Craig J. Pearson

Potential yield of maize, determined by rates of growth and development of ears

Abstract Zea mays L. hybrids which differed in their expression of prolificacy were grown in field experiments and subjected to treatments which altered the amounts of nitrogen and solar radiation available to each plant. Detailed measurements were made of growth of the tassel and ears to determine the mechanisms through which plants adjusted their grain-yield to the environment. In preliminary field experiments, increases in plant population from 5 to 20 m−1 caused increased asynchronous flowering and reduced grain-yield per plant (but not per ha), due mainly to a 47% decrease in kernel number per plant. In the main field experiment, grain-yield of XL 80 (semi-prolific) and XL 81 (single-eared) were reduced by nitrogen deficiency and defoliation by 65 and 40% respectively, relative to controls. Yield per plant was reduced through (a) 1.3-2.7-fold reductions in number of potential kernels; (b) increases in asynchronous flowering; and (c) a 25% reduction in the number of spikelets differentiated by the first ear when grown under low N. Asynchronous flowering of the first ear increased because development of the ears was delayed relative to the tassel, and the relative elongation rate of silks was reduced. Treatment effects on ear growth were greater at lower ear positions. Spikelet number appeared to be related to the size of the ear, and was not influenced directly by treatments. We conclude that the processes which determine kernel number are highly sensitive to low levels of intercepted radiation or N supply.

Daily changes in nitrate influx, efflux and metabolism in maize and pearl millet

Maize (Zea mays L.) and pearl millet (Pennisetum americanum (L.) Leeke) seedlings were exposed to [15N]nitrate for 1-h periods at eight times during a 24-h period (16–8 h light-dark for maize; 14–10 h for millet). Influx of [15N]nitrate as well as its reduction and translocation were determined during each period. The efflux of previously absorbed [14N]nitrate to the uptake solution was also estimated. No marked diurnal changes in [14N]nitrate efflux or [15N]nitrate influx were evident in maize. In contrast, [14N]nitrate efflux from millet increased and eventually exceeded [15N]nitrate influx during the late dark and early light periods, resulting in net nitrate efflux from the roots. The dissimilarity of their diurnal patterns indicates that influx and efflux are independently regulated. In both species, [15N]nitrate reduction and 15N translocation to shoots were curtailed more by darkness than was [15N]nitrate influx. In the light, maize reduced 15% and millet 24% of the incoming [15N]nitrate. In darkness, reduction dropped to 11 and 17%, respectively. Since the accumulation of reduced-15N in shoots declined abruptly in darkness, whereas that in roots was little affected, it is suggested that in darkness [15N]nitrate reduction occurred primarily in roots. The decrease in nitrate uptake and reduction in darkness was not related to efflux, which remained constant in maize and did not respond immediately to darkness in pearl millet.

Daily changes in nitrate uptake and metabolism in Capsicum annuum

The diurnal pattern of nitrate uptake by Capsicum annuum L. cv. California Wonder in a constant environment is described by a Fourier harmonic, with the maximum uptake in the middle of the photoperiod and the minimum in the middle of the dark period. Comparison of the uptake pattern with that of nitrate reductase (EC 1.6.6.1.) activity suggests against a direct control of one process by the other. This was confirmed by the observation that the pattern of nitrate reductase activity was not altered by restricting nitrate uptake to one hour per day. Translocation of 15N from the roots is much greater in the lightperiod than in the dark period. Reduction of 15N in the leaves occurs in the lightperiod but very little is reduced in the dark period. Amino acid levels showed marked daily fluctuations but in the roots neither amino acids, sucrose, fructose, glucose nor malate showed fluctuations. The amino acid composition of roots and leaves differed: glutamine+glutamate were relatively more important in leaves than in roots whereas alanine was a more important constituent of roots than of leaves.

Effects of nitrogen supply and population density on plant development and yield components of irrigated sunflower (Helianthus annuus L.)

Abstract During two seasons the growth and development of an F1 hybrid of sunflower ( Helianthus annuus L.) cv. Hysun 30 was tested at nitrogen supply rates between 0 and 500 kg N ha −1 and at three population densities: 5, 10 or 15 plants m −2 . Shoot dry matter and N content per plant increased with increases in applied N and decreased with increases in population density. The relationship between relative growth rates and relative N uptake rates could be described by a single quadratic function over all N and populations treatments in both experiments. Early in crop development in dense populations there was a partitioning of dry matter into the stem at the expense of the production of leaf primordia, decreasing the number of leaves. Throughout crop development stem dry weight % shoot dry weight remained greater in plants at high than at low population densities. Floret numbers per capitulum were also less but this was as a result of limited receptacle expansion during floret production and not due to a smaller generative area on the floral meristem. After floret initiation, dry matter and nitrogen accumulation rates decreased in all organs in dense populations. Population density had no effect on the duration of expansion of individual leaves but rates of expansion were lower in dense populations. Low N supplies increased the duration but decreased the rates of leaf expansion. Increased N supply could compensate for the effects of dense populations except where direct population effects occurred, e.g. on floret numbers and rates of leaf expansion. There was no response of N concentration (nitrogen % organ dry weight) in separate organs to population but there was a positive response to increase in N application rates. Nitrogen distribution was independent of dry matter distribution between organs. The percentage of initiated florets that produced filled seeds was less (38%) on 15 plants m −2 than on 5 plants m −2 (68%). Seed number, single seed weight, oil yield per plant and harvest index were increased by increasing nitrogen application and decreased by increasing population density. Oil % seed dry weight was not affected by population but was decreased by increasing N application rates. Fitted response surfaces describing seed number per plant, single seed weight and oil yield per plant in terms of plant N status and population density are given and may be useful for predicting crop yield.

Photosynthate translocation in Capsicum annuum.

SummaryThe leaves of Capsicum annuum L. cv. California Wonder function as efficient carbon exporting organs for a large part of their life span. Young and old leaves export the same percentage of fixed carbon (74% within 24 hours) although fixation rates per unit area of young leaves may be three times that of old leaves. A constant feature of translocate distribution is a supply to the roots and to the terminal fruit of the main axis from both the main axis leaves and branch leaves. Carbon supply to branch fruit is flexible in that main axis leaves supply carbon to the very small fruit, but the subtending branch leaves supply more as the fruit grows and the branch node becomes more self-sufficient. Fruit are capable of carbon fixation but translocation to terminal fruit from main axis leaves supplies about 7.5 times as much carbon as direct fixation.

Litter fall and litter layer decay in coastal scrub at Sydney, Australia

SummarySeasonal litter fall and changes in dry weight and minerals within the litter layer were sampled throughout one year. The annual total litter fall was 4.9 t per hectare of which 70% was leaf fall. Litter fall was highest in spring and early summer, the falls of each component (leaf, wood, etc.) and species being markedly seasonal but not all in phase. The annual amounts of minerals cycling as litter fall ranged from 44 kg N to 2 kg P per hectare. Mineral concentrations of the litter fall underwent seasonal variation and in the case of potassium were related to rainfall. The litter layer, mean biomass 19 t ha-1, declined in weight at a rate of 2.4% per week between spring and summer. The mean turnover time for the dry weight of the litter layer was 3.8 years and for the mineral approx. 1 year (K, Cl) or in excess of 3 years (P, Ca, S, Mg and N).

Course of grain development and its relationship to black region appearance in Pennisetum americanum

Abstract Genotypic differences in grain dry weight accumulation, the correlation of appearance of a dark region on the grain with maximum grain dry weight, and changes in chemical composition of the developing grain of Pennisetum americanum were examined in a field experiment at Camden, Australia. The length of the grain filling period and the rate of grain fill in the linear phase were closely correlated with final grain dry weight differences between genotypes. Hybrids exhibited higher rates of grain fill than parental lines. Black region development was a reliable indicator of the achievement of maximum grain dry weight. The level of storage carbohydrate, soluble and NaOH-extractable protein increased in a manner similar to the grain dry weight increase, except that there was some catabolism of storage carbohydrates and increase in soluble carbohydrates at the time of black region appearance.

Changes in root growth within successive wheat crops in a cropping cycle using minimum and conventional tillage

Abstract The aim was to describe, through a cropping cycle of four years of successive wheat, root growth following different times of sowing using minimum or conventional tillage in the central west of New South Wales. The hypothesis was that, due to transient differences in soil structure at sowing, early growth of roots would be retarded under minimum tillage, but that eventually root growth would be better under minimum tillage than under conventional cultivation because of long-term changes in soil structure and infiltration characteristics. Minimum tillage did cause total root length to be less in the seedling stage in the first three years, whereas root length was the same in later growth in the early years, and greater under minimum tillage than under conventional cultivation in the later two years of the cropping cycle. Tillage, time of sowing or cultivar did not affect the relative distribution of roots with depth; treatments also did not affect maximum root depth, which increased during the development of each crop to an asymptote of only 75 cm. Minimum tillage resulted in higher rates of water infiltration to the soil at the end of the cropping cycle, and in higher soil water and greater crop water use during the cycle; minimum tillage did not result in statistically higher grain-yields compared with conventional cultivation. Relatively low grain-yields and low water-use efficiencies, particularly in years of above-average rainfall, implied over-riding constraints probably associated with poor soil structure, on crop performance.

Development and source-sink effects on single leaf and canopy carbon dioxide exchange in maize

Abstract Patterns of canopy net photosynthesis of irrigated maize showed maximum rates at anthesis and differences between genotypes during grain filling. Genotypic differences were due to maximum net photosynthesis (at irradiances above 1800 μE m −2 s −1 ) of individual leaves at the top of the canopy as well as to more rapid leaf senescence of the older genotype in late grain filling. The cause of the differences in rates of net photosynthesis and quantum yield appeared to be stomatal. In a second experiment source-sink balance was manipulated by detopping and ear removal 10 days after anthesis Detopping increased, and ear removal decreased, net photosynthesis by about 20% within 1 h. The increase in net photosynthesis due to detopping was related to stomatal aperture on the lower leaf surface and the decrease due to ear removal was associated with accumulation of carbohydrates in the leaf subtending the ear. Short-term changes in net photosynthesis were not related to nitrate or total nitrogen concentrations within the leaf.

Over-riding effects of temperature and soil strength on wheat seedlings under minimum and conventional tillage

Abstract Soil temperature and soil strength are the most important soil factors affecting the germination and emergence of seedlings when soil moisture status and aeration are not limiting. Wheat cultivars Hartog and Rosella were shown in six experiments on conventional and minimum tillage treatments in the central-west of NSW in 1987–1988. Measurements of soil temperature, soil water content and soil strength in the vicinity of the seeds were taken for 20 days during seedling of the shoot (first leaf) and soil temperature and soil strength when all environmental factors were changing and interacting. Times to first seedling emergence and to median emergence were delayed by 7 and 9 days, respectively, as soil temperature decreased from 17 to 5°C, whereas the median emergence time was delayed by an extra 5 days for a two-fold increase in soil strength. The use of such information to derive a predictive model for wheat emergence is discussed.