No abstract

Leaf chamber studies were conducted on single attached leaves of Pennisetum typhoides (Burm.) S. & H. (bulrush millet) to identify and describe the processes regulating photosynthesis.

The short-term response of leaf extension rates and leaf water potential (Ψ1) to controlled diurnal changes in the environment of a pasture species, prairie grass (Bromus catharticus) was followed over a soil drying cycle. Consistent relationships between rates of leaf extension and leaf water status were found only when measurements had been made under a common environment or when the effects of the environmental differences were allowed for by comparing the response of desiccated plants to that of well watered control plants under the same conditions. In the early stages of desiccation, leaf extension rates were extremely sensitive to reduction in Ψ1. Water potentials of only 2-3 bars below that of well watered control plants were sufficient to depress leaf extension rates by 50%. However, as desiccation became more severe, leaf extension rates became much less responsive to further reductions in Ψ1. We infer that it will be possible to resolve some of the apparent discrepancies among various reports on the sensitivity of leaf extension rates to desiccation when allowance can be made for the actions of other important influences, such as temperature in this experiment, and when Ψ1 at the site of measurement can be related unequivocally to Ψ1 at the region of elongation.

Recovery of rates of leaf emergence, leaf extension, tiller number, leaf number, leaf area and shoot dry weight per plant following water deficits lasting for between 10 and 28 days was measured in Bromus catharticus in a controlled environment. The rate of leaf extension recovered to the rate of control plants within 4-6 days of rewatering and then exceeded the control rate for up to 28 days. During this time, four or five new leaves emerged. The maximum rate of leaf extension for individual leaves during recovery was up to 20% higher than rates typical for leaves of the same insertion on well watered control plants. During an initial recovery phase which followed the 28-day water deficit, and which lasted 20 days from rewatering, the rates of increase in tiller number, leaf number, and leaf area per plant were greater than control rates by 38, 48 and 51%, respectively. Rates for these components of yield then normalized. Relatively small differences among treatments were measured for rate of shoot dry matter accumulation. Final shoot dry weight was approximately proportional to the number of 'non- drought' days, which were defined as those days on which the leaves showed no sign of wilting. This also corresponded to day-time leaf water potentials higher (less negative) than - 1400 kPa.

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.