Simplified cryopreservation of sweet potato [ Ipomoea batatas (L.) Lam.] by optimizing conditions for osmoprotection

Shoot tips of sweet potato were successfully cryopreserved using an encapsulation vitrification method. Encapsulated shoot tips were pre-incubated in liquid Murashige-Skoog medium containing 30 g/l sucrose for 24 h, then precultured in sucrose-enriched medium (0.3 M sucrose) for 16 h. Shoot tips were osmoprotected with a mixture of 2 M glycerol and 1.6 M sucrose for 3 h before being dehydrated with a highly concentrated vitrification solution (PVS2) for 1 h at 25 degrees C. The encapsulated and dehydrated shoot tips were transferred to a 2 ml cryotube, suspended in 0.5 ml PVS2, and plunged directly into liquid nitrogen. Rapidly warmed shoot tips developed normal shoots and roots in 21 days without any morphological abnormalities after plating on a recovery medium. High levels (average of about 80%) of shoot formation were obtained for three cultivars of sweet potato. This encapsulation vitrification method appears promising for cryopreservation of sweet potato germplasm.

Effects of air current speed on gas exchange in plant leaves and plant canopies

To obtain basic data on adequate air circulation to enhance plant growth in a closed plant culture system in a controlled ecological life support system (CELSS), an investigation was made of the effects of the air current speed ranging from 0.01 to 1.0 m s-1 on photosynthesis and transpiration in sweetpotato leaves and photosynthesis in tomato seedlings canopies. The gas exchange rates in leaves and canopies were determined by using a chamber method with an infrared gas analyzer. The net photosynthetic rate and the transpiration rate increased significantly as the air current speeds increased from 0.01 to 0.2 m s-1. The transpiration rate increased gradually at air current speeds ranging from 0.2 to 1.0 m s-1 while the net photosynthetic rate was almost constant at air current speeds ranging from 0.5 to 1.0 m s-1. The increase in the net photosynthetic and transpiration rates were strongly dependent on decreased boundary-layer resistances against gas diffusion. The net photosynthetic rate of the plant canopy was doubled by an increased air current speed from 0.1 to 1.0 m s-1 above the plant canopy. The results demonstrate the importance of air movement around plants for enhancing the gas exchange in the leaf, especially in plant canopies in the CELSS.

[Effects of NaCl stress on seedling growth and IAA metabolism of sweet potato and its relation to salt-tolerance]

Seedlings of three sweet-potato (Ipomoea batatas) cultivars with different salt-tolerance were stressed with different concentrations of NaCl during the seedling stage. Plant growth, leaf area growth, and IAA content decreased with increasing NaCl concentration, while the activities of POD and IAA oxidase increased. The extent of plant growth, leaf area growth and IAA content decrease and the extent of POD and IAA oxidase activity increase were larger in sensitive cultivar than those in moderately tolerant and tolerant cultivars. In addition, the content of chlorogenic acid increased under NaCl stress. Chlorogenic acid content reached its peak value at 170 mmol.L-1 of NaCl in sensitive and moderately tolerant cultivars and 225 mmol.L-1 of NaCl in tolerant cultivar. Chlorogenic acid content decreased with increasing NaCl concentration, but it was higher than that of the control.

Molecular characterization of a senescence-associated gene encoding cysteine proteinase and its gene expression during leaf senescence in sweet potato

The structure and expression of a senescence-associated gene (SPG31) encoding a cysteine proteinase precursor of sweet potato have been characterized. The coding region of the gene consists of two exons encoding an enzyme precursor of 341 amino acids with conserved catalytic amino acids of papain. Examination of the expression patterns of the SPG31 gene in sweet potato by Northern blot analyses reveals that the transcripts of SPG31 are specifically induced in the senescing leaves but not in other organs. The differential accumulation of the mature SPG31 protein in the senescing leaves was further identified by two-dimensional electrophoresis of leaf proteins and N-terminal sequencing. This result suggests the important role played by SPG31 in proteolysis and nitrogen remobilization during the leaf senescence process. Furthermore, treatment of mature green leaves with ethylene for 3 d resulted in a high-level induction of SPG31 transcripts. Ethylene-regulated expression of SPG31 is consistent with the presence of a number of putative ethylene-responsive elements in the 899-bp SPG31 promoter region.

The effect of gravity on surface temperature and net photosynthetic rate of plant leaves

To clarify the effects of gravity on heat/gas exchange between plant leaves and the ambient air, the leaf temperatures and net photosynthetic rates of plant leaves were evaluated at 0.01, 1.0, 1.5 and 2.0 G of 20 seconds each during a parabolic airplane flight. Thermal images of leaves were captured using infrared thermography at an air temperature of 26 degrees C, a relative humidity of 15% and an irradiance of 260 W m-2. The net photosynthetic rates were determined by using a chamber method with an infrared gas analyzer at an air temperature of 20 degrees C, a relative humidity of 50% and a photosynthetic photon flux of 0.5 mmol m-2 s-1. The mean leaf temperature increased by 1 degree C and the net photosynthetic rate decreased by 13% with decreasing gravity levels from 1.0 to 0.01 G. The leaf temperature decreased by 0.5 degree C and the net photosynthetic rate increased by 7% with increasing gravity levels from 1.0 to 2.0 G. Heat/gas exchanges between leaves and the ambient air were more retarded at lower gravity levels. A restricted free air convection under microgravity conditions in space would limit plant growth by retarding heat and gas exchanges between leaves and the ambient air.