EFFECT OF Cu AND Mn TOXICITY ON CHLOROPHYLL FLUORESCENCE AND GAS EXCHANGE IN RICE AND SUNFLOWER UNDER DIFFERENT LIGHT INTENSITIES

Copper (Cu) and manganese (Mn) are essential micronutrients for plants, but toxic at high concentrations. Responses of rice (Oryza sativa L.) and sunflower (Helianthus annuus L.) to toxic concentrations of Mn and Cu (up to 100 μM) were studied under three light intensities including low (LL, PPFD=100), intermediate (IL, PPFD=500) and high (HL, PPFD=800) light intensities in hydroponic medium. Rice plants showed higher susceptibility than sunflower to both heavy metals concerning dry matter of shoot and root. Growing under higher light intensity strengthened the effect of Cu toxicity while ameliorated that of Mn, the latter was attributed to the lower Mn accumulation of HL plants in both shoot and root. Chlorophyll content of leaves was influenced negatively only by Cu treatment and that at the highest concentration in the medium (100 μM). Similar with growth results, reduction of net assimilation rate (A) was higher in HL than LL plants treated by excess Cu, but in contrast to growth response, reduction was more prominent in sunflower than rice. Excess Mn-induced reduction of A was similar between LL and HL plants and was greater in sunflower than rice. Reduction of A was partly attributable to stomatal limitation, but non-stomatal mechanisms were also involved in this reduction. Copper and Mn treatment did not change the optimal quantum efficiency of PSII in dark-adapted chloroplasts (Fv/Fm ratio), but Fv/F0 was influenced particularly by Cu treatment, the reduction was higher in rice than sunflower and in HL compared to LL plants. Regarding excess Cu and Mn-mediated alterations in chlorophyll concentration, Fv/F0 and Tm values, it was suggested that, Cu and Mn toxicity depress the leaf photosynthetic capacity primarily by causing a significant alteration of the composition and functional competence of the photosynthetic units rather a reduction in the number of photosynthetic units (PSUs) per unit leaf area

Consistent alleviation of abiotic stress with silicon addition: a meta-analysis

1. Hundreds of single species studies have demonstrated the facility of silicon (Si) to alleviate diverse abiotic stresses in plants. Understanding of the mechanisms of Si-mediated stress alleviation is progressing, and several reviews have brought information together. A quantitative assessment of the alleviative capacity of Si, however, which could elucidate plant Si function more broadly, was lacking. 2. We combined the results of 145 experiments, predominantly on agricultural species, in a meta-analysis to statistically assess the responses of stressed plants to Si supply across multiple plant families and abiotic stresses. We interrogated our database to determine whether stressed plants increased in dry mass and net assimilation rate, oxidative stress markers were reduced, antioxidant responses were increased and whether element uptake showed consistent changes when supplied with Si. 3. We demonstrated that across plant families and stress types, Si increases dry weight, assimilation rate and chlorophyll biosynthesis and alleviates oxidative damage in stressed plants. In general, results indicated that plant family (as a proxy for accumulator type) and stress type had significant explanatory power for variation in responses. The consistent reduction in oxidative damage was not mirrored by consistent increases in antioxidant production, indicative of the several different stress alleviation mechanisms in which Si is involved. Silicon addition increased K in shoots, decreased As and Cd in roots and Na and Cd in shoots. Silicon addition did not affect Al, Ca or Mn concentration in shoots and roots of stressed plants. Plants had significantly lower concentrations of Si accumulated in shoots but not in roots when stressed. 4. Meta-analyses showed consistent alleviation by Si of oxidative damage caused by a range of abiotic stresses across diverse species. Our findings indicate that Si is likely to be a useful fertilizer for many crops facing a spectrum of abiotic stresses. Similarities in responses across families provide strong support for a role of Si in the alleviation of abiotic stress in natural systems, where it has barely been explored. We suggest this role may become more important under a changing climate and more experiments using non-agricultural species are now needed

Stressed out symbiotes:hypotheses for the influence of abiotic stress on arbuscular mycorrhizal fungi

Abiotic stress is a widespread threat to both plant and soil communities. Arbuscular mycorrhizal (AM) fungi can alleviate effects of abiotic stress by improving host plant stress tolerance, but the direct effects of abiotic stress on AM fungi are less well understood. We propose two hypotheses predicting how AM fungi will respond to abiotic stress. The stress exclusion hypothesis predicts that AM fungal abundance and diversity will decrease with persistent abiotic stress. The mycorrhizal stress adaptation hypothesis predicts that AM fungi will evolve in response to abiotic stress to maintain their fitness. We conclude that abiotic stress can have effects on AM fungi independent of the effects on the host plant. AM fungal communities will change in composition in response to abiotic stress, which may mean the loss of important individual species. This could alter feedbacks to the plant community and beyond. AM fungi will adapt to abiotic stress independent of their host plant. The adaptation of AM fungi to abiotic stress should allow the maintenance of the plant-AM fungal mutualism in the face of changing climates. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00442-016-3673-7) contains supplementary material, which is available to authorized users

The effects of selenate and sulphate supply on the accumulation and volatilization of Se by cabbage, kohlrabi and alfalfa plants grown hydroponically

The effect of Selenium (Se) supplementation at five levels of 0 (control), 5, 10, 15, 20 ìM in plants supplied with one of four concentrations of sulphur (S) including 0.05, 0.25, 0.5 and 1.0 mM was investigated in two varieties of Brassica oleracea (cabbage and kohlrabi) and alfalfa (Medicago sativa L.) in a hydroponic experiment. In severely S deficient plants (0.05 mM), Se acted as a toxic element, alfalfa was the most susceptible plant that died at this treatment. However, in plants supplied with near adequate (0.5 mM) or adequate (1.0 mM) S, Se acted as a growth promoting element. The most pronounced stimulation of growth was observed in cabbage and the lowest in alfalfa. Increasing S concentration in the medium, reduced Se uptake and transport. In contrast, S uptake and transport increased in response to Se addition. Se volatilization was higher in alfalfa than cabbage and kohlrabi when expressed on unit shoot dry weight or leaf area basis, but not when expressed per plant. Results suggested that Se supplementation of plants supplied with adequate S, not only had beneficial effects on plants growth but also can have application in enrichment of livestock fodder and human food.

Comparison of water stress and UV radiation effects on induction of CAM and antioxidative defense in the succulent Rosularia elymaitica (Crassulaceae)

Growth and photosynthetic characteristics, inducibility of the CAM pathway and the functioning of the antioxidant defense system were investigated in Rosularia elymaitica (Crassulaceae) under drought and UV stresses. Drought did not substantially affect the growth of the plants, but it significantly reduced leaf thickness as well as osmotic potential, water potential and relative water content. In contrast, UV radiation treatment affected neither growth nor the water relations of leaves. Water limitation for 12 days caused a significant increase in nighttime PEPC and NAD-MDH activity and an increase in Δtitratable acidity relative to well-watered plants. The nighttime CO2 net assimilation rate increased significantly in drought-stressed plants but was still negative, resembling a C3-like pattern of gas exchange. Twenty days of UV treatment, increased Δtitratable acidity slightly and increased only daytime PEPC activity, and did not affect other parameters of carbon metabolism. As judged by maintenance of membrane integrity and stable amounts of H2O2 under UV stress, the antioxidant defense system effectively protected the plants against UV radiation. In contrast, oxidative stress occurred under severe drought stress (20 days of withholding water). Except for higher daytime APX activity in the UV-treated plants, enzyme activity in the control and in the drought- and UV-stressed plants did not show any diurnal fluctuation during 24 h. Temporal changes in Δtitratable acidity and ΔPEPC activity coincided closely with those of antioxidant enzymes; both started to increase after 12 days of drought stress. These results indicate that drought stress but not UV radiation induced the CAM-cycling pathway in R. elymaitica