GrainGenes

Nuclear magnetic resonance (NMR) offers a quick and noninvasive method for characterizing water status in tissues. In the pulsed NMR spectrometer, the sample containing nuclei of nonzero spin like ¹H is kept in a steady magnetic field that polarizes protons of the sample, resulting in a net magnetization along the field direction. A radio frequency field, applied perpendicular to the steady field direction for a few micro seconds, flips the magnetization away from the steady field, resulting in the induction of voltage in the radio frequency coil. The induced voltage is called the NMR signal. This signal decays with a characteristic time constant, T₂, called the spin-spin relaxation time. The magnetization again builds up along the steady field direction with the time constant, T₁, called the spin-lattice relaxation time. These relaxation times give information about the molecular mobility of the water protons present in the sample. NMR spin-lattice relaxation T₁ times of leaf protons of many cereals were found to be related to classical physiological water relation variables like leaf water content, relative water content, leaf water potential, and osmotic potential. Also, in wheat leaf water, the T₁ of a drought tolerant cultivar was significantly higher than that of a susceptible cultivar under different growing condition. In the present study, this relationship was verified by measuring the leaf water T₁, of a large number of wheat cultivars known to vary in drought tolerance when grown in the field as both irrigated and rainfed crops.

Twenty wheat cultivars, differing in their performance under rainfed conditions, were grown in the field in a sandy loam soil, at IARI, New Delhi, under both irrigated and rainfed conditions. The recommended agronomic practices were used, and a randomized block design was followed with three replicated in each cultivar. Two to three samples, consisting of 4-5 leaves (second mature leaf from the top), were taken from each replicate at the tillering stage (about 45 days after sowing) for measurement of T₁. The measurements were done on a Bruker Minispec pc 20 Pulsed NMR spectrometer using a 90-t-90 pulse sequence.

The leaf water T₁s of the 20 cultivars under both growing conditions and their rankings are given in Table 1. The last five cultivars in the table are early maturing and recommended for highly fertile and irrigated conditions and may possess less tolerance to water stress. WH 157 is recommended for saline soils and is expected to have tolerance to drought as well. Pissi local is a heat-tolerant selection from central India. NI 5493 and WL 711 are medium maturing and recommended for both rainfed and irrigated cultivation. HD 2285 is recommended for late-sown, irrigated conditions and possesses heat tolerance during grain development. The remaining 10 cultivars are late maturing and recommended for cultivation under low fertility and rainfed conditions. The rankings obtained for the cultivars, based on T₁ values under the two growing conditions, tallied well when tested by the Spearman rank correlation analysis (r = 0.909, P = 0.001). Therefore, we concluded that the relative drought tolerance of wheat genotypes can be evaluated by measuring the leaf water spin lattice relaxation time, T₁, at the tillering stage of the crop when grown under either irrigated or rainfed conditions.

Table 1. Leaf water spin lattice relaxation time, T₁, at tillering stage

for wheat cultivars grown under irrigated and rainfed conditions.

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S. Irrigated Rainfed

No. Cultivar T₁ (msec) Rank T₁ (msec) Rank

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1. Mukta 445 1 434 1

2. IWP 72 431 2 417 2

3. Hyb 65 401 3 365 5

4. WH 157 399 4 376 4

5. C 306 399 5 384 3

6. K 8027 393 6 360 7

7. Pissi local 390 7 355 10

8. HD 2285 381 8 324 14

9. WL 711 379 9 357 9

10. WL 410 365 10 346 12

11. Raj 3077 364 11 366 6

12. PBW 65 361 12 358 8

13. DL 153-2 355 13 349 11

14. HD 2428 355 14 330 13

15. NI 5439 345 15 319 16

16. WH 147 344 16 296 18

17. Sonalika 339 17 303 17

18. PBW 154 335 18 321 15

19. HD 2329 329 19 293 20

20. HD 2009 328 20 296 19

CD at 5 % 12.0 14.2

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Indian Agricultural Research Institute

Regional Station, Wellington, 643 231, Tamil Nadu, India.

R.N. Brahma, R. Asir, A. Saikia, and M. Sivasamy.

Comparative yields of Unnath (Improved) C 306 and C 306.

Unnath C 306, developed by M.K. Menon and possessing Agropyron elongatum-derived linked genes Sr24 and Lr24, has been found to be resistant to stem and leaf rusts. The comparison of this improved variety with original C 306, which is highly susceptible to stem and leaf rusts but resistant to stripe rust, is difficult at Wellington, because this location is a hotspot for all three rusts. Therefore, the study was carried out using 'Tilt' (Propiconazole), an effective chemical for the three rusts, over two rabi seasons.

The test varieties were sown in 4 x 4 meter plots, each replicated five times. The plots comprised treated Unnath C 306, untreated Unnath C 306, treated C 306, and untreated C 306. The chemical (0.1 %) was first applied at the time of initial appearance, followed by three applications at intervals of 15 days. Water-sprayed plots served as untreated checks. Plot yields of the two varieties were recorded and expressed as Quintal/ha. The incidence of all three rusts was high.

The chemical was found to be effective against the three rusts. In both seasons, the mean yields observed in treated Unnath C 306 and C 306 and untreated Unnath C 306 were significantly higher than that of untreated C 306. The mean yields recorded in treated and untreated Unnath C 306 and treated and untreated C 306 were not statistically significant. This showed that the transfer of A. elongatum-derived linked genes Sr24 and Lr24 had no adverse effect on grain yield in Unnath C 306.

Control of wheat rusts with Bayleton (Triadimefon).

The effectiveness of Bayleton was evaluated against the three wheat rusts in comparison to Dithane M-45 (dithiocarbamate) under heavy natural epiphytotic conditions at Wellington.

The variety Kalyansona, highly susceptible to three rusts, was sown in 4 x 4 meter plots, each replicated five times. The chemicals Bayleton (0.1 %) and Dithane M-45 (0.2 %) were applied first at the time of the initial appearance of the rusts, followed by three applications at intervals of 15 days. Water-sprayed plots served as checks. The comparative effectiveness of the two chemicals was assessed on the basis of plot yields expressed in quintal/ha.

The chemical Bayleton was found effective against all three rusts and gave better performance in the control of rusts compared to Dithane M-45, resulting in significantly higher grain yields. No complete control of all the rusts was observed. Bayleton-treated plots also gave very high seed quality.

Sher-E-Kashmir University of Agricultural Sciences and Technology

Shukast, Regional Agriculture Research Station, R. S. Pura (Jammu)-181102, India.

J.S. Bijral and T.R. Sharma.

Chromosome pairing in Triticum boeoticum - T. aestivum hybrids.

The pool of potentially useful genes has shrunk alarmingly in recent years, primarily because of the replacement of highly variable land races with high yielding pure line varieties. As a consequence, breeders are now turning to the immediate progenitors of wheat as a supplementary source of useful genes.

Of the various wild species, Triticum boeoticum L. (2n = 2x = 14, A^bA^b) is highly resistant to Karnal bunt (Gill 1990) and is reported to be capable of discriminating high K/Na ratio - a trait closely associated with salt tolerance (Gorham et al. 1991). Furthermore, being a diploid species, T. boeoticum possesses a specific genetic advantage over tetraploid and hexaploid wheats and is easier to manipulate because of the presence of only seven nonhomologous groups of chromosomes (Waines 1983). Introgression of useful genetic variation of T. boeoticum into cultivated wheat T. aestivum (2n = 6x = 42, AABBDD) would considerably improve productivity and stability in diverse wheat growing environments. The present communication reports meiotic chromosome pairing in T. boeoticum-T. aestivum hybrids. Also discussed are the possibilities of direct genetic transfers from T. boeoticum to hexaploid wheat.

T. boeoticum (Acc. No. 4667, seed kindly provided by Dr. H.S. Dhaliwal, Punjab Agricultural University, Ludhiana) was crossed as a female with a hexaploid wheat cv. C 306 (a widely adapted and agronomically superior wheat variety) under field conditions. Embryo culture or hormone application was not necessary, and the crossed seeds were harvested at maturity. For meiotic analysis, the spikes of the interspecific hybrids were fixed in acetic acid-alcohol (1:3 v/v), and anthers were squashed in 2 % acetocarmine.

Chromosome pairing at the first meiotic division in the tetraploid (A^bABD) hybrids averaged 9.0 I + 9.42 II (ring) + 0.03 III per meiocyte. The number of univalents, bivalents, and trivalents ranged from 2-16, 6-13, and 0-1, respectively. Because T. boeoticum is shown to have contributed only six chromosomes (possibly not 4A) to T. aestivum (Miller et al. 1981), tetraploid hybrids (assuming perfect pairing) were expected to give a mean meiotic chromosome pairing of 7.0 bivalents and 14 univalents. The increased level of pairing observed in the tetraploid hybrids is indicative of homoeologous pairing.

Pairing fertility of the T. boeoticum-aestivum hybrids in conjunction with the successful production of BC₁ seed indicates the possibility of direct genetic transfer of useful traits from T. boeoticum to common wheat.

References.

Gill KS. 1990. Crop Improvement 17:1-8.

Gorham J, Bustol A, Young EN, and Wyn Jones RG. 1991. Theor Appl Genet 82:729-736.

Miller TE, Shepherd KW, and Riley R. 1981. Cereal Res Comm 9:327-329.

Waines JG. 1983. In: Proc 6th Int Wheat Genet Symp, Kyoto, Japan. Pp. 153-158.

Interspecific hybridization between T. aestivum and T. urartu.

Karnal bunt (Neovosia indica (Mitra) Mundkur) has become a serious disease of common wheat in the Indian subcontinent. Apart from reducing the quality of the wheat grain, this disease also results in considerable yield losses. Because the disease is soil-, air-, and seed-borne, its control through fungicides is not fully effective. Obviously, development of resistant cultivars appears to be the only economic and effective method of disease control. However, the extremely low genetic variability for Karnal bunt resistance in the bread wheat stocks (Aujala et al. 1992) limits the development of resistant wheat cultivars. Fortunately, T. urartu (2n = 2x = 14, A^uA^u), a wild species and donor of the A genome in bread wheat (Dvorak et al. 1992), is reported to be highly resistant to Karnal bunt (Dhaliwal et al. 1986; Royer and Rytter 1988; Pannu et al. 1994) and, therefore, constitutes a valuable source for transfer of resistance gene(s) into common wheat. Keeping this in mind, T. urartu was crossed with two hexaploid wheat cultivars, C 306 and Chinese Spring (ph1b mutant), and meiotic chromosome pairing in the interspecific hybrids and possibilities of direct transfer of Karnal bunt resistance from T. urartu to common wheat are reported in the present communication.

Triticum aestivum cvs. C 306 (a widely adapted and agronomically superior wheat cultivar) and Chinese Spring (ph1b mutant, seed kindly supplied by Dr. G.S. Sethi, HPKV, Palampur) were crossed as female with T. urartu (Acc. No. 5357, seed kindly provided by Dr. H.S. Dhaliwal, PAU, Ludhiana). Neither hormone application nor embryo culture was necessary, and the crossed seeds were harvested at maturity. For meiotic studies, the spikes of the hybrid plants fixed in acetic acid-alcohol (1:3 v/v) removed to 70 % alcohol, and the anthers were squashed in 2 % acetocarmine.

The hybrid plants were partially fertile and exhibited reasonable seed set on selfing or backcrossing to C 306 and HD 2329. A few F₂ and BC₁ plants derived from `C 306 x T. urartu' and `(CS(ph1b mutant) x T. urartu) x HD 2329' cross combinations showed pronounced leaf necrosis. However, the basis for the occurrence of such a necrosis remains to be explained and merits further investigation.

Meiotic chromosome pairing in the interspecific hybrids between T. aestivum cv. C 306 and T. urartu was characterized by the formation of 10.0 II + 8.0 I in 38.2 % of the pollen mother cells (PMCs) followed by 9.0 II + 10.0 I and 11.0 II + 6.0 I in 35.5 % and 14.7 % of the PMCs, respectively. An additional 11.8 % of the cells revealed the

presence of 12.0 II + 4.0 I. The overall mean chromosome association was 9.9 II + 8.2 I per meiocyte and the ranges of bivalents and univalents were 8-12 and 4-12, respectively. On the other hand, hybrids between Chinese Spring (ph1b mutant) and T. urartu showed a mean meiotic chromosome pairing of 8.8 II + 0.3 III + 9.9 I per meiocyte. Of the 25 PMCs analyzed, 7 (27 %) showed the presence of multivalent (trivalents) associations. The ranges of the univalents, bivalents, and trivalents were 3-16, 5-13, and 0-1, respectively.

Partial fertility of the interspecific hybrids, coupled with the ease of obtaining the BC₁ seeds, suggests the possibility of direct transfer of Karnal bunt resistance from T. urartu to common wheat.

References.

Aujala SS, Sharma I, and Gill KS. 1992. Indian J Agric Sci 10:171-172.

Dhaliwal HW, Gill KS, Singh P, Multani DS, and Singh BB. 1986. Crop Improvement 13:107-112.

Dvorak J, Di TP, Zhang HB, and Pesta R. 1992. Genome 36:21-31.

Pannu PPS, Singh H, Datta R, and Dhaliwal HS. 1994. FAO/IPGRI Plant Genetic Resources Newslet 97:47-48.

Royer MH and Rytter J. 1988. Plant Dis 72:133-134.