CONTRIBUTION OF LEAF EXPANSION, RATE OF LEAF APPEARANCE, AND STOLON BRANCHING TO GROWTH OF PLANT LEAF-AREA UNDER WATER-DEFICIT IN WHITE CLOVER

Response of white clover (Trifolium repens L.) leaf area to water defi cit depends on three processes: individual leaf expansion, phytomer pr oduction, and stolen branching. We have evaluated these processes in f ive clones under a wide range of conditions. Four experiments of 60 to 120 d duration were conducted: two in a growth chamber, one in a gree nhouse, and one outdoors, Water supply was managed in such a way that predawn leaf water potential and day-time leaf relative water content (RWC) were maintained at constant values during deficit periods of 20 to 68 d. This was achieved by growing plants either in soil columns wi th irrigation controlled by tensiometers, or in vermiculite, on top of foam columns with a controlled level of water inside the foam. Leaf a ppearance rate (LAR) and leaf initiation rate remained constant with t ime at a given RWC, but decreased with decreases in RWC. A reduction i n individual leaf area (LA) was observed in all leaves which experienc ed water deficit during part of the expansion period. Stolon branching decreased considerably with relatively mild deficits. Similar relatio nships between relative reductions in RWC (compared with control plant s), and relative reductions in LA, LAR and stolen number were observed in all experiments. Short deficits (1-2 wk) caused a 20 to 30% reduct ion in stolen number and individual LA but no change in leaf number pe r stolen. Longer deficits with mild, intermediate, or severe intensiti es caused major reductions in stolen number, individual leaf area, and number of leaves per stolen, respectively, with respect to the reduct ion in plant leaf area. In addition to individual leaf area, phytomer production and stolen branching make major contributions to drought-in duced reductions in leaf area depending upon the type of water deficit .