Maataloustieteellinen A ikakauskirja Vol. 63: 379—389, 1991 Effect of moderate and severe drought stress on the pre-anthesis development and yield formation of oats P. PELTONEN-SAINIO University of Helsinki, Department of Crop Husbandry, SF-00710 Helsinki, Finland Abstract. Greenhouse experiments were conducted at the University of Helsinki, Depart- ment of Crop Husbandry in 1989 and 1990 to provide information on the effects of drought stress on the developmentalrate of the panicle, formation and abortion of florets, and forma- tion of yield components in two oat cultivars, both adapted to the northern marginal growing conditions. Moderate water deficit and severe drought stress did not affect the developmental rate of the panicle, but the pre-anthesis and post-anthesis architecture changed considerably; leaves unrolled later, the number of green leaves was lower, the internodes lengthenedless, and the inflorescence was shorter as a consequence of water deficit. Drought stress decreased the number of fertile florets in the panicle significantly, in the lower clusters of branches in particular. In the cultivar “Puhti”, 57 % of the fertile florets aborted as a consequence of moderate drought stress and 89 % as a result of severe drought stress when comparedto well-watered individuals, while in “Virma”, a cultivar of significant- ly higher yield potential, 80 °lo and 90 % of the fertile florets wilted, respectively. In addition, water deficit caused considerable losses in all the morphological characteristics measured. Further implications of these results for yield formation are discussed. Index words: Avena saliva L., water deficit, development, yield potential, morphology, yield components Introduction Genome as well as environment determine the progress of development of cereal plants. The development of oats is known to be af- fected by temperature and photoperiod (e.g. Coffman and Frey 1961, Forlin 1987). Wa- ter deficit is, however, associated with high temperatures a reason why drought stress is commonly noted as hastening not only ripening but also pre-anthesis inflorescence de- velopment. Within this context, the question of water stress is of special interest in Finnish growing conditions, as early summer drought (Mukula and Rantanen 1989), occurring ex- actly during the development of inflorescence, is a recurrent problem in southern and south- western areas especially. In addition, oats re- 379 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=N0qZHEVK_r278Ugt.NieG2QSILCO0HkCLlQ2CGA.PMn_8axjLfZTC56qUBLkQzAicnsj-wgexOJReB5pz1sRsuPLC4MFUjLTumIhfETsCxLyCZlFwBIl5Vylg5MDBDrZi6jJgHnIXtBPnFeLAOj9DTtOxkGdbmWnddmq55AEoNmfeOki70G3slLOTXT-q4OFQD9najQYnKxqyOo35OK-FOVMM5lo_dj6L4tE_Mag_nKMj9VfcGbe3SRmZauBmMCZT__h9NS62YH8yvT1qdYjJCkeIRg0UjE0875CPwZ7aMXnde4yYIWIO7jZ quire more water than other cereals in order to produce a kilogram of dry-matter (Geisler 1970), so that water deficit may severely limit the yield formation of oats. The objectives of the present study were to evaluate the effects of moderate and severe water deficit on 1) the developmental rate of the panicle, 2) the formation and abortion of fertile florets, and 3) the formation of the oat stand and the yield components in two culti- vars, “Puhti” and “Virma”, differing in yielding ability. Materials and Methods Greenhouse experiments were conducted at the University of Helsinki, Department of Crop Husbandry, in 1989 and 1990 in order to assess the effect of drought stress on the development and yield formation of two oat cultivars, “Puhti” (introduced in 1978) and “Virma” (introduced in 1988). Growing conditions Before sowing, the seeds were pre-germi- nated on blotting paper for two days in 20° C in order to assure steady emergence. Alto- gether 13 pre-germinated seeds were settled into each pot (volume 5 liters, 0 20 cm) con- taining peat-vermiculite mixture (1:1), which was watered evenly before portioning to the pots. After emergence the number of oat in- dividuals per pot was verified. From the un- rolling of the first leaf to pollination, thepots were grouped into three treatments with 40 pots in each: 1) well-watered, as the pots were frequently top-watered and the pF-value was continu- ously near field capacity (pF 2) 2) moderately water stressed, as the pF in- creased to 3.5 before top-watered 3) severely water stressed, as pF increased to 4, near wilting point, until top-watered, and leaves evidently lost turgor. As the oats pollinated, these three treat- ments were divided into the following treat- ments: 4) continuously well-watered (pF 2) 5) continuously moderate water deficit (pF <3.5) 6) continuously severe water deficit (pF <4) 7) well-watered at pre-anthesis stage (pF 2), moderately drought stressed after pollina- tion (pF 8) moderate pre-anthesis water deficit (pF well-watered after pollination (pF 2) 9) severe pre-anthesis water deficit (pF well-watered after pollination (pF 2). The oats were fertilized every other week in connection with watering. The daylength was 18 hours, which corresponds to the daylength at latitude 60°N during the early part of the summer. The day and night temperatures were adjusted to 20°C and 17°C, respectively. Measurements The development and growth of the pani- cle and the morphological traits were evalu- ated every other day from the one-leaf stage to pollination. The developmental stage of in- florescence was determined from eight in- dividuals per treatment according to the let- ter scale (Table 1) introduced by Afors et al. (1988), which is a modified version of the developmental scale of Waddinoton et al. (1983). A certain developmental stage was reached when more than half of the individu- als (i.e., five from eight) had attained the stage in question. Simultaneously, macro-morpho- logical traits, such as the total number of leaves on the main stem, the number of green leaves on the main stem, the height of the up- permost ligule measured from the coleoptile node (cm), the height of the uppermost node measured from the coleoptile node (cm), and the length of inflorescence (mm), were also measured. When the oats pollinated, the number of fertile florets, separately from each different cluster of branches, was evaluated from ten randomly sampled individuals per treatment. 380 The fertility of the florets, i.e., the develop- ment of both carpel and stamens, was inves- tigated using a stereo microscope and a scal- pel. When ripened oat individuals from each treatment were gathered by hand and carefully analyzed separately. The number of secondary tillers was measured from 100 plants per treat- ment. In addition, the plant height (cm), num- ber of grains per panicle (empty hulls were ex- cluded), single grain size (mg), panicle weight (g), phytomass (g), and harvest index (%) were measured separately from the main stem and secondary tillers. Statistical analyses The significance of the differences in mor- phological traits and yield components be- tween the watering treatments was examined by a one-way analysis of variance using the computer program MSTAT 4.0. The relation between the developmental stage and the effective temperature sum for different water- ing treatments was studied using linear regres- sion analysis (MSTAT 4.0). Results Neither moderate water deficit nor severe drought stress affected the developmental rate of inflorescence in the cultivars examined when compared to the well watered group. Both “Puhti” and “Virma” pollinated simul- taneously (Fig. 1) when the effective temper- ature sum was 770°C (the base temperature being 5°C). As a consequence of moderate and severe water deficit, oats, however, ri- pened some 10 days earlier, thereby resulting in a considerably shortened grain-filling period. When severely drought stressed, half of the panicle remained in the leaf sheath in both cul- tivars. In addition, the more severe the water deficit was, the shorter the inflorescence (Fig. 2). The effect of water deficit on the length of inflorescence was not observed until the de- velopmental stages O-P. Table 1. Micro-morphological scale for cereals. Letter 1 Description l ' 2 code A Vegetative cupola stage B Transition apex C Early double ridge stage D Double ridge stage E Triple mound stage (not detected in wheat) F Glume primordium present G Lemma and floret primordium present H Stamen primordium present I Pistil primodium present K Carpel primodium present L Carpel surrounded by enlarged stamens M Stylar canal closing, ovarian cavity still open above N Stylar canal remaining as a narrow opening, two short style primordia present O Styles begin elongating, stillprojecting (not de- tected in wheat) P Styles strongly elongating Q Stigmatic branches differentiating as swollen cells on styles R Stigmatic branches elongating (not detected in wheat) S Unicellular hairs differentiatingon ovary walls, stigmatic branches still elongating T Hairs on ovary wall strongly elongating, stig- matic branches as well U Stigmatic branches form a tangles mass (not de- tected in oats) V Style and stigmatic branches erect, stigmatic hairs differentiating X Stigmatic hairs well-developed, branches spread- ing outward Y Styles curved outward and stigmatic branches spread wide, pollen grains on well-developed stigmatic hairs 1 according to Afors et al. (1988) 2 according to Waddinoton et al. (1983) Drought did not affect the total number of leaves on the main stem. When watered fre- quently, leaves unrolled earlier, but the max- imum number of leaves was, however, eight in all treatments of both cultivars. Water def- icit resulted in a reduced number of green leaves on the main stem already during the pre-anthesis development, but the effect of se- vere drought stress was not greater than that 381 of moderate drought stress. The difference be- tween well-watered and moderately/severely water stressed main stems in the number of green leaves was observed already at the four- leaf stage, when the inflorescence was at the triple mound stage (E). Drought stress retarded the lengthening of internodes. Differences between treatments in the height of the uppermost node were observ- able when carpel began to differentiate (stage K, Fig. 3). In addition, differences between treatments in the length of the uppermost lig- ule as measured from the coleoptile node were observable when “Virma” reached the de- velopmental stage G and “Puhti” stage K. The oat cultivars examined, “Puhti” and Fig. I. Effect of water deficit on apical development (1 = developmental stage A, 2 = 8,... 23 = Y). 382 “Virma”, differed significantly (P<0.01) from each other in yield potential. The num- ber of fertile florets per panicle of the culti- var “Puhti” was 72 when well-watered, while “Virma” produced 103 fertile florets. The number of fertile florets was 42 in “Puhti” and 50 in “Virma” under moderate drought stress, while under severe drought stress it was 36 and 42, respectively (Table 2). Despite the higher floret number of “Virma”, the rachis was some 2 cm shorter than in “Puhti”. Examining the most sensitive parts of the panicle for the effects of water deficit, it was found that “Puhti” produced an equal num- ber of florets on the four uppermost clusters of branches, while “Virma” produced the Fig. 2. Effect of water deficit on length of inflorescence (1 = developmental stage A, 2 = 8,... 23 = Y) 383 384 same floret number on the two uppermost the greater and more significant the reduction clusters of branches in all treatments. Below in the number of fertile florets as a conse- that, the lower the cluster of branches was, quence of moderate and severe water deficit Fig. 3. Effect of water deficit on lengthening of internodes (1 = developmental stage A, 2 = 8,... 23 = Y) (Table 2). When well-watered, about one third of the fertile florets aborted in both cultivars (Table 3). “Virma”, a cultivar with clearly higher yield potential, suffered more than “Puhti” from water deficit; under moderate drought stress 80 % of the fertile florets of “Virma” wilted, and under severe drought stress even 90 %. In “Puhti”, the proportion of aborted florets was 57 % and 89 %, respec- tively. The oat stand was more abundant when watered frequently. When well-watered before pollination and then drought stressed at the post-anthesis phase, the number of grains per panicle and the harvest index were not clear- ly reduced in “Puhti”, but single grain size, panicle weight, and phytomass decreased slightly (Table 4). Reductions of cultivar “Vir- ma” in all the yield components examined were stronger. The differences between treat- ments in the morphological characteristics ex- amined were highly significant. The ability of the oat cultivars to compen- sate for yield losses by the production of secondary tillers was limited. The number of secondary tillers in “Puhti” was two as a con- sequence of both moderate drought and se- vere water deficit, and two and three in “Vir- ma”, respectively (Table 4). When drought stressed at the pre-anthesis phase and fre- quently watered after pollination, the produc- tion of secondary tillers didnot increase. Both Table 2. Number of fertile florets in different clusters of branches in panicle. Cluster of branch 1 Puhti Virma well- moderate watered drought well- moderate watered drought severe severe drought drought NF ratio NF ratio NF ratio NF ratio NF ratio NF ratio 1 1 100 1 100 1 100 2 100 2 100 2 100 3 100 3 100 3 100 4 100 4 100 4 100 9 100 7 78 6 67 14 100 9 64 7 50 18 100 8 44 7 39 21 100 8 38 6 29 2 3 4 5 6 7 8 Total number of florets 72 100 42 58 36 50 ns 1 100 1 100 1 100 ns ns 2 100 2 100 2 100 ns ns 3 100 2 67 2 67 * ns 6 100 4 67 3 50 •• • 10 100 6 60 7 70 16 100 10 63 9 56 28 100 12 43 9 32 *** 37 100 13 35 9 24 103 100 50 49 42 41 1 the uppermost branch corresponds to number 1 NF = number of fertile florets ns = non-significant difference between treatments, * = ** = ••• = P