Abscisic Acid Induces Leaf Age-dependent Chlorosis by Limiting Nitrogen Distribution to Mature Leaves in Arabidopsis

Abscisic acid (ABA) is involved in leaf senescence under water stress. The senescing effect is generally attributed to the up-regulation of hydrolytic enzymes involved in chlorophyll breakdown or the stimulation of ethylene production. Here, we propose a new mechanism for the ABA-induced senescence, in which preferential nitrogen (N) distribution is involved. Arabidopsis (Arabidopsis thaliana) plants were treated with 0, 0.1, or 1 mM ABA at the rosette stage with 7–8 leaves. The magnitude of chlorosis was pronounced with ABA concentration and leaf maturity; 1 mM ABA reduced leaf chlorophyll index (SPAD reading) of the first (oldest) leaf by 44% within 24 hours of treatment and by up to 78% thereafter, whereas neither 0.1 nor 1 mM ABA induced chlorosis on leaves younger than the third leaf. As opposed to the general assumption, ethylene stimulation by ABA was not observed regardless of leaf maturity. Uptake and distribution of N were traced using ¹⁵N-labeled KNO₃ added to the growth medium immediately after ABA treatment. Similarly to the chlorophyll data, leaf N concentration was reduced by ABA only in the first leaf. Isotopic analysis of ¹⁵N revealed that this N loss was due to the reduced distribution of newly absorbed N. These results suggest that ABA accumulation limits distribution of N into non-growing mature leaves, thereby inducing leaf-age dependent chlorosis. This preferential N distribution may be a stress adaptation mechanism to conserve nutrients for new growth.