DIVISION OF VEGETATION SCIENCE | Annual Report 1996

We have been carrying out studies covering a wide range of vegetational components in agro-ecosystems in order to increase food production in a sustainable way and to maintain rich biodiversity. The division in FY 1996 had 9 researchers and some newcomers as postdoctoral fellow of the Science and Technology Agency of Japan. The activities of the Vegetation Division were as follows:

One of the impacts on biodiversity, the preservation of which is a common concern of humankind,is desertification caused by excessive grazing, especially in semi-arid regions. The research of the Vegetation Conservation Laboratory has been continued to prevent and reverse vegetation degradation by desertification in Inner Mongolia, China. In restoration experiments, some plots using sand fixation measures were constructed and restoration processes were monitored.

Other international cooperative operations had been conducted under JICA sponsorship in the Philippines on the structure and species diversity of cogon grasslands in ultisol areas, and on the assessment of allelopathic activity of cogon and other Philippine plants, and the recommendation of Mucuna pruriens and Glyricidia sepium as control strategies for cogon fields.

On the contrary,in the agro-ecosystems of our country, biodiversity has been recently threatened by the invasion of various alien weeds from foreign countries with the increasing agro-materials trade. The workshop entitled "International Workshop on Biological Invasions of Ecosystem by Pests and Beneficial Organisms," sponsored by Japan International Science and Technology Exchange Center, was held in cooperation with the Division of Entomology on February 25-27,1997. The researchers of this division presented reports especially on "Relationships between habitat conditions and life history characteristics of invasive plants" and on "Alien plant invasions into forage crop fields and pastures in Japan".

Spatial and Temporal Variation in Photon Flux Density on Rice (Oryza sativa L.) Leaves

The light environment is highly variable in plant canopies.To obtain advanced knowledge on spatial and temporal variations of photosynthetic photon flux density (PFD) on leaf surfaces in field conditions, we developed a PFD measuring system which can detect detailed PFD condition by using small photodiodes (active area:1.7mm²) attached directly on the surface of leaf blades of rice (Oryza sativa L.). Considerable variation in PFD was found among leaves with different orientation and inclination even at the same height of the canopy. Under sunny conditions, the fluctuation patterns of PFD was mainly determined by the orientation and inclination of leaf surfaces, but showed little dependency on the daily course of PFD in the open, e.g., peak PFD values were observed in the morning at east-oriented location. Moreover, drastic and irregular fluctuations in PFD were observed frequently, even within a short time. Under overcast conditions, however, PFD on the leaf surface depended highly on the PFD in the open in spite of the leaf orientation and inclination. The study suggests that the orientation and inclination should be considered as the critical factors that influence PFD regime (spatial variation and daily total PFD) within the rice canopy. And it also suggests that the transient light environment should be considered because direct light, which is the main component of solar radiation in sunny condition, is intercepted irregularly by other leaves. These variation in incident PFD on leaf surfaces may greatly affect leaf photosynthesis, or total canopy photosynthesis. (Fig.1)

The objective of this study is to clarify the sensitivity of weeds to enhanced UV-B radiation.We compared the growth and phenology of some weedy species to artificial UV-B radiation regulated so as to be 1.7 times as much as measurements at Tsukuba in winter (Fig.2). (1) The growth experiments using three populations of Plantago asiatica L. in different latitudes: the results showed that variation in weight ratio (the ratio of the plant weight at UV-B treatment to the weight at control)coincided with two types of ecotypes (summer green type and evergreen type), not but the difference in latitude. (2) The growth experiments using four annual plants (Chenopodium album L., Persicaria vulgaris Webb, et Moq., Digitaria ciliaris Koel and Commelina communis L.): visible foliar injury by UV-B irradiation occurred in all plants. The typical foliar symptoms were rolls in the edges of leaves at the upper surface of leaves. And all plants grew less with UV-B treatment than the control, showing that the effect of UV-B significantly reduced the weight of plants. C. album, P. vulgaris, D. ciliaris, short-day plants,did not exhibit delayed flowering after UV- B treatment. However, C. communis, a day-neutral plant, exhibited significantly delayed flowering after UV-B treatment.

Crop production is often suppressed by Alminum (Al) toxicity in acidic soils. Al has a high ability to bind with cellular components of roots, and usually shows little translocation to the upper parts of plants. Most plants contain not more than 0.2 mg Al g^-1 dry weight. However, some plants, known as "Al accumulators,"may contain more than 10 times this level of Al without any Al injury. Hydrangea (Hydrangea macrophylla) is a well-known Al-accumulating plant,and accumulates as much as 5mg Al g^-1 dry weight in the leaves within several months. This suggests that internal detoxification mechanisms are required for tolerance to high Al in hydrangea. To clarify the mechanisms, we investigated chemical forms of Al present in the cell of hydrangea. The leaves of hydrangea contained as much as 15.7 mmol Alkg^-1 fresh weight, and more than two-thirds of the Al was found in the cell sap. Using ²⁷Al-nuclear magnetic resonance (NMR), the dominant peak of Al was observed at a chemical shift of 11 to 12 ppm in both intact leaves and the extracted cell sap, which is in good accordance with the chemical shift for the 1:1 Al-citrate complex. Purification of cell sap by molecular sieve chromatography (Sephadex G-10)combined with ion-exclusion chromatography indicated that Al in fractions with the same retention time as citric acid contributed to the observed ²⁷Al peak in the intact leaves. The molar ratio of Al to citric acid in the crude and purified cell sap approximated 1. The structure of the ligand chelated with Al was identified to be citric acid. Bioassay experiments showed that the purified Al complex from the cell sap did not inhibit root elongation of corn (Zea mays L.) and the viability of cells on the root tip surface was also not affected. These results indicate that the internal detoxification of Al is achieved by the formation of a strong Al-citrate complex, a nontoxic form of Al, in hydrangea leaves. (Fig.3)

Fig.1 Micro -photodiode attached to the surface of leaf blade

Fig.2 Photograph of UV-B irradiation system using fluorescent sunlamps wrapped in cellulose diacetate film for UV-B treatment and Lumirror film for control.

Fig.3 Hydrangea ( Hydrangea macrophylla )