We irrigated field-grown celery (Apium graveolens L. var. dulce [Mill.] Pers. 'Tall Utah') with four concentrations of saline water, NSC (nonstressed control), SW1, SW2, and SW3, corresponding to EC of 0.5, 4.4, 8.5, and 15.7 dS·m-1, respectively, plus a nonirrigated control (NIC) and investigated the effects of the treatments on water relations, yield and ion content. In addition, we compared simultaneously plant response to both salt and drought stress by using a modified version of the threshold-slope model. Increasing salinity of the irrigation water reduced fresh and dry weights of the shoots, but increased the dry matter percentage in shoots. The marketable yield was moderately affected by salinity (25% reduction at EC 8.5 dS·m-1). In contrast, a severe water stress dramatically decreased the marketable yield from 23 t·ha-1 (average of the irrigated treatments) to <7 t·ha-1 (nonirrigated control). Na+ and Cl- concentrations increased in salinized plants whereas nitrogen content, K+, Ca2+, and Mg2+ concentrations decreased upon salinization. Midday leaf water potentials (Ψt) decreased from -1.48 MPa (0.5 dS·m-1) to -2.05 MPa (15.7 dS·m-1) and - 2.17 MPa (nonirrigated control), though the reduction in leaf cellular turgor was less severe. The maintenance of high leaf cellular turgor was positively correlated to a decrease in osmotic potential and to an increased bulk modulus of elasticity. These results indicate that it is possible to irrigate celery with saline water (up to 8.5 dS·m-1) with acceptable losses in marketable yield and confirmed that in the field, this species has the ability to efficiently regulate water and ion homeostasis. In the absence of irrigation, celery plants were unable to cope with the drought stress experienced, although this was comparable, in terms of soil water potential, to the one caused by saline irrigation.
Three-year-old `Braeburn' apple trees (Malus domestica Borkh.) on MM106 rootstock were studied in a glasshouse to assess the effects of deficit irrigation on fruit growth, water relations, composition, and the vegetative growth of the trees. Trees were assigned to one of three treatments. The control (C) was fully watered. The first deficit treatment (D1) was deficit-irrigated from 55 days after full bloom (DAFB) until final harvest at 183 DAFB. The second deficit treatment (D2) was deficit-irrigated from 105 to 183 DAFB. Compared to C, the D1 and D2 trees developed a lower photosynthetic rate, leaf water potential (Ψl), and stomatal conductance (gs) during the stress period. Trunk-circumference growth was reduced in both D1 and D2 trees, but leaf area and shoot length were reduced in D1 only. Total soluble solids increased in both D1 and D2 fruit. Fructose, sorbitol, and total soluble sugar concentrations were higher in D1 fruit than in C and D2. Titratable acidity and K+ levels were higher in D1 fruit than C and D2. For D1, lowering of fruit water potential (Ψw) was accompanied by a decrease in osmotic potential (Ψs), and therefore turgor potential (Ψp) was maintained throughout the sampling period. Regardless of fruit turgor maintenance, the weight of D1 fruit was reduced from 135 DAFB. Weight, sugar concentration, and water relations of D2 fruit were not affected by deficit irrigation. This indicates that fruit water relations and sugar concentration are modified if water deficit is imposed from early in the season. However, if water deficit is imposed later in the season it has less impact on the composition and water relations of the fruit.
. Nonetheless, there are some commonalities. Tolerance (to high salt levels in plant tissues) almost always requires the combination of several different traits: accumulation and compartmentation of ions for osmoticadjustment; the synthesis of compatible
, osmoticadjustments, and saturation level of cell membrane lipids ( Huang et al., 2014 ; Shinozaki and Yamaguchi-Shinozaki, 2007 ), these responses can be overwhelmed during climatic extremes. Drought stress may inhibit photosynthesis and cause an energy
water loss, which is governed by stomatal behaviors and osmoticadjustment among various other physiological factors ( Farooq et al., 2009 ; Kramer and Boyer, 1995 ). Osmoticadjustment helps to maintain the cell water balance with active accumulation
metabolic processes even under cellular water deficit. Drought tolerance may be accomplished through various mechanisms such as osmoticadjustment (OA), which involves accumulation of solutes to maintain cellular turgidity. Drought tolerance has been
Net photosynthesis (Pn) of two ecotypes of redbud (Cercis canadensis L.) was studied following growth under high temperatures and increasing drought. Although mexican redbud [C. canadensis var. mexicana (Rose) M. Hopkins] exhibited greater Pn than eastern redbud (C. canadensis var. canadensis L.), Pn decreased at a similar rate under water deficit stress for both ecotypes. Mexican redbud also had greater instantaneous water use efficiency [net photosynthesis: transpiration (WUE)] than eastern redbud. Differences in both Pn and WUE might have been due to differences in leaf thickness. The optimum temperature for potential photosynthetic capacity (37 °C) was unaffected by irrigation or ecotype. Tissue osmotic potential at full turgor was more negative in eastern redbud, but was unaffected by drought stress in either ecotype. Soluble carbohydrate content was higher in eastern redbud, and in both ecotypes, d-pinitol was the major soluble carbohydrate and was considerably more abundant in the water-stressed plants. Total polyol content (myo-inositol + ononitol + pinitol) was also greater in the water-stressed plants. Both ecotypes were very tolerant of high temperatures and drought.
Mechanisms of sugar accumulation in response to drought stress in Satsuma mandarin (Citrus unshiu Marc.) fruit were investigated. Predawn leaf water potentials averaged -0.35MPa for well-watered, -0.60 MPa for moderately drought-stressed, and -1.00 MPa for severely drought-stressed glasshouse-grown 3-year-old trees. Fruit peel turgor and fruit growth of the moderately drought-stressed trees recovered to a similar value to that of the well-watered trees. Photosynthetic rates and stomatal conductance of both moderately and severely drought-stressed trees were significantly lower than those of the well-watered plants. However, the total sugar content per fruit of moderately drought-stressed trees was the highest among the drought treatments. A 13C-labeling experiment showed that 13C distribution in fruit grown under the moderately drought-stressed condition was the highest. These findings indicate that sugar accumulation in fruit was caused by an increase in translocation of photosynthates into fruit, especially into the juice sacs, under drought stress.
Responses of five bottomland tree taxa to drought and flooding were studied to identify those adapted to urban environments. During one experiment, containerized `Franksred' red maple [Acer rubrum L. `Franksred' (trademark = Red Sunset)], sweetbay magnolia (Magnolia virginiana L.), black tupelo (Nyssa sylvatica Marsh.), bald cypress [Taxodium distichum (L.) Rich.], and pawpaw [Asimina triloba (L.) Dunal.] were treated with various irrigation regimes for up to 118 days. Net assimilation rate (NAR) and relative growth rate (RGR) were reduced more by flooding than by drought for plants of all taxa, except pawpaw, which showed similar NAR and RGR during flooding and drought. Only sweetbay magnolia and bald cypress maintained positive NAR and RGR during flooding, and sweetbay magnolia was the only taxon that did not produce significantly less leaf surface area, shoot dry mass, and root dry mass during flooding and drought. Apparent morphological mechanisms of stress resistance included an increase in specific mass of leaves (mg·cm-2) during drought for red maple and bald cypress and a 385% increase in the root: shoot mass ratio for droughted plants of pawpaw. Leaf water relations of drought- and flood-stressed `Franksred' red maple and sweetbay magnolia were determined in a second experiment. Predawn and mid-day leaf water potential (ψ) decreased with decreasing root-zone matric potential for both taxa, and transpiration rate was reduced by drought and flooding. Pressure-volume analysis showed that leaves of `Franksred' red maple responded to drought by shifting symplastic water to the apoplast. Leaves of drought-stressed sweetbay magnolia adjusted osmotically by reducing osmotic potential (ψπ) at full turgor by 0.26 MPa. Our results suggest that sweetbay magnolia and bald cypress will perform well at urban planting sites where episodes of drought and flooding regularly occur.
that respond to the low water availability, such as osmoticadjustment (OA), which consists of reducing cellular damage by accumulating osmolytes or compatible solutes ( Chaves et al., 2003 ). The aforementioned osmolytes are metabolites that accumulate