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Maize, barley and sunflower plants were grown in the field, well supplied with water and nutrients. During growth, net CO2 exchange and transpiration of the crops at varying ambient CO2 concentrations and irradiance were determined by infra-red gas analysis. In maize the net photosynthetic rate (Pn) was linearly related to the irradiance (/) and independent ofthe ambient CO2 concentration (Ca). The transpiration rate {Ej) was also linearly related to / but decreased strongly with increasing Ca. In sunflower and barley Pn increased and Ej decreased with increasing Ca. A mean stomatal conductance and intercellular CO2 concentration (Ci) were calculated. In all three species the internal CO2 concentration was independent ofthe irradiance. In maize it was also independent of Ca, but in sunflower and barley C, was proportional to Ca with a ratio of 0.6.It is concluded that differences in stomatal behaviour are only partly species-specific and depend mainly on growing conditions. The importance of stomatal regulation for crop growth under conditions of water shortage and CO2 depletion is discussed.

Takakura, T., Goudriaan, J. and Louwerse, W., 1975. A behaviour model to simulate stomatal resistance. Agric. Meteorol., 15: 393-404.The purpose of the present study is to make a behaviour model which might have a different structure from the real system but acts in the same way as the real system does in the region considered. In the present model, the basic working hypotheses are a functional relationship between stomatal resistance and internal C0 2 concentration and an effect of leaf temperature on internal resistance.It is found that stomatal resistance in the model responds to changes in light, external C0 2 concentration and leaf temperature in a way which is experimentally confirmed.At the pre&ent stage of work on plant growth simulation, the model may be good . enough to account for the rather complicated interactions that govern stomatal movement, although some clear phenomena can not be explained.

Aboveground net photosynthesis and respiration of maize cv. LG11 was determined in the field between mid-June and end-Oct. at regular intervals (1-2 weeks) at 12.5 and 22.5 degrees C by measuring the CO2 uptake or release in mobile crop enclosures. The actual growth rate of the crop was determined from fortnightly harvests. Temp. dependence of photosynthesis was highest in the early (until mid-July) and very late (early Oct.) stages of crop growth, showing a reduction of about 50% at the higher irradiances (>400 W/msuperscript 2). In the period of major DM production (mid-July to Sep.) the reduction was only 12-15%. Assuming maintenance respiration to become constant for cobs and grain exceeding a yield of 1000 kg/ha and for stems exceeding 2500 kg/ha, the measured and calculated dark respiration at 22.5 degrees matched fairly well. At 12.5 degrees the calculation, using the same assumptions, significantly overestimated dark respiration during the first part of the growing period. The carbon balance sheet showed that from the total amount of CO2 absorbed by the crop (equivalent to 30.7 t DM/ha), 30% was lost by aboveground respiration and 50% was invested in aboveground harvestable material. The remaining 20% was assumed to be transported to plant parts below ground. Substantial losses of DM at the end of the growing season did not occur. (Abstract retrieved from CAB Abstracts by CABI’s permission)