Eight cycles of selection for drought tolerance in lowland tropical maize. III. Responses in drought-adaptive physiological and morphological traits

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<ul><li><p>Field Crops Research, 31 (1993) 269-286 269 Elsevier Science Publishers B.V., Amsterdam </p><p>Eight cycles of selection for drought tolerance in lowland tropical maize. III. Responses in drought- </p><p>adaptive physiological and morphological traits </p><p>J. Bolafios, G.O. Edmeades and L. Martinez CIMMYT, Mexico D.F., Mexico </p><p>(Accepted 29 January 1992) </p><p>ABSTRACT </p><p>Bolafios, J., Edmeades, G.O. and Martinez, L.M., 1993. Eight cycles of selection for drought tolerance in tropical maize. III. Responses in drought-adaptive physiological and morphological traits. Field Crops Res., 31: 269-286. </p><p>Selection for grain yield under severe drought stress has often been considered inefficient because the estimate of heritability of grain yield has been observed to decline as yields fall. Under these conditions secondary traits may increase selection efficiency, provided they have adaptive value, high heritability, and are easy to measure. Increased relative stem and leaf elongation rate (RLE), delayed foliar senescence, reduced canopy temperatures and reduced anthesis-silking interval (ASI) were used to augment efficiency of selection for grain yield under drought during eight cycles of recurrent full- sib selection in the lowland tropical maize (Zea mays L.) population, 'Tuxpefio Sequla'. Six cultivars comprising Cycles 0, 2, 4, 6 and 8 of Tuxpefio Sequta, and a check cultivar were grown for two con- secutive years at Tlaltizap~tn, Mexico, under three moisture regimes that provided a well-watered control, a severe moisture stress during flowering, and a severe stress during grain-falling. Previous reports have documented significant improvements in grain yield and ASI in this population. When observed under drought, no significant differences were detected among cultivars in RLE or canopy- air temperature differentials, nor in chlorophyll per unit leaf area during grain-filling (an indicator of foliar senescence). Cultivars did not differ in seasonal predawn or diurnal courses of leaf water po- tential, in leaf osmotic potential, in capacity to adjust osmotically, nor in their seasonal profiles of soil water content with depth to 140 era. Selection significantly altered final plant height, total leaf num- ber, and tassel primary branch number by -0.9%, -0.5% and -2.6% cycle-~, respectively. Obser- vations on root growth in 2-m deep pots showed that eight cycles of selection had reduced root bio- mass in the upper 50 cm by 33%, consistent with a significant change of - 1.2% cycle -~ in vertical root-pulling resistance. The lack of direct and correlated changes in traits related to plant water status due to selection suggests that in this population heritabflities of such traits are low, or that the traits are only weakly associated with grain yield under severe moisture stress. The present study indicates that improved drought tolerance in Tuxpeiio Sequla was due to increased partitioning of biomass towards the developing ear during a severe drought stress that coincided with flowering, rather than to a change in plant water status. </p><p>Correspondence to: G.O. Edmeades, CIMMYT, Apdo. Postal 6-641, 06600 Mexico D.F., Mexico. </p><p>0378-4290/93/$05.00 1993 Elsevier Science Publishers B.V. All fights reserved. </p></li><li><p>270 J. BOLAI~IOS ET AL. </p><p>I N T R O D U C T I O N </p><p>Selection for grain yield under environmental stress, as compared to selec- tion under unstressed conditions, has often been considered less efficient be- cause of reductions in the estimate of heritability of grain yield as environ- mental variance rises and observed genetic variance falls (Rosielle and Hamblin, 1981; Blum 1988; Johnson and Geadelmann, 1989). The use of secondary traits can increase selection efficiency under these conditions, pro- vided traits have a clear adaptive value under stress, relatively high heritabil- ity, have a significant genetic correlation with grain yield, and are easy to measure (Falconer, 1960 ). Physiologists and ideotype breeders have long ad- vocated the judicious incorporation of secondary traits within breeding pro- grams (Donald, 1968; Edmeades et al., 1987; Blum, 1988; Ludlow and Mu- chow, 1990; Rasmusson, 1991 ). </p><p>Desirable secondary traits associated with improved performance under drought include those which allow plants to gain access to and absorb a greater volume of soil water, to reduce rates of water loss, and to maintain high phys- iological activity at low water potentials (Turner, 1986; Ludlow and Mu- chow, 1990). For example, the capacity to maintain leaf expansion and to retain green leaf area under drought stress is expected to increase light inter- ception and radiation-use efficiency, and result in increased productivity. Several reports have shown that the expansion of leaves is very sensitive to moisture deficits and responds rapidly to changes in leaf water status (Acev- edo et al., 197 I, 1979; Michelena and Boyer, 1982). In tropical maize culti- vars, Sobrado (1986) found a strong relationship between leaf expansion rate under drought and pre-dawn leaf turgor potential, elongation ceasing at tur- gor potentials of less than - 0.2 MPa. Water stress accelerates the senescence of lower leaves in maize, and cultivars with increased capacity for osmotic adjustment have been shown to have delayed leaf senescence under drought (Wright et al., 1983; Hsiao et al., 1984). Other studies comparing the re- sponse of different maize cultivars to drought have shown that those able to maintain higher leaf water potential (leaf ~u) under drought also produced more biomass and grain yield (Ackerson, 1983; Lorens et al., 1987). These reports suggest that selection for an increased rate of leaf and stem extension (RLE) and delayed senescence under drought would improve crop water sta- tus via increased rooting volume and increased capacity for osmotic regulation. </p><p>Transpiration consumes energy and cools the leaf, so under conditions of similar energy influx, differences in canopy temperature (CT) reflect varia- tion in rates of transpiration (Jackson, 1982; O'Toole et al., 1984; Blum, 1988 ). Transpiration rate may itself be an indirect measure of the volume of plant-available soil moisture. Portable hand-held infrared thermometers have been used to measure CT and thus provide an indirect field measure of de- hydration avoidance and plant water status in several crop breeding programs (Jackson, 1982; Blum, 1988 ). Selection for reduced CT under drought there- </p></li><li><p>SELECTION FOR DROUGHT TOLERANCE IN LOWLAND TROPICAL MAIZE III 271 </p><p>fore could be expected to result in an increase in root length density and root- ing depth. </p><p>Recurrent selection for improved performance under drought in the tropi= cal maize population 'Tuxpefio Sequia' was begun in 1975. The superior frac= tion of the population was identified by a selection index comprising in part increased grain yield and a reduced anthesis=silking interval (ASI) under drought. Other traits included in the index were increased RLE, reduced CT and delayed foliar senescence, observed under drought stress. Correlations between grain yield under drought and these secondary traits were usually significant among full=sib families in the early cycles of selection (Fischer et al., 1989), suggesting that they had a positive adaptive value under drought. A comparison of experimental varieties selected from within Cycle 0 for an index of these traits plus grain yield, or for grain yield alone, confirmed the value of the multi=trait selection procedure. Evaluation of the first three selec- tion cycles under severe stress showed that RLE, retention of green leaves and CT had significantly improved with selection, suggesting that these traits have a relatively high heritability (Fischer et al., 1989). </p><p>Previous papers in this series described direct and correlated responses to eight cycles of recurrent selection for improved performance under drought in this same population (Bola~os and Edmeades, 1993a,b). Selection re= suited in significant increases in grain yield in droughted environments vary- ing in yield potential from 0.3 to 8 Mg ha- 1. Drought was generally timed to coincide with flowering and grain-filling, and yield gains were associated with improved partitioning toward the developing ear at flowering. This in turn resulted in a marked reduction in ASI under drought and an increase in har- vest index over a range of moisture stress conditions. It seemed likely that reduced ASI was partly the consequence of higher plant ~ during flowering, as silk extrusion is extremely sensitive to plant water deficits (Westgate and Boyer, 1986). Although selection did not significantly affect total biomass production, it resulted in a statistically significant but quantitatively unim- portant improvement in intercepted radiation use efficiency under moisture stress (Bola~os and Edmeades, 1993a,b), perhaps due to delayed foliar senescence. </p><p>The objective of this aspect of the study was to evaluate the direct and cor- related responses in drought-adaptive physiological and morphological traits to eight cycles of recurrent selection for drought tolerance in Tuxpefio Sequia. The traits examined relate mainly to plant water status and the maintenance of canopy function, but correlated morphological responses to selection are also reported. </p><p>M A T E R I A L S A N D M E T H O D S </p><p>Selection procedures for improving drought tolerance of Tuxpefio Sequia have been described in detail elsewhere (Edmeades et al., 1987; Fischer et al., </p></li><li><p>272 J. BOLAI~OS ET AL. </p><p>1989; Bolafios and Edmeades, 1993a). In each cycle 30-40% of the full-sib families were selected using an index which aimed to: maintain maturity and grain yield constant under well-watered conditions; increase grain yield, RLE and green leaf longevity under drought; and decrease ASI and CT under drought (Fischer et al., 1989). </p><p>Prior to evaluation, fresh seed of bulks of Tuxpefio Sequia Cycle 0, 2, 4, 6 and 8 (Co, C2, C4, C6 and C8), and the check, Pop. 21 C6, were prepared (Bolafios and Edmeades, 1993a). These six cultivars were then evaluated in four replications of a randomized complete block design under three mois- ture regimes during the rain-free winter seasons (November-April) of 1986/ 87 ('87 season) and 1987/88 ('88 season) on the CIMMYT Experiment Sta- tion at TlaltizapAn, Mexico. Site characteristics, crop husbandry, experimen- tal design, and data analysis procedures have been described previously (Bo- lafios and Edmeades, 1993a,b). The three water regimes used during evaluation were: (a) well-watered (WW), with water applied approximately every 10 days; (b) intermediate stress (IS) or stress during grain-filling, where irrigation was permanently suspended 65-75 days after planting (DAP); and (c) severe stress (SS), obtained by irrigating only until 44-53 DAP, and al- lowing the crop to complete its life cycle on stored soil water. </p><p>Measurements of secondary traits in field experiments. All measurements were taken on well-bordered plants. Leaf and stem elongation rate was measured over two sequential 7-day periods in the SS and WW treatments during both seasons, beginning 3 weeks prior to flowering. The youngest visible leaf in the whorl on eight plants per plot was marked by cutting 5 cm offthe leaf tip. The height from the ground to the cut tip of the vertically extended leaf was mea- sured, and one week later the measurement repeated on the same leaf. A sec- ond set of measurements was initiated immediately using the current young- est visible leaf in the whorl. The absolute increment in height under drought was divided by the corresponding increment under well-watered conditions for the same entry and replicate to give the relative leaf and stem extension rate (RLE). Analysis showed no significant interactions among seasons, mea- surement periods or cultivars, so data for RLE were averaged across measure- ment periods and years. </p><p>Canopy-to-air temperature differentials (CT d) were determined by taking 10 readings in the center of each plot with a hand-held infrared thermometer (Model AG-42, Telatemp Corp., Fullerton, CA) having a field of view of 2.5 Measurements were performed at 81, 97 and 108 DAP in '87, and at 76 and 92 DAP in '88 (i.e., when radiation interception &gt; 80%), between 12:00- 15:00 h on hot, clear, windless days. The instrument was pointed away from the sun at an angle of depression of 20and at an angle of incidence to the row of 20 . The field of view included only fully exposed foliage, avoiding tassels when they were emerged. We attempted to remove time trends in the energy </p></li><li><p>SELECTION FOR DROUGHT TOLERANCE IN LOWLAND TROPICAL MA1ZE Ill 273 </p><p>balance of the crop during the measurement period by averaging results from two observers, each of whom proceeded in opposite directions through the plots. Data were averaged across sampling dates and years for stressed (SS, IS) and WW environments. </p><p>Differences in leaf senescence were quantified by measuring lamina chlo- rophyll concentrations (#g c m - 2 ) near the center of 12 ear leaves per plot at three sampling times during mid- to late grain-filling. A portable chlorophyll photometer (Design Electronics, Palmerston North, New Zealand) de- scribed by Hardacre et al. (1984) was used. It was calibrated using leaves with a wide range of chlorophyll concentrations (20-70 gg c m - 2 ) , and instru- ment readings were regressed on values obtained from the same leaves by chemical extraction (Arnon, 1949). No cultivarmoisture environment or cultivar sampling date interactions were observed, so data were averaged across moisture environments and sample dates. </p><p>Vertical root-pulling resistance, the maximum force required to pull a plant vertically from the soil (Beck et al., 1987), was measured at 50% anthesis on six plants per plot in each of the water regimes. Each measured plant was separated by at least one undisturbed plant. Measurements were made with a mechanical root puller connected to the lower stalk by a chain and a 200-kg capacity torsion balance. Values were averaged across moisture regimes. </p><p>Plant height (here defined as the distance from the ground to the ligule of the uppermost leaf) was measured after anthesis on 10 plants per plot in the WW treatment. During vegetative growth the fifth and tenth leaves of 10 plants per plot were identified by cutting off the leaf tip. Final leaf number (FLN) was recorded on these plants 3 weeks after flowering, and the number of pri- mary tassel branches (TBN) of l 0 additional plants per plot was determined at the same time. Cultivar moisture environment interactions for TBN or FLN were non-significant, so data for each were averaged across environments. </p><p>Pre-dawn (05:00-07:00 h ) leaf ~ of Co, C4, and C8 under SS and WW treat- ments was measured several times 7-14 days apart, beginning at 80 DAP in '87 and at 56 DAP in '88. On four successive days ~was measured at 2-hourly intervals from 06:00-19:00 h to describe diurnal trends. In each plot four you...</p></li></ul>

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