Increased Plant Water Stress Research Articles

Red Clover Seed Production: II. Plant Water Status on Yield and Yield Components

Limited information on crop water stress response is available for red clover (Trifolium pratense L.) seed production. The purpose of this study was to quantify the effects of plant water status on red clover flower production, seed yield, and seed yield components. Five supplemental irrigation treatments were applied in 1990 and 1991 to first and second year red clover grown on a Woodburn silt loam (fine‐silty, mixed, mesic Aquultic Argixeroll) near Corvallis, OR. A nonirrigated control was also maintained. Increased plant water stress reduced the duration of season‐long bud and flower production, stem length, potential seed yield, and seed yield. Application of water soon after haying increased total above‐ground phytomass but increased seed yield less than watering at peak flowering. Root deterioration from a root and crown rot disease complex reduced second‐year seed yield. Root rot index increased with increased levels of plant water stress, indicating that supplemental water applications reduced root rot severity. The reduction in seed yield resulting from increased plant water stress was primarily caused by a decrease in floral fertility, and less conclusively, by reductions in flower number per unit area. One irrigation filling the active soil profile during peak flowering provided adequate water to maintain seed production and achieve high water‐use efficiency. Contrary to popular belief, it was shown that red clover was not adversely affected by high levels of available soil water in a well drained soil. Also, yield losses to root disease in the second year of seed production can be greatly reduced by supplemental irrigation compared to non‐irrigated clover.

Distribution and Survival of Eggs of Summerform Pear Psylla (Homoptera: Psyllidae) Affected by Leaf Midvein

Distribution of pear psylla eggs on pear seedlings and cues affecting oviposition were monitored. Most eggs were deposited on the youngest foliage; few eggs were placed on the oldest, leathery leaves. Highest densities occurred adjacent to the midvein or in leaf serrations. Densities were very low on the leaf blade away from the midvein. Oviposition was induced in areas normally avoided (e.g., leaf blade) by providing strips of tape as artificial structures resembling midveins. Masking of particularly prominent midveins with strips of tape resulted in reduction of oviposition in these normally preferred sites. These results suggest that the midvein provided positive mechanical cues to ovipositing females. Effects of egg location on survival and development of eggs was quantified. Survival rates of eggs were higher for eggs deposited along the midvein than those deposited on the leaf blade. Survival decreased with increasing plant water stress. Development rates of eggs were not affected by location or stress.

Adaptation of Grasses to Water Stress-Leaf Rolling and Stomate Distribution

Leaf dimension, degree of leaf rolling or folding, and stomatal densities on adaxial and abaxial leaf surfaces were measured on herbarium specimens of 39 grass species from a range of dry to wet habitats in western Canada. Stomata were counted on vinyl leaf impressions taken from the herbarium specimens. Representative surfaces also were examined using scanning electron microscopy. All species from dry habitats had narrow rolled or folded leaves (4 mm or less). The proportion of stomata on the abaxial surface of species from dry habitats ranged from 0 to 65%, but 56% of the species were strongly amphistomatous. The results were compatible with a conceptual model predicting that rolling, amphistomatous leaves would be selected for habitats in which water supply and demand fluctuate widely on seasonal or diurnal time scales. The evolutionary adaptation of grasses involves a syndrome of physiological, anatomical, and morphological characteristics. Grass leaf structure is closely coupled with major physiological processes such as photosynthesis, water relations, and energy balance. For example, vascular bundle and mesophyll cell arrangement are related to the type of photosynthetic pathway (Black et al., 1973; Hattersley & Watson, 1975), which, in turn, can influence niche separation in grasses (Monson et al., 1983). Specialized leaf tissue structure in some grasses results in leaf rolling, which strongly influences water and energy balances by changing the characteristic leaf dimension and the conductance of heat, water vapor, and carbon dioxide (Ripley & Redmann, 1976). This paper concentrates on the relationship between leaf rolling and stomatal distribution and conductance. The anatomy and mechanism of leaf rolling in grasses have been studied for over a century (Tschirch, 1882; Shields, 1951). Loss of turgor in the bulliform cells on the adaxial (upper) surface generally is considered to induce rolling. Shrinkage of the adaxial subepidermal sclerenchyma and mesophyll, due to water loss, also contributes to involution; rolling can occur in leaves that lack bulliform cells (Shields, 1951). Some grasses have permanently rolled or folded leaves. Leaves of native grasses from semi-arid grassland roll in response to increased plant water stress during dry periods (Ripley & Redmann, 1976). More mesic grasses such as cereal crops also exhibit leaf rolling when exposed to water stress (Hurd, 1976; O'Toole et al., 1979). Leaves of Sorghum bicolor roll and unroll in response to diurnal changes in plant water status, provided stress is not too severe (Begg, 1980). The early textbooks on plant ecology generally explained rolling as a xeromorphic adaptation for reducing transpiration by protecting the stomata, which were considered to be concentrated on the upper surface (Warming, 1909; Weaver & Clements, 1929; McDougall, 1949). Unfortunately, references to data supporting this explanation were not given, but the idea probably originated with early work on ecological plant anatomy. Tschirch (1882) classified grasses as meadow type, with flat leaves, or steppe type, with rolling leaves. Lewton-Brain (1904) grouped British grasses into four categories representing progressively more drought resistant types: (1) leaves with flat upper surfaces, amphistomatous, (2) leaves with ribbed upper surfaces, amphistomatous, (3) leaves with ridged upper surfaces, rolled with drying, epistomatous, and (4) leaves permanently rolled or folded, epistomatous. I was unable to find published comparative data on relative stomatal distributions on grass leaves in relation to rolling or drought resistance that would verify the generalizations described above. Parkhurst (1978) pointed out that the limited data in the literature show no clear trends 1 This research was supported by a Natural Science and Engineering Research Council of Canada grant. Thanks go to G. Shaw who did the electron microscopy work using facilities in the Department of Biology, University of Saskatchewan. V. L. Harms contributed valuable discussion regarding drought resistance classification. 2 Department of Crop Science and Plant Ecology, University of Saskatchewan, Saskatoon S7N OWO, Canada. ANN. MISSOURI BOT. GARD. 72: 833-842. 1985. This content downloaded from 207.46.13.153 on Fri, 05 Aug 2016 06:11:51 UTC All use subject to http://about.jstor.org/terms

Plant and Soil Water Potentials following Fire in a Northern Mixed Grassland

Highlight: Leaf water potential, osmotic potential, and soil moisture were measured in mixed grassland during the growing season following an October fire. Plant and soil water stress increased near the end of the May-August period in both burned and control sites. Potentials in nothern wheatgrass and junegrass become lower in burned than in control plots as the growing season progressed. Increased plant water stress was judged sufficient to account for the reductions in productivity which have been observed after fire.

The Use of Controlled Environments in Research

Abstract Everyone who grows plants is aware of the vital role of the environment and at almost every meeting of horticulturists papers are presented describing the effects of environmental factors on plant growth. However, the environment includes so many interrelated factors (light, water, temperature, CO2, mineral nutrition, air pollutants, and pests) that it often is difficult to decide which one is producing an observed effect on growth. For example an increase in temperature often is accompanied by an increase in transpiration which may produce plant water stress. Thus it is difficult to separate the direct effect of increasing temperature on biochemical processes from the indirect effects produced by increasing plant water stress. The only way in which the effects of environmental factors on growth and yield can be analyzed effectively is by growing plants in controlled environment laboratories (phytotrons) where a particular component of the environment can be varied while all the others are held constant.

Growth, Water Relations and Yield of Wheat

Two crops of Sherpa wheat grown in successive years, but under contrasting seasonal conditions, were subjected to comprehensive environmental and biological measurement. An analysis is made of the changing state of water in the soil-plant system and of the consequent growth and the development of grain yield. The early pattern of growth was strongly influenced by moderate restriction in the availability of soil water (> - 200 J kg -1) and was associated with a marked shift in the allocation of growth resources in favour of root development. In both years, but more so in the second, grain filling proceeded under rapidly increasing plant water stress and senescence of productive photosynthesizing area. An analysis of the quantitative consistency of the biological and environmental data is attempted by the application of a published model of crop growth to the experimental data.

The Effect of Age, Pre‐conditioning, and Water Stress on the Transpiration Rates of Douglas‐Fir (Pseudotsuga menziesii) Seedlings of Several Ecotypes

AbstractSeedlings of Douglas‐fir from seed of a number of mesic and xeric origins were grown in growth chambers and a nursery to various ages up to 16 weeks. Measurements were made to determine the effect of seedling age, growth chamber and nursery pre‐conditioning, and seed source on transpiration rates under closely controlled laboratory conditions. Additional experiments were conducted on seedlings of two contrasting ecotypes to determine the effect of different pre‐conditioning combinations of plant and soil water potential on seedling transpiration rates. Results show that well‐watered seedlings of two mesic ecotypes show no decline in transpiration rates per unit leaf area up to 16 weeks of age while corresponding seedlings of three exeric ecotypes do decline. The growth chamber pre‐conditioning results in lower seedling transpiration rates and more decline in seedling transpiration rates with increasing plant water stress than for nursery pre‐conditioning. In a similar way, the xeric ecotype seedlings have more decline in transpiration rates with increasing plant water stress than do the mesic ecotype seedlings. Soil water potential influences transpiration rates through pre‐conditioning effects. Seedlings which have experienced prior soil moisture stress decrease transpiration more in response to low plant water potentials than do plants which have experienced no soil moisture stress. These behavioral characteristics illustrate adaptive means by which seedlings conserve water through the interaction of genetic and preconditioning mechanisms.

Seasonal Pattern of Net Photosynthesis of Artemisia tridentata

Gas exchange studies were carried out on Artemisia tridentata during the course of a growing season using microclimatically controlled cuvettes and infrared gas analysis. A definite seasonal pattern of net photosynthesis emerged. This pattern was influenced by the interaction of four major factors: plant water potential, leaf temperature, irradiation, and stage of phenological development. In spring and early summer, when plant water stress was minimal, photosynthesis rate was mainly correlated with leaf temperature and irradiation. During mid and late summer, increased plant water stress and phenological changes assumed at least equal importance with temperature and irradiation in limiting net photosynthesis. Indeed, plant water potential, mainly through its influence on stomatal aperture, rs‘, was probably the single most important factor influencing assimilation rate of this species on a seasonal basis. However, variations in mesophyll resistance to CO2 flux, rm‘, in response to temperature, water stress, or phenological changes also were involved. Sagebrush photosynthesis under field conditions was highest in late May and early June, and declined thereafter, minimum rates occurring in August during the driest period. Optimal temperatures for net photosynthesis were higher later in the season, indicating a change in gas exchange capacity more suitable to the warmer temperatures later in the season.

SEASONAL PATTERN OF NET PHOTOSYNTHESIS OF ARTEMISIA TRIDENTATA

Gas exchange studies were carried out on Artemisia tridentata during the course of a growing season using microclimatically controlled cuvettes and infrared gas analysis. A definite seasonal pattern of net photosynthesis emerged. This pattern was influenced by the interaction of four major factors: plant water potential, leaf temperature, irradiation, and stage of phenological development. In spring and early summer, when plant water stress was minimal, photosynthesis rate was mainly correlated with leaf temperature and irradiation. During mid and late summer, increased plant water stress and phenological changes assumed at least equal importance with temperature and irradiation in limiting net photosynthesis. Indeed, plant water potential, mainly through its influence on stomatal aperture, rs‘, was probably the single most important factor influencing assimilation rate of this species on a seasonal basis. However, variations in mesophyll resistance to CO2 flux, rm‘, in response to temperature, water stress, or phenological changes also were involved. Sagebrush photosynthesis under field conditions was highest in late May and early June, and declined thereafter, minimum rates occurring in August during the driest period. Optimal temperatures for net photosynthesis were higher later in the season, indicating a change in gas exchange capacity more suitable to the warmer temperatures later in the season.

Stomatal responses to changes in temperature at increasing water stress

The response of stomata to a gradual increase in temperature at increasing plant water stress was studied in a hot desert habitat (Negev, Israel) in the field, but under controlled temperature and humidity conditions. Four native species (Zygophyllum dumosum, Artemisia herba-alba, Hammada scoparia, Reaumuria negevensis) and one cultivated plant (Prunus armeniaca) were used in these studies. The stomatal response to temperature was compared with the response in well-irrigated plants of the same species.At low water stress, the diffusion resistance for water vapour decreased in response to a gradual increase in temperature. Transpiration increased accordingly. This response was reversible. All species responded in the same way. The opening of stomata with increasing temperature was apparently independent of the stomatal response regulated by atmospheric humidity. At high plant water stress, the stomatal response was reversed, i.e., the stomata closed when temperature was gradually increased. This stomatal closure was also independent of the closure regulated by atmospheric humidity. The plant water potential at which the stomatal response to temperature was reversed, differed among the species investigated.