In addition, most of the Cu was sequestered in root tissues. Therefore, the likelihood of Cu over-accumulation in fruit is low. According to US Department of Agriculture, annual per capita consumption of fresh cucumbers in the United States is 3.0 kg in 2013, which means average daily cucumber consumption is ∼8.2 g per person-day . The average cucumber water content is 95%, so the daily consumption is 0.41 g dry weight per person-day. Thus, daily personal Cu intake from cucumber used in this study would be 10.0, 11.2, 11.8, and 12.9 μg from control, low, medium and high treatments. According to the Food and Nutrition Board at the U.S. Institute of Medicine of the National Academies, the recommended average requirement for Cu is 700 μg per person-day, with a tolerable upper intake level of 10 mg per person-day.Therefore, Cu intake from consumption of cucumber exposed to nCu enriched soil would be within the recommended Cu levels, even at the higher application level. Hence, consumption of nCu treated cucumbers, even at the high level, represents no significant added risk to consumers. Fruit quality is affected by, among others, sugars and fatty, amino, and carboxylic acids. The profile alteration of these nutrients may result in flavor and nutritional supply changes induced by exposure to nCu.Leaves growing in sunny locations have comparatively high photosynthetic capacities, Rubisco activity, rates of electron transport, and rates of dark respiration . Some species are restricted to sunny or shady locations, and the leaves of these plants are often genetically adapted to their characteristic light environment. The leaves of other species, including those that are naturally exposed to particularly variable light environments,fodder system acclimate to local conditions . Acclimation to extended changes in light enhances net assimilation and nitrogen use efficiency while decreasing vulnerability to high light stress . Either anatomical or biochemical mechanisms may be involved in acclimation .
The local light environment influences the morphological development of leaves in many species, resulting in comparatively thick leaves in bright locations . Fully expanded leaves have a limited capacity for morphological change , and acclimation by these leaves requires biochemical changes in carboxylation, electron transport, and light harvesting, as well as modifications to chloroplast structure and orientation . Monocotyledons with basal meristems, long leaves, and dense canopies may represent a case where photosynthetic acclimation by biochemical change is particularly advantageous. The grass Lolium multiflorum exhibits a strong capacity for local photosynthetic acclimation along the length of a leaf . The leaves of plants like Lolium are produced in dark or dim conditions at the base of plants, and, over time, are pushed to the upper part of the canopy. Typha latifolia , atall monocot that forms dense and highly productive monospecific stands in wetlands , may provide an even more extreme example. T. latifolia ramets originate from rhizomes that are buried in sediment, submerged under water, and often shaded by a dense layer of litter and existing plants. Initial leaf growth is supported by carbohydrates that are either mobilized from rhizomes or translocated from older leaves. Depending on sediment thickness and water depth, and the density of the litter layer and existing canopy, the lower 50–100 cm of a Typha leaf may experience almost total darkness . These characteristics make Typha a useful experimental system for investigating the acclimation capacity of morphologically mature leaves. Basal growth in Typha allows the separation of leaf age from light environment; the oldest segments of Typha leaves are exposed to the brightest light, as opposed to plants with apical meristems, where the youngest leaves are in bright conditions. We investigated the photosynthetic capacity of T. latifolia leaves over time following step changes in shading at different locations along leaves. We hypothesized that morphologically mature Typha leaves have a strong ability for local acclimation, and that individual leaf segments acclimate to the local light level autonomously from the rest of the leaf.T. latifolia rhizomes and crowns were collected in April 2004 at the San Joaquin Freshwater Marsh , located in Orange County, California . Rhizomes with an average weight of 49.7 ± 2.5 g were planted in 20-l pots filled with sand. Plants were grown in a greenhouse under either low light with a mean PPFD of 2.7 mol m−2 d−1 and a maximum of 159 mol m−2 s−1, which was created with 80% neutral shade cloth, or high light with a mean PPFD of 19.4 mol m−2 d−1 and a maximum of 990 mol m−2 s−1.
Each pot had one plant, and the pots were widely spaced. The lower segments of leaves were unshaded by either neighboring plants or upper leaf segments, and the light levels were approximately constant along the length of leaves. The water level in the pots was maintained 5 cm above the sand surface with daily additions of deionized water. The pots were fertilized every 2 weeks with Flora Grow and Flora Micro following the manufacturer’s instructions . The pots were drained before fertilization to avoid salt buildup.Two-month-old sun and shade grown plants with several fully expanded leaves were placed on a bench under high light, and a pair of fully expanded leaves from each plant were selected for experimentation. Individual leaf segments between 20 and 45 cm from the tip were exposed to either sun or shade during the 15-day transfer experiment using cylinders of 80% neutral shade cloth, creating the full combination of segments exposed to constant low light , constant high light , low to high light , or high to low light . Additionally, a set of segments on the same leaves were exposed to either constant high light or low to high light . All treatment combinations and locations were replicated six times. The photosynthesis rate under bright light , stomatal conductance and dark respiration rate were measured every two or three days for two weeks in the middle of the sun and shade segments , on six replicate plants using a portable gas exchange system . Afull sun was measured at a PPFD of 2000 mol m−2 s−1 and Rd was measured in darkness after allowing 3–5 min for equilibration. Leaf temperature was controlled at 25 ◦C and reference CO2 concentration at 370 mol mol−1. The leaf to air vapor pressure deficit ranged from 0.6 to 1.5 kPa. Photosynthetic light response curves were measured after leaves had fully acclimated to a change in light . The light response curves were fit using a non-rectangular hyperbola . Afull sun was calculated as the photosynthetic rate at 2000 mol m−2 s−1; Amax was calculated by extrapolating the regression to infinite light; Rd was calculated as the y-intercept; the apparent quantum yield was calculated as the slope extrapolated to darkness. The light response curves were started at high light , and assimilation was measured in response to stepwise PPFD decreases until full darkness.
Stomatal conductance decreased gradually in response to light decreases, and increased gradually in response to light increases. This sluggish stomatal response either led to lower rates of photosynthesis for light curves run from dim to bright conditions relative to curves run from bright to dim conditions, or forced unreasonably long equilibration times. Moreover,fodder system for sale midday field and greenhouse observations showed that leaves exposed to a continuous PPFD of 2000 mol m−2 s−1 for ∼15 min exhibited a steady CO2 assimilation. We therefore opted to carry out light curves from bright to dark conditions, but acknowledge that lags in stomatal adjustment may have resulted in somewhat higher Ci for the light curves than would have been observed for fully equilibrated leaves. Nonetheless, we emphasize that our study is comparative, and the key is consistency across treatments; we executed the light curves the same way for all treatments and leaf segments. Nitrogen concentration , and leaf mass per area , were measured on the leaf segments used for gas exchange. Nitrogen was determined using the micro Kjeldahl technique; samples were oven dried, ground in a Wiley mill, weighed, digested, and nitrogen concentration was determined with an auto analyzer .We characterized the vertical gradients of light and photosynthetic characteristics during midday sunny conditions in August 2004. The PPFD profile was measured through the canopy at 48 different locations in the SJFM using a horizontal quantum sensor mounted on a 2 m handheld pole. Each profile consisted of ten individual measurements recorded with a data logger at 0.0, 0.6, 1.2, and 3.0 m above the sediment surface. The 3.0 m measurement was above the canopy. LAI was measured at the base of the canopy with a LI-COR LAI-2000, assuming non-clumped leaves and without distinguishing between live leaves and litter. Photosynthetic light response curves were measured on three segments of fully expanded leaves from 5 different plants. The cross section of leaves changed from flat at the tip to triangular at the base, and it was not possible to seal the chamber on leaf segments further than 100 cm from the tip.The parameters derived from the light response curves, the nitrogen content, and the leaf mass per area, were compared between treatments using Univariate ANOVA or t tests. The effect of light treatment was analyzed by Student’s t-test. Univariate ANOVAs and Tukey tests were used to compare Afull sun, Amax, gs and Rd between the light treatments within each sampling period. The effects and interactions of treatment and time following transfer were analyzed with multivariate analysis of variance ; this analysis corrected F values due to temporal auto correlation. MANOVA does not require the response variables to be equally correlated, assuming an unstructured variance–covariance matrix . The effect of leaf position on the photosynthetic parameters of leaves growing in natural conditions was analyzed with three paired t-tests, because of the high variation among leaves.
Statistical analyses were performed with JMP software version 7.0 and Minitab statistical software version 15.Fully expanded T. latifolia leaf segments exhibited strong acclimation to a change in light. The light response curves of individual segments of shade grown leaves that were transferred to high light were similar to those of segments that remained in high light throughout the experiment . This acclimation was highly localized; the response curves of SH-SH segments did not change, even as adjacent SH-SU segments acclimated to high light. Likewise, the response curves of SU-SU segments did not change, even as adjacent SU-SH segments acclimated to shade . Photosynthesis in bright light , maximum photosynthetic capacity , dark respiration , and stomatal conductance differed significantly between sun and shade segments following acclimation, regardless of initial growth conditions . In contrast, intercellular CO2 concentration and apparent quantum yield showed no significant differences. When compared within plants that started in sun, Afull sun and Amax of SU-SU segments differed significantly from SU-SH segments. The same pattern was found for Rd and gs, but not ˚y and Ci, where no significant differences among treatments were found. When light treatments were compared within plants that started in the shade, SH-SU segments and SH-SH segments were significantly different for most parameters except ˚y and Ci .Acclimation to a change in light occurred over a 10 to 15 day period. The MANOVA showed significant effects of light treatment and time, as well as an interaction, on leaf gas exchange . The rate of Afull sun by SH-SU segments increased over time, and was significantly greater than that of adjacent SH-SH segments beginning on day. The Afull sun observed for the SH-SU segments after ∼10 days was comparable to that of SU-SU segments . Broadly similar, or somewhat faster, responses were observed for the SU-SH treatments; Afull sun decreased, reaching a rate that was comparable to that of SH-SH segments . Sun grown segments exposed continuously to high light alsoshowed a decrease in Afull sun over time , though this trend was smaller than that observed for the SU-SH segments, and Afull sun by the SU-SH segments was significantly less than that by the SU-SU segments beginning on day 8. Stomatal conductance paralleled the changes in Afull sun; the gs of SH-SU segments increased over time, becoming significantly different from that of SH-SH segments on day 4, and reaching a maximum after ∼10 days that was comparable to that of SU-SU segments . Likewise, the gs of SU-SH segments decreased significantly, reaching a rate that was comparable to that of SH-SH segments .