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Root-shoot interactions in irrigated white clover

S.J. Blaikie, D.J. Connor1, W.K. Mason and T.F. Neales1

Kyabram Research Institute, RMB 3010, Kyabram, Victoria 3620
1 University of Melbourne, Parkville, Victoria 3052

Low shoot production of irrigated pastures in the Goulburn Valley is attributed to adverse conditions in the root zone which impede the growth and differentiation of the root system. The experiment described below aimed to define the correlation between root and shoot growth in irrigated white clover and to examine the degree to which a shortage of soil water and defoliation (two common field stresses) disturb this relationship.

Methods

Clonally propagated white clover (Trifolium repens cv. Haifa) was grown in pots in a greenhouse. Treatments were (T1, T3) well-watered - pots regularly watered to dripping with nutrient solution to avoid plant water stress; (T2, T4) limited water - pots received 0.5 the volume given to treatment (T1, T3) at each watering; (T1, T2) full canopy; (T3, T4) defoliated - all leaves were excised after three weeks growth. The treatments were factorially arranged and replicated five times. Initially plants had 2-3 leaves. Destructive harvests were carried out after 2,3,4,5 and 6 weeks.

Results and discussion

The allometric relationship between root and shoot dryweight is presented in Figure 1. The straight line (R'=0.99) for T1 shows that growth of the two systems was highly correlated. Water shortage and defoliation disturbed this relationship by initially reducing shoot relative to root growth. In all treatments the plants reacted to these stresses by reducing the relative growth of roots until equilibrium was re-established and normal growth, as defined by T1, resumed. By the end of the experiment the plants in T2 and T3, that were subject to either of the stresses alone, had regained equilibrium although those in T4, that were subject to both stresses, were still adjusting. These results suggest that to improve the yield of white clover shoots, root growth and differentiation must be unimpeded. Apart from any effects on root function, stresses that reduce shoot growth are likely to cause a compensatory reduction in the growth of the. roots until equilibrium is re-established, with the consequence that plant yield is reduced.

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