Leaf water

Frequently overlooked as a nutrient, water can have a major influence on the growth of immature insects (Scriber and Slansky, 1981). Water content alterations in artificial diets (Reese and Beck, 1978) and in excised cherry leaves (Scriber, 1977) affect the metabolic costs, efficiencies and growth rates of Lepidoptera. In spite of a variety of behavioral, physiological and ecological adaptations to acquire adequate water and avoid desiccation, low leaf water content has remained a major evolutionary hurdle for most phytophagous insects (Southwood, 1972). 

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Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

Fig. 2. Rates of CO2 assimilation,. 4, and leaf conductances, g, as functions of intercellular partial pressure of CO2, p in Zea mays on various days after withholding watering. Measurements made with 9.5,19.0,30.5, and 38.0 Pa ambient partial pressure of CO2, 2 mmol m" s" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure differences between leaf and air. Closed symbols represent measurements with 30.5 Pa ambient partial pressure of COj. Leaf water potentials were 0.05, - 0.2, - 0.5 and - 0.8 MPa on day 0, 4, 11 and 14, respectively (after Wong et al., 1985).

This point was made clearly by Wong, Cowan Farquhar (1985). They imposed slow water stress on 30-day-old Zea mays plants grown in 45 1 plastic bins during early spring where glasshouse vapour pressure deficit was generally about 1.0-1.5 kPa. During the 14 day period the predawn leaf water potential declined from - 0.5 to - 0.8 MPa. Rate of assimilation. 

Bates, L.M. Hall, A.E. (1981). Stomatal closure with soil water depletion not associated with changes in bulk leaf water status. Oecologia, 50, 62-5. 

Boyer, J.S. (1970). Differing sensitivity of photosynthesis to low leaf water potential in corn and soybean. Plant Physiology, 46, 236-9. 

Potter, J.R. Boyer, J.S. (1973). Chloroplast response to low leaf water potentials. II. Role of osmotic potential. Plant Physiology, 51, 993-7. 

Fig. 9. Leaf water potential and turgor, abaxial stomatal conductance, and ABA content of abaxial epidermis of leaves of Commelina plants which were grown with their root systems divided between two pots. Water was either applied daily to both halves of the root system (A) or was withheld from one half of the root system after day 1 of the experimental period (A). Points for water relations and conductance are means s.e. Modified from Zhang, Schurr Davies (1987).

Fig. 10. Leaf water potential and abaxial stomatal conductance (upper figure), and water potential and turgor of secondary and tertiary root tips (lower figure) of maize plants growing in 1 m deep soil columns, watered daily (A) or not watered after day 0(A). The roots were sampled from the upper 20 cm of the soil column. Plants were 20 days old at the beginning of the experimental period. Points are means s.e. Modified from Zhang Davies (1989).

Jones, H.G. (1985). Physiological mechanisms involved in the control of leaf water status Implications for the estimation of tree water status. Acta Horticulturae, 171, 291-6. 

Westgate, M.E. Boyer, J.S. (1985f>). Carbohydrate reserves and reproductive development at low leaf water potentials in maize. Crop Science, 25, 762-9. 

Fig. 8. The relationship between leaf water potential (V>l) mono-dehydroascorbate reductase activity in barley leaves during a 7-day drying period, r = 0.72, P<0.001 (from Smirnoff Colombe, 1988).

O Toole, J.C. Moya, T.B. (1978). Genotypic variation in maintenance of leaf water potential in rice. Crop Science, 18, 873-6. 

Clarke, J.M. Townley-Smith, T.F. (1986). Heritability and relationship to yield of excised leaf water retention in durum wheat. Crop Science, 26, 289-92. 

Unfavorable temperatures, especially when suddenly imposed, have been shown to cause leakage of ions and metabolites from the root to the surrounding medium (188). In addition, sudden changes of temperature cause a lowering of conductivity to water of roots and inhibition of ion transport (188). This can indirectly affect the carbon economy of the whole plant by leading to more negative leaf water potentials, partial stomatal closure, and hence a slowing of net CO2 assimilation. 

Leaf water potential and osmotic potential were measured using a Wescor Dewpoint Microvoltmeter (Model HR-33) coupled with C-51 and C-52 sample chambers. Two plants from each group were sampled each day by taking two 7-mm diameter leaf disks from each plant, one for water potential and one for osmotic potential. Plants from which leaf disks were obtained were discarded. The water potential of a leaf disk was read following a 2-hr equilibration period in a sample... 

The reported (14) mechanisms of action of allelochemlcals Include effects on root ultrastructure and subsequent Inhibition of Ion absorption and water uptake, effects on hormone-induced growth, alteration of membrane permeability, changes In lipid and organic acid metabolism, inhibition of protein synthesis and alteration of enzyme activity, and effects on stomatal opening and on photosynthesis. Reduced leaf water potential Is one result of treatment with ferulic and p-coumaric acids (15). Colton and Einhellig (16) found that aqueous extracts of velvetleaf (Abutllon theophrastl Medic.) Increased diffusive resistance In soybean fGlycine max. (L.) Merr.] leaves, probably as a result of stomatal closure. In addition, there was evidence of water stress and reduced quantities of chlorophyll In Inhibited plants. 

Evans, L. S., and I. P. Ting. Ozone sensitivity of leaves Relationdiip to leaf water content, gas transfer resistance, and anatomical characteristics. Amer. J. Bot. 61 592-597, 1974. 

Atmospheric CO2 first moves through the stomata, dissolves into leaf water and enters the outer layer of photosynthetic cells, the mesophyll cell. Mesophyll CO2 is directly converted by the enzyme ribulose biphosphate carboxylase/oxygenase ( Rubisco ) to a six carbon molecule that is then cleaved into two molecules of phosphoglycerate (PGA), each with three carbon atoms (plants using this photosynthetic pathway are therefore called C3 plants). Most PGA is recycled to make ribulose biphosphate, but some is used to make carbohydrates. Free exchange between external and mesophyll CO2 makes the carbon fixation process less efficient, which causes the observed large C-depletions of C3 plants. 

Phenolic acids interfere with many major physiological processes of higher plants (35). These disruptions of function include an alteration of plant water balance. We found depression of leaf water potential to be an early indicator of allelochemical stress from ferulic and p-coumaric acids (42). Likewise one mechanism of allelopathic action by cultivated sunflower, velvetleaf Abutilon theophrasti Medic.), Koahia [Koahia saoparia (L.) Schrad.], and several other weeds was water stress (43-45). Since some allelochemicals interfere with plant-water relationships, it seemed logical that their action might be most critical at times when plants are under water stress from other causes. 

Wilman, D. and Wright, P.T. (1978) Dry-matter content, leaf water potential and digestibility of three grasses in the early... 

Anonymous. Final report on the safety assessment of Mentha piperita (peppermint) oil, Mentha piperita (peppermint) leaf extract, Mentha piperita (peppermint) leaf and Mentha piperita (peppermint) leaf water. Int J Toxicol 2001 20(3) 61. 

Dried. S divinorum leaves were obtained from Kava Kauaii (Hawaii). To test for antimicrobials, two crude extractions of dried, powdered Salvia divinorum leaf were made. For the first extraction, 100 ml hot distilled water was used to extract 5.016 g dried leaf. Water was heated to 95 degrees Celsius and temperature of the leaf-water mixture maintained for 15 minutes. Filtering yielded 18.5 ml brown fluid. In the second extraction, 4.113 g leaf was extracted at room temperature with 25 ml acetone producing 14.0 ml of a deep, bright green solution. 

Tam et al. (1996) investigated the uptake from water of a series of chlorinated benzenes by various tissues (leaves, petals, stems, roots) of the soybean plant. For two of the seven compounds investigated, they obtained the following apparent equilibrium leaf-water (BAFi]eafv,) and root-water (BAFimom) bioaccumulation factors (or bioconcentration factors since uptake is only from water, see Fig. 10.5) ... 

Winter, K. Gademann, R. (1991). Daily changes in C02 and water vapor exchange, chlorophyll fluorescence, and leaf water relations in the halophyte M. crystallinum during the induction of CAM in response to high NaCl salinity. Plant Physiology 95, 768-76. 

A number of workers (63, 64, 65) have suggested that there Is a critical. threshold leaf water potential in the -10 to -12 bar range at which the ABA levels of leaves start to increase. However, more recent evidence indicates that zero turgor is the critical parameter at which the ABA content starts to increase (66, 67). 

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