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Cucumber plants were cultured in a greenhouse and subjected to either well-watered or water deficit conditions that reduced leaf water potential to-0.6 MPa. Leaf gas exchange measurements were conducted using an open gas exchange system. Carbon dioxide assimilation (A) attained saturation at a photon flux density (PFD) of 1000 μmol·m-2·s-1 (400-700 nm). There were no significant differences in A at ambient temperatures between 16 and 34C. Water use efficiency decreased rapidly with increasing vapor-pressure deficits to 2.5 kPa. Water stressed plants had lower stomata1 conductances and CO2 assimilation rates. The decrease in A was only partially due to stomata1 closure. The A vs. intercellular CO (Ci) relationship for stressed leaves revealed a change in the CO, compensation point, and that nonstomatal factors were contributing to the decrease in A in stressed plants. Thus, feedback inhibition of A may have occurred through photoassimilate accumulation. The concentrations of sucrose and raffinose were higher, and the concentration of stachyose was lower in leaves of stressed than of well-watered plants.
Percent loss of conductivity of sweet cherry (Prunus avium `Emperor Frances') under dry conditions was determined by measuring hydraulic conductivity before and after high pressure perfusion removal of xylem embolisms. Cut stems were allowed to dry for 0 to 8 hours, recut underwater and the rate of flow of solution through the stems measured under positive pressure (∼8.0 kPa). Hydraulic conductivity (Kh)was then calculated, and typical values for well hydrated stems were 6 × 10-9 m4MPa-1s-1. Embolisms were then dissolved by high pressure perfusion (125kPa) and the subsequent flow rate measured. A second Kh was then calculated and the difference in Kh values before and after the high pressure treatment used as a measure of % loss of conductivity (or % xylem embolism). A curve of the `vulnerability' to xylem embolism was generated by plotting % loss of conductivity against initial stem water potential. The curve shows the stems undergo xylem embolism as soon as stem water potential reaches -1.5MPa, and at stem water potentials of -3 MPa, the stems are over 80% embolized. This cultivar appears to be vulnerable to embolism relative to other studied woody species, and xylem dysfunction likely is a problem early in a drying period. However, a particular rootstocks ability to supply water during a dry period and a cultivar's ability to limit water loss by stomatal closure will dictate the exact water potentials in the stem and thus its level of embolism.
Abstract
Changes in surface characteristics of the adaxial and abaxial cuticular membranes of expanding ‘Red-haven’ peach [Prunus persica (L.) Batsch] leaves were followed and related to wettability and to retention and penetration of foliar-applied compounds. Epicuticular wax deposition increased with leaf development until full expansion, the maximum being 50 to 65 μg cm−2. Although wax fine-structure was not prominent on expanding leaves, localized areas of crystalline wax occurred on both surfaces of fully expanded leaves. The 2 surfaces differed markedly in the composition of the epicuticular wax. Although triterpenoid acids, alkanes and primary alcohols were present on both surfaces, sterols and esters were predominant on the adaxial surface. Marked changes in composition occurred with leaf age. The contact angles formed by aqueous droplets on the adaxial surface increased with leaf development and attained the highest value only after full leaf expansion; on the abaxial surface the contact angle increased sharply and approached a maximum at 70 to 80% full expansion. Retention of aqueous solutions decreased with leaf expansion. Permeability of the cuticular membrane, as indexed by transpirational loss and penetration of 14C-NAA (naphthaleneacetic acid), decreased progressively with increasing leaf age. These findings are discussed in relation to spray application and penetration of foliar-applied chemicals.
`Imperial Gala' on M.9 EMLA, MM.111 and Mark rootstocks were planted in a rain exclusion shelter. Two drought stress periods lasting approximately 1 month each were imposed during 1991. Water was supplied at 2 liters per day per tree before and after each drought cycle while water was withheld from half of the trees during the drought stresses. Maximal and variable chlorophyll fluorescence and fluorescence quenching were significantly reduced by the drought stress with M.111 generally affected first and with the largest difference between drought and control followed by Mark and then M.9. Leaf and stomatal conductance, assimilation and transpiration usually occurred first and were lowest for M.9 followed by Mark and then M.111 during the first stress cycle while Mark responded more rapidly and to a greater extent than M.9 and M.111 during the second stress. Water potential was lower for the stressed trees during both stress periods but osmotic and turgor potentials were reduced only during the first stress period. Changes in water relations were noticed first and to a greater extent for Mark followed by M.9 with M.111 exhibiting the least sensitivity and differences.
15Nitrogen-ammonium nitrate was applied to four `Mutsu' apple (Malus ×domestica Borkh.) trees 40 days before harvest of 1996 (summer supplied nitrogen, SUN) and four others at full bloom in 1997 (spring supplied nitrogen, SPN) to evaluate the effect of application timing on N partitioning in mature trees. At leaf fall the largest amount of SUN was partitioned to roots and 2- to 4-year-old wood; the largest amount of SPN was partitioned to fruit and leaves and only a small amount detected in the roots. SUN did not increase N concentration in fruit or modify fruit firmness and soluble solids concentration, although it contributed to building up N reserves in the perennial woody organs. In 1997, as a result of the different timings of N supply, two sources of labeled N were distinguished and monitored in the vegetative organs: 1) the remobilized N, taken up in summer of 1996, stored in winter and then translocated to the growing tissues; 2) the newly absorbed N, taken up and moved to the canopy after the 1997 spring supply. Both fractions of remobilized and newly uptaken labeled N contributed to leaf and fruit N. Remobilized 15N was provided principally by roots which, from August to leaf fall, decreased their percentage of 15N by ≈18%, replacing the labeled with unlabeled N to maintain a constant concentration of total N.
European red mite (Panonychus ulmi) populations were monitored in a tart cherry (Prunus cerasus `Montmorency') orchard and the effects on photosynthesis determined. Mites levels were controlled in some trees by miticide applications to establish different cumulative mite*days in the trees. Photosynthetic inhibition caused by insect injury was also simulated by spraying other trees with 78 ppm Terbacil at one of four different times during the season, The mite*days accumulated in 1993 ranged from 937 to 2205, however, there were no differences in single leaf or whole tree CO2 assimilation, chlorophyll a fluorescence, or chlorophyll levels among the different levels of mite damage. Likewise, there were no differences in these same parameters among the Terbacil-treated trees except that photosynthesis was reduced on treated trees for 10-14 days, after which photosynthesis recovered to the level of the controls. There were no differences in yield or fruit quality among any treatments, and cold hardiness and return fruiting characteristics will be measured.
Mannitol, a sugar alcohol that appears to serve as an osmoprotectant/compatible solute to cope with salt stress, is synthesized in celery (Apium graveolens L.) via the action of a NADPH dependent mannose-6-phosphate reductase (M6PR). To evaluate the abiotic stress effects of mannitol biosynthesis, we transformed celery with an antisense construct of the celery leaf M6PR gene under control of the CaMV 35S promoter. Unlike wild type (WT) celery, independent antisense M6PR transformants did not accumulate significant amounts of mannitol in any tissue, with or without salt stress. In the absence of NaCl, and despite the lack of any significant accumulation of mannitol that is normally the major photosynthetic product, antisense transformants were mostly phenotypically similar to the WT celery. However, in the presence of NaCl, mature antisense transgenic plants were significantly less salt-tolerant, with reduced growth and photosynthetic rates, and some transformant lines were killed at 200 mM NaCl, a concentration that WT celery can normally withstand. Although mannitol biosynthesis is normally enhanced in salt-treated WT celery, no such increase was observed in the antisense transformants. Like our previous gain of function results showing enhanced salt tolerance in Arabidopsis plants transgenic for a sense M6PR construct, these loss of function results, using an antisense construct in celery, demonstrate a major role for mannitol biosynthesis in developing salt-tolerant plants.
One-year-old `Imperial Gala' on Mark, M.9 EMLA, and MM.1ll; and `Indian Summer' on MM.lll and MM.106 rootstocks were planted in a rain exclusion shelter in May 1991. All trees were irrigated. Half the trees were drought stressed and received no water for two, 30-day drought cycles. Four trees from each scion ×rootstock×irrigation combination were excavated in mid-October. Nonstructural carbohydrate reserves of stems and roots were determined. Cold hardiness, determined by visual examination of tissue after controlled freezing, was influenced by rootstock, drought, and stem age. Concentrations of several carbohydrates were correlated with cold hardiness. Regression models of carbohydrate concentrations on cold hardiness were significant. Removal of root tissue, which was cold sensitive and had high carbohydrate levels, altered the regression equations. Rootstock significantly influenced root concentrations of sorbitol, sucrose, and starch. Root sorbitol increased following drought stress. Mark and MM.106 roots had the largest increases in sorbitol. Irrigated Mark roots had very low levels of sorbitol.