K. S. Fischer

Growth and yield of sorghum lines extracted from a population for differences in osmotic adjustment

From 47 S2 lines which had been extracted from a random mated population of sorghum, eight lines for a glasshouse experiment and four lines for a field experiment were divergently selected for variation in osmotic adjustment, and were grouped into two, High and Low osmotic adjustment (OA). Both the glasshouse and field experiments examined whether osmotic adjustment modified the plants’ response to soil water deficit and also whether grain sink demand for assimilates, varied by removal of 50% spikelets, affected osmotic adjustment. In each experiment, there were well-watered control and water stress treatments. In both experiments, the dawn osmotic potential in the High OA group was always lower than in the Low OA group under water limiting conditions, and the difference was significant after anthesis. The difference in osmotic potential was about 0.1 MPa in the field and up to 0.25 MPa in the glasshouse. In the glasshouse experiment, removal of 50% spikelets at anthesis significantly decreased osmotic potential during grain filling, suggesting that osmotic adjustment is influenced by the availability of assimilates in the leaves. Under well-watered conditions, the two groups behaved very similarly in terms of maximum leaf area, green leaf area retention during grain filling, total dry matter production, grain yield and grain number in both experiments. Under water-limiting conditions, the High OA group produced larger maximum leaf area and had better leaf retention during grain filling. Despite similar water use, total dry matter was also significantly higher in the High OA group though the difference was small. Grain number was also greater in this group in both experiments, whereas grain yield was significantly higher in the High OA group in the field, but not in the glasshouse where severe water stress developed more rapidly. It is concluded that the adverse effect of water stress can be reduced by adopting sorghum genotypes with high osmotic adjustment. However, selection for high osmotic adjustment needs to ensure that osmotic adjustment is not solely due to small head size.

Field Phenotyping Strategies and Breeding for Adaptation of Rice to Drought

This paper is a section of the book “Drought phenotyping in crops: from theory to practice” (Monneveux Philippe and Ribaut Jean-Marcel eds, published by CGIAR Generation Challenge Programme. Texcoco, Mexico). The section describes recent experience in drought phenotyping in rice which is one of the most drought-susceptible crops. The section contains genetic and genomic resources for drought adaptation and methods for selection of drought-resistant varieties in rice. In appendix, there is experience from Thailand on integration of direct selection for grain yield and physiological traits to confer drought resistance.

Effect of drought during early reproductive development on growth of cultivars of groundnut (Arachis hypogaea L.). I. Utilization of radiation and water during drought

Abstract Cultivars of groundnut ( Arachis hypogaea L.) were subjected to a period of reduced soil water supply during early reproductive development. During the period of water deficit, total biomass production of two Virginia type cultivars (Virginia Bunch and Q18801) was greater than that of a Spanish type cultivar (McCubbin), even though all cultivars used similar amounts of radiation and water. Hence, the radiation use efficiency and transpiration efficiency of Q18801 were significantly greater than those of McCubbin. The radiation use efficiency of the stressed crops was only about 45% of those that were fully irrigated. Throughout the period of water deficit, noon leaf water potential was lowest in McCubbin in both treatments. In the well-watered treatment, this was associated with a higher leaf temperature and lower leaf conductance, than in the Virginia cultivars. Under increasing soil water deficit, the leaves of McCubbin tended to wilt, while the Virginia cultivars displayed active leaf folding. Thus, the ratio of the fraction of radiation intercepted by the canopy to leaf area index (LAI) was always lower in the Virginia type cultivars. For a given LAI, this phenomenon may have allowed these cultivars to decrease the effective atmospheric demand within the canopy, while maintaining radiation interception at saturation for photosynthesis. The consequence of this, given that the supply of water from the roots did not differ, was that Q18801 was able to maintain a higher LAI and a greater crop transpiration efficiency (ratio of biomass production to transpiration) than McCubbin. The existence of differences among cultivars in transpiration efficiency under drought may prove useful in improving adaptation of groundnut to these environments.

Genotypic variation in osmotic adjustment in grain sorghum. II.* Relation with some growth attributes

Two sets containing large numbers (23 and 47 entries) of sorghum genotypes were grown in the glasshouse to examine the effect of osmotic adjustment on water extraction, dry matter growth and grain yield. Water stress was developed in two periods, one before and one after anthesis. The results were similar in the two experiments despite a large difference in the genetic background of the plant material. Since osmotic potential did not differ significantly among genotypes before water stress was induced, osmotic potential obtained under stress was used directly to indicate the genotype’s ability to adjust osmotically. Osmotic adjustment was positively associated with green leaf area retention during grain filling and to root length density at 70 cm depth. Genotypes with high osmotic adjustment used more water during the second drying period. As a result, total dry matter was well related to osmotic adjustment during grain filling, but grain yield was negatively associated with osmotic adjustment in one experiment and not significantly related in the other. When comparison was made for lines which had similar leaf water potential during early stages of growth but which differed in osmotic adjustment during grain filling, there was still a positive effect of osmotic adjustment on total dry matter. This suggests that the positive effect was not caused by large plants extracting more water during early stages of growth, but was due to the difference in line’s ability to extract water during grain filling.

Effect of drought during early reproductive development on growth of cultivars of groundnut (Arachis hypogaea L.). II. Biomass production, pod development and yield

Abstract Cultivars of groundnut ( Arachis hypogaea L.) subjected to a period of reduced water supply during early reproductive development differed in growth responses during and following the period of water deficit. A Virginia type cultivar with a high harvest index (HI) under non water-limiting conditions, Q18801, yielded higher than Virginia Bunch (another Virginia type) and McCubbin (a Spanish type). During the period of water deficit, all cultivars produced a similar number of pegs and pods, but greater proportions of these were converted to pods in Q18801 and McCubbin than in Virginia Bunch. In all cultivars, water deficit delayed the start of the period of rapid pod growth by about 15 days and hence extended the time required to reach maturity. After rewatering, the number of pegs and pods and the leaf area index of Virginia Bunch and McCubbin increased rapidly. In contrast, Q18801 partitioned more assimilate to pods, achieving a higher average growth rate of individual pods, and consequently a higher total yield of pods and seed. While only three cultivars were examined, the implication of this result is that selecting cultivars with increased HI (via rapid pod growth at the expense of excess canopy growth) under irrigated conditions may also increase yields following a drought during early reproductive development.

Control of Grain Yield by Sink Capacity and Assimilate Supply in Various Rice (Oryza sativa) cultivars

The objective of this study was to identify whether grain yield in four contrasting rice cultivars is limited by supply of assimilate to fill the grains or by sink capacity to accept the assimilate. Grain yield was limited mostly by sink capacity, with little variation in single grain weight among cultivars, but an old cultivar showed some ability to adjust single grain weight. Sink capacity was very sensitive to variation in assimilate supply immediately after anthesis. Reduction in assimilate supply in the anthesis to early grain filling period reduced filled grain percentage and grain yield, particularly in high yielding cultivars with a large number of grains per panicle.

Effect of drought during pod filling on utilization of water and on growth of cultivars of groundnut (Arachis hypogaea L.)

Abstract Cultivars of groundnut (Arachis hypogaea L.) were grown well watered or well watered except for a period during pod filling when water was withheld. When water was withheld from 84 days after sowing (DAS) to maturity, pod and seed yield was reduced by 30% in the Virginia type cultivars (Virginia Bunch and Robut-33) and by 45% in a Spanish type cultivar, McCubbin. The Virginia type cultivars also extracted water from a greater depth. Robut-33, a cultivar with a high harvest index (HI), produced the greatest yield under both well-watered and water-stressed conditions. In all cultivars, potential pod number (PN) had been almost achieved prior to the start of the period of water deficit. However, low pod number (apparently through abscission of the youngest pods), rather than small pod size, was mainly responsible for the decrease in yield. Part of the yield advantage of Robut-33 lay in initiating a large number of pods prior to the period of water deficit. This greater synchrony of development compared with the other Virginia type cultivar (Virginia Bunch) created a greater sink for assimilate prior to the period of water deficit. During the first 3 weeks of the period of water deficit, Robut-33 also had the highest crop growth rate (CGR) and was thus able to produce the most pod biomass. The characteristics of early and rapid pod growth and high HI were more important in determining yield under this water deficit than was the amount of water extracted from the soil.

Genotypic Variation in Osmotic Adjustment in Grain Sorghum. I. Development of Variation in Osmotic Adjustment under Water-limited Conditions

Development of genotypic variation in osmotic adjustment was examined in two glasshouse experiments using two sets of sorghum material. In the first experiment, 47 S2 lines extracted from a randomly mated population were used, whereas in the other, inbred parents and their 15 hybrids were compared. In both experiments, water deficit was induced in two periods, one before anthesis and the other after anthesis for most genotypes. In both experiments osmotic potential at the beginning of the first drying period was similar among genotypes and therefore osmotic potential obtained under water deficit was used for the comparison of osmotic adjustment among genotypes. In the first drying period of both experiments, when stress was milder, about 40% of the variation in osmotic adjustment was accounted for by difference in leaf water potential. When the effect of water potential was removed by covariance analysis, there was significant genotypic variation in osmotic adjustment in the second experiment, but not in the first experiment. On the other hand, in the second drying period, when stress was more severe, the effect of leaf water potential on osmotic adjustment was small. There was significant genotypic variation in osmotic adjustment in both experiments after the water potential effect was removed by covariance analysis. Osmotic adjustment in the second drying period was also negatively correlated with grain sink/source ratio (number of grains/leaf area) in the first set of materials. The comparison of osmotic adjustment among hybrids and their parents showed that, in this particular set of genotypes, the female parents were more important than the male in determining osmotic adjustment of the hybrids. The genotypic variation was associated with performance under water deficit in the field. It is concluded that there is considerable genotypic variation in osmotic adjustment in the genetic material examined. Osmotic adjustment is, however, correlated with water potential and grain sink/source balance, and hence the selection for osmotic adjustment needs to ensure that high value is not due simply to low water potential or small head size.