Tolerance to water stress in boron-deficient tea (Camellia sinensis) plants
The effects of boron (B) deficiency and water stress were studied in tea plants (Camellia sinensis [L.] O. Kuntze) grown in growth chambers in perlite irrigated with a nutrient solution. Dry matter production was reduced significantly by both low B supply and water stress. Shoot-root translocation of B declined in water-stressed plants. In addition, the re-translocation of B was impaired under drought, which was reflected in a significantly lower ratio of B content of young to old leaves in both B-deficient and B-sufficient plants. Leaf photochemical parameters were negatively influenced by B deficiency and water stress in the old but not in the young leaves. Although B-deficient plants were more conservative in relation to water loss following elevated stomatal limitation, their water potential was lower than in B-sufficient plants irrespective of the watering regime. Under the combined effects of B deficiency and water stress, the reduction in the CO2 assimilation rate was more prominent than that under a single stress factor. The reduction of the net assimilation rate (A) in B-deficient plants due to water stress and in water-stressed plants due to low B supply were not accompanied by significant changes in the stomatal conductance, suggesting an involvement of non-stomatal factors. The activity of antioxidant enzymes and proline content increased under B deficiency and water stress conditions. Our results suggested that, in young leaves that have been developed under water stress, an acclimation to water stress conditions occurred that was well reflected in their more stable photochemistry, water relations and an efficient antioxidant defence system compared with the older leaves.
Photochemistry and gas exchange in cold conditions in Zn-deficient red cabbage (Brassica oleracea L. var. capitata f. rubra) plants
The responses of red cabbage (Brassica oleracea L. var. capitata f. rubra) plants to a low Zn supply and cold conditions (10°/7°C day/night temperature) were investigated in a hydroponic growing medium. A low Zn supply caused a significant reduction of shoot and root dry weight - up to 55% and 45% for the control and 62% and 52% for cold-treated plants, respectively. The total soluble carbohydrates and starch declined in Zn-deficient plants. Exposure to low temperatures, however, led to a decline in starch but an increase in soluble sugars. In Zn-sufficient plants, low temperatures increased the excitation capture efficiency of open photosystem II (PS II) reaction centres (RCs) (F'v/F'm), the quantum yield of PS II (ΦPSII), the electron transport rate (ETR) and the proportion of active chlorophyll associated with the RCs of PS II (Fv/F0). Low temperatures did not affect net CO2 uptake in Zn-sufficient plants, though a reduction of stomatal conductance occurred. The results demonstrated that although cold-treated plants were slightly more susceptible to Zn deficiency, cold treatment caused greater shoot biomass (up to 32%) in plants supplied with adequate Zn. The adaptation of red cabbage plants to cold conditions is attributable to improved photochemical events in the leaves, a maintenance of the net CO2 assimilation rate, lower water loss and the accumulation of anthocyanins as antioxidants.
Tea (Camellia sinensis) is an important beverage crop cultivated in the tropics and subtropics under acid soil conditions. Increased awareness of the health-promoting properties of the tea beverage has led to an increase in its level of consumption over the last decades. Tea production contributes significantly to the economy of several tea-cultivating countries in Asia and Africa. Environmental constrains, particularly water deficiency due to inadequate and/or poorly distributed rainfall, seriously limit tea production in the majority of tea-producing countries. It is also predicted that global climate change will have a considerable adverse impact on tea production in the near future. Application of fertilizers for higher production and increased quality and quantity of tea is a common agricultural practice, but due to its environmental consequences, such as groundwater pollution, the rate of fertilizer application needs to be reconsidered. Cultivation of tea under humid conditions renders it highly susceptible to pathogens and pest attacks. Application of pesticides and fungicides adversely affects the quality of tea and increases health risks of the tea beverage. Organic cultivation as an agricultural practice without using synthetic fertilizers and other chemical additives such as pesticides and fungicides is a sustainable and eco-friendly approach to producing healthy tea. A growing number of tea-producing countries are joining organic tea cultivation programmes in order to improve the quality and to maintain the health benefits of the tea produced.
The effect of silicon (Si) supplementation (0.35 g Na2SiO3 kg-1 soil, 2.73 mmol Si kg-1 soil) was studied in drought-stressed pistachio (Pistacia vera L. ‘Ahmadaghaii’) plants under field conditions. Silicon treatment significantly increased plant dry weight and relative water content under drought stress. The application of Si for drought-stressed plants improved the maximum quantum yield of PSII. A reduction in the net assimilation rate due to drought stress was alleviated by Si application, accompanied by an increase in stomatal conductance. Silicon treatment resulted in higher catalase and superoxide dismutase activities and lower lipid peroxidation in the leaves of drought-stressed plants. The results suggest that supplementation of water-deficient pistachio plants with Si alleviates the adverse effects of drought due to its enhancement of photochemical efficiency and photosynthetic gas exchange, as well as an activation of the antioxidant defence capacity in this species.