The gaseous exchange characteristics of two clones of P. radiata were compared. Clone 457, which had a low survival rate in the field, showed a transpiration rate up to twice that of clone 456, which had a high survival rate. Transpiration in both clones was not markedly affected by leaf-air vapour pressure differences varying between 0.4 and 1.4 kPa. The stomatal resistance of clone 457 was almost half that measured for clone 456 under all experimental conditions; however, increasing the leaf-air vapour pressure difference resulted in increased stomatal resistances in both clones. The photosynthetic response to CO2 concentration was also determined. The measured rates for clone 457 were twice those of clone 456 and both clones gave maximum rates of apparent photosynthesis above about 700 ng cm-3 internal CO2 concentration (corresponding to about 1400 ng cm-3 external concentration). The differences in photosynthetic rate could be accounted for by the greater than twofold difference in mesophyll resistance between the clones. Mesophyll resistance was not affected by altering the leaf-air vapour pressure difference. It is suggested that a factor in determining the relative survival rate of the two clones is the greater ability of clone 456 to limit transpiration water loss and that this more than compensates for the reduced rate of photosynthesis of this clone compared with clone 457.

No abstract

Maize plants (hybrid XL45) were exposed to three temperature treatments of different duration: a 'growth' temperature (extending from sowing to the day before photosynthetic measurements were made), an 'acclimation' temperature (exposure to a given temperature for 24 h) and a 'measurement' temperature (the temperature during the photosynthetic measurement, 30-45 min duration). The measurement temperature had the greatest single effect on photosynthetic rate, which increased 2-3-fold between 16 and 35�C. The highest rate of apparent photosynthesis was measured in plants grown at 25�C, acclimated at 35�C and measured at 35�C (2.1 mg m-� s-�). The relative contribution of stomatal and residual resistances to the total resistance to CO2 uptake changed little with temperature treatment, but the absolute magnitude of these resistances was strongly affected by temperature. Stomatal resistances ranged from 200 to 700 s m-�. The stomatal resistance dominated, accounting for 65-80% of the total resistance to apparent photosynthesis. Chlorophyll concentrations changed in response to both growth and acclimation temperatures. Those grown at 16�C had the lowest concentration, those grown at 35�C the highest. The chlorophyll concentration changed over a 24-h acclimation period in both expanding and fully expanded leaves, increasing when plants were transferred to higher temperature and decreasing when the transfer was to lower temperature.

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