Transpirational, Leaf Area, Stomatal and Photosynthetic Responses to Gradually Induced Water Stress in Four Temperate Herbage Species

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Abstract
A series of physiological parameters was monitored under glasshouse conditions on micro-swards of four temperate herbage species. The micro-swards were grown on large soil cores so that the slow onset of water stress usually encountered in the field was simulated when water was withheld. Generally, water use per unit cover continued at a high rate even when water stress was causing considerable leaf death. Leaf diffusive conductance fell only gradually and did not reach minimum values until after much of herbage on the swards was dead. Stomatal closure in all species reduced water use per unit foliage cover by only 20-30%. Under increasing water stress, gross photosynthesis resembled stomatal conductances in remaining substantial (>50% of controls) even when relative water contents had fallen to below 80%. Both water use and gross photosynthesis were reduced more as a result of the reduced leaf area available for gaseous exchange than by the influence of stomatal action. Leaf moisture retention curves were similar for all species, there being a loss of c. 10% of relative water content per unit leaf water potential, down to drier than -5 MPa. It was concluded that the generally inferior herbage yields of white clover under dryland field conditions could be due in part to its relatively high leaf diffusive conductances when under water stress, while the better than average tall fescue yields under similar conditions may be attributed in part to its ability to roll its leaves tightly when water stress prevails.