Mycorrhizal plants often have greater tolerance to drought than nonmycorrhizal plants. This study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi inoculation on growth, grain yield and mineral acquisition of two winter wheat (Triticum aestivum L.) cultivars grown in the field under well-watered and water-stressed conditions. Wheat seeds were planted in furrows after treatment with or without the AM fungi Glomus mosseae or G. etunicatum. Roots were sampled at four growth stages (leaf, tillering, heading and grain-filling) to quantify AM fungi. There was negligible AM fungi colonization during winter months following seeding (leaf sampling in February), when soil temperature was low. During the spring, AM fungi colonization increased gradually. Mycorrhizal colonization was higher in well-watered plants colonized with AM fungi isolates than water-stressed plants. Plants inoculated with G. etunicatum generally had higher colonization than plants colonized with G. mosseae under both soil moisture conditions. Biomass and grain yields were higher in mycorrhizal than nonmycorrhizal plots irrespective of soil moisture, and G. etunicatum inoculated plants generally had higher biomass and grain yields than those colonized by G. mosseae under either soil moisture condition. The mycorrhizal plants had higher shoot P and Fe concentrations than nonmycorrhizal plants at all samplings regardless of soil moisture conditions. The improved growth, yield and nutrient uptake in wheat plants reported here demonstrate the potential of mycorrhizal inoculation to reduce the effects of drought stress on wheat grown under field conditions in semiarid areas of the world.

Water needed for expansion is believed to enter plant tissue in response to a growth-induced water potential gradient that occurs as turgor is reduced during relaxation of cell walls or in response to increased solutes. Under water stress, the cotton flower petal continues to expand when all leaves on the plant are wilted and new leaf expansion has ceased in the shoot tips. This study was undertaken to determine if water for expansion entered the petal in response to a gradient or to increased solutes. Water potentials of cotton petal, leaf, bract and fruit were determined pre-dawn and midday in dryland and irrigated field plots. The mechanism by which petal expansion occurs appears not to be associated with a growth-induced water potential gradient or to increased solutes because the gradient is reversed from that needed to drive expansion. The water potential of the petal tissues was consistently higher than that of the subtending leaves and bracts both during and after anthesis, and under different water stress conditions. How this reversal in water potential gradient is established and maintained should provide insight into mechanisms involved in growth during water stress.

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The concept that ethylene is an endogenous growth regulator has evolved in the past few years (16). This concept has been strengthened by recent findings that internal concentrations of endogenous ethylene in vegetative tissues reach physiologically active levels (3,12,13). These internal concentrations have been directly related to the corresponding production rates of excised tissues (13), although parallel data on production rates of intact plant tissues are not available. Using excised abscission zones from primary bean leaves, Jackson and Osborne (9) presented evidence that the timing of abscission of explant petioles can be related to the extent of ethylene production adjacent to the separation zone. They suggest that the ethylene production is coupled to a particular stage of senescence. Further, they proposed that in natural leaf abscission ethylene initiates the biochemical sequences leading to separation, but the mechanism is not clear. Beyer and Morgan (4) have recently shown that ethylene production by and internal levels in detached cotton cotyledons increases as auxin transport declines. Amounts of exogenous ethylene necessary to induce abscission and inhibit auxin transport were similar. They propose that the rise in ethylene production and decline in auxin transport capacity are causally related and that reduced auxin transport is one of the ethylene mediated actions which precede induction of hydrolytic enzymes in the separation layer. Their measurement of ethylene involved whole cotyledons and was not restricted to the petiole. McAfee and Morgan (13) found internal ethylene levels and production rates were several times higher in petioles than leaf blades. Since auxin must be translocated through the petiole to the abscission zone, this observation strengthens the proposed role of ethylene in auxin transport inhibition preceding cotton leaf abscission (4). Alternatively, high rates of ethylene production by petiole tissue near the abscission zone may directly trigger the biochemical changes preceding separation independent from the proposed effect on auxin supply to the abscission zone.Aside from abscission related to senescence, little attention has been directed to other problems of natural abscission, particularly those involving environmental stresses. Plant water deficits may induce both leaf and boll abscission from cotton under field conditions (5, 6, 18). In most instances, actual separation follows relief from the deficit and rehydration of the abscission zone. This communication describes the effect of a brief period of water deficit on ethylene production by intact cotton petioles. MATERIALS AND METHODSCotton plants (Gossypium hirsutum L. var TM-1) used in this study were grown in pots containing sand in a greenhouse and were between 75 and 90 days of age. Each pot contained two plants of equal size, each with 15 to 17 leaves on the main stem. The use of paired plants made possible the determination of leaf water potential on one plant, while ethylene production was measured on the ...