Zea mays hybrid XL66 was grown with irrigation in sand in the field and subjected to different rates of nitrogen supply from floral initiation to anthesis and again from anthesis to maturity. Nitrogen supply during spikelet differentiation affected grain number markedly. Nitrogen supply during grain filling had a small but significant effect on the rate of grain filling and an effect on the rate of nitrogen entry to the grain, causing a 1.3-fold change in percentage grain nitrogen. Increasing the nitrogen supply produced larger plants, but the distribution of nitrogen and dry weight among plant parts were similar and the harvest indices for dry weight (0.46) and nitrogen (0.66) were the same over all nitrogen treatments. We found no evidence that shoot size per se controlled grain number or rate of grain growth; rather, fertilizer management during spikelet differentiation had most effect on yield of XL66.

We measured the effects of temperature on the germination, emergence and growth during early tillering of Italian ryegrass (Lolium multiflorum), prairie grass (Bromus catharticus) and tall fescue (Festuca arundinacea). Radicle emergence (the percentage of seeds with emerged radicles) and 'field' emergence (the percentage of seeds with shoots > 1 cm long) were reduced at 35�C (constant, all lines), or 30/25�C (12/12 h, prairie grass and Tama Italian ryegrass only). All lines took longer to germinate at 15 and 10�C (constant) than at higher temperatures, and final percentage germination of tall fescue lines was lower at 10�C (constant) than at other temperatures. Italian ryegrass had the fastest shoot extension, and consequently the fastest 'field' emergence. Seed respiration rates were higher in fescue than in the other lines at 4 days from imbibition, and rates rose in all lines at 8 days. Over the first 20 days after sowing, growth rates were closely related (r = 0.87-0.99) to weights of seed and caryopses, when account was taken of perenniality (which was associated with relatively slow growth) and polyploidy (which was associated with growth faster than might be expected from seed size alone), and a multivariate relationship including temperature was developed (R2 = 0.88). Growth rates of shoots at 20 days after sowing were ranked Tama Italian ryegrass > prairie grass > Ucivex Italian ryegrass > tall fescue, and 24/19 > 21/16 > 18/13 > 30/25 > 15/10�C. Growth during tillering, up to 42 days after emergence, was ranked Italian ryegrass > prairie > tall fescue and 21/16 = 18/13 > 24/19 = 15/1O�C. Relative growth rates and relative rates of tillering were independent of temperature in Italian ryegrass, but were lower at 15/10 and 24/19 than at 21/16 and 18/13�C in prairie grass and tall fescue. Prairie grass had higher yield per tiller but lower tiller numbers than Italian ryegrass; tall fescue had both lower yield per tiller and lower tiller number. Italian ryegrass was therefore the species best adapted to variations in environment and competition at sowing.

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.

Stomatal apertures and methanol-, water-, and hydrochloric acid-soluble carbohydrates and malate were measured in the epidermis, mesophyll, and midvein of leaves of C. cyanea R. Br. and V. [aba L. over a period of 26 hr in one experiment and over 9 hr during the photoperiod in a second experiment.Metabolite concentrations were the same in mesophyll and vein and followed typical diurnal patterns of accumulation and depletion. Concentrations in the epidermis showed little association with those in the mesophyll. The accumulation during the morning of epidermal sugars plus malate, at rates equivalent to 1·4 and 3 . 6 p.g mg-1 hr-1 in C. cyanea and V. [aba, was accompanied by a fall in polysaccharides with no net change in total metabolites, whereas rises in mesophyll sugars and malate of 3·6 and 6'9 p.g mg-1 hr-1 were associated with increases in polysaccharides.Associations were found between aperture and the concentration of epidermal malate.The differences between minimum and maximum epidermal malate concentrations (about 2· 6 and 6·1 p.g mg-1 in C. cyanea and V. [aba) were sufficient to balance the potassium changes found by other authors to occur during stomatal opening in Vicia.

Loss of yield in rice owing to exposure to suboptimal (<20�C) temperatures is a widespread phenomenon; in Australia there is circumstantial evidence that cold damage is substantial and that it can occur at any time during the growing season. Damage may take the form of reduced potential yield, abnormal spikelet development, and reduced spikelet fertility. In extreme cases plants may be completely infertile. We are using a conceptual model based on the daily cycle of temperature to quantify a cooling index and the impact of cold. The daily impact of cold varies with the phenology of the crop (i.e. some stages of development are particularly cold-sensitive), and the total or cumulative impact depends on the summation of thermal time below a critical minimum temperature for cold damage. We are in the process of quantifying the variables of the model for 2 cultivars of rice. This aims at prediction of yield loss in response to seasonal variation in temperature, and identifying the likely benefit which will arise from using particular parent lines in later breeding for cold tolerance.