Indole acetic acid and abscisic acid levels in new shoots and fibrous roots of citrus scion-rootstock combinations

Citrus rootstocks are important for the growth of scion varieties, but it is not clear how they regulate scion vigor. We studied three citrus rootstock cultivars; ‘Flying Dragon’ (Poncirus trifoliata var. monstrosa) a dwarfing rootstock, ‘Swingle’ citrumelo (P. trifoliata x Citrus paradisi) a vigorous rootstock, and trifoliate orange (P. trifoliata) as the control. Tender buds from new shoots of ‘Eureka’ lemon were cleft grafted on etiolated rootstock seedlings. Eighteen months after grafting, the dry matter of each part of the young grafts was measured. Top weight was the greatest on ‘Swingle’ citrumelo and smallest on ‘Flying Dragon’, but there was no distinct difference in root growth among the rootstocks. Endogenous indole acetic acid (IAA) and abscisic acid (ABA) were measured in the new shoots and fibrous roots. The IAA level in the new shoots was highest in ‘Swingle’ citrumelo and lowest in ‘Flying Dragon’. The ABA level in the new shoots was highest in ‘Flying Dragon’ and lowest in ‘Swingle’ citrumelo. Both the IAA and ABA levels in the fibrous roots were highest in the strains of Poncirus trifoliata and lowest in ‘Swingle’ citrumelo. The vigorous rootstock ‘Swingle’ citrumelo had the highest T–R and IAA-ABA ratios in the new shoots.