Auxin has long been thought to have only a limited role in the growth of leaves. Avery (1935) first reported that treatment of young tobacco (Nicotiana tabacum L.) leaves with auxin produced epinasty (dorsoconvex curvature) resulting from growth within the midrib and that auxin was without effect on the lateral veins and interveinal tissues. Went and Thimman (1937) also detected auxin-induced elongation growth of the midrib and the lateral veins and confirmed that auxin treatment failed to increase the surface of the mesophyll. In summarizing the early auxin investigations, these authors also concluded that all of the available data "fit in with the view that auxin causes elongation of leaf veins, while the growth of the mesophyll depends on other factors" (Went and Thimman, 1937). This idea that leaf development is controlled by chemically distinct vein growth factors or "caulocaline" and mesophyll growth factors or "phyllocaline" (Went, 1951) was supported by the observation that treatment with adenine (Bonner and Haagen-Smit, 1939) and cytokinins (Engelke et al., 1973) promoted mesophyll growth but not veinal growth.

The role of auxin in controlling leaf expansion remains unclear. Experimental increases to normal auxin levels in expanding leaves have shown conflicting results, with both increases and decreases in leaf growth having been measured. Therefore, the effects of both auxin application and adjustment of endogenous leaf auxin levels on midrib elongation and final leaf size (fresh weight and area) were examined in attached primary monofoliate leaves of the common bean (Phaseolus vulgaris) and in early Arabidopsis rosette leaves. Aqueous auxin application inhibited long-term leaf blade elongation. Bean leaves, initially 40 to 50 mm in length, treated once with α-naphthalene acetic acid (1.0 mm), were, after 6 d, approximately 80% the length and weight of controls. When applied at 1.0 and 0.1 mm, α-naphthalene acetic acid significantly inhibited long-term leaf growth. The weak auxin, β-naphthalene acetic acid, was effective at 1.0 mm; and a weak acid control, benzoic acid, was ineffective. Indole-3-acetic acid (1 μm, 10 μm, 0.1 mm, and 1 mm) required daily application to be effective at any concentration. Application of the auxin transport inhibitor, 1-N-naphthylphthalamic acid (1% [w/w] in lanolin), to petioles also inhibited long-term leaf growth. This treatment also was found to lead to a sustained elevation of leaf free indole-3-acetic acid content relative to untreated control leaves. Auxin-induced inhibition of leaf growth appeared not to be mediated by auxin-induced ethylene synthesis because growth inhibition was not rescued by inhibition of ethylene synthesis. Also, petiole treatment of Arabidopsis with 1-N-naphthylphthalamic acid similarly inhibited leaf growth of both wild-type plants and ethylene-insensitive ein4 mutants.

The effect of auxin on the physiology of protoplasts from growing oat (Avena sativa L.) coleoptiles was investigated. Protoplasts, isolated iso-osmotically from peeled oat coleoptile segments, were found to swell steadily over many hours. lncubated in 1 mM CaCI,, 1 O mM KCI, 1 O mM 2-(morpholino)ethanesulfonic acid/lJ-bisItris(hydroxymethyl)methylaminolpropane, pH 6.5, and mannitol to 300 milliosmolal, protoplasts swelled 28.9% f 2.0 (standard error) after 6 h. Addition of 10 p~ indoleacetic acid (IAA) increased swelling to 41.1 YO 2 2.1 (standard error) after 6 h. Swelling (in the absence of IAA) was partially dependent on K+ in the bath medium, whereas auxin-induced swelling was entirely dependent on K+. Replacement of mannitol in the bath by Clc increased swelling (in the absence of IAA) and eliminated auxin-induced swelling. Swelling with or without IAA was inhibited by osmotic shock and was completely reversed by 0.1 mM NaN,. Sodium orthovanadate, applied at 0.5 mM, only gradually inhibited swelling under various conditions but was most effective with protoplasts prepared from tissue preincubated in vanadate. Our data are interpreted to suggest that IAA increases the conductance of the plasma membrane to K+.