Genotypic Variation for Tolerance to Transient Drought During the Reproductive Phase of Brassica rapa

Brassica rapa L. is a genetically diverse parent species of the allotetraploid species, oilseed rape (B. napus) and a potential source of drought tolerance for B. napus. We examined the effect of a 13‐day drought stress period during the early reproductive phase, relative to a well‐watered (WW) control, on subsequent growth and development in nine accessions of B. rapa and one accession of Brassica juncea selected for their wide morphological and genetic diversity. We measured leaf water potential, stomatal conductance, water use, and leaf and bud temperatures during the stress period and aboveground dry weight of total biomass at maturity. Dry weight of seeds and reproductive tissue were not useful measures of drought tolerance due to self‐incompatibility in B. rapa. The relative total biomass (used as the measure of drought tolerance in this study) of the 10 accessions exposed to drought stress ranged from 47 % to 117 % of the WW treatment and was negatively correlated with leaf‐to‐air and bud‐to‐air temperature difference when averaged across the 13‐day stress period. Two wild‐type (B. rapa ssp. sylvestris) accessions had higher relative total and non‐reproductive biomass at maturity and cooler leaves and buds than other types. We conclude that considerable genotypic variation for drought tolerance exists in B. rapa and cooler leaves and buds during a transient drought stress in the early reproductive phase may be a useful screening tool for drought tolerance.     

Changes in soil chemistry associated with the establishment of forest gardens on eroded,acidified grassland soils in Sri Lanka

Topsoil properties were determined in forest gardens established about 20 years ago on eroded grassland soils (abandoned tea lands) in the wet zone of the Sri Lankan highlands. They were compared with adjacent, eroded grasslands (abandoned tea lands) on strongly weathered soils vs soils at earlier stages of pedogenic development in a two-way analysis of variance. Soil pH in forest gardens was, on average, 6.1, nearly one unit higher than in the adjacent grasslands. In the garden soils, the cation exchange capacity (CEC measured at pH 4.8) was nearly double, exchangeable calcium concentrations five times and exchangeable magnesium three times as high as in the grasslands soils. Total soil N content was found to be nearly 40% higher in the gardens. Topsoil gravel contents in the gardens were less than half as high as in the grasslands. The increases in exchangeable bases and N in gardens, relative to grasslands, were attributed to increased nutrient retention and acquisition. Higher retention was partly due to the higher CEC_pH4.8, and probably to reduced erosion and increased, continuous fine root density in the garden topsoils. Higher field CEC in gardens was likely to result from generally higher C contents and from the reversal of acidification, presumably caused by base accumulation and decomposition processes. Our results suggest that forest garden establishment on degraded grasslands can lead to accumulation of mobile nutrients in the topsoil, probably due to increased nutrient retention, subsoil uptake and litter input exceeding nutrient uptake by the standing biomass.     

Drought Stress in Grain Legumes during Reproduction and Grain Filling

Water scarcity is a major constraint limiting grain legume production particularly in the arid and semi‐arid tropics. Different climate models have predicted changes in rainfall distribution and frequent drought spells for the future. Although drought impedes the productivity of grain legumes at all growth stages, its occurrence during reproductive and grain development stages (terminal drought) is more critical and usually results in significant loss in grain yield. However, the extent of yield loss depends on the duration and intensity of the stress. A reduction in the rate of net photosynthesis, and poor grain set and grain development are the principal reasons for terminal drought‐induced loss in grain yield. Insight into the impact and resistance mechanism of terminal drought is required for effective crop improvement programmes aiming to improve resistance to terminal drought in grain legumes. In this article, the impact of terminal drought on leaf development and senescence, light harvesting and carbon fixation, and grain development and grain composition is discussed. The mechanisms of resistance, management options, and innovative breeding and functional genomics strategies to improve resistance to terminal drought in grain legumes are also discussed.     

Synthetic seed production by encapsulating nodal segment of <Emphasis Type="Italic">Capparis decidua</Emphasis> (Forsk.), in vitro regrowth of plantlets and their physio biochemical studies

Synthetic seed technology is an emerging and broadly used technique in the field of plant biotechnology to conserve economically important plants. In the present study, nodal segments of Capparis decidua were entrapped in calcium alginate gel matrix to produce firm and uniform synthetic seeds. 3% sodium alginate and 75 mM calcium chloride were found best for encapsulation. Among all the concentrations and combinations of thidiazuron (TDZ) either singly or with indole -3- acetic acid (IAA) augmented in Murashige and Skoog medium used, TDZ (5.0 µM) + IAA (0.5 µM) was found most effective in conversion of synthetic seeds into plantlets as 79% plantlets were developed on this combination with 13.2 ± 0.87 shoots and 5.5 ± 0.40 cm shoot length after 8 weeks of culture. Further, synthetic seeds stored at low temperature (4 °C) can retain their viability up to 4 weeks and showed maximum conversion rate (93%) into plantlets, when placed back to regeneration medium. Root formation was also occurred in the same regeneration medium and roots were healthy. Plantlets were successfully hardened in culture room in plastic cups filled with sterile vermiculite and after 4 weeks, they were transferred to greenhouse where they exhibited normal growth with 80% survival. Growth parameters were evaluated in micropropagated plants and compared with the seedlings of same age. Effect of different days of acclimatization were also recorded on various physio-biochemical activities and showed a positive response that can be interpreted as better protection mechanism of micropropagated plants against the stress possibly generated due to reactive oxygen species when transferred to ex vitro environment.     

Estimation of genetic components of variation for salt tolerance in chickpea using the generation mean analysis

Chickpea (Cicer arietinum L.) is known to be salt-sensitive and in many regions of the world its yields are restricted by salinity. Recent identification of large variation in chickpea yield under salinity, if genetically controlled, offers an opportunity to develop cultivars with improved salt tolerance. Two chickpea land races, ICC 6263 (salt sensitive) and ICC 1431 (salt tolerant), were inter-crossed to study gene action involved in different agronomic traits under saline and control conditions. The generation mean analysis in six populations, viz. P₁, P₂, F₁, F₂, BC₁P₁ and BC₁P₂, revealed significant gene interactions for days to flowering, days to maturity, and stem Na and K concentrations in control and saline treatments, as well as for 100-seed weight under salinity. Seed yield, pods per plant, seeds per plant, and stem Cl concentration were controlled by additive effects under saline conditions. Broad-sense heritability values (>0.5) for most traits were generally higher in saline than in control conditions, whereas the narrow-sense heritability values for yield traits, and stem Na and K concentrations, were lower in saline than control conditions. The influence of the sensitive parent was higher on the expression of different traits; the additive and dominant genes acted in opposite directions which led to lower heritability estimates in early generations. These results indicate that selection for yield under salinity would be more effective in later filial generations after gene fixation.     

Direct plant regeneration from nodal explants of <Emphasis Type="Italic">Balanites aegyptiaca</Emphasis> L. (Del.): a valuable medicinal tree

This report describes in vitro shoot induction and plant regeneration from nodal segments of Balanites aegyptiaca on Murashige and Skoog (MS) medium fortified with 6-benzyladenine (BA), thidiazuron (TDZ) and kinetin (Kin) (0.5–20.0 μM). MS medium supplemented with BA (12.5 μM) was the most effective in inducing bud break and growth and also in initiating multiple shoot proliferation. However, the optimal level of TDZ supplementation to the culture medium was 5.0 μM. Shoot cultures were established by repeatedly subculturing the original nodal explants on the same medium. Highest number of shoots (11.5 ± 0.7) and shoot length (5.0 ± 0.2 cm) were achieved when cultures were subcultured on MS medium supplemented with 12.5 μM BA and 1.0 μM α-naphthalene acetic acid (NAA). The shoots regenerated from TDZ supplemented medium when subcultured to hormone free MS basal medium considerably increased the rate of shoot multiplication and shoot length by the end of fifth subculture. Rooting of the shoots was achieved on MS medium augmented with 1.0 μM indole-3-butyric acid (IBA) plus 0.5% activated charcoal followed by their transfer to half strength MS basal medium. The in vitro raised plantlets with well developed shoots and roots were successfully established in earthen pots containing garden soil and were grown in greenhouse with 70% survival rate. The results of this study provide the first successful report on in vitro direct plant regeneration of B. aegyptiaca.     

Productivity and water use of alfalfa and subsequent crops in the semiarid Loess Plateau with different stand ages of alfalfa and crop sequences

Alfalfa (Medicago sativa L.) plays an important role in crop–livestock mixed farming on marginal land in the semiarid Loess Plateau. However, the duration, yield performance and water use of long-term alfalfa stands and choice of appropriate subsequent crops are not clear. A 5-year field experiment was conducted at Zhonglianchuan, Gansu Province, China from 2001 to 2005. Productivity and water use were determined and compared between (1) three alfalfa stands that were 1–5 (A1–5), 6–10 (A6–10) and 11–15 (A11–15) years old during the trial; (2) alfalfa using conventional cultivation and a water-harvesting technique (RA1–5); and (3) conventional crop rotation (CK) and four 5-year crop sequence rotations sown after 10-year-old alfalfa had been ploughed, being millet–wheat–potato–pea–potato (MWLPL); millet–corn–corn–wheat–wheat (MCCWW); millet–potato–wheat–corn–corn (MLWCC) and millet–fallow–pea–potato–pea (MFPLP). Forage yield peaked in 7-year-old alfalfa (5740 kg ha⁻¹), but 9-year-old alfalfa had the maximum forage yield profit (4477 kg ha⁻¹ y⁻¹) in terms of whole growing years. Soil water use efficiency (WUE_S in terms of forage yield and soil water use) of alfalfa increased dramatically up to the 11th year, and then leveled off from year 12 to 15. Forage yield and WUE_B/ET (WUE in terms of aboveground biomass and evapotranspiration) of alfalfa were significantly higher using water harvesting compared with conventional cultivation, but were significantly lower than CK. Soil water content did not change in CK as stand age increased, but it decreased in conventional alfalfa stands. After 10 years of alfalfa, a fallow year was not necessary before planting annual crops as soil water was greatly restored after sowing subsequent annual crops. Yield of some crops in the four crop sequence rotations did not differ significantly from CK. MWLPL and MLWCC had more aboveground biomass than MCCWW and MFPLP but the choice of crop sequence needs to be further considered.     

Comparative effect of drought duration on growth,photosynthesis, water relations,and solute accumulation in wild and cultivated barley species

Drought is a major factor limiting crop production worldwide. Barley is a well‐adapted cereal that is largely grown on dry marginal land where water and salinity are the most prevalent environmental stresses. This study was carried out to investigate the effects of drought stress and subsequent recovery on growth, photosynthetic activity, water relations, osmotic adjustment (OA), and solute accumulation of wild (Hordeum maritimum) and cultivated barley (H. vulgare L.). In a pot experiment, 60 d old seedlings were subjected to drought stress for 0, 7, 14, 21, or 28 d, and then re‐watered to recover for up to 21 d. Plants were harvested at the end of each of these drought/recovery treatments. Drought significantly reduced fresh and dry weights at the whole‐plant level, photosynthetic activities, and solute and water potentials, while increasing leaf Na⁺ and K⁺ concentrations. The adverse effects of drought on growth were more marked in cultivated barley than in wild barley and the reverse was true for photosynthetic activities. During recovery, all wild barley seedlings completely recovered. For cultivated barley seedlings, rehydration had a beneficial effect on growth and photosynthesis, independent of treatment duration, but complete recovery did not occur. The reduction in leaf solute potential at full turgor in drought‐stressed barley, relative to the control, suggests active OA which was more significant in wild barley than in cultivated barley. OA was mainly due to the accumulation of inorganic (K⁺ in cultivated barley and Na⁺ in wild barley) and organic (soluble sugars and proline) solutes. The results suggest that OA is an important component of the drought‐stress adaptation mechanism in wild barley, but is not sufficient to contribute to drought tolerance in cultivated barley. In the latter species, the results show that even short periods (as little as 7 d) of water deficit stress had considerable long‐term effects on plant growth.     

Control of chlorophyll formation and sprouting of tubers by oil dipping in five potato cultivars of Pakistan

Five potato cultivars of autumn crop, dipped in corn oil, were observed for 90 days at room temperature under natural light. Chlorophyll formation took place in minute quantities in treated tubers of ‘Désirée’ and ‘Prima’, and a little more in ‘Primura’, ‘Montana’ and ‘Multa’, whereas all control tubers became completely green after 90 days. Sugars were significantly lower and starch higher in treated tubers than in controls. No sprout-development was observed in treated tubers of all cultivars, but 50–70% control tubers had sprouted during this period. Peeling-losses were minimum in treated tubers and were 67.5–85.4% in controls. The oil-dipped potato tubers of all cultivars could safely be stored for 60 days without quality deterioration under ordinary conditions of temperature and light.