Growth, water transport and transpiration in root-restricted plants of bean, and their relation to abscisic acid accumulation

Abstract

The effects of root zone confinement on growth, water transport and transpiration, and their association with abscisic acid (ABA) accumulation in Phaseolus vulgaris L., were studied. Transfer of 6-day-old seedlings, at time zero (TZ), to aerated nutrient solutions, in 10-ml and non-restricting-volume containers, divided the plants for treatment as restricted root-zone plants (RRPs) and non-restricted root-zone plants (NRPs). Growth restraint in the RRPs was not detectable 2 days after TZ, but was very pronounced after a further 8 days. Analysis of ABA was carried out 2 and 12 days after TZ. In the earlier analysis, a much higher level of ABA was detected in the leaves and the roots of the RRPs than in those of the NRPs, preceeding any expression of growth suppression; in the later analysis, the concentrations of ABA in the leaves and the stems of the RRPs were much greater than in those of the NRPs, while in the roots, the previous difference had disappeared. Water transport from the roots, expressed for the whole root system and per unit weight of root biomass, was much lower in both intact and detopped RRPs, than in NRPs. The reduced transport of water from the RRP roots was pronounced 12 days after TZ, in spite of the disappearance of the formerly detected difference between the two treatments in root-accumulated ABA. The transpiration rate per unit leaf area was similar in both treatments, irrespective of the higher level of ABA in the foliage of the RRPs. The leaf water potentials of RRPs and NRPs, as measured 12 days after TZ were also similar. The root-exuded ABA, as measured 12 days after TZ reached values of 0.13 and 0.25 ng ABA/g fresh wt. root/h, in the RRPs and NRPs, respectively. It was, therefore, concluded that the higher level of ABA in the RRP leaves was not a consequence of an enhanced transport from the restricted roots. The present work led to the conclusion that root-zone restriction might promote ABA accumulation in the root and the shoot. The possible influence of such accumulation on other processes in RRPs, such as growth suppression, water transport and transpiration, is in question.