Nutrient responses of seedlings of wild and cultivated Oryza species

Field Crops Research, 6 (1983) 205--218 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

NUTRIENT RESPONSES OR Y Z A SPECIES

OF SEEDLINGS

205

OF WILD AND CULTIVATED

M.G. COOK and L.T. EVANS

CSIRO, Division of Plant Industry, P.O. Box 1600, Canberra City, A.C.T 2601 (Australia) (Accepted 4 October 1982)

ABSTRACT Cook, M.G. and Evans, L.T., 1983. Nutrient responses of seedlings of wild and cultivated Oryza species. Field Crops Res., 6: 205--218. Lines of wild and cultivated species of Oryza from both Asia and Africa were grown in daylight at 27/22°C under a variety of nutritional conditions. The concentration of Fe, N or the complete nutrient solution was varied, as was pH and the aeration of the root medium. The wild species and primitive cultivars were more susceptible to an assumed Fe deficiency than were advanced cultivars of O. sativa. All groups showed a broadly comparable response to increasing concentration of nitrogen or of complete nutrient solution. There was no evidence that growth and photosynthesis of either indica or japonica rice were more adversely affected in the advanced than in the primitive cultivars or wild species by the lowest concentrations of nutrients or of nitrogen. In all lines leaf area per plant was strongly reduced at lower nitrogen concentrations, and there was an increase in the proportion of seedling dry weight in the roots, which was highest throughout in advanced cultivars. Photosynthetic rate was closely correlated, across all species and,lines, with N content per unit leaf area (r = 0.85) and also with specific leaf weight (r = 0.78).

INTRODUCTION In a comparison of the growth and development of the two cultivated s p e c i e s o f r i c e , O r y z a sativa a n d O. glaberrima a n d t h e i r w i l d r e l a t i v e s in t h e C a n b e r r a p h y t o t r o n , i t w a s f o u n d t h a t all l i n e s , p a r t i c u l a r l y t h o s e o f t h e w i l d s p e c i e s , g r e w p o o r l y u n d e r t h e u s u a l c o n d i t i o n s f o r p l a n t g r o w t h in t h e p h y t o t r o n . T h e s e i n c l u d e d g r o w t h in a e r a t e d c o n d i t i o n s , s u p p o r t o f p l a n t s in a p e r l i t e a n d v e r m i c u l i t e m i x t u r e in f r e e l y d r a i n e d p o t s , a n d w a t e r i n g w i t h a m o d i f i e d H o a g l a n d ' s n u t r i e n t s o l u t i o n o f p H c l o s e t o 6. In the first part of this paper, the changes made to the standard growing c o n d i t i o n s u s e d in t h e p h y t o t r o n in o r d e r t o e l i m i n a t e t h e c h l o r o s i s e n c o u n t e r e d in y o u n g s e e d l i n g s a n d t o i n d u c e g o o d g r o w t h in all s p e c i e s a r e b r i e f l y d e s c r i b e d . S i n c e c h l o r o s i s h a s o f t e n b e e n f o u n d in r i c e g r o w n in s o l u t i o n c u l t u r e a n d is g e n e r a l l y a s s u m e d t o b e d u e t o i r o n d e f i c i e n c y {e.g. O k a j i m a , 1 9 6 5 ) , the modifications were particularly directed towards increasing the availability of iron. 0378-4290/83/$03.00

© 1983 Elsevier Science Publishers B.V.

206 The second part examines whether the more advanced (high yielding) cultivars are more susceptible in their seedling growth and photosynthesis to low levels of nutrient supply, especially nitrogen. An examination of the responses of each species and type of cultivar to variations in nutrient concentration was considered essential before making physiological comparisons between species and lines (see Cook and Evans, 1983), particularly because leaf nitrogen content is known to affect photosynthesis in rice to a marked extent (Takano and Tsunoda, 1971; Yoshida and Coronel, 1976). MATERIALS AND METHODS Seeds obtained from the International Rice Research Institute (IRRI), Los Bafios, the Philippines included representatives of the Asian and African cultivated species O. sativa and O. glaberrima and their wild and weedy relatives O. rufipogon, O. nivara and O. sativa-spontanea (wild perennial, annual and weedy Asian species respectively) and O. barthii and O. stapfii {wild annual and weedy African species). The evolutionary relationships between them are described by Chang (1976) and illustrated by Cook and Evans (1983), who give details of most of the lines used. Some additional lines were included in the first experiment described below, to make a total of 48 lines, while subse. quent experiments used only a small selection of lines.

Standard growing conditions in the phytotron These were employed in growing samples for seed production and observation in the first (quarantine) generation and in Experiment T1. Following germination in petri dishes containing filter paper and distilled water, 4 or 5 day old seedlings were planted out in 12.5 cm diameter pots in a 50/50 mixture of perlite and vermiculite and watered with a modified Hoagland solution (N, 211.7; P, 32.2; K, 235.9; Ca, 160.9; Mg, 48.3; S, 66.7; Fe, 5.007; Na, 3.61; B, 0.105; C1, 0.143; Co, 0.005; Cu, 0.013; Mn, 0.111; Mo, 0.012; Zn, 0.02 mg 1-1 ; pH = 5.8) each morning and water each afternoon. The pots were allowed to drain freely. Plants were kept in a naturally lit glasshouse with daylength extended by 16 h by incandescent lamps and maintained at a temperature of 27°C for 8 h during the day and 22°C at night.

Seedling growth improvement Changes in the watering system were tested in several experiments, each incorporating previously found improvements. In Experiment T3, growth in freely drained pots was compared with that where pots were placed in a large tray filled to a depth of 3 cm with solution; nutrient solution and water were applied to the pots in each case. In Experiment T4, nutrient solution and water were applied either to the pots as in Experiment T3 or added directly into the

207

tray, topping it up twice daily. Experiment T6 compared growth using the vermiculite/perlite mixture with that using vermiculite only; nutrient and water were added directly into the tray. In subsequent experiments, pots were filled with vermiculite only and stood in a tray of solution with only the appropriate nutrient solution added directly into the tray twice daily. Trays were drained and refilled twice weekly. The effect on growth of lowering the pH of the nutrient solution was tested both in the standard free-draining system (Experiment T1) and using the filled trays (Experiment T6). Two experiments examined iron nutrition directly. Firstly {Experiment T3), the availability of Fe was tested by comparing growth with and without a thrice weekly application of Fe citrate (5 mg 1-1) to the foliage using an atomiser spray with Tepol as a wetting agent. In Experiment T14, nutrient solutions at 0.25 and 0.125 times the standard concentration were compared with and without an increase in the Fe concentration (as Fe EDTA) to 2 mg 1-1 (from either 1.25 or 0.625 mg 1-1). At least one representative of each species was included, with two O. nivara lines and three O. sativa cultivars: JP5 and IR8 {intermediate and advanced indicas) and Tainan 3 (an advanced japonica).

Responses to nutrient and nitrogen concentration Experiment T15. With the proportion of Fe doubled and the pH adjusted to 4.8 with HC1, nutrient was supplied at 0.5, 0.25, 0.125 or 0.0625 times the standard concentration. Leaf area, whole plant dry weight and tiller number were measured at harvest 45--49 days from sowing and the uppermost fully expanded leaves (leaf 5, 6 or 7) of four plants per line were used for measurement of net photosynthetic rates prior to harvest. All Asian species were represented, O. nivara by three lines, and primitive (Molagasamba G18), intermediate (JP5) and advanced indicas (IR8 and Pelita I/1) and an advanced japonica (Tainan 3) were included. Experiment T16. Using the nutrient solution favoured at IRRI, in which N is supplied as NH4NO3 only (Yoshida et al., 1976), the N concentration was varied in two-fold steps from 5 to 160 mg 1-1. The pH was reduced to counteract the alkalinity of vermiculite. Net photosynthesis measurements were taken just prior to harvest on the uppermost fully expanded leaves (leaf 6, 7 or 8). The N content of the measured leaf portions was determined. Each line was harvested when the slowest growing treatment had six leaves on the main shoot. Lines included most of those used in Experiment T15, excluding one O. nivara line, JP5 and Pelita I / l , but including one each of O. barthii and O. glaberrima. The treatments described in the above two sections are summarised in Table I.

208 TABLE I Summary of experimental details Experiment Support medium

Watering system

Nutrient concentration (× standard)

pH

Fe (mg l- ' )

T1

perlite/ vermiculite

standard

1.0

5.8, 5.0

T3

perlite/ vermiculite

standard vs 3 cm bath, through pots a

1.0

5.8

T4

perlite! vermiculite

3 cm bath through pots vs into tray a

1.0

5.8

T6

mixture vs vermiculite

3 cm bath, into tray a

0.5

4.0, 4.5 5.0, 5.5

T14

vermiculite

3 cm bath, into tray b

0.125, 0.250

4.5

0.625, 1.25, 2.0

T15

vermiculite

3 cm bath, into tray b

0.0625--0.5

4.8

0.625--5.0

T16

vermiculite

3 cm bath, into tray b

N (mg l-' ) 5--160

4.8

2

5 5 +_ Fe citrate spray 5

10

aNutrient added (through pots or into tray) each morning, water each afternoon. bNutrient only supplied.

Photosynthesis measurements F o r e a c h m e a s u r e m e n t , 5 c m l e n g t h s o f f o u r l e a v e s w e r e e n c l o s e d in a p e r s p e x a s s i m i l a t i o n c h a m b e r w h e r e t h e p h o t o n f l u x d e n s i t y w a s 8 0 0 p E m -2 s -1 from fluorescent tubes augmented by incandescent lamps. The temperature o f t h e l o w e r s u r f a c e o f t h e l e a v e s w a s ca. 2 7 ° C . M e a s u r e m e n t s w e r e m a d e u s i n g a n i n f r a - r e d gas a n a l y s e r , w i t h a i r d r a w n f r o m t h e e x t e r n a l a t m o s p h e r e t h r o u g h a m i x i n g t a n k , a t a r a t e o f 4 - - 6 1 m i n -1.

Nitrogen determinations Leaf tissue was digested by a modified Kjeldahl method and then assayed f o r N u s i n g a n a u t o m a t i c a n a l y s e r as d e s c r i b e d b y _ W i l l i a m s a n d T w i n e ( 1 9 6 7 ) . S t a t i s t i c a l d a t a as p r e s e n t e d in t h e t e x t d e n o t e X + s . e . m , o f t w e l v e r e p l i c a t e plants.

209 RESULTS

Seedling growth and mortality under standard conditions Three germinated seedlings were planted in each of 21 pots per line, the intention being to obtain growth rate comparisons while thinning to one plant per pot. However, many seedlings developed severe chlorosis by the third or fourth leaf stage and subsequently died, precluding such harvesting in many lines. The percentage mortality averaged over each species or group within O. sativa is shown in Table II. It is clear that the more advanced O. sativa cultivars, including the advanced japonica X indica cultivar Century Patna 231, suffered least under these conditions. Less advanced O. sativa cultivars had higher mortality and exhibited more severe symptoms, but all were far superior to the wild and weedy Asian species whose mortality reached 80--90%. None T A B L E II

Seedling mortality and interval between successive leaves on surviving plants under standard growing conditions, compared with leaf production under modified conditions Species or g r o u p

Conditions Standard

Modified a

Mortality (%)

Days leaf ~~

Days leaf- 1

O. rufipogon

89

10.1

4.5

O. nivara

79

8.7

4.0

O. spontanea

77

9.2

4.2

O. sativa indica Primitive Upland Intermediate Taiwan Advanced

35 45 14 25 12

9.3 9.0 6.8 6.7 6.7

4.2 4.5 4.1 4.3 3.9

5

7.1

4.6

japonica Old Advanced

24 11

6.7 6.4

3.8 3.9

javanica

98

9.0

4.7

92

8.4

4.5

japonica x indica

O. barthii O. stapfii

68

9.8

4.1

O. glaberrima

49

10.7

4.4

a D a t a f r o m C o o k a n d E v a n s ( 1 9 8 3 ) . T h e r e was zero m o r t a l i t y u n d e r m o d i f i e d c o n d i t i o n s .

210 of the African lines performed well, but with them also the wild and weedy species were the most susceptible. The rate of main shoot leaf production, calculated from the interval between the expansion o f leaves 2 and 9 in surviving seedlings, was taken as a measure o f growth. This is compared in Table II with the leaf intervals obtained b y C o o k and Evans (1983) under improved conditions for growth. Under standard conditions the rate of leaf appearance was less than that obtained in the later experiment even in advanced cultivars, b u t particularly so in primitive cultivars and wild species. Seedling mortality was eliminated in the later experiment.

Growth improvement (a) Watering system. When Emata Yin, an intermediate indica cultivar, was grown with pots standing in 3 cm of nutrient solution (Experiment T3), plant dry weight at harvest 32 days from sowing was significantly greater than for plants in free-draining pots (54.9 + 5.7 cf. 36.5 + 4.9 mg). Substantial improvements were made with t w o further changes. By adding nutrient and water directly into the tray under the pots rather than through the pots (Experiment T4) the growth o f Tainan 3, Taichung Native 1 and an O. glaberrima line was significantly enhanced (e.g. from 57 +- 7 to 180 + 31 mg per plant of Tainan 3 at 43 days). Using the better watering system it was then found (Experiment T6) that Tainan 3 and another cultivar performed better when pots were filled with vermiculite only. For Tainan 3, plant dry weight (41 days from sowing) and R G R (over the preceding 9 days) were 172 + 14 mg and 0.082 g g-~ day -1 compared with 74 + 4 mg and 0.046 g g-~ day -~ for plants grown in a mixture with perlite. (b) pH. The foregoing treatments utilized nutrient solutions of standard composition and pH, and growth was poor even with the improved conditions. In experiment T1 seedlings of O rufipogon, O. glaberrima and Taichung Native 1 were grown in standard conditions and given nutrient solution at either pH 5.8 or pH 5.0. Although average dry weights for surviving plants of O. rufipogon were not greatly increased b y reducing the pH to 5.0, survival was enhanced dramatically, from 14 to 71% at 50 days from sowing. The other t w o lines survived well under b o t h conditions but growth was substantially greater at the lower pH, dry weights at final harvests being 3.0 and 1.6 times that at pH 5.8 in O. glaberrima and Taichung Native 1, respectively. In Experiment T6 plants were given nutrient solution adjusted to pH's ranging from 4.0 to 5.5. The near-standard pH 5.5 yielded the poorest growth in all cases and, at least with vermiculite, pH 4.0 gave the best results. Data for Tainan 3 with pH 4.0 and vermiculite only were 407 mg dry weight at the final harvest (41 days) and R G R 0.169 g g-1 day-1 (cf. preceding paragraph). (c) Fe supply. In one early experiment (T3) plants of Emata Yin were spray-

211 ed with either Fe citrate solution or water. By t he final harvest at 6 weeks, plant d r y weight o f Fe-treated plants was m or e than twice as great as control plants when plants were grown using trays, and was also increased under t he freely-drained system. When growth and p h o t o s y n t h e t i c rates of a n u m b e r o f lines representing all of the species were c om par e d at 0.25 and 0.125 times the standard nutrient c o n c e n t r a t i o n with or w i t hout the addition of Fe to a c o n c e n t r a t i o n of 2 mg 1-~ ( E x p e r i m e n t T14), it was f ound that the wild and weedy Asian species were most affected by the lack o f additional iron at the lower nutrient concentration. Table III shows t h a t for plants grown ~n the standard nutrient solution at the 0.125 X c o n c e n t r a t i o n t he p h o t o s y n t h e t i c rate o f the wild species was only about one third of t hat at the 0.25 X concentration, whereas in the O. sativa lines there was little difference between the t w o treatments. A similar distinction bet w e e n species was evident in dry weights at harvest, with th e weed y O. sativa-spontanea and the African species lying between them. Where Fe c o n c e n t r a t i o n was raised however, the African and wild Asian species grew almost as well in the m or e dilute nutrient solution. TABLE III Net photosynthetic rate and seedling dry weight at the lower nutrient concentration (0.125 x standard) expressed relative to that at the higher nutrient concentration (0.25 x standard), and the effect of extra Fe EDTA (Experiment T14) Species

Wild Asian O. sativa-spontanea O. sativa African

Net photosynthesis (%)

Dry weight (%)

Standard Fe

Standard Fe

Extra Fe

35

28 36 61 49

94 102 76 82

93

Response to nutrient and nitrogen concentration In E x p e r i m e n t T15 there were no marked differences between the responses o f wild and cultivated lines to decreasing nutrient concent rat i on when the p r o p o r t i o n o f Fe had been doubled. In all cases plant dry weight and leaf area decreased in parallel as nutrient concent r a t i on fell from half the standard strength whereas p h o t o s y n t h e t i c rates measured on the upperm ost fully e x p a n d ed leaf remained relatively constant Until the concentration fell below 0.125 × standard (Fig. 1). The indica lines t ended t o show the smallest decline in p h o t o s y n t h e t i c rate. P h o t o s y n t h e t i c rates changed in the same direction as the specific leaf weight (SLW) o f th e leaves used for photosynthesis measurements. This applied to the change with nutrient c o n c e n t r a t i o n within lines as well as to comparisons be-

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tween lines and species (Fig. 2), but O. rufipogon had SLW's which were higher relative to net photosynthetic rates than the other species. For the pooled data the correlation was highly significant (r = 0.78, P < 0.001), and was even higher (r = 0.86) if O. rufipogon was excluded. This perennial differed from the other species in Experiment T16 also when leaf N contents were examined. Differences between species were evident in the proportions of dry matter in root and shoot but their responses to changing nutrient supply were for the most part similar, the % dry weight in the roots increasing in response to decreasing nutrient concentration. The % dry weight in the roots at half the standard nutrient concentration ranged between 21 and 27%, while at the lowest concentration (0.0625 X standard) the range was 29 to 37%. Seedlings of advanced indica cul£ivars displayed the greatest investment in roots and the least in leaf laminae throughout, while O. nivara lines showed the reverse. Since it is known that the photosynthetic rates of rice leaves are highly dependent on leaf nitrogen content (e.g. Takano and Tsunoda, 1971; Yoshida and Coronel, 1976), an experiment (T16) was conducted with nine lines

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Fig. 2. Relationship between SLW and net photosynthesis of portions of the uppermost fully expanded leaves in O. rufipogon (~), O. nivara (V), O. sativa-spontanea (o), primitive (u), intermediate (~) and advanced (m) indicas and advanced japonica (o) plants grown using a rang~e of nutrient concentrations. Experiment T15.

representing all species except O. stapfii to determine at what concentration the photosynthetic rate approached its maximum. Although the differences were not as marked as in Experiment T15, plant size as expressed by leaf area and plant dry weight again fell more rapidly than did the net photosynthetic rate of the uppermost leaves as the N concentration of the solution was reduced (Fig. 3). Plant size decreased as N concentration fell below 80 mg 1-1, which approximated the N level in the 0.5 X standard concentration in Experiment T15 (106 mg 1-1}, whereas in all but O. barthii net photosynthetic rate fell only at N concentrations below 40 mg 1-~. As in Experiment T15 there were no major differences between species in these responses: the apparent increases in plant size from 160 to 80 mg 1-1 solutions in O. rufipogon and O. glaberrima are not significant, these species being represented by one line only. A clear linear relation was obtained between the net photosynthetic rate and leaf N content (Fig. 4; r = 0 . 8 5 ; P < 0.001). With the exception of O. rufipogon, there were no differences evident between species in this relation, but the O. sativa cultivars tended to have higher values of both factors than the other species, consistent with their higher SLW. The leaf N content of the O. rufipogon line did not vary consistently with either photosynthesis or solution N. For the other species the leaf N content and photosynthetic rates were similar at the uppermost two or three supply levels, suggesting that the N supply was no longer limiting photosynthesis at those concentrations. Several factors contributed to the decrease in leaf area and plant dry weight with decreasing N supply. The area of leaf 6 was found to decrease slightly in all lines, to about 83% of that in the highest N treatment, and such a decrease probably applied to other leaves. The number of main stem leaves was affected to a similar degree, decreasing from 7.3 leaves on average to 6.2 at the lowest N

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Fig. 3. Effect of N concentration in the nutrient solution on net photosynthesis of uppermost leaves (X), plant dry weight (o), leaf area (v) and tiller number (z~). Experiment T16. level. T h e rate o f tiller p r o d u c t i o n , and h e n c e tiller leaves, decreased as indic a t e d b y tiller n u m b e r at harvest in Fig. 3. T h e level o f N in t h e n u t r i e n t s o l u t i o n had a c o n s i d e r a b l e i n f l u e n c e o n t h e partitioning o f dry m a t t e r b e t w e e n r o o t and s h o o t as has b e e n f o u n d for o t h e r species (e.g. B r o u w e r , 1 9 6 6 ) . T h e p e r c e n t a g e o f dry w e i g h t in t h e r o o t s is s h o w n in Fig. 5; it increased rapidly f r o m a m e a n o f 1 8 t o 38% at t h e e x p e n s e o f b o t h t h e l e a f l a m i n a e ( f r o m 46 t o 33%) and t h e r e m a i n d e r o f t h e s h o o t , as t h e N c o n c e n t r a t i o n decreased. All species r e s p o n d e d in substantially t h e s a m e m a n n e r over t h e s a m e c o n c e n t r a t i o n range. T h e r e w e r e i n s u f f i c i e n t representatives o f each t o draw c o n c l u s i o n s a b o u t t h e overall d i f f e r e n c e s in percentage, b u t c o m p a r i s o n w i t h o t h e r data - - including that o f E x p e r i m e n t T 1 5 -- c o n firms t h e higher p r o p o r t i o n o f r o o t f o u n d in O. sativa. W h e n m a n y m o r e lines

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Nitrogen content of leaf (rag N dm "~ )

Fig. 4. Relationship between leaf N content and net photosynthesis of portions of the uppermost fully expanded leaves of O. rufipogon (~), O. nivara (V), O. sativa-spontanea (0), primitive (o) and advanced ( - ) indica, advanced japonica ( . ) , O. barthii (+) and O. glaberrima (×) plants grown using nutrient solutions with a wide range of N concentration. Experiment T16.

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Fig. 5. Effect of N concentration in the nutrient solution on the proportion of plant dry weight in the roots of O. rufipogon (~), O. nivara (V), O. sativa-spontanea (0), O. sativa ( - ) and African species (*). Experiment T16.

were grown under a single nutrient regime (Cook and Evans, 1983), the advanced dwarf and semi-dwarf indica lines had the highest proportion of root, while O. barthii and O. nivara had the lowest. The changes with N supply in the proportion of dry weight in the roots were evidently a function of N level and not of plant dry weight since in the major seedling experiment (Cook and

216 Evans, 1983), the proportion of root varied by only a few percent with a maximum of about 24% through the four harvests which encompassed the range o f dry weights obtained in the present experiment. The more rapid decline of plant dry weight relative to photosynthetic rate as the supply of N fell (Fig. 3) was probably associated with the increased investment of dry matter in the r o o t at the expense of the photosynthetic area. DISCUSSION The results o f the experiments altering growth conditions and those providing ranges of nutrient and nitrogen concentrations allowed us to select a regime favourable for all species and lines, with which we could make useful comparisons between them {Cook and Evans, 1983). The nutrient solution adopted for regular use was the standard solution diluted by half, with the Fe concentration doubled to its original level and the pH reduced to 4.5 with HC1. As described previously, pots were filled with vermiculite and stood in a tray of nutrient solution which was replenished twice daily and drained and refilled twice weekly. While plants of the first generation were not assayed for nutrient content, it seems probable that Fe deficiency was responsible for the chlorosis and poor growth in the standard nutrient conditions. The s y m p t o m s resembled those of Fe chlorosis. Lowering the pH, which increases the solubility of Fe in aerated solution, was found to improve growth, as was the supply of Fe citrate to the foliage or the addition of Fe to the nutrient solution. The reasons for the greater susceptibility of wild and primitive lines of Oryza to Fe deficiency in these experiments were not investigated, but the relatively smaller size of their root system (Fig. 5) may have been a contributing factor (cf. also Cook and Evans, 1983). All species and groups -- whether wild, weedy or domesticated, whether primitive or advanced -- showed a broadly comparable response to changes in the concentration of complete nutrient solution (Fig. 1) or of nitrogen alone (Fig. 3). There was certainly no evidence that growth and photosynthesis of the advanced cultivars of either indica orjaponica rice were more adversely affected than those of the primitive cultivars or wild species by the lowest concentrations of nitrogen or all nutrients. There were however certain differences in response to N concentration amongst the O. sativa cultivars which are compatible with the fact that the greater yield response to N of advanced dwarf cultivars results from the avoidance of lodging and mutual shading to which tall, large leaved cultivars are prone. Comparing the high yielding ("high nitrogen response") cultivar IR8 with the tall primitive cultivar Molagasamba G18, the latter showed the greater leaf area response, with an 8 fold difference between extreme N concentrations compared with a four fold change in IR8. Molagasamba G18 also showed a greater response in terms of tiller number, plant dry weight and r o o t / s h o o t ratio, b u t a smaller response in leaf N content and net photosynthetic rate.

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These results agree with those described by Tanaka (1965) who claimed that "low-nitrogen-response" varieties in fact respond most as isolated plants b u t in the field fail to increase yield with nitrogen fertilization because of early overcrowding. Bearing in mind that in Experiment T15, where the concentration of the complete nutrient solution was varied, the N concentration varied from 13.3 to 106 mg 1-1, there was a strong resemblance between the curves of plant dry weight, leaf area and percentage root in that experiment and in Experiment T16 when N concentration only was altered (Figs. 1 cf. 3). It is, therefore, likely that N supply was the major limiting factor in T15. The close correlation between leaf N content and photosynthetic rate (Fig. 4) confirms other results. Yoshida and Coronel (1976) working with one cultivar, IR8, found a linear correlation between leaf N content and photosynthetic rate as they changed in response to differences in solution N concentration. Close correlations have also been found between varieties and species grown under one set of cultural conditions (Takano and Tsunoda, 1971; Ohno, 1976). There were sufficient lines and N supply levels in the present experiment to show that, except for the single representative of the perennial O. rufipogon, the same relation between leaf N content and photosynthetic rate holds for all of the species and lines. ACKNOWLEDGEMENTS

We are grateful to Kati Bretz and the Phytotron staff for their technical assistance throughout these experiments, to Drs. R.M. Gifford, I.F. Wardlaw and R.W. King for helpful discussions of the results and to Dr. T.T. Chang for suggesting a suitable set of accessions and for providing seed of them.

REFERENCES Brouwer, R., 1966. Root growth of grasses and cereals. In: F.L. Milthorpe and J.D. Ivins (Editors), The Growth of Cereals and Grasses. Butterworths, London, pp. 153--166. Chang, T.T., 1976. The origin, evolution, cultivation, dissemination and diversification of Asian and African rices. Euphytica, 25: 425--441. Cook, M.G. and Evans, L.T., 1983. Some physiological aspects of the domestication and improvement of rice. Field Crops Res., 6: 219--238. Ohno, Y., 1976. Varietal differences of photosynthetic efficiency and dry matter production in Indica rice. Tech. Bull. 9, Trop. Agric. Res. Cent., 72 pp. Okajima, H., 1965. Environmental factors and nutrient uptake. In: The Mineral Nutrition of the Rice Plant. Johns Hopkins, Baltimore, MD, pp. 63--73. Takano, Y. and Tsunoda, S., 1971. Curvilinear regression of the leaf photosynthetic rate on leaf nitrogen content among strains of Oryza species. Jpn. J. Breed., 21: 69--76. Tanaka, A., 1965. Plant characters related to nitrogen response in rice. In: The Mineral Nutrition of the Rice Plant. Johns Hopkins, Baltimore, MD, pp. 419--435. Williams, C.H. and Twine, J.R., 1967. Determination of nitrogen, sulphur, phosphorus, potassium, sodium, calcium and magnesium in plant material by automatic analysis. Tech. Pap. 24, CSIRO Aust. Div. Plant Ind., 16 pp.

218 Yoshida, S. and Coronel, V., 1976. Nitrogen nutrition, leaf resistance, and leaf photosynthetic rate of the rice plant. Soil Sci. Plant Nutr., 22: 207--211. Yoshida, S., Forno, D.A., Cock, J.H. and Gomez, K.A., 1976. Laboratory manual for physiological studies of rice. International Rice Research Institute, Los Bafios, Philippines, 83 pp.