Soil compaction. A role for ethylene in regulating leaf expansion and shoot growth in tomato?

The role of ethylene in regulating growth in tomato (Lycopersicon esculentum Mill.) during compaction stress was examined using wild-type (cv Ailsa Craig) and transgenic (ACO1(AS)) genotypes; the latter has a reduced capacity to produce ethylene. Ethephon or silver ions were applied to increase ethylene production or block its action. Shoot growth in both genotypes was comparable in uncompacted (1.1 g cm(-3)) and uniformly compacted soil (1.5 g cm(-3)). However, a 1.1/1.5-g cm(-3) split-pot treatment invoked marked genotypic differences: growth was reduced in cv Ailsa Craig but was comparable to uncompacted control plants in ACO1(AS). As xylem sap abscisic acid levels were similar, abscisic acid was not responsible for inhibiting growth in cv Ailsa Craig. These genotypic differences in growth were accompanied by increased ethylene evolution in cv Ailsa Craig, suggesting that the ability of ACO1(AS) to maintain growth in the split-pot treatment reflected its lower ethylene levels, a view supported by the observation that excising the roots in the compacted compartment reduced ethylene evolution and restored shoot growth in cv Ailsa Craig. Treatment with silver restored shoot growth in cv Ailsa Craig, whereas treatment with ethephon reduced growth in ACO1(AS). Thus, ethylene apparently has a key role in determining growth when tomato plants encounter differential soil compaction.

The role of cytosolic potassium and pH in the growth of barley roots

In an earlier paper we showed that in fully developed barley (Hordeum vulgare L.) root epidermal cells a decrease in cytosolic K+ was associated with an acidification of the cytosol (D.J. Walker, R. A. Leigh, A.J. Miller [1996] Proc Natl Acad Sci USA 93: 10510-10514). To show that these changes in cytosolic ion concentrations contributed to the decreased growth of K+-starved roots, we first measured whether similar changes occurred in cells of the growing zone. Triple-barreled ion-selective microelectrodes were used to measure cytosolic K+ activity and pH in cells 0.5 to 1.0 mm from the root tip. In plants growing from 7 to 21 d after germination under K+-replete conditions, the mean values did not change significantly, with values ranging from 80 to 84 mM for K+ and 7.3 to 7.4 for pH. However, in K+-starved plants (external [K+], 2 &mgr;M), the mean cytosolic K+ activity and pH had declined to 44 mM and 7.0, respectively, after 14 d. For whole roots, sap osmolality was always lower in K+-starved than in K+-replete plants, whereas elongation rate and dry matter accumulation were significantly decreased after 14 and 16 d of K+ starvation. The rate of protein synthesis in root tips did not change for K+-replete plants but declined significantly with age in K+-starved plants. Butyrate treatment decreased cytosolic pH and diminished the rate of protein synthesis in K+-replete roots. Procaine treatment of K+-starved roots gave an alkalinization of the cytosol and increased protein synthesis rate. These results show that changes in both cytosolic pH and K+ can be significant factors in inhibiting protein synthesis and root growth during K+ deficiency.