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 deficit depends on three processes: individual leaf expansion, phytomer production, and stolen branching. We have evaluated these processes in five 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 greenhouse, 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 with irrigation controlled by tensiometers, or in vermiculite, on top of foam columns with a controlled level of water inside the foam. Leaf appearance rate (LAR) and leaf initiation rate remained constant with time at a given RWC, but decreased with decreases in RWC. A reduction in individual leaf area (LA) was observed in all leaves which experienced water deficit during part of the expansion period. Stolon branching decreased considerably with relatively mild deficits. Similar relationships between relative reductions in RWC (compared with control plants), and relative reductions in LA, LAR and stolen number were observed in all experiments. Short deficits (1-2 wk) caused a 20 to 30% reduction in stolen number and individual LA but no change in leaf number per stolen. Longer deficits with mild, intermediate, or severe intensities caused major reductions in stolen number, individual leaf area, and number of leaves per stolen, respectively, with respect to the reduction in plant leaf area. In addition to individual leaf area, phytomer production and stolen branching make major contributions to drought-induced reductions in leaf area depending upon the type of water deficit.