Plants raised from shoot-tip cultures of apple (Malus x domestica Borkh ) were grown in divided pots so that approximately half the root system could be exposed to soil drying whilst the remainder was well watered. Water was withheld from half the root system in this way for 24 d. During this time, the daily increment in leaf area declined to 65% of that of control plants, which had all their roots in well watered soil Both leaf expansion and, more particularly, leaf initiation were inhibited. Water loss per plant also fell to 70%% of the rate exhibited by control plants. No significant differences were detected between the leaf water status of the partially dried plants and that of the well watered ones
After this drying period, the half-dried plants were treated during the night of the 24th day as follows; one group had its roots in dry soil rewatered, another had them excised, and a third continued half dry Both the rewatered group and the group with roots excised showed significant recoveries in leaf growth rates compared with the group kept half dry.
The inhibition of leaf growth by drying soil is discussed with regard to root-to-shoot signalling The alleviation of this inhibition by the excision of roots in dry soil, is taken as evidence of a positive inhibitor, produced by drying roots, influencing shoot growth.