Kazuyoshi Nada

Ameliorative effect of polyamines on the high temperature inhibition of in vitro pollen germination in tomato (Lycopersicon esculentum Mill.)

Effect of polyamines on in vitro pollen germination at high temperatures in tomato (Lycopersicon esculentum Mill.) was investigated. Pollen germination and tube growth were significantly inhibited at 33°C and 35°C compared to those at 25°C. This inhibition was reversed by the addition of spermidine or spermine in the germination medium. Spermidine at 0.5 mM was slightly more effective than spermine at 0.05 mM. Spermidine at 0.05 and 0.5 mM and spermine at 0.05 mM slightly increased pollen germination rates at 25°C. Spermidine at 5 mM and spermine at 0.5 M were inhibitory to pollen germination, regardless of incubation temperatures. Spermidine also promoted germination of pollen grains incubated at 38°C for 1 and 3 h and then at 25°C for the rest of the 20 h incubation period. The effect was higher at 0.5 mM than at 0.05 mM. Treatment of spermidine to intact flowers 1 day before anthesis was also effective in ameliorating the high temperature inhibition of in vitro pollen germination on the polyamine-free medium. Here, the optimum concentration was 5 mM. These results demonstrate that polyamines can counteract the inhibitory effects of high temperature on pollen germination. They also suggest that the endogenous level of polyamines in germinating pollen grains is an important factor for the pollen germinability at high temperature.

The relationship between canopy structure and high productivity in napier grass, Pennisetum purpureum Schumach

Abstract The vigorous growth and high production potential of napier grass ( Pennisetum purpureum Schumach) have been reported at various sites in the tropics and also in the temperature zone, including Japan. A large structural change of the canopy with growth is a prominent feature of this species. This study investigated the high productivity of napier grass in relation to structural features of the canopy. Young shoots (cv. Merkeron) were transplanted in a field and grown for 102 days. The final dry matter of aboveground parts was 2.7 kg m −2 and the maximum crop growth rate (CGR) was 58.0 g m −2 d −1 . As leaf area index (LAI) increased from 0.7 to 15.4 with growth, light extinction coefficient ( K ) of the canopy decreased gradually from 1.1 to 0.38 due to elongation and erection of stems. K varied primarily as a function of stem inclination and length. The structural change of the canopy from planophile to erectophile with growth protects lower leaves from heavy shading, allowing the canopy to approximate an optimum LAI throughout the growth period. This is regarded as the main cause of high productivity of this species.

Cloning, biochemical identification, and expression analysis of a gene encoding S-adenosylmethionine decarboxylase in navel orange (Citrus sinensis Osbeck)

Summary Polyamines have been shown to be involved in a variety of plant physiological processes such as growth, development, and stress responses. As S-adenosylmethionine decarboxylase (SAMDC) is the rate-limiting enzyme for polyamine biosynthesis, efforts were made to isolate the gene encoding SAMDC from ‘Newhall’ navel orange (Citrus sinensis Osbeck). RT-PCR, in combination with a bioinformatics approach, produced a cDNA (CsSAMDC) containing an intact open reading frame (ORF) of 1,095 bp. The deduced amino acid sequence shared high homology (>75%) with four selected plant SAMDCs and possessed several highly-conserved amino acid residues or motifs. Expression of CsSAMDC in Escherichia coli gave rise to a protein of the expected size, which was shown to have biochemical activity via a SAMDC assay. The expression pattern of the CsSAMDC gene in ‘Newhall’ callus tissue broadly matched the growth of callus cells and the fluctuation in endogenous polyamine levels. Levels of transcription of CsSAMDC in ‘Newhall’ shoots were enhanced by treatment with 10% (w/v) polyethylene glycol. In addition, the steady state level of CsSAMDC mRNA varied in genotypes with varying levels of tolerance to salinity or to low temperature stress, as revealed by the earlier induction of CsSAMDC mRNA at the initial stages of either stress treatment in tolerant genotypes. These results suggest that CsSAMDC is a stress-responsive gene and that temporal changes in its expression profile may be part of the mechanism responsible for genotypic variations in stress tolerance.

Fruit photosynthesis in Satsuma mandarin

To clarify detailed characteristics of fruit photosynthesis, possible gas exchange pathway and photosynthetic response to different environments were investigated in Satsuma mandarin (Citrus unshiu). About 300 mm(-2) stomata were present on fruit surface during young stages (∼10-30 mm diameter fruit) and each stoma increased in size until approximately 88 days after full bloom (DAFB), while the stomata collapsed steadily thereafter; more than 50% stomata deformed at 153 DAFB. The transpiration rate of the fruit appeared to match with stoma development and its intactness rather than the density. Gross photosynthetic rate of the rind increased gradually with increasing CO2 up to 500 ppm but decreased at higher concentrations, which may resemble C4 photosynthesis. In contrast, leaf photosynthesis increased constantly with CO2 increment. Although both fruit and leaf photosynthesis were accelerated by rising photosynthetic photon flux density (PPFD), fruit photosynthesis was greater under considerably lower PPFD from 13.5 to 68 μmolm(-2)s(-1). Thus, Satsuma mandarin fruit appears to incorporate CO2 through fully developed and non-collapsed stomata, and subject it to fruit photosynthesis, which may be characterized as intermediate status among C3, C4 and shade plant photosynthesis. The device of fruit photosynthesis may develop differently from its leaf to capture CO2 efficiently.