Exploring fine tuning between phytohormones and ROS signaling cascade in regulation of seed dormancy, germination and seedling development.

In higher plants, seed is a propagule which ensures dissemination and survival of species. Developmental phases of a seed comprise embryogenesis, maturation and germination paving a way to its final fate i.e. seedling establishment. The final stage of seed maturation is marked by dehydration, acquisition of dessication tolerance and induction of dormancy. A precise Abscisic acid (ABA) to Gibberellins (GA) ratio, accumulation of miRNA 156, low level of reactive oxygen species (ROS) and enzyme inactivity govern seed dormancy. This also prevent pre harvest sprouting of the seeds. Overtime, stored seed mRNAs and proteins are degraded through oxidation of specific nucleotides in response to ROS accumulation. This degradation alleviates seed dormancy and transforms a dormant seed into a germinating seed. At this stage, ABA catabolism and degradation accompanied by GA synthesis contribute to low ABA to GA ratio. GA as well as ROS acts downstream, to mobilize reserve food materials, rupture testa, enhance imbibition and protrude radicle. All these events mark seed germination. Further, seedling is established under the governance of auxin and light. ABA and GA are master regulators while auxin, cytokinins, ethylene, jasmonic acid, brassinosteroids act through interdependent pathways to tightly regulate seed dormancy, germination and seedling establishment. In this review, the role of phytohormones and ROS in accordance with environmental factors in governing seed dormancy, promoting seed germination and thus, establishing a seedling is discussed in detail.

Plant growth regulators preserved the longevity of cut stems of Chrysanthemum morifolium by orchestrating physio-biochemical and anatomical responses.

The marketability of cut flowers is determined by their postharvest quality parameters. Among these parameters, vase life is most crucial and different plant growth regulators (PGRs) play a significant role in regulating vase life. With the objective, to regulate vase life and other quality parameters of commercially important cut stems of chrysanthemum cv. White Star, a study was planned to have an insight into the role of different PGRs in orchestrating underlying physio-biochemical and anatomical responses. Three PGRs viz., Benzyl adenine (50, 100, 150 and 200 µM BA), Thidiazuron (5, 10, 15 and 20 µM TDZ) and Salicylic acid (50, 100, 150 and 200 µM SA) were used as pulsing solution. The results revealed that PGR especially 10 µM TDZ delayed leaf and floret senescence as compared to control. The visual observations (retention of green colour of leaves and white of florets) were correlated to enhanced water absorption, relative water content, membrane stability index, total soluble sugars and total soluble proteins ; higher content of chlorophyll in leaves and lower content of carotenoids and anthocyanins in florets. Further, the antioxidant enzymes (peroxidase and catalase) activities were also higher in PGR-treated stems than in control. These results were further supported by anatomical studies that indicated minimum blockage in xylem and maintenance in turgidity of cells as revealed through the size of tissues (radius of pith) of treated stems (control-118.48 mm and 10 µM TDZ-177.94 mm). Although all PGR concentrations significantly maintained relative water content, membrane stability index, respiratory substrates and antioxidant activity for a longer time, the low concentrations of TDZ had the most relevant impact on longevity. The longevity of stems treated with10 µM TDZ was up to 23 days as compared to 13 days in control. So 10 µM TDZ could be used as a chemical tool to improve chrysanthemum longevity and increase its commercial potential.

Effect of soil cadmium on growth, photosynthesis and quality of Raphanus sativus and Lactuca sativa.

Cadmium (Cd) raises serious concerns as its accumulation in the plant not only affect the growth and quality of plant but also threaten the health of consumers. In this research, two vegetables, i.e., radish (Raphanus sativus L.) and lettuce (Lactuca sativa L), were planted in pots having soil treated with Cd as Cd (NO3)2 at different doses (25, 50, 100 and 200 mg Cd kg-1 soil ) to investigate the influence of cadmium on their growth, photosynthetic attributes and quality. Cadmium retarded plant growth as dry weight of radish roots decline by 87% and leaves by 83% following 200 mg Cd kg-1 soil application and the corresponding values for lettuce were 64 and 69% respectively. Significant reductions in various photosynthetic parameters viz., leaf area per plant, total chlorophyll, Chl a and Chl b content were also recorded with Cd applications. The Cd treatments resulted in loss of membrane integrity as revealed by significant increase in electrolyte leakage in leaves of both vegetables. There was significant increase in Cd accumulation in radish and lettuce with all applications but no visual symptoms of Cd toxicity were noticed with 25 and 50 mg Cd kg-1 soil application except for yield differences, illustrating that Cd accumulate in this crop without visual evidence of its presence. However, toxicity symptoms in the form of interveinal chlorosis of the leaf lamina, followed by necrosis and leaf rolling, were clearly evident with 100 and 200 mg Cd kg-1 soil application. Apparently, Cd causes harm due to its phytotoxic effects and high accumulation in edible parts of radish and lettuce without any visible symptoms that constitutes a substantial hazard to human health.

Influence of cadmium on dry matter yield, micronutrient content and its uptake in some crops.

The study was conducted to evaluate the accumulation pattern of cadmium (Cd) and its tolerance in different crops grown on sewage irrigated soils with differentCd levels. Growth, yield, uptake and tolerance of maize, raya, berseem and spinach were assessed to different levels of Cd ranging from 0-40 mg kg(-1) soil. Significant reduction in dry matter yield in raya was observed at 20 mg Cd kg(-1) soil where as for other crops it was 10 mg Cd kg(-1) soil. The quadratic models seem to give the best description of variation in dry matter yield with Cd levels as revealed by significant coefficient of determination (R2 > or = 90). Cadmium content and uptake varied in the following order: raya > spinach > maize > berseem. Raya could tolerate high levels of Cd as compared to other crops. Conversely, berseem and maize which showed reduced ability to absorb or translocate Cd for genetic reasons compared to raya and spinach be preferred for growing on sewage irrigated soils or area having increased Cd-levels. The relationship of Cd with other micronutrients was variable.

Manganese influx and its utilization efficiency in wheat.

Manganese deficiency in wheat has become an important nutritional disorder particularly in alkaline calcareous soils where rice-wheat rotation is followed. This experiment was aimed to study the mechanism of Mn efficiency during various developmental stages in six wheat cultivars grown at two Mn levels viz. 0 and 50 mg Mn kg(-1) soil (Mnapplied as MnSO4 x H20) in pots. The Mn vegetative efficiency calculated on the basis of shoot dry weight at anthesis indicated HD 2967 and PBW 550 (bread wheat) as Mn efficient and durums as Mn inefficient. The efficient cultivars recorded highest values for influx, uptake, shoot dry weight, leaf area/plant, SPAD index, F(v)/F(m)ratio and root length that explained their higher efficiencies whereas inefficiency of durum cultivars was attributed to their smaller roots and lower influx. Under Mn deficiency, PDW 314 and PDW 291 retained 68% and 64%, respectively, of total Mn uptake in vegetative parts (stem and leaves) and lowest in grains 7% and 5%, respectively, whereas PBW 550, BW 9178 and HD 2967 retained 29, 37 and 34% in vegetative parts, and 21, 17 and 15 % in grains, respectively at maturity. Higher utilization efficiency of efficient genotypes also indicated that increased Mn uptake with Mn supply produced more efficiently grains in efficient genotypes but vegetative parts in inefficient genotypes. Hence Mn efficiency of a cultivar could be explained by longer roots, higher uptake, influx and efficiency index during vegetative phase and higher grain yield and utilization efficiency during generative phase.

Genotypic variation in partitioning of dry matter and manganese between source and sink organs of rice under manganese stress.

KEY MESSAGE: Genetic variability in dry matter and manganese partitioning between source and sink organs was the key mechanism for Mn efficient rice genotypes to cope with Mn stress. Considerable differences exist among cereal genotypes to cope manganese (Mn) deficiency, but the underlying mechanisms are poorly understood. Minimal information regarding partitioning and/or remobilization of dry matter and Mn between source and sink organs exists in rice genotypes differing in Mn efficiency. The present study was aimed to assess the growth dynamics in terms of dry matter and Mn remobilization in the whole plant (leaves and tillers as source and panicles and grains as sink) during the grain development in diverse rice genotypes. The efficient genotypes accumulated higher dry matter than inefficient genotypes under low Mn level. The translocation index i.e., uptake in grain/total uptake was 0.11 in efficient genotype (PR 116) and 0.04 in inefficient genotypes (PR 111). The efficient genotype had higher grain Mn utilization efficiency of 0.71 in comparison to 0.48 of inefficient genotype indicating that in efficient genotype, Mn in grain produces more dry matter than inefficient genotypes. The efficient genotypes also had higher flag leaf area and nitrate reductase activity. The source of efficient genotypes contributed to a greater extent to developing sink but further mobilization to grain was hindered by panicle. The panicle of inefficient genotypes had higher per cent of Mn uptake than efficient genotypes indicating that Mn was least mobilized from panicle to grain in inefficient genotypes. The lower per cent uptake of Mn in efficient genotypes indicated that Mn was mobilized from panicle to developing grain and this led to higher Mn translocation index in grain of efficient genotypes. The uptake partitioning revealed that source of all genotypes mobilized the Mn towards the sink to almost same extent but it was the panicle where highest per cent uptake per plant was in inefficient genotypes and lowest in efficient genotypes. The lowest per cent uptake in panicle of efficient genotypes revealed that it supported developing grain to have highest translocation index.

Role of nitric oxide in cadmium-induced stress on growth, photosynthetic components and yield of Brassica napus L.

Experiments were carried out to study the effect of cadmium (Cd) and exogenous nitric oxide (NO) on growth, photosynthetic attributes, yield components and structural features of Brassica napus L. (cv. GSL 1). Cadmium in the growth medium at different levels (1, 2 and 4 Mm) retarded plant growth viz. shoot (27%) and root (51%) length as compared to control. The accumulation of total dry matter and its partitioning to different plant parts was also reduced by 31% due to Cd toxicity. Photosynthetic parameters viz., leaf area plant(-1) (51%), total Chl (27%), Chl a / Chl b ratio (22%) and Hill reaction activity of chloroplasts (42%) were greatly reduced in Cd-treated plants. Cd treatments adversely affected various yield parameters viz., number of branches (23) and siliquae plant(-1) (246), seed number siliqua(-1) (10.3), 1000-seed weight (2.30g) and seed yield plant(-1) (7.09g). Different Cd treatments also suppressed the differentiation of various tissues like vessels in the root with a maximum inhibition caused by 4mM Cd. Exogenous application of nitric oxide (NO) improved the various morpho-physiological and photosynthetic parameters in control as well as Cd-treated plants.