Root growth response to defoliation in two Agropyron bunchgrasses: field observations with an improved root periscope

Summary Root growth responses to defoliation were observed in the field with an improved root periscope technique, which is described. The grazing tolerant, Eurasian bunchgrass, Agropyron desertorum, was compared with the very similar but grazing sensitive, North American bunchgrass, A. spicatum. Root length growth of clipped A. desertorum was about 50% of that of intact plants, while root elongation of clipped A. spicatum continued relatively unabated during ninety days of regrowth following severe defoliation. The reduced root growth in A. desertorum was correlated with the allocation of relatively more resources to aboveground regrowth, thus aiding reestablishment of the root: shoot balance. This balance was apparent in similar root mortality patterns of clipped and control A. desertorum plants in the season following defoliation. In clipped A. spicatum, however, root mortality increased in the winter following the season in which the clipping was done and continued into the subsequent growing season. Reduction of root growth following defoliation appears to be an effective mechanism to aid reestablishment of the photosynthetic canopy and the root: shoot balance. As such it contributes to both herbivory tolerance and maintenance of competitive ability.     

Effects of Drought and Defoliation on Bud Viability in Two Caespitose Grasses

Axillary bud number, bud respiratory activity, and photosyntheticcanopy re-establishment after defoliation were determined fortwo bunchgrass species, Agropyron desertorum and Agropyron spicatum,which were exposed to draughted, natural or irrigated conditions.These field treatments were repeated annually on the same plantsfor the period 1984–1986. Bud respiratory activity wasexamined using the tetrazolium test, which was validated withthe vital stain Evan's blue, at the end of the study In spring of the third year, the number of axillary buds ontillers of both species was lowest in the drought treatment.Most of these buds, and those observed immediately after defoliationin 1985, were metabolically active. These results indicate thatafter mid-season defoliation under drought, when no re-growthoccurred, the re-growth capacity was not limited by bud numberor viability. After 3 years of defoliation, tiller number andgrowth in both species were reduced in the following springunder all water regimes. This reduction was present 1 year earlierin the drought treatment than in the treatments with higherwater availability. Permanent dormancy or death of the replacementaxillary meristems can explain this plant response. Continueddefoliation of the tillers under drought would reduce the photosyntheticarea further, and probably affect the persistence of these speciesin the community Agropyron desertorum, Agropyron spicatum, crested wheatgrass, bluebunch wheatgrass, drought, defoliation, re-growth, bud viability, tetrazolium, Evan's blue     

Morphological Analysis of Tillering in Agropyron spicatum and Agropyron desertorum

The caespitose grasses Agropyron spicatum and Agropyron desertorumexhibit a striking difference in tillering response followingexperimental clipping treatment, with plants of A. desertorumproducing up to 18 times more tillers. The two species are similarin many aspects of their phenology and physiology. Previousexamination of current photosynthate production and levels ofstored carbohydrates indicate only slight differences betweenthe species. The possible role of three anatomical/morphologicalconstraints in controlling tillering was examined. No evidencefor such constraints was found. A basal cluster of buds is presenton the parent tillers. The mean bud number per tiller was similarfor both species and the range (3–9) was identical. Nearlyall of the bud apical meristems appeared anatomically viablethroughout the growing season and vascular development occurredto within 250 to 490 µm of the various bud apices of bothspecies. Both normal fall tillers and summer tillers producedunder clipping treatment originated from the largest, most distalbuds of the basal cluster of buds. However, precocious, morphologicallydistinctive, second-order tillers occasionally grew out fromthe smaller, most basal buds of some elongating fall tillers. Agropyron spicatum, Agropyron desertorum, bluebunch wheatgrass, crested wheatgrass, bud, tiller, tillering ability, meristematic potential, vascular development, regrowth     

Growth and carbon allocation of Agropyron desertorum following autumn defoliation

Summary Growth and carbon allocation of a cool season tussock grass, Agropyron desertorum, following defoliation of newly initiated tillers in the autumn of 1988 and 1989 were investigated. Tiller density and mortality, reproductive shoot density, root density, biomass, individual tiller weight, carbon allocation, and soil water depletion were used to evaluate the response of A. desertorum to autumn grazing. Tiller recruitment was lower in the autumn-defoliated treatment in both years compared with the control because of the cessation of tiller development following autumn defoliation. Autumn defoliation also significantly reduced the movement of ¹³C to the roots in 1988 but not in 1989. Soils were cooler and drier in 1989. Other plant growth measurements and soil water depletion rates were not different between treatments. Autumn defoliation in 1988 did not influence tiller recruitment in the following autumn. Two consecutive years of autumn defoliation did not affect tiller overwinter mortality or peak standing crop in 1990.     

Comparative demography of co-occurring introduced and native tussock grasses: persistence and potential expansion

Summary Demographic characteristics associated with the maintenance and growth of populations, such as seed dynamics, seedling emergence, survival, and tiller dynamics were examined for two tussock grasses, the native Agropyron spicatum and the introduced Agropyron desertorum in a 30-month field study. The introduced grass was aerially sown onto a native grassland site. Seed production of the introduced grass was greater than the native grass in both above- and below-average precipitation years. Seeds of A. spicatum were dispersed when they mature, while A. desertorum retained some seeds in inflorescences, and dispersed them slowly throughout the year. This seed retention allowed some seeds of the introduced grass to escape peak periods of seed predation during the summer and allowed seeds to be deposited constantly into the seed bank. Carryover of seeds in the seed bank beyond one year occurred in the introduced grass but not in the native species. For both species, seedling emergence occurred in both autumn or spring. Survival rates for A. desertorum were higher than A. spicatum when seedlings emerged between November and March. Survival rates of cohorts emerging before November favored A. spicatum whereas survival rates did not differ between species for cohorts emerging after March. Individuals of both species emerging after April were unable to survive the summer drought. Demographic factors associated with seeds of A. desertorum seemed to favor the maintenance and spread of this introduced grass into native stands formerly dominated by A. spicatum.     

The timing and degree of root proliferation in fertile-soil microsites for three cold-desert perennials

Summary Root proliferation in nutrient-rich soil patches is an important mechanism facilitating nutrient capture by plants. Although the phenomenon of root proliferation is well documented, the specific timing of this proliferation has not been investigated. We studied the timing and degree of root proliferation for three perennial species common to the Great Basin region of North America: a shrub, Artemisia tridentata, a native tussock grass, Agropyron spicatum, and an introduced tussock grass, Agropyron desertorum. One day after we applied nutrient solution to small soil patches, the mean relative growth rate of Agropyron desertorum roots in these soil patches was two to four times greater than for roots of the same plants in soil patches reated with distilled water. Most of the increased root growth came from thin, laterally branching roots within the patches. This rapid and striking root proliferation by Agropyron desertorum occurred in response to N-P-K enrichment as well as to P or N enrichment alone. A less competitive bunchgrass, Agrophyron spicatum, showed no tendency to proliferate roots in enriched soil patches during these two-week experiments. The shrub Artemisia tridentata proliferated roots within one day of initial solution injection in the N-enrichment experiment, but root proliferation of this species was more gradual and less consistent in the N-P-K and P-enrichment experiments, respectively. The ability of Agropyron desertorum to proliferate roots rapidly may partly explain both its general competitive success and its superior ability to exploit soil nutrients compared to Agropyron spicatum in Great Basin rangelands of North America.     

Competitive ability is linked to rates of water extraction

Summary The relative competitive abilities of Agropyron desertorum and Agropyron spicatum under rangeland conditions were compared using Artemisia tridentata ssp. wyomingensis transplants as indicator plants. We found A. desertorum to have substantially greater competitive ability than A. spicatum as manifested by the responses of Artemisia shrubs that were transplanted into nearly monospecific stands of these grass species. The Artemisia indicator plants had lower survival, growth, reproduction, and late-season water potential in the neighborhoods dominated by A. desertorum than in those dominated by A. spicatum. In similar, essentially monospecific grass stands, neutron probe soil moisture measurements showed that stands of A. desertorum extracted water more rapidly from the soil profile than did those of A. spicatum. These differences in extraction rates correlate clearly with the differences in indicator plant success in the respective grass stands. Nitrogen and phosphorus concentrations in Artemisia tissues suggested these nutrients were not limiting indicator plant growth and survival in the A. desertorum plots.     

Characteristics of successful competitors: an evaluation of potential growth rate in two cold desert tussock grasses

Summary Within the first few weeks after seedling emergence, Agropyron desertorum, a more competitive tussock grass, had a much higher mean relative growth rate (RGR) than Agropyron spicatum, a very similar, but less competitive species. However, beyond the early seedling stage, the two grasses had a remarkably similar whole-plant RGR in hydroponic culture and aboveground RGR in glasshouse soil, if root temperatures were above approximately 12°C. At soil temperatures between 5 and 12°C, A. desertorum exhibited a 66% greater aboveground RGR than A. spicatum (P<0.05). Both species responded similarly to warming soil temperatures. In the field, however, tiller growth rates were generally similar. Neither species showed marked tiller elongation until a couple of weeks after snowmelt, by which time soil temperatures, at least to a depth of 10 cm, were above 12°C for a significant portion of the day. Aboveground biomass accumulation over a three-year period indicated that both grasses had similar potential growth rates whereas Artemisia tridentata ssp. vaseyana, a common neighbor planted in the same plots, had a much greater potential growth rate. The greater competitive ability of adult A. desertorum, as compared to A. spicatum, cannot be attributed to appreciable differences in potential growth rates.     

Carbon import among vegetative tillers within two bunchgrasses: assessment with carbon-11 labelling

Summary Carbon allocation among bunchgrass tillers was examined with carbon-11 (¹¹CO₂) steady state labelling. Labelled carbon was continuously transported from parent tillers to anatomically attached daughter tillers at a time when morphological characteristics indicated that tiller maturation had occurred. Steady state levels of import into monitored daughter tillers increased within 30 min of either defoliation or shading. Import levels decreased within 30 min of the removal of shading, but remained accelerated throughout an 84 h observation period following defoliation. A second defoliation further increased carbon import into a monitored tiller above the previously accelerated level resulting from the initial defoliation. Carbon import by vegetative tillers in the two bunchgrass species examined may be most appropriately viewed as a series of potentially accelerated import levels above a low level of continuous import.     

Nitrogen-15 partitioning within a three generation tiller sequence of the bunchgrass Schizachyrium scoparium: response to selective defoliation

Summary Nitrogen partitioning among three generations of tillers within the bunchgrass Schizachyrium scoparium var. frequens was investigated in a controlled environment as a potential mechanism of herbivory tolerance. Nitrogen-15 was transported from the labelled primary tiller generation to both shoots and roots of nondefoliated secondary and tertiary tiller generations within 24 h. Partial defoliation increased shoot nitrogen concentration of secondary and tertiary generation tillers by 110 and 120%, respectively, 24 h following defoliation. Shoot nitrogen concentration was preferentially increased by partial defoliation of tertiary generation tillers throughout the 120 h experimental period, but diminished to concentrations comparable to nondefoliated tillers within shoots of the secondary generation at 72 h. In contrast to nitrogen concentration, the total amount of nitrogen imported by secondary and tertiary generation tillers decreased 62 and 73%, respectively, 24 h following partial defoliation and did not attain values comparable to respective nondefoliated tillers. Consequently, preferential nitrogen concentration occurred in response to partial tiller defoliation without an increase in total nitrogen import based on the reduction in the total nitrogen requirement per tiller generation associated with defoliation. Estimates of both the total amount of nitrogen import and nitrogen concentration are necessary to accurately interpret the dynamics of intertiller nitrogen allocation.     

A test of compensatory photosynthesis in the field: Implications for herbivory tolerance

Summary The occurrence of compensatory photosynthesis was examined in the field for all foliage elements on two Agropyron bunchgrass species that differ in their evolutionary history of grazing pressure. This is the first reported field study of compensatory photosynthesis in individual foliage elements of graminoids. Compensatory photosynthesis was defined as an increase in the photosynthetic rates of foliage on partially defoliated plants relative to foliage of the same age on undefoliated plants. Compensatory photosynthesis did occur in many individual foliage elements during at least part of their ontogeny. For both species, compensatory photosynthesis was related primarily to delayed leaf senescence and increased soluble protein concentrations, but not to an improvement in the water status of clipped plants. Soluble protein concentration increased in all foliage elements. A delay in senescence on clipped plants was documented for the two oldest, fully-expanded leaves that were present when the plants were initially clipped, but the initiation and senescence of all other foliage elements were not affected by the clipping treatments. Photosynthetic water use efficiency and photosynthetic rates per unit soluble protein of foliage on partially defoliated plants were not increased following the clipping treatments. Although A. desertorum and A. spicatum were exposed to different levels of grazing pressure during their evolutionary history, the phenology, water status, and gas exchange rates of foliage were very similar both for undefoliated as well as partially defoliated plants. Thus, we conclude that compensatory photosynthesis does not appear to be an important ecological component of herbivory tolerance for these species.     

Spatial arrangement of tiller replacement in Agropyron desertorum following grazing

Summary The spatial arrangement of tiller replacement was assessed on grazed and ungrazed tussocks of Agropyron desertorum (Fisch. ex Link) Schult. for three annual cycles. Frequency distributions of the number of replacement tillers per single progenitor were also determined. Tiller replacement was usually greater on the perimeter of tussocks than within the core, with or without grazing. Replacement was inversely related to grazing intensity, both on the perimeter and within the core of tussocks. Heights of replacement tillers on the perimeter or within the core seldom differed. Furthermore, grazing seldom affected the number of replacement tillers per progenitor. Greater tillering on the perimeter than within the core indicates that the tussocks were expanding. Apparently, grazing neither enhances tussock expansion and subsequent disintegration, nor does it necessarily lead to patches of tillers (multiple tillering per progenitor) within tussocks of A. desertorum.     

Impact of early root competition on fitness components of four semiarid species

Summary Plant demographic and root exclusion approaches were used to examine the influence of roots of adult Artemisia tridentata, Agropyron desertorum, and Agropyron spicatum individuals on seedling survival of four C₃ semiarid species, three perennials, Ar. tridentata, Ag. desertorum, Ag. spicatum, and an annual, Bromus tectorum. Furthermore, height of Ar. tridentata seedlings and seed production of B. tectorum were assessed. The probability of a seedling being alive significantly depended on the seedling species, the neighboring adult species, and on the depth to which root competition was excluded. As seedlings, survival of Agropyron species did not differ, whereas survival of Ar. tridentata seedlings was higher than Ag. desertorum and was similar to Ag. spicatum. Bromus tectorum maintained significantly higher survival rates than perennial seedlings. Established individuals of Ar. tridentata reduced seedling survival more than established individuals of either Agropyron species. Seedling survival significantly increased with greater depth of root exclusion for the perennials but did not significantly affect seedling survival of B. tectorum. Height of Ar. tridentata seedlings and seed production of B. tectorum significantly increased with depth of root exclusion. Seed production of B. tectorum was highest when competing with Ag. desertorum and was lowest with Ar. tridentata. Root competition decreased the seed population of B. tectorum in the next generation even though it had no impact on survival. Competition in the upper soil horizon occurs between seedlings and established adults early in the growing season and potentially restricts root growth of seedlings. In arid and semiarid ecosystems, soil moisture is depleted from the upper horizons first, resulting in the death of seedlings that do not have access to moisture.     

SYNTHETIC HYBRIDS OF NEW WORLD AND OLD WORLD AGROPYRONS. IV. TETRAPLOID AGROPYRON SPICATUM F. INERME X TETRAPLOID AGROPYRON DESERTORUM

Emasculated and unemasculated crosses of tetraploid A. spicatum f. inerme X A. desertorum yielded four hybrids. The hybrids were morphologically intermediate between the parent species but resembled A. desertorum more closely than A. spicatum. Both parents behaved cytologically as autoploids. Mean chromosome associations of 0.04 I, 8.60 II, 0.01 III, and 2.67 IV were observed at diakinesis in the 28-chromosome A. spicatum. The A. desertorum parent contained 30 chromosomes, 2 of which were likely supernumeraries, and averaged 0.03 I, 9.85 II, and 2.57 IV at diakinesis. Three hybrids contained 30 chromosomes, and one had 29. The most common chromosome association in the 30-chromosome hybrids was 2 I and 14 II; and the average was 3.00 I, 13.40 II, 0.06 III, and 0.01 IV. A. spicatum and A. desertorum chromosomes were usually distinguishable from each other in the hybrid cell on the basis of size. All pairing in the hybrids was attributed to autosyndesis within parental genomes. A. spicatum, A. desertorum, and their hybrids were represented by genome formulas of SSSS, CCCC, and SSCC, respectively. The hybrids produced 5 to 439 seeds under open pollination. Three controlled crosses between the hybrids yielded 2, 5, and 23 seeds, respectively, on 10 maternal spikes in each cross. The prospects of developing a fertile, cytologically stable allotetraploid species from the hybrids appear favorable.     

Performance of two Picea abies (L.) Karst. stands at different stages of decline

Summary The annual replacement of tillers of Agropyron desertorum (Fisch. ex Link) Schult., a grazing-tolerant, Eurasian tussock grass, was examined in the field following cattle grazing. Heavy grazing before internode (culm) elongation seldom affected tiller replacement. Heavy grazing during or after internode elongation, which elevates apical meristems, increased overwinter mortality of fall-produced tillers and reduced the number and heights of these replacement tillers. Unexpectedly, tussocks grazed twice within the spring growing season tended to have lower overwinter tiller mortality, greater tiller replacement, and larger replacement tillers than tussocks grazed only once in late spring. These responses of twice-grazed tussocks, however, were still less than those of ungrazed tussocks or tussocks grazed moderately in early spring. The presence of ungrazed tillers on partially grazed tussoks did not increase the replacement of associated grazed tillers relative to tillers on uniformly grazed plants. This result indicates that resource sharing among tillers, if present, is short-lived or ecologically unimportant in this species. Although A. desertorum is considered grazing-tolerant, tiller replacement on heavily grazed tussocks, particularly those grazed during or after internode elongation when apical meristems were removed, was usually inadequate for tussock maintenance. These observations at the tiller (ramet) level of organization in individual tussocks (genet) may explain the often noted reduction in stand (population) longevity with consistent heavy grazing.     

Interspecific and intraspecific variation in tree seedling survival: effects of allocation to roots versus carbohydrate reserves

We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species –Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina– with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots. Received: 14 December 1999 / Accepted: 2 June 1999     

Light field heterogeneity among tussock grasses: Theoretical considerations of light harvesting and seedling establishment in tussocks and uniform tiller distributions

Although the tussock growth form of caespitose graminoids is widespread, the effect of this growth form on light interception and carbon gain of tillers has received little attention. Daily incident photosynthetic photon flux density (PFD_inc) and carbon gain in monospecific stands of tussock grasses were compared with those of a hypothetical distribution with the equivalent tiller density per total ground area, but evenly distributed rather than clumped in tussocks. This was computed for two tussock grasses Pseudoroegneria spicata (Pursh) A. Löve (bluebunch wheatgrass) and Agropyron desertorum (Fisch, ex Link) Schult. (creasted wheatgrass) at different plant densities. Daily PFD_inc and net photosynthesis (A) were greater if tillers were distributed uniformly rather than clumped in tussocks, except when the density of tussocks was so great as to approach a uniform canopy. When tussock density per ground area was low, much of the difference between tussock and uniform tiller densities in PFD_inc and A was due to shading within the tussocks; up to 50–60% of the potential carbon gain was lost in A. desertorum due to shading within tussocks. In a matrix of tussocks, the light field for establishing seedlings was very heterogeneous; potential A ranged from 7 to 96% relative to an isolated seedling. The mean of daily PFD_inc and A for seedlings in a tussock stand were nearly identical to the values in corresponding stands of uniform tiller distributions. It is hypothesized that the loss of A resulting from clumping tillers into tussocks is offset by benefits of protecting sequestered belowground resources from invasion by seedlings of competitors.     

Effects of bison grazing on Andropogon gerardii and Panicum virgatum in burned and unburned tallgrass paririe

Summary Responses to clipping and bison grazing in different environmental contexts were examined in two perennial grass species, Andropogon gerardii and Panicum virgatum, on the Konza Prairie in northeastern Kansas. Grazed tillers had lower relative growth rates (RGR) than clipped tillers following defoliation but this difference was transient and final biomass was not affected by mode of defoliation. Grazed tillers of both species had higher RGR throughout the season than ungrazed tillers, resulting in exact compensation for tissue lost to defoliation. However, A. gerardii tillers which had been grazed repeatedly the previous year (1988) had reduced relative growth rates, tiller biomass and tiller survival in 1989. This suggests that the short-term increase in aboveground relative growth rates after defoliation had a cost to future plant growth and tiller survival.In general, the two species had similar responses to defoliation but their responses were altered differentially by fire. The increase in RGR following defoliation of A. gerardii was relatively greater on unburned than burned prairie, and was influenced by topographic position. P. virgatum responses to defoliation were similar in burned and unburned prairie. Thus grazing, fire, and topographical position all interact to influence tiller growth dynamics and these two species respond differently to the fire and grazing interaction. In addition, fire may interact with grazing pattern to influence a plants' grazing history and thus its long-term performance.     

Tillering in Tussock Grasses in Relation to Defoliation and Apical Bud Removal

Experiments with five caespitose grass species from temperateand tropical environments showed that the number of lateralshoots (tillers) which emerged following defoliation was notincreased by leaving a greater residual leaf area. Increasedavailability of photosynthate (and perhaps other resources)was effective, however, in increasing the rate of growth anddegree of flowering of new lateral shoots in one tropical species,Panicum maximum. In two temperate Agropyron tussock grasses, decapitation (apicalbud removal) did not stimulate lateral shoot growth. This indicatedthat apical dominance was not a factor preventing growth oflateral buds just prior to inflorescence emergence on the parenttillers. However, defoliation, where both terminal buds andfoliage were removed from the parent tillers stimulated lateralbud growth. Hormones other than those produced by the apicalbud or light quality or intensity may control lateral bud growthin these species. In contrast to the temperate species, lateralbud growth was stimulated by both decapitation and defoliationin the three tropical species. This response is consistent withthe model of correlative inhibition by apical dominance. Agropyron desertorum, Agropyron spicatum, Heteropogon contortus, Panicum maximum, Themeda triandra, crested wheatgrass, bluebunch wheatgrass, black speargrass, green panic grass kangaroo grass, apical dominance, tillering, regrowth, grazing, tussock grasses     

Expression of dehydration-stress-related genes in the crowns of wheatgrass species [Lophopyrum elongatum (Host) A. Love and Agropyron desertorum (Fisch. ex Link.) Schult.] having contrasting acclimation to salt, cold and drought

Agropyron desertorum and Lophopyrum elongatum were grown in a control environment or acclimated in high‐salt (daily exposure to 75 or 150 m M NaCl for 6 d), cold (6/2 °C for 14 d) or drought environments (watering withheld for 6 d). Lophopyrum elongatum was constitutively tolerant to salt and also acclimated more to salt than did A. desertorum whereas A. desertorum acclimated more to cold and drought. Dehydrin and non‐specific lipid transfer protein (nsLTP) mRNA sequences and polypeptides increased more, during acclimation to cold and drought, in A. desertorum than in L. elongatum crowns. Expression of immunologically identified dehydrin polypeptides was much higher in drought‐acclimated A. desertorum than in any other species/treatment combination. The most strongly expressed were 42 and 20 kDa. No change in dehydrin or nsLTP polypeptides were detected in the crowns during acclimation to salt. Overall, there was stronger acclimation to dehydrative stresses, at the molecular level, in A. desertorum than in L. elongatum crowns. Differences in dehydrin and nsLTP mRNA and polypeptide expression during acclimation to different stresses indicated that plants sense the differences between different primary potential causes of cellular dehydration.