Inhibition of Light-Stimulated Leaf Expansion by Abscisic Acid

Abscisic acid (ABA) applied to intact bean (Phaseolus vulgaris)leaves or to isolated leaf discs inhibits light-stimulated cellenlargement This effect may be obtained with 10^–4 molm^–3 ABA, but is more significant at higher concentrations.The inhibition of disc expansion by ABA is greater for discsprovided with an external supply of sucrose than for discs providedwith KC1, and may be completely overcome by increasing the KC1concentration externally to 50 mol m^–3. Decreased growthrate of ABA-treated tissue is not correlated with loss of solutesfrom growing cells, but is correlated with a decrease in cellwall extensibility. ABA does not prevent light-stimulated acidificationof the leaf surface, and stimulates the acidification of theexternal solution by leaf pieces. However, the capacity of thecell walls to undergo acid-induced wall loosening is diminishedby ABA-treatment. The possibility that ABA acts directly byinhibiting growth processes at the cellular level, or indirectlyby causing stomatal closure, is discussed. Key words: Phaseolus vulgaris, ABA, Inhibition, Leaf expansion     

Cellular Processes Limiting Leaf Growth in Plants under Hypoxic Root Stress

The effects of root hypoxia on leaf growth of a Populus trichocarpa? deltoides hybrid have been assessed. Clonal plants were subjectedto hypoxic root conditions in pot culture by flooding and insolution culture by gassing with nitrogen. The rate of leafexpansion declined within 8 h and was suppressed for the durationof the treatment. Final leaf size was reduced by 35% to 60%compared to aerated plants. Final epidermal cell size and numberdepended both on the developmental stage of the leaf at theonset of stress and on the duration of the treatment. No differencesin bulk leaf water potential were measured between the hypoxicand aerated plants. Cell wall extensibility was lower, leafsolute potential was more negative and turgor potential washigher in leaves of hypoxia-treated plants than of aerated plants.These data suggest that leaf growth of hypoxia-stressed plantsis limited by cell wall extensibility. The mechanism by whichthe root stress induces changes in leaf cell wall characteristicsis not known. Key words: Populus, flooding     

Acclimation of leaf growth to low water potentials in sunflower

Abstract Leaf growth is one of the most sensitive of plant processes to water deficits and is frequently inhibited in field crops. Plants were acclimated for 2 weeks under a moderate soil water deficit to determine whether the sensitivity of leaf growth could be altered by sustained exposure to low water potentials. Leaf growth under these conditions was less than in the controls because expansion occurred more slowly and for less of the day than in control leaves. However, acclimated leaves were able to grow at leaf water potentials (Ψ₁) low enough to inhibit growth completely in control plants. This ability was associated with osmotic adjustment and maintenance of turgor in the acclimated leaves. Upon rewatering, the growth of acclimated leaves increased but was less than the growth of controls, despite higher concentrations of cell solute and greater turgor in the acclimated leaves than in controls. Therefore, factors other than turgor and osmotic adjustment limited the growth of acclimated leaves at high ψ₁ Four potentially controlling factors were investigated and the results showed that acclimated leaves were less extensible and required more turgor to initiate growth than control leaves. The slow growth of acclimated leaves was not due to a decrease in the water potential gradient for water uptake, although changes in the apparent hydraulic conductivity for water transport could have occurred. It was concluded that leaf growth acclimated to low ψ₁, by adjusting osmotically, and the concomitant maintenance of turgor permitted growth where none otherwise would occur. However, changes in the extensibility of the tissue and the turgor necessary to initiate growth caused generally slow growth in the acclimated leaves.     

Role of K+ in leaf growth: K+ uptake is required for light-stimulated H+ efflux but not solute accumulation

The stimulation of dicotyledonous leaf growth by light depends on increased H⁺ efflux, to acidify and loosen the cell walls, and is enhanced by K⁺ uptake. The role of K⁺ is generally considered to be osmotic for turgor maintenance. In coleoptiles, auxin‐induced cell elongation and wall acidification depend on K⁺ uptake through tetraethylammonium (TEA)‐sensitive channels (Claussen et al., Planta 201, 227–234, 1997), and auxin stimulates the expression of inward‐rectifying K⁺ channels ( Philippar et al. 1999) . The role of K⁺ in growing, leaf mesophyll cells has been investigated in the present study by measuring the consequences of blocking K⁺ uptake on several growth‐related processes, including solute accumulation, apoplast acidification, and membrane polarization. The results show that light‐stimulated growth and wall acidification of young tobacco leaves is dependent on K⁺ uptake. Light‐stimulated growth is enhanced three‐fold over dark levels with increasing external K⁺, and this effect is blocked by the K⁺ channel blockers, TEA, Ba⁺⁺ and Cs⁺. Incubation in 10 mm TEA reduced light‐stimulated growth and K⁺ uptake by 85%, and completely inhibited light‐stimulated wall acidification and membrane polarization. Although K⁺ uptake is significantly reduced in the presence of TEA, solute accumulation is increased. We suggest that the primary role of K⁺ in light‐stimulated leaf growth is to provide electrical counterbalance to H⁺ efflux, rather than to contribute to solute accumulation and turgor maintenance.     

The Influence of Water Deficit on the Factors Controlling the Daily Pattern of Growth of Phaseolus Trifoliates

Phaseolus seedlings were grown in liquid culture under controlledtemperature and irradiance and measurements were made of dailyvariation in growth of the first trifoliate leaves. Leaf growthrate was significantly enhanced within a few hours of the startof the light period. Over a similar time, a small decrease inleaf turgor and an increase in cell wall plasticity were recorded.Slowly declining growth rates as the light period progressedmay have been caused by decreases in turgor during this time.When water availability to the leaves was restricted by growingthe plants for several days in nutrient solution maintainedat a low temperature (12°C), the daily pattern of growthof the trifoliates was changed quite markedly. Dark-growth rateswere slightly enhanced, while light-growth rates were significantlyreduced when compared to growth rates of plants well-suppliedwith water (roots at 20°C). Relative ‘plateau’growth rates of plants well-supplied (ww) with water or sufferinga restricted supply (ws) in the light (L) and in the dark (D)were as follows: ww L > ws D > ww D > ws L. In thelight, turgors of the two groups of plants were similar, suggestingthat the reduced growth rate of the cooled plants resulted froma change in cell wall structure and/or properties. Immediatelybefore the lights were switched on, plants grown with a restrictedwater supply showed relatively high turgors in the trifoliatesand these were presumably responsible for the enhanced growthrates at this time. Restriction of water availability may haveslightly increased the plasticity of cell walls and decreasedthe yield threshold for growth. The control of leaf growth inplants developing water deficit is discussed. Key words: Leaf growth turgor, Cell wall plasticity, Water deficit, Light