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Response of cassava to row spacing and plant population density (0.62 plants m−2 in 180-cm rows; 1.23 plants m−2 in 90-, 180-, 270-, and 270- plus 90-cm (i.e. paired rows); and 2.46 plants m−2 in 90- and 180-cm rows), and to soybean intercrop at two row spacings of cassava (90 and 270 cm) was studied at a high latitude (27°S) in south-east Queensland, Australia, where low temperature limits a growing season to 9 months. Detailed observations were made in sole crops on leaf canopy structure and light penetration in the three row spacings at the medium density to allow an estimation of light availability for an intercrop between cassava rows. The low plant density or the 270-cm row plants produced the lowest total dry matter and tuber yield at harvest, while the two higher densities or the two narrower rows produced similar total and tuber dry weight. Intercropped cassava produced a similar tuber yield to the sole crop at the corresponding spatial arrangement, but total dry matter was lower in the former. Leaf area index was similar among the 90-, 180- and 270-cm row spacings in sole crops throughout the growth period. However, leaf area was unevenly distributed horizontally for a longer time as row spacing increased. This resulted in light penetrating the inter-row space for a longer period in wider rows in sole crops, more than 50% full sunlight reaching soil level for 90, 120 and 130 days after planting in the 90-, 180- and 270-cm rows, respectively. This light environment would be available for an intercrop if cassava growth is not affected by the intercrop. The results for cassava intercropped with soybean show that in fact cassava growth was reduced by the associated soybean, and hence light available for the soybean growth would have been more than that estimated above.

The development of yield in cassava, either as a sole crop or intercropped with quick-maturing soybean sown 1, 5 or 9 weeks after cassava planting, or a succession of two soybean sowings 1 and 14 weeks after planting, was followed at a high latitude (27°S) where the cassava growing season is limited to 9 months by winter temperatures. Competition, at least largely for nitrogen, restricted the growth of cassava, but after soybean harvest, leaf-area index increased such that there was little difference in interception of radiation among crops, and consequently growth rates and thus amounts of assimilate potentially available for tuber growth were similar. Competition from earlier-sown soybean greatly reduced branching by cassava. The reduced number of branches were sufficient to provide adequate leaf-area index, but were a reduced sink for assimilates during the main period of tuber growth. The slightly reduced assimilate supply available in early intercropped cassava was offset by the increased partitioning to tubers. As a result, soybean intercropping did not reduce tuber yield, except slightly in the case of double-intercropped cassava, but provided an additional yield of grain. Land equivalent ratio was particularly high at about 1.6 when soybean was sown within 5 weeks of cassava planting. When soybean was sown 9 weeks after cassava planting, land equivalent ratio was reduced to about 1.3 as a result of lower soybean yield.

The responses in the growth of peanuts (Arachis hypogaea L. cv. Florunner) to plant density and to spatial ratio (ratio of inter-row: intra-row spacing) in a tropical monsoonal environment were investigated. Biological yield (above-ground biomass plus pods) was unresponsive to spatial ratio over the range 1:1-1:7:19, but it increased markedly (12 600-16 900 kg ha−1), with increasing density up to the maximum density of 588 000 plants ha−1. In contrast, economic yield (commercially recoverable pods and kernels) was relatively unresponsive to plant density in the range of 88 000–394 000 plants ha−1, after which a marked decline was recorded (6 500 kg ha−1 pods or 4 900 kg ha−1 kernels, falling to 5 700 kg ha−1 pods or 4 300 kg ha−1 kernels). Economic yields were maximized at square plant-arrangements (6 900 kg ha−1 pods or 5 200 kg ha−1 kernels), but were relatively uneffected by increasing plant rectangularity from spatial ratios of 1:2.15-1:7.19 (6 100 kg ha−1 pods or 4 600 kg ha−1 kernels). The increase in biological yield with increasing plant was primarily attributed to differences in the amount of photosynthetically active radiation (PAR) intercepted (I), which was evident during both vegetative growth and kernel development. In contrast, I was unresponsive to spatial ratio. Neither density nor spatial ratio affected the average efficiency of conversion (Ec) of I to biological yield during vegetative growth (2.6-3.0 g MJ−1) nor during the entire life-cycle (2.1–2.3 g MJ−1). Average Ecm during kernel development was 55–60% of that during vegetative growth. Thus the higher economic yield of the square plant-arrangement was not associated with changes in I and Ec. Rather it was associated with greater partitioning of dry matter to reproductive yield and a greater proportion of total pods recovered at harvest.

Earlier experiments on cassava/soybean intercropping showed that early-maturing soybean reduced the growth of cassava, but that after soybean maturity partitioning of assimilates favoured tuber growth so that tuber yield was not reduced below that of sole cassava. An experiment was conducted to test the hypothesis that this favourable partitioning was due mainly to limited N availability to the intercropped cassava, and also to examine if performance of cassava/soybean intercropping was affected by the level of N application. There were three N treatments in both sole and intercropped cassava; no application, and application of 80 kg N ha-1 either at planting or at day 85 when soybean was harvested in intercropping. The experimental area was irrigated frequently to minimize the development of plant water stress. In sole-cropping, application of N at planting enhanced leaf area and dry-matter production during early stages of growth, but the effects did not persist until the final harvest. Dry-matter partitioning to tubers was reduced, and in consequence tuber yield tended to be less in this treatment than in the no-N control, although not significantly. Application at day 85 had negligible effects on dry-matter production and partitioning. The adverse effect of soybean on the growth and morphology of intercropped cassava was similar to, but more severe than, that of the no-N application in sole crop. Total dry-matter of intercropped cassava was always less than that of sole cassava in any N treatment. Lateral-branch production and leaf turnover were reduced by the presence of soybean, and the consequent reduction in shoot demand for assimilates resulted in an increased proportion of assimilates partitioned to tubers. When N was applied at planting, harvest index was higher in intercropped than in sole cassava, and tuber yield was similar in the two crops. Intercropping under no N application made only a slight further improvement in harvest index over the corresponding sole cassava, while severely reducing total dry-matter production. It appears, therefore, that the tuber-yield advantage of cassava/soybean intercropping is likely to be small under low availability of soil N.

Our previous work has shown that early-maturing soybean is suitable for intercropping with vassava at a high latitude (27°S) in south-east Queensland, Australia, as it does not effect the tuber yield. The present study examines whether later-maturing cultivars of soybean with higher yield potential might be more productive. Plant arrangement for cassava was the same in sole crop and in intercrop, while two soybean rows in every six rows were replaced by a row of cassava in intercropping. All soybean cultivars dominated intercropped cassava, and their dry-matter growth and seed yield were not affected by competition with cassava. Growth of cassava was, on the other hand, severely restricted by intercropped soybean, particularly by late-maturing types. After removal of early-maturing soybean, cassava recovered quickly to produce high leaf-area and effectively intercepted solar radiation. Consequential high total dry-matter production, combined with high assimilate allocation to tubers, resulted in tuber yield at the final harvest similar to that in sole cassava. After the removal of late-maturing soybean, however, recovery was poor, and with a short growing season remaining, tuber yields were only 50–60% of that of sole cassava. In addition to their adverse effect on cassave growth, late-maturing cultivars were not suitable as an intercrop because of low harvest indices and low light-conversion efficiency (dry matter produced per unit intercepted radiation), although total light interception during the whole growth of cassava/soybean intercrop was similar to that of sole cassava. The low overall light-conversion efficiency in intercropping with late-maturing cultivars was due to very low dry-matter production of soybean during pod-filling when light interception was still high.

Soybean ( Glycine max (L.) Merr.) has been shown to acclimate to, and produce high seed yield in, continuously saturated soil. In trials conducted to clarify the physiological mechanisms underlying this potentially valuable trait, soybean cv. Fitzroy was grown on beds in two years at Lawes, S.E. Queensland, and irrigated either continuously (saturated soil culture, SSC) or after 60 mm cumulative open-pan evaporation (conventional irrigation, CI). Shoot growth was initially lower in SSC but subsequently greater than that in CI, particularly during seed growth. Root and nodule growth, nitrogenase activity, and leaf conductance to water vapour were all greater in SSC. Seed yields were 24% and 52% higher in SSC in the two years, and these responses were associated with greater numbers of pods through reduced pod abortion, and higher individual seed weights, in SSC. Nitrogen assimilation by shoots in SSC during seed growth was approximately double that in CI, and during senescence the loss of leaf area and nodule function, and the withdrawal of nitrogen from the leaves, was delayed. It is suggested that enhanced leaf water-status in SSC resulted in greater photosynthesis and sustained nodule during pod growth, which in turn enabled continued nitrogen supply to the seeds and thus delayed the impairment of leaf function which follows nitrogen depletion.

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. *Part III, Aust. J. Agric. Res., 26: 11 (1975).

Cassava is a potential tuber crop for northern Australia where water stress is likely to occur during some stages of growth. Field and glasshouse experiments were conducted to examine the response of cassava to soil water deficits which developed at various stages. The field experiments, covering 10-month growth durations from planting in spring to harvesting in winter, showed that water stress occurring in summer or winter had small effects, but in autumn severely reduced the final yield. Autumn was the time of maximum bulking of underground storage organs in well-watered plants, and water stress which reduced assimilate production also reduced bulking. Temperature at this time was suboptimal for canopy development, and leaf area which was reduced during the stress did not increase after its relief, affecting further the growth of storage organs. Similarly, in the glasshouse experiment, plants recovered rapidly during early stages of growth, but when stress occurred later leaf area was reduced greatly, and recovery after its termination was poor. In all experiments, water deficits affected yield of storage organs but not the pattern of assimilate distribution, resulting in similar harvest indices among the plants of different watering treatments. It is concluded that the reduction in cassava yield (cv. M Aus 7) is caused by the reduction in total biomass production, and that stress occurring later in the season is most detrimental to yield because of the additional effect of reduced ability of old plants to recover leaf area after the stress is relieved.

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. *Part II, Aust. J. Agric. Res., 22: 39-47 (1971).

The effect of water deficit on floral initiation, leaf development and yield of commercial leaf of tobacco planted in the winter was studied in northern Queensland. The deficits were induced by different watering treatments at transplanting in combination with a range of times during which irrigation was withheld after transplanting. In addition, the effect of two periods of water deficits on flower and leaf development was investigated in a pot experiment under controlled environment conditions. Water deficits after transplanting delayed floral initiation and allowed more leaves to be produced. Moreover, the size of most leaves of commercial value was increased by water deficit; for example, withholding water for 28 days after transplanting increased the area and dry weight of commercial yield per plant by 33%. Results from the pot experiment demonstrated that the larger leaf area resulted from an extended period of linear growth and a greater rate of expansion after water deficits were relieved.

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—specifically 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 10-6g cal-1. *Part IV, Aust. J. Agric. Res., 26: 25 (1975).

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%.