Environmental stresses such as drought (Lauteri et al., 1997), salt stress (Bongi and Loreto, 1989), and leaf aging (Loreto et al., 1994) reduce conductance to CO 2 diffusion in the leaf mesophyll (mesophyll conductance). No information exists about possible increases of mesophyll conductance, such as when the stresses are alleviated. One obstacle to the investigation of this possibility is that mesophyll conductance reduction is frequently associated with the impairment of biochemical and photochemical characteristics of the leaf. The former is generally permanent, whereas the latter may recover slowly. However, it was recently shown that low salt accumulation (leaf Na concentration less than 15 mg g Ϫ1 ) primarily affects the conductance to CO 2 diffusion in spinach (Spinacia oleracea L.) leaves (Delfine et al., 1998). A coordinate reduction in stomatal and mesophyll conductance decreased the chloroplast CO 2 concentration of salt-stressed spinach. This, in turn, caused an inhibition of photosynthesis that was not associated with changes in biochemical or photochemical capacity when salt accumulation in the leaves was two to three times that of the controls.Mesophyll conductance reduction is also frequently associated with changes in leaf anatomy (Longstreth and Nobel, 1979;Bongi and Loreto, 1989;Evans et al., 1994;Syvertsen et al., 1995). This is likely to be a permanent effect, at least when leaf thickness is involved. However, low salt accumulation did not increase but slightly decreased the thickness of spinach leaves (Delfine et al., 1998). On the other hand, salt accumulation caused a 25% reduction of the intercellular spaces in the mesophyll of spinach leaves with respect to the controls. This could have caused a more tortuous path for CO 2 directed toward the chloroplast and was suggested to be responsible for the observed photosynthesis reduction associated with low mesophyll conductance in salt-stressed leaves (Delfine et al., 1998).The objectives of this work were to understand, under conditions that do not affect relevantly the biochemical and photochemical capacity of salt-stressed leaves, and are not able to change leaf anatomy significantly: (a) whether the reduction of mesophyll conductance can be reversed by alleviating the salt stress, and (b) how important changes in mesophyll conductance are in determining photosynthesis limitation. MATERIALS AND METHODS Plant Material and Experimental ConditionsFour groups of 30 spinach (Spinacia oleracea L. cv Matador) plants were grown in 3-dm 3 pots containing a mixture of soil, peat, and sand (1:1:1). When five to six leaves were fully expanded, the first group of plants (control) was grown under optimal water conditions by daily restoring the water lost through evapotranspiration. Evapotranspiration was estimated by weighing the pots daily. The second group of plants (salt stressed) was irrigated for 50 d with saline water (containing 1% [w/v] NaCl) when evapotranspiration was restored. The third group of plants (rewatered) was irrigated with saline w...

Spinach (Spinacia oleracea L.) leaves stressed by irrigation with water containing 1% (w/v) NaCl for 20 days had low conductance to CO2 diffusion both at the stomata and in the mesophyll. Mesophyll anatomy changed in salt-stressed leaves, which could have accounted for the decreased mesophyll conductance. Ribulose- 1,5-bisphosphate carboxylase/oxygenase in vitro activity and content were not affected by up to 20 days exposure to salinity but decreased when leaves were exposed to salt stress for longer than 20 days. Salt accumulation also caused a drop of Ca and Mg which might have decreased membrane stability and chlorophyll content, respectively. Measurements of chlorophyll fluorescence indicated that the 20-day-long salt stress did not directly affect photochemistry. We conclude that salinity reduces photosynthesis primarily by reducing the diffusion of CO2 to the chloroplast, both by stomatal closure and by changes in mesophyll structure which decreases the conductance to CO2 diffusion within the leaf. The capacity for carbon metabolism is eventually reduced but that occurs after substantial decreases in the conductance to CO2 diffusion.

Stress‐induced senescence is frequently alluded to, but interactive effects of water and N deficits on leaf longevity and on physiological changes associated with leaf aging have received little attention. A 2‐yr field experiment with maize (Zea mays L.) was conducted on Yolo loam (fine‐silty, mixed, nonacid, thermic Typic Xerorthents) with four treatments: N fertilized (180 kg N/ha) with (NI) and without (ND) irrigation; and no N applied with (OI) and without (OD) irrigation. Size and longevity measurements were made of leaves at all nodal positions, and leaves 5, 11, and 15 (counting up from stem base) were monitored for changes in photosynthetic capacity (PC), N concentration, and chlorophyll content (CHL). Leaf PC was correlated with percent N during senescence in all treatments (r2 = 0.86), but an occasional midday depression in PC, unrelated to leaf N status, was observed in severely water stressed plants. Chlorophyll and percent N were also correlated (r2 = 0.80), but initial onsets of decline in these two parameters often differed by several days. Nitrogen deficits were less severe in 1983 than 1982, but in both years lack of applied N had an early impact and reduced longevity of lower leaves. In 1982, N deficiency caused a 50% reduction in leaf peak values of percent N, CHL, and PC, and the decline in leaf 5 was most rapid in low N plants. Water stress intensified as stored soil water was gradually depleted, and the most rapid decline and earliest complete senescence of upper leaves (11 and 15) occurred in the ND treatment. These results indicate that early stress effects on leaf area and ear development affected subsequent demands for water and N, which in turn altered patterns of leaf senescence.

-The aim of this study was to monitor the effect of foliar application of humic acid on plant growth, photosynthetic metabolism and grain quality of durum wheat grown in a Mediterranean-type climate. Four fertilization treatments were applied: a non-fertilized control, a crop fertilized with foliar application of humic acid, a crop fertilized with mineral N on soil at sowing, tillering and stem elongation, and a crop fertilized with foliar application of N (ammonium-nitrate solution). Aboveground plant mass accumulation was measured throughout two growing seasons, and grain quality parameters were tested at harvest time. Gas exchange, leaf protein content and Rubisco activity were monitored at different stages of plant development. Differences between years were often relevant due to weather conditions. The foliar application of humic acid caused a transitional production of plant dry mass with respect to unfertilized control and split soil N application. This effect was also evident for grain yield, spike fertility and grain protein content during the two years of the study. Humic acid never affected photosynthesis or stomatal conductance, while Rubisco activity and leaf protein content showed intermediate responses between unfertilized control and split soil N application. We conclude that humic acid had limited promoting effects on plant growth, grain yield and quality, and photosynthetic metabolism of durum wheat crops grown in a typical Mediterranean-type agro-ecosystem of southern Italy, with respect to split soil N application.humic acid / durum wheat / gas exchange / nitrogen / protein / Rubisco / grain quality

Stress effects on leaf senescence are important for maize (Zea mays L.), since production of new photosynthetic area is genetically pre‐determined. Interactive effects of water and N deficits on water potential components, green leaf area (GLA) duration, and N and biomass allocation during grain filling were evaluated in a 2‐yr field study conducted on Yolo loam (fine‐silty, mixed, nonacid, thermic Typic Xerorthents) under four treatments: 180 kg N/ha applied at planting, with (NI) and without (ND) irrigation; and no N fertilizer, with (OI) and without (OD) irrigation. Leaves of low N plants had reduced capacity for solute accumulation, and both water and N deficits caused 0.1‐ to 0.5‐MPa reductions in midday leaf turgor. Total GLA was smallest in the OD treatment, but senescence during grain filling was greatest in the ND treatment because of a high transpiration demand and a high reproductive sink demand for N. Ear removal reduced the rate of post‐anthesis leaf N decline of all treatments, and plants in the ND treatment showed a significant reduction in leaf senescence rate when ears were prevented. Grain yield of the OD treatment was <50% of NI yield in both years; ND and OI treatments had intermediate reductions. Although the timing and intensity of stress differed between treatments and years, both water and N deficits reduced leaf longevity, and GLA duration from anthesis to harvest was linearly related (r2 = 0.92) to final grain dry weight.

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Physiological characteristics, growth, and biomass production of rainfed and irrigated bell pepper [Capsicum annuum L. var. anuum (Grossum Group) `Quadrato d'Asti'] plants were measured in the semiarid conditions of a Mediterranean summer to determine if drought stress effects are transient and do not affect plant growth and crop yield or are persistent and adversely affect plant growth and crop yield. A low midday leaf water potential indicated the occurrence of transient drought stress episodes in rainfed plants during the first 2 months of the study. Later on, predawn water potential also increased, indicating a persistent drought stress condition despite the occurrence of some rainfall. Photosynthesis was reduced when stress conditions developed, but the reduction was transient and limited to the central part of the day during the first 2 months. As plants aged, however, the impact of drought stress on photosynthesis was not relieved during the overnight recovery period. Stomatal conductance was reduced both during transient and permanent stress conditions while CO2 transfer conductance (i.e., conductance to CO2 inside the leaf) was only reduced when photosynthesis inhibition was unrecoverable. However, chloroplast CO2 concentration was higher in rainfed than in irrigated leaves indicating that CO2 availability was not limiting photosynthesis. Nonphotochemical quenching of fluorescence increased significantly in rainfed leaves exposed to permanent stress indicating the likely impairment of ATP synthesis. Transient inhibition of photosynthesis did not significantly affect leaf area index and biomass production, but growth was significantly reduced when photosynthesis was permanently inhibited. Fruit dry weight was even higher in rainfed plants compared to irrigated plants until drought stress and photosynthesis reduction became permanent. It is suggested that bell pepper growth without supplemental irrigation over the first part of the vegetative cycle does not impair plant growth and may even be useful to improve yield of early fruit.

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