate the maximum amount of the material assimilated in the pre-anthesis period that could have gone to grain production, and thus the percentage of grain material derived from this earlier period. The estimate was 12%.

A previous study of one grass and one legume at early vegetative stages has been extended to include a further nine species of each. It confirms the superior relative growth rates (RW) of grasses, arising from very high net assimilation rates (EA), in turn dependent on high photosynthetic rates (P). Leaf area ratios (FA) varied over a similar range in both groups. RW values of grasses in the second week after sowing varied from 0.41 to 0.55 gg-1 day-1; for legumes, they were 0.31-0.36. Corresponding EA values were 1.5-1.9 g dm-2 wk-1 in grasses, and 1.1-1.8 in legumes. Within both grasses and legumes, comparisons between species can be made with less certainty. In grasses, variation in FA combined with generally uniform EA values resulted in variable RW values. Both FA and EA varied in the legumes but were usually compensatory, leading to uniformity in RW. Watson and Hayashi's method of separating EA into photosynthetic and respiratory components was used. Differences both between and within grasses and legumes were shown, but there are probably important errors in this technique, particularly where there are discrepancies between light and dark respiration.

Lettuce and tomato seeds were soaked in solutions of sodium molybdate in an attempt to provide the whole of the molybdenum requirements of these species. Dry weight comparisons were made between plants grown from soaked seed in molybdenum-deficient solutions, and those grown from untreated seed in complete solution. The requirement of tomatoes was met by soaking seeds in solutions containing 5000 and 50,000 p.p.m. of molybdenum for 8 hr, but satisfactory results were not obtained with lettuce. The ability of soaked seeds to retain molybdenum against outward diffusion during germination is important. Well-washed, soaked seeds may lose much in this way. The implications of such loss to the soil are discussed.

Hamil grass and Siratro were grown as single plants in pots in controlled environment cabinets at a temperature favouring high growth rates and with adequate water and mineral nutrients. Leaf areas and dry weights of plant parts were recorded in two experiments, to which some of the plants were common. In the first experiment, weekly samples until 4 weeks from sowing provided data for the calculation of relative growth rates, net assimilation rates, and leaf area ratios. The relative growth rate of Hamil grass was almost twice that of Siratro and appears to be the highest yet recorded for any plant. The higher growth rate for the grass resulted from a high net assimilation rate, although the leaf area ratio was lower. The second experiment used Watson and Hayashi's method, in which plants are kept in darkness for a varying number of days to separate net assimilation rate into photosynthetic and respiratory components. Data were collected at 2 and 4 weeks from sowing. The high net assimilation rate of the grass was shown to result from a higher photosynthetic rate, despite a much greater respiration rate than in the legume. The technique used for estimating photosynthesis and respiration is discussed in relation to some of the assumptions that it makes.

A complete fertilizer was applied to a native heath community in the coastal lowlands of south-eastern Queensland. Changes in species composition, and the structure of the community 14 years later are reported. The application of fertilizer allowed the establishment of non-endemic plants, notably Imperata cylindrica and Baccharis halimifolia, at the expense of many of the original components. It also allowed Angophora woodslana, which originally existed as a fire-maintained coppice, to develop into a low tree layer, thereby completely changing the structure of the community. Details of changes in species composition and density are recorded.