Water deficit and plant competition effects on growth and water-use efficiency of white clover (Trifolium repens, L.) and ryegrass (Lolium perenne, L.)

The combined effects of soil water deficit and above and below ground inter specific plant competition on the growth, water-use efficiency (WUE), and m easured carbon isotopic composition (delta C-13) values of white clover and ryegrass were studied. White clover and ryegrass were grown in specially d esigned crates 1) individually; 2) in shoot competition; or 3) in shoot + r oot competition and either well-watered or at a moderate or severe soil wat er deficit. The effects of shoot + root competition on shoot dry matter gro wth were substantial and benefited both white clover and ryegrass when well -watered or at a moderate soil water deficit, while severely reducing white clover shoot dry matter growth at severe soil water deficit. Plant competi tion did not affect the WUE of white clover or ryegrass. As soil water defi cit increased, the WUE of white clover did not change whereas the WUE of ry egrass increased and was greater than that of white clover. This was attrib uted to the lower leaf water conductance of ryegrass which conserved water and maintained growth longer compared to white clover. A stronger correlati on existed between soil water deficit and measured delta C-13 values for ry egrass at each plant competition level (P <0.001) than existed for white cl over (individual: P <0.01; shoot + root: P <0.001; shoot: P <0.10). Unlike white clover, the relationship between measured delta C-13 values and shoot dry matter growth indicated that C assimilation for ryegrass was dependent on type of plant competition. That WUE remained constant for white clover while measured delta C-13 values increased as soil water deficit increased, suggests that the role below ground respiration rate played in determining delta C-13 values increased. The WUE of white clover appears to be indepen dent of the nature of the competition between plants and the soil water def icit level at which it is grown, whereas for ryegrass, the addition of root competition to shoot competition should lead to increases in its WUE.