Pot cultured wheat plants, cv. Asakaze-komugi, were grown under sufficient soil water till ripening period. They were then grouped into two soil moisture treatments, 75% (control) and 40% (water stressed) of field capacity, respectively. Photosynthesis and transpiration of various plant parts i.e. ear, top internode, leaf sheath of flag leaf, leaf blades of two or three uppermost leaves were measured using a small chamber and the effects of water stress on these physiological processes were examined. The results obtained are summarized as follows : 1. Photosynthesis and transpiration of each plant part were strongly depressed by the water deficit treatment. The extent of depression in photosynthesis and transpiration was largest immediately after the start of the water deflcit treatment. However, as the water deficit condition was prolonged, the effect diminished (Figs. 1-3). 2. The extent of depression in photosynthesis by water deficit was larger in the leaf blades than in the ear, leaf sheath and the stem. Comparing leaf blades at different positions, depression was larger in the lower leaf blades than in the upper ones. Thus, it was concluded that photosynthetic sensitivity to water deficit varied among plant parts (Figs. 4-7). 3. The sensitivity of plant parts to water deficit in photosynthesis was closely correlated with that of transpiration. This suggested that the photosynthesis sensitivity of a certain plant part to water deflcit is related to the sensitivity of the stomatal behavior of that part.

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In the previous paper, it was pointed out that the extent of depression in photosynthesis by soil water deficit treatment was different from part to part of the wheat plant. The objective of the present study was to find out the physiological basis for this phenomenon from the viewpoint of water potential and CO₂ diffusion resistances, i.e. leaf resistance (r₁) and mesophyll resistance (r_m) using the same materials as those for the previous paper. The results are summarized as follows : 1. Water potential of each plant part was decreased by water deficit treatment. The extent of decrease in water potential was the same in every plant part. It was concluded form this finding that the difference in photosynthetic sensitivity among plant parts is not be attributable to the difference in the extent of decrease in water potential of plant parts (Figs. 1, 2). 2. Both r₁ and r_m were increased by the water deficit treatment. The extent of increase in r₁ and r_m was largest just after the start of the water deficit treatment. However, when the water deficit condition was prolonged, r₁ and r_m tended to decrease and come close to those of unstressed plants (Figs. 6-9). 3. Both r₁ and r_m increased to a larger extent in lower leaves than in upper ones by the water deficit treatment (Figs. 3-4). The extent of increase in r₁ and r_m resulted from water deficit was always larger in lower leaves than in upper ones (Figs. 5-9). Since there was no difference in the extent of decreasse in water potential by the water deficit treatment between lower and upper leaves, both stomata and the photosynthetic mechanism of lower leaves seemed to have greater sensitivity to decrease of water potential than those of upper leaves.

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The rice plants, cultivated in paddy field, were cut in 6 periods (I-VI), on every 7 days during the maximum tiller number stage (July 17, 1981 : I) to the first heading stage (Aug. 21 : VI). The cutting was conducted at 15cm (H) or 7.5cm (L) in height above the ground. Regrowth plants from the stubbles were investigated on 2 and 5 weeks after cutting, and their yield components were examined after harvest (Oct. 23). 1. The stems which had been grown after cutting and attained to heading were classified into two types as follows : (1) SS (survived stem) : stems which survived in spite of the cutting and attained to heading, and (2) NS (new stem) : tillers newly developed from axillary buds of the cut stems. 2. Regrowth in I-III plots accomplished by development of SS, and V and VI plots are by NS (Table 2). In IV-H plot, shoot apices (young panicles) were cut off in only 5.5% of the whole stems (Table 1), and the regrowth was carried out by both of NS and SS. On the other hand, shoot apices of 65.4% stems were not cut off in IV-L plot, but they did not grow up after cutting, consequently their regrowth were consisted of only NS. 3. After cutting, number of stems per stubble reduced in I-III and IV-L plots, because of death of non-productive tillers (Fig. 3). In V-H and VI-H plots, number of stems increased after cutting because some of the cut stems bore two or three NS. 4. Number of fully ripened grains per panicle was fewer according to delaying the cutting time, and it was lesser in L than H plot (Table 3). And no grain yield was obtained from the panicles of NS in V-L and VI plots. The yield of brown rice became inferior through delaying the cutting time and lower cuttings (Table 4).

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The objective of the present study was to obtain fundamental knowledge to monitor remotely the physiological and ecological status of crops in fields. In this paper the relationships between photosynthetic rate (Pn), transpiration rate (Tr), vapor pressure deficit (VPD), and other meteorological data such as photosynthetically active radiation (PAR) were investigated under field conditions. A model relating Pn and Tr was presented. The influence of chlorophyll concentration (Chl) of leaves on Pn was also examined. 1. A theoretical model to interrelate Pn and Tr was presented, which was based on the biophysical processes for gas and vapor transfer via stomata and boundary layer (Eq.1-6). P_n = a[C_a-C_i][Tr/VPD₁] (6) where, a is a physical constant, C_a, C_i are ambient and substomatal CO₂ concentration, respectively. The Pn is a product of the difference of those two CO₂ concentrations and Tr/VPD as indicated in the eq. (6). 2. Close positive correlations between Pn and Tr/VPD were obtained for both corn and soybean as the results of experiments under field conditions. The correlation coefficients were around 0.9 and consistently higher than those coefficients between Pn and Tr in all cases. This relationship held under a wide range of environmental and crop conditions such as air temperature, PAR, VPD, soil water content, crop variety, chloropyll concentration of leaves, developmental stage, leaf position, rolling or wilting of leaves and time of measurements (Tables 1, 2). 3. Linear regression equations were obtained on the above relationships for both corn and soybean. According to those equations the difference of ambient and substomatal CO₂ concentrations remains constant, because Pn is proportional to Tr/VPD. The difference of external and internal CO₂ concentration was estimated as 159ppm for corn and 51ppm for soybean from the regression coefficients of those equations (Figs. 1∼2). Since the possibility of remote estimation of transpiration rate Tr had been already shown by INOUE¹⁰⁾, the photosynthetic activity could also be estimated remotely by means of combination of remotely sensed data and meteorological data connecting above relationships. 4. As a result of the regression analysis the chlorophyll concentration (Chl) of a leaf had as large positive influence as PAR on Pn under water stress free conditions. The partial regression coefficients for Chl were around 0.7. On the other hand the effect of VPD on Pn increased negatively under water stress conditions. The partial regression coefficients were around -0.5 (Table 3).

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The varietal difference of stomatal aperture in rice seedlings was studied in the phytotron, in relation to the cool temperature susceptibility of Tongil group (an indica-japonica hybrid group) varieties. Eight varieties including 4 japonica and 4 Tongil group were used. They were Sangpungbyeo (cool temperature tolerant), Daecheongbyeo (medium), Odaebyeo (medium) and Hwasongbyeo (medium) as japonica type, and Suwon 339 (moderately susceptible), Iri 377 (moderately susceptible), Samgangbyeo (susceptible) and Yongmunbyeo (susceptible) as Tongil group. The seedlings were grown in a natural light room of the phytotron upto the early hardening stage. The temperature treatments were started at the middle hardening stage (approximately 2-leaf stage) in the artificially lit room, under about 30, 000 lux illumination (0700-1900 hours) and at about 60 percent relative humidity. The set temperatures were (1) 20°C, (2) 16°C, (3)20°C → 16°C from the 6th day, (4) 20°C → l2°C from the 8th day and (5) 16°C → 12°C from the 8th day. Several fAactors affecting the estimation of stomatal aperture of rice seedlings in the artificially lit room were examined (Tables 1-4, Figs. 1 and 2). The following results were obtained in relation to the varietal difference of stomatal aperture in rice seedlings : (1) Stomatal aperture was larger in the Tongil group varieties than in the japonica type, and varietal difference was observed also within the japonica type or the Tongil group varieties (Tables 1-6 and Figs. 3-6). (2) More tolerant varieties of cool temperature generally showed smaller stomatal aperture, except Daecheongbyeo which showed particularly small aperture (Figs. 3-6). (3) Stomatal aperture decreased at lower temperature, and the degree of decrease was smaller in the Tongil group varieties (Figs. 3 and 4). Accordingly, the varietal diffecrence was larger at lower temperature. (4) Preliminary experiments showed that reactions in the stomatal aperture to the temperature shift from 20°C to 16°C, 20°C to 12°C and 16°C to 12°C were all different (Figs. 5 and 6). (5) Stomatal aperture showed a diurnal change, higher in the afternoon, in the experimental condition in the artificially lit room (Figs. 3 and 4). These results implies that stomatal aperture has a high reciprocal correlation with the cool temperature tolerance of rice seedlings, and that thus the estimation of stomatal aperture would contribute to the elucidation of physiological mechanism of cool temperature tolerance in rice seedlings.

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The objective of this study was to investigate the effects of planting density on the dry-matter (DM) production and yield of maize for whole-plant silage in the north-marginal area in Japan (where accumulated temperature from June to September is 1946°C). Experiments were conducted for 5 years from 1978 to 1982, based on the same design. Wase-homare (early hybrid) was grown at four planting densities from 40, 000 to 100, 000 plants/ha in 1978∼1980 and from 58, 000 to 103, 000 plants/ha in 1981 and 1982 (Table 1). DM weights in each organ and leaf area were measured at the 4th-, 7th-, 11th-leaf fully developed stages, silking stage, and, 3 and 6 weeks after silking. DM yields and percentage of barren plants were measured at harvesting date. The results obtained were as follows : I. As planting desity increased, top growth rates (TGR) during the vegetative growth period were increased. Contrarily, TGR during the ear-filling period were decreased rapidly, resulted in the minimum at the highest planting density of about 100, 000 plants/ha during the latter half of the ear-filling period (Fig. 1). It was due to rapid decrease of net assimilation rates (NAR) during the ear-filling period in high planting density causing increase of mutual shading and specific leaf area (SLA) (Table 2). 2. The optimum leaf area index (LAI), under which maximum TGR obtained, during the ear-filling period, was about 3.0 in all the years (Fig. 5). The planting density which gave the optimum LAI of about 3.0 during the ear-filling period was obtained at the planting density of 73, 000 plants/ha (Fig. 1). 3. The maximum ear growth rates (EGR) during the latter half of the ear-filling period were obtained at 60, 000∼80, 000 plants/ha, and EGR decreased at the higher planting density than the above. Furthermore, the declines of EGR at the higher planting density were promoted in the cold years of 1980 and 1981 (Fig.2). 4. The incidence of barren plant was obviously increased with increasing planting density (Table 2). It reached above 20% as NAR during the latter half of the ear-filling period decreased below 2.0g/m²/day (Fig. 4). 5. Although the highest ear DM yields were obtained at the medium planting density as 60, 000∼80, 000 plants/ha, stover DM yields and total DM yields increased with increasing planting density. Percentage of dry-matter in whole-plant did not differ significantly among planting densities in most years. Ear/Total ratio was decreased and maturity was delayed with increasing planting density (Table 3). 6. It was concluded that the optimum planting desity was about 70, 000∼75, 000 plants/ha for high yield and high quality of maize for whole-plant silage in the north-marginal area.

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Using seven parent varieties of Japonica-Indica hybrid bred in Korea, 16 aman, 17 aus, 10 boro, 15 bulu and 15 tjereh rice varieties, excised seminal roots (root-tips) were cultured in modified White's medium containing 0% or 0.2% casamino acids. The excised roots were cultured at 28°C in the dark for two weeks. The results obtained were as follows. 1. In Japonica type parents, the excised seminal root growth was better in the 0.2% casamino acids lot than in the 0% lot. In Indica type parents, on the other hand, main root length was almost same between the two lots of casamino acids concentration in each of three varieties, while the length was shorter in the 0.2% casamino acids lot than in the 0% lot in two varieties. But, dry root weight was larger in the former than in the latter in every variety (Table 1). 2. In all the varieties belonging to aman and tjcreh ecotypes, main root length was shorter in the 0.2% casamino acids lot than in the 0% lot. In each ot aus, boro and bulu ecotypes, the length in the above half of varieties was shorter in the 0.2% casamino acids lot than in the 0% lot and it was opposite in the rest. Dry root weight, on the other hand, was larger in the 0.2% casamino acids lot than in the 0% lot in all the varieties in every ecotype (Table 2).

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The changes in the distribution of phytin-containing particles (phytin particles) were microscopically studied in rice ovary tissues during the development of embryo and endosperm. Similarity in the phytate nature of these particles was confirmed by histochemical and ultrastructural procedures^{7, 25)}. Phytin particles appeared within the aleurone layer and scutellar parenchyma 5 or 6 days after anthesis. This is followed by an increase in number and also by the formation of relatively large particles (3 to 7 μm in diameter). In addition to their presence in these storage tissues, phytin particles were also temporarily found in different parts of ovary tissues after anthesis, as follows : (1) the peripheral regions of pericarp, for a period of 10 days, (2) parenchymatous cells in dorsal vascular bundles for about 20 days, (3) nucellar epidermis and nucellar projection and the modified aleurone cells abutting the embryo or the suspensor cells for about 10 days, (4) the outer layer of starchy endosperm from day 8 to day 20, and (5) scutellar epithelium until about 25 days. With regards to the route of flowing materials into the developing endosperm, the presence of two suggested transport pathways from dorsal vascular bundles into endosperm, i. e. 'the dorsal pathway' and the nucellar epidermal pathway'^{6, 8, 9, 23)}, was confirmed by our observations with the distribution changes of phytin particles in close association with the ovary histogenesis. The observations also suggests the routes into the developing embryo, namely, (1) from nucellar tissue and modified aleurone layer to suspensor, (2) from the modified aleurone layer and starchy endosperm to the embryo surface, and (3) from endosperm to scutellar epithelium. These processes seem to operate within 10 days after anthesis, while the scutellar epithelial process mainly play a role later. Phytin particles frequently occurred in the epidermal or peripheral regions of tissues, such as pericarp and endosperm etc., in contrast to starch grains, which appeared rather in the inner parts of tissues. Accumulation of mineral elements in aleurone cells being a peripheral zone, could not only be a result of deposition of large amounts of mirlerals, supplied from source organs through the supposed transport pathways via the nucellar projection and nucellar epidermis extending around the aleurone layer, but could also be influenced by probable interactions in deposition processes of minerals and carbohydrates. Furthermore, the sink activity of young embryos appeared superior in terms of tissue development based on higher accumulation of mineral elements when compared to the modified aleurone layer which was adjacent to the embryo. The depleted layer in endosperm near the epithelial surface of the scutellum was formed due to the disappearence of starch grains and degradation of endosperm cells during embryo development. These phenomena could be explained in terms of the 'pulling' function of embryo, based on its hypothetically young developmental age, high metabolic activities in protein synthesis, ion absorption and accumulation.

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Japonica typc cv. Kinmaze and Indica type cv. IR8 were grown on water culture. Seasonal changes in photosynthesis and root respiration of the both cultivars were measured by assimilation chamber method and Warburg manometric method, respectively. At the time of maximum tillering, the roots were submerged in a solution on nine different of enzyme inhibitors at three concentrations (10^-2, 10^-3, 10^-4M) and in a reductive solution of -300mV, prepared by addition of soluble starch in water culture solution in order to inhibit root respiration. Photosynthetic rate was measured prior to treatment as well as for a period during treatment. Cultivarietal differences grown under the same nutrient condition revealed that IR8 had high quantity of roots, much more leaf area per plant and lower activity in photosynthetic rate per leaf area throughout its whole growing stages (Figs. 1, 2). The roots of cv. Kinmaze showed higher per-cent in nitrogen and carbohydrate (starch + total sugar) compared with cv. IR8. Further it also showed higher rate of root respiration than that that of IR8 at ripenning period (Tables 1, 2). The degree of inhibition of root respiration by treatments of enzyme inhibitors, reductive solution and 70°C hot water almost coincided with the degree of decline in photosynthesis of the treated plant (Fig. 5). The influence of As₂O₃, NaN₃ and DNP in 10^-2M on the drop in root respiration was remarkable (Table 5). Submerging of root in the reductive solution resulted in a sharp decline in photosynthetic rate at the time of maximum tillers, however significant influence at mid-period of ripenning was not recognized in the same treatment. Degree of inhibition of root respiration by the reductive solution was different according to the stage of growth and root age, i.e., aged roots showed lower rate in respiration and a slight drop compared with vigorous roots at ripenning period (Fig. 6). Transmittance of β-ray through the leaf blade suggested that rapid decline of the photosynthesis by the root submerged in the inhibitor solution was caused by leaf water deficit due to decrease of root activity for water absorption (Fig. 7).

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The objectives of this series of papers are to elucidate the differences in the root morphology among rice varieties which belong to different ecotypes. and thus to contribute to analyse the morphological and functional relations between roots and aerial parts of rice plant. This paper reports the results of estimations on several morphological characters of crown roots for 53 rice varieties belonging to different ecotypes (Table 2). Examinations were carried in the experimental field (clay loam) located in Konosu city, Saitama prefecture. 1. The morphological characters were estimated in portion of 1∼5cm from the base of crown root. The third node counted from the highest rooting node was selected for the estimation, according to results of preliminary experiments. 2. The depth of tillage, the amount of applied fertilyzers or the climate of cultivation year did not affect greatly the morphological characters of crown roots (Tables 1 and 2), and accordingly the varietal orders in the characters were very close among the data obtained from experiments of different cultivation conditions or under different climatic conditions (Fig. 2). 3. The varieties used were divided into two groups, according to the thickness of crown root stele : the one group had a smaller transectional area of crown root steles (Group A in Fig. 3), and the other a larger one (Group B in Fig. 3). The former group included japonica, indica, Chincse indica, and some javanica lowland varieties. The latter included some javanica, American long grain lowland varieties and upland varieties (Fig. 3). 4. Japanese lowland varieties had thinner crown roots and thinner crown root steles, except that Hokkaido varieties and Tanginbozu had comparatively thicker ones in Group A (Fig. 3). 5. The transectional area of stele was highly correlated to the total transectional area of metaxylem II in the stele of a crown root among different varieties (Fig. 4). They were highly correlated also to number of metaxylem I (Fig. 5) and metaxylem II (Fig. 6) in the stele of a crown root. Varieties which had a comparatively small transectional area of crown root stele such as japonica or indica lowland generally had smaller and fewer vascular bundles in the crown root (Figs. 4 to 6). Some javanica varieties, however, had a thinner stele, but a larger number of vascular bundles (Figs. 5 and 6). 6. Varieties with thicker stele (Group B in Fig. 3) showed larger transectional area of metaxylem II per hill (that is, the product of the transectiolal area of metaxylem II per crown root and the number of crown root per hill). In varieties with thinn steles (Group B in Fig. 3), japonica lowland varieties showed smaller transectional area of metaxylem II per hill than indica lowland (Table 3). The ratio of the transectional area of metaxylem II to leaf area per hill, which is an index of the balance of water conductibility and transpiration, showed a similar order among varieties to total transectional area of metaxylem II per hill (Table 3). This order coincides with the order in transpiration ability reported by previous literature. These results stated above implies that the morphological analyses of rice roots in relation to the varieties will contribute to understand not only morphological characters of roots but also physiological or functional relations between aerial parts and roots, and ecotypical differences of them in rice plants.

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Relationships between the growth direction of primarer roots (Fig. 1) and shoot growth were studied by treating the shoot in various ways for controlling the amount of assimilates translocated from shoot to root system (Table 1). Experiment 1 : With shading and defoliation, the growth of shoot was suppressed and the percentage of horizontally growing primary roots was higher than the control. The removal of tillers, on the other hand, let the growth of the main stem more vigorous and the percentage of primary roots growing obliquely or vertically was higher than the control (Figs. 2 and 3). Experiment 2 : Under high planting density, the growth of shoot was suppressed and the percentage of horizontally growing primary roots was higher than the control. Meanwhile under low planting density, the growth of shoot was more vigorous and the percentage of obliquely or vertically growing primary roots was higher than the control (Table 2, Fig. 4). The growth direction of primary roots is strongly influenced by the shoot growth in each case mentioned above ; i.e. the better growth of shoot increases the proportion of obliquely or vertically growing primary roots, and on the contrary the poor growth of shoot increases the proportion of horizontally growing primary roots. These facts seem to support our assumption that the amount of assimilates translocated from shoot to root system might control the growth direction of primary roots in rice plants.

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Two cultivars of potted rice plants were given various treatments, such as, 10 cm depth inundation, high water parcolation (30 mm/day) and excision of lower leaves below top three leaves, in order to change the respiratoy rate of roots. Photosynthesis and dark respiration were measured by the assimilation chamber method under the given conditions of temperature range from 20°C to 40°C during 4 hours at the time of young panicle formation, heading and mid-period of ripening. Immediately after this measurement, the roots were sampled with thoroughly washing out soil and the root respiration in normal air was measured by chamber method. At the time of young panicle formation, the respiratory rate of younger roots from upper three nodes at the base of stem showed much higher activity than that of older roots on lower nodes, however, such diflbrences were not noticed during the period after heading. The ratio of gross photosynthesis of 30°C to 40°C showed a high positive correlation to the respiratory rate of root ; the plants with lower rate of respiration exhibited strongly depressed photosynthesis at high temperature of 40°C perhaps due to thermoactivc closure of stomata. The depression observed at high temperature appears to be caused by water deficit in leaves due to inferior water absorption, since the transpiration efticiency at 30°C was quite dependent on the respiratory rate of roots. Multiple regression analysis of root respiration, nitrogen and sugar content in root revealed that the respiratory rate of root had high positive correlation with the nitrogen as well as the sugar content of the roots. The nitrogen content in root was swayed by nitrogen concentration at the top whereas the sugar content was parallel to net photosynthetic, activity of the plant which in turn is capable of expressing by nitrogen content in leaves of the plant.

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Present experiment was conducted to clarify the effect of natural daylength on the floral induction of rice plants, using four Japanese cultivars : Sasanishiki (less sensitive to photoperiod), Tosan No.38 (moderately sensitive to photoperiod), Hatsushimo (photoperiod-sensitive) and Zuiho (photoperiod-sensitive). The plants sown on two different dates were treated by natural day length or short day 1 or 10 days at 10 days intervals (Table 1, Fig. 1). The dates of young panicle initiation and heading were compared with those of the plants grown under long day condition. In Sasanishiki sown on 1 May, short day effect was obtained by the natural daylength treatment for 10 days started from 2O June when astronomical daylength is the longest in ycar. In Tosan No. 38 sown on 1 May, the time of earliest natural daylength treatment showing short day effect was not claified. The earliest natural daylength treatment showing short day effect started on 30 June in Sasanishiki sown on 22 May, on 10 July in Tosan No. 38 sown on 31 May, on 20 July in Hatsushimo sown on 1 May and sown on 10 June, on 30 July in Zuiho sown on 1 May and on 9 August in Zuiho sown on 10 June, respectively (Figs. 2, 3, 4 and 5). From these results, the dates when rice cultivars begin short day response to natural daylength in Osaka can be estimated as fellows : 28 June in Sasanishiki sown on 22 May, 15 July in Tosan No. 38 sown on 31 May, 20 July in Hatsushimo sown on 1 May, 25 July in Hatsushimo sown on 10 June, 28 July in Zuiho sown on 1 May and 7 August in Zuiho sown on 10 Junc (Table 3). It is discussed that critical daylength is the daylength controlling heading date of Japanese rice cultivars.

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It is recognized in various species that triacontanol (TRIA) increases the growth of seedlings at very low concentrations. However, very fcw reports so far have actually shown increases in dry weight or yield in grain crops under field conditions. A field experiment was, therefore, carried out in 1983 and 1984 at the University Farm of Tokyo Univcrsity of Agriculture, Atsugi, Kanagawa Pref., using two cultivars of rice, with the aim to clarify the cffect of foliar-applied TRIA in colloidal dispersion at concentrations between 0.1 and 10 ppb. Main results obtained are as follows : 1. In 9 out of 16 TRIA-treated plots, increases of 5% to 14% (significant at 5% level in the latter) over control were found in the yield of hulled rice, i.e., brown rice (Tables 7 and 8). 2. As for the effect of TRIA on yield componcnts, increases in the number of ears and 1000-grain weight of hulled rice were often observed (Table 7). 3. The effective concentration of}TRIA for increasing the yield was found to be 0.2-10ppb and the most effective tilnc of application was at the early tillering stage or the end of nursery stage (Tables 7 and 8). 4. Plant length, total leaf area and total dry weight in early growth stages showed a tcndency to be slightly inhibited by TRIA-treatment at concentrations 0.2 to 10ppb (Tables 3 and 4). 5. At the activc ripening stage, however, not only the totalleaf area but also the total root mass were found to be larger in the TRIA-treated plants than in the non-treated control (Tables 4 and 5). It is highly possible that these characters have contributed heavily to increasing the yield through promoting the dry matter accumulation after heading (ΔW) in the TRIA-treated plants (Fi9. 1) . 6. It was found that ΔW was positively correlated with NAR (Fig.2) and that the latter in turn was negatively correlated with SLA (Fig. 3). These may be interpreted to suggest that photosynthetic rate during the active ripcning period was accelerated in the TRIA-treated plants, thus leading to arl increased yield.

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In order to examinc the varietal differences in photosynthetic rate, transpiration rate and leaf conductance for leaves of rice plants, rates of photosysthesis and transpiration of fifty cultivars as shown in Table 1 were simultaneously measured under the controlled conditions : 1800 μmol·m^-2·s^-1 light intensity, 30.4°C leaf temperature, 14.5 mbar vapour pressure difference and 340 μl·l^-1 CO₂ concentration. The measurements were performed three times during the period from maximum tiller number stage to panicle heading stage. We designate these as periods I, II, and III with period I being the maximum tiller number stage, period II being midway between periods I and III, and period III being the heading stage. From the data obtained in this experiment, water use elficiency, leaf conductance, mesophyll conductance and intercellular CO₂concentration (Ci) were calculated. In addition, chlorophyll content, nitrogen content and specific leaf area (SLA) were measured. Using these parameters, the causes of differences in photosynthesis for the different rice cultivars w'ere analyzed in relation to the diffusion pathway of carbon dioxide from the atmosphere to the chloroplasts and to its biochemical activity. The results obtained were as follows : 1. The maximum rate of photosynthesis was 51 mgCO₂·dm^-2·hr^-1 in 'Century Patna 231' and the minimum rate was 22 mgCO₂·dm^-2·hr^-1 in 'Senbon asahi' rice cultivars. The mean value for fifty rice cultivars was 39.8, 36.2 and 33.1 mgCO₂·dm^-2·hr^-1 for periods I, II and III, respectively. The coefficients of variation for each of these periods was 12.1, 11.1 and 13.4%, respectively (Table 1). 2. The transpiration rates lbr the cultivars ranged from 2.21 to 4.88 with an average of 3.70, 3.59 and 3.40 gH₂O·dm^-2·hr^-1 for periods I, II and III, respectively. The cocfficients of variation for each period were 12.6, 15.5 and 16.4%, respectively (Table 1). 3. From the linear regression analysis, there were close correlations between photosynthetic rate and leaf or mesophyll conductance. There were loose correlations, however, between photosynthetic rate and chlorophyll content, nitrogen content, or SLA. Intercellular CO₂ concen-tration, which is considered to be onc of the parameters strongly regulating the photosynthetic rate, was not associated with photosynthetic rate in the rice cultivars (Table 2). 4. Using the least squares method, we tried to obtain some information on the relationships between photosynthetic rate and leaf or mesophyll conductance in the rice cultivars. Both of the relationships were approximated by a fburth order polynomial. It appeared from the former relationship that the photosynthetic rate has an optimum leaf conductance ; that is, for values up to 1 cm·s^-1 of leLlf conductance, photosynthetic rate increased with increasing in leaf conductance. Beyond tlle value, photosynthetic rate decreased (Fig. 1-A). On the other hand, photosynthetic rate was linealy correlated with mesophyll conductance (Fig. 1-B). Consequently, it is clear that differences in photosynthetic rate among the rice cultivars examined was due to the differences in mesophyll conductance.

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Isoprothiolane (diisopropyl-1, 3-dithiolan-2-ylidenemalonate, IPT) is a compound originally developed as a systemic fungicide to control rice blast disease. VVe investigated the growth regulating effect of IPT on rice seedling, especially on root growth. The results obtained are summarized as follows : 1. When the germinated rice (cv. Nipponbare) seeds were induced on agar medium containing various concentrations of IPT at 25°C, it remarkably promoted the growth of seminal root dependent on the conc(tntration from 10^-6 to 10^-4M. IPT also promoted notably the growth of crown root, too, at 10^-5 and 10^-4M. On the other hand, the plant hcight and second leaiA sheath were inhibited by IPT at high concentrations above 10^-4M (Fig.2). 2. The promoting effct of seminal root growth by IPT shit-ted low concentration sides by reducing the incubation temperature in steps (20, 18 and 16°C). That is, 10^-6 and 10^-5M IPT promoted the seminal root growth without eflbctiveness for plant height at 16°C. We also observed that this compoLlnd hastened the elongation of crown roots (Figs. 3, 4). 3. The in vitro root-tip (1cm long) culture in modified Kawatas' liquid medium exhibited that IPT also promoted the elongation of excised seminalroot at 10^-6 and 10^-5M, and inhibited its clongation and secondary root development at high concentration above 10^-4M (Figs. 5, 6). The original part ofinoculated root-tip tissue cultured in the medium containing IPT remained to be white color with brown in control even after three weeks. This excised root part which was immersed in hydrochloric acid-phloroglucinol mixture fbr the detection of lignification faded rose color compared with control one ( Figs. 6, 7). From these results, it is clear that IPT is directly associated with root metabolism and promotes its growth. IPT may take part in maintenance and buildup of root activity under low temperature in rice seedlings.

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