M.M. Ludlow

Expression of osmotic adjustment and dehydration tolerance in diverse rice lines

Abstract The magnitude of genotypic variation in osmotic adjustment and lethal osmotic potential (dehydration tolerance) was determined for 61 lines of Oryza sativa and eight accessions of Oryza spp. grown in a controlled environment under a slowly developing stress. The lines were adapted to a wide range of hydrological conditions, and were of both Japonica and Indica type. Genotypic variation in lethal osmotic potential (−4 to −10 MPa) and in osmotic adjustment (0.4 to 1.5 MPa) was greater than has previously been demonstrated in rice. In general, lines with Japonica background had poor dehydration tolerance and low osmotic adjustment, while Indica lines had greater dehydration tolerance and greater osmotic adjustment. Range in osmotic adjustment and dehydration tolerance for the eight Oryza spp. studied, was within that found in Oryza sativa so it appears that these species are not a source of further genetic variation. We conclude that there is good potential for increasing dehydration tolerance and osmotic adjustment of current rice cultivars. However, breeders will need to be mindful of the best combination of traits and the appropriate response strategy for the target environment.

Water extraction by grain sorghum in a sub-humid environment. I. Analysis of the water extraction pattern

Abstract Under water-limiting conditions, water extraction by a dryland crop is limited by the depth of the root system, and by the rate and degree of water extraction. The water extraction pattern of 6 crops of grain sorghum under continous soil drying in a sub-humid sub-tropical environment was analysed in terms of two components: the rate of descent of the extraction front down the soil profile (extraction front velocity), and the time required to extract 90% of the extractable water from each depth after the extraction front arrived (1/kl90). Extractable water content (θa), at each depth, was defined as the difference between the stable water content (θ) at the start of extraction and the lower asymptote of the exponential decay curve of θ versus time (lower limit). The crops varied in genotype, level of evaporative demand, degree of tillering, and plant population density, and were grown on the same soil type over two seasons. The aim of the study was to test the stability of the extraction front velocity, θa and of 1/kl90 under different agronomic and environmental conditions, to assess their usefulness for modelling water extraction of sorghum. Extraction front velocity varied little among the 6 crops with an overall average of 3.43 cm day−1. The extraction front of crops that were grown under long, terminal drying cycles continued to descend until early grain-filling, reaching a maximum depth of 190 cm. The value of 1/kl90 in the upper 100 cm of the profile varied considerably across crops. It is shown that this variation can be explained by variation in root length density and level of evaporative demand. For crops exposed to a long, terminal drying cycle, the actual water extracted below 150 cm depth was less than θa in the soil layers above 150 cm. This was due to both a lower θa below 150 cm, associated with low root length density, and also insufficient time between the arrival of the extraction front and maturity, for the crop to extract all the water above the lower limit.

Water extraction by grain sorghum in a sub-humid environment. II. Extraction in relation to root growth

Abstract An experiment was conducted under continuous drying in the field to examine the relationship between soil water extraction and root growth of grain sorghum. Extraction was analysed in terms of two components: (1) the time when the extraction front reached a depth, defined as the moment when soil water content (θ) begins to decline exponentially with time and, (2) the decline of θ with time at each depth after the extraction front arrives. A second aim was to test if severe assimilate limitation, caused by shading the crop during late vegetative growth, would influence the rate of root front penetration and root length accumulation. The root front penetrated from sowing at a constant downward rate of 2.72 cm day−1. Down to 100 cm depth, the extraction front lagged behind the root front; thereafter the extraction and root fronts descended together, to reach the same maximum depth of 190 cm shortly after anthesis. Total root length also reached a maximum at that time. Shading had no effect on the rate of root front penetration and length accumulation, despite reducing above-ground growth by 70%. Once the root front arrived at a given depth, root proliferation continued until about 40-20% of the extractable water in the layer remained. As a consequence of the roots proliferating ahead of the extraction front, a small amount of water was taken up before the extraction front arrived and the extraction rate at each depth reached a maxium when half of the extractable water remained. As a consequence, the decline in θ was better described by a sigmoidal, than an exponential curve, particularly in the middle layers of the profile. The applicability of the sigmoidal curve to describe the decline in θ with time is limited, however, to the special situation where root length increases during water extraction.

Variation among accessions of pigeonpea (Cajanus cajan) in osmotic adjustment and dehydration tolerance of leaves

Abstract Pigeonpea accessions were screened for variation in osmotic adjustment and dehydration tolerance of leaves. Twenty-two accessions tested in the growth chamber exhibited substantial osmotic adjustment at a leaf water potential of −5 MPa compared with the maximum values reported for other crops. However, no difference in osmotic adjustment was found between two accessions in the field, but moderate variation (0.7-1.3 MPa) was found among twenty-two accessions grown under controlled conditions. In addition, moderate variation in dehydration tolerance was found among the accessions; lethal leaf water potentials ranged from −6.8 to −8.2 MPa, although the level of tolerance was high compared with other grain legumes. In view of the general genetic diversity of the accessions, it was concluded that the probability of finding greater variation in these traits is small. Moreover, because pigeonpea has high osmotic adjustment and high dehydration tolerance compared with other crops, we would not give a high priority to attemptong improvement of drought resistance by increasing the magnitude of these two traits.

Modelling root growth of grain sorghum using the CERES approach

Abstract A simple model is described, based on the approach used in the CERES crop growth models, which simulates the depth of rooting and root length density in each soil layer for grain sorghum growing under soil drying. The model has five main components: (1) daily accumulation of root length is proportional to above-ground biomass growth, (2) the root front descends at a constant rate from sowing until early grain-filling, (3) daily accumulation of root length in water non-limiting conditions is partitioned among the occupied soil layers in an exponential pattern with depth, (4) proliferation of root length is restricted in any layer if the extractable soil water in that layer declines below a threshold, and (5) a fixed proportion of existing root length is lost due to senescence each day. The parameter values for the relationships were derived from analysis of measured depth distributions of root length from crops of grain sorghum grown in the sub-humid subtropics of Australia, on oxisol and vertisol soil types. The soils had no physical or chemical restrictions to root growth. The model was validated using four independent data sets. Overall, the model simulated the root distribution with depth well, but predictions of accumulated root length were less reliable. One of the most sensitive parameters affecting the modelled distribution of root length with depth was the factor used to partition daily accumulation of root length among the occupied layers, and the value of this parameter varied between well-watered and water-limited crops. The study shows that it is possible to model root growth of field crops using only five simple relationships, with inputs that are already used in most crop growth models.

Genotypic variation of osmotic adjustment and desiccation tolerance in contrasting sorghum inbred lines

Abstract Variation in maximum osmotic adjustment and desiccation tolerance were determined before anthesis for 21 lines of Sorghum bicolor (L.) Moench, selected on the basis of their capacity for osmotic adjustment and for their putative drought resistance. A reproducible and controlled imposition of gradual water stress was achieved by withholding water from plants grown in a controlled environment with a constant evaporative demand. Even though the rate of imposition of stress was lower in the second of two experiments, the maximum level of osmotic adjustment expressed by the lines was fairly consistent across both. The highest osmotic adjustment was 1.71±0.06 MPa for TAM422 in Exp. 2, while the lowest was 0.78±0.09 MPa for QL27 in Exp. 1. The difference in maximum osmotic adjustment between the highest and the lowest lines was 0.75 and 0.87 MPa, respectively, for Exp. 1 and 2. There was also variation in desiccation tolerance among the 20 lines in Exp. 2; 58% to 68% for lethal relative water content and −3.1 to − 3.9 MPa for lethal leaf water potential. Mean values of lethal realtive water content and lethal leaf water potential were, respectively, 62% and −3.4 MPa. Maximum osmotic adjustment was inversely related to desiccation tolerance; lethal relative water content and lethal leaf water potential increased linearly as maximum osmotic adjustment increased. Thus lines with high osmotic adjustment died at a higher relative water content and lower leaf water potential, than those with low osmotic adjustment. Despite their reduced tolerance of desiccation, lines with high osmotic adjustment survived 10 days longer. In both experiments, a high level of repeatability of line mean discrimination was identified for osmotic adjustment (0.75±0.15 and 0.96±0.09, for Exps. 1 and 2, respectively). The repeatability on a line mean basis across experiments was also high (0.84±0.07), as was the genetic correlation between the line means in both experiments (0.86±0.08). The high levels of repeatability suggest that the screening procedure developed in this study in robust, and that it could be used to evaluate the inheritance of osmotic adjustment in breeding populations.

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.

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.

Influence of rate of development of water deficit on the expression of maximum osmotic adjustment and desiccation tolerance in three grain sorghum lines

Abstract An experiment was conducted to quantify the effect of rate of development of water deficit (Rψ) on the expression of maximum osmotic adjustment (OA) and desiccation tolerance (DT) in three grain sorghum lines, TAM422, Tx2813 and QL27. DT was determined as the lethal relative water content (RWCL) and lethal leaf water potential (ψL) of leaves, i.e. the value of these traits just prior to tissue death. Five rates of development of water deficit were generated by using different soil volumes in which the decrease in leaf water potential ranged from 0.093 MPa day−1 to 0.153 MPa day−1. The expression of maximum OA increased markedly with a reduced Rψ, for all three lines. In contrast, DT decreased only slightly with a reduced Rψ. The three lines differed for expression of maximum OA and DT. Both TAM422 and Tx2813 had higher maximum OA, higher RWCL and lower ψL than QL27. There was a significant line-by-Rψ interaction for the expression of OA and DT. The interaction was much smaller than the effect of lines for the expression of maximum OA. Therefore, the absolute level of OA for the three sorghum lines and the magnitude of the difference between them changed with the Rψ without affecting their ranking. As with maximum OA, the interaction for RWCL was smaller than the line effect. The RWCL of TAM422 and QL27 was stable across rates of development of water deficit. The results indicated that low Rψ increased the expression of OA and decreased ψL but had little effect on RWCL. While line-by-soil-volume interactions were observed, these were largely explained by the increase in magnitude of the variation for the traits as Rψ decreased and were not the result of changes in rank of the lines among soil volumes. Therefore, the current screening strategy is considered to be appropriate for examining genetic variation for OA and DT in grain sorghum. Opportunities exist for using smaller pots and soil volumes where large numbers of genotypes are examined.

Effect of drought during early reproductive development on the dynamics of yield development of cultivars of groundnut (Arachis hypogaea L.)

Abstract In a glasshouse experiment, cultivars of groundnut ( Arachis hypogaea L.) were grown well watered throughout or were grown well watered except when water was withheld for a period of 3 weeks during either of two periods during early reproductive development. When water was withheld from 46 to 67 days after sowing (DAS), Robut-33, the cultivar with the highest harvest index (HI), produced a greater yield than either Virginia Bunch or McCubbin. This was attributed to the maintenance of a relatively higher number of pods, compared to the other cultivars. When water was withheld from 61–78 DAS, the Virginia type cultivars (Virginia Bunch and Robut-33) produced a greater yield than the Spanish type cultivar (McCubbin). Frequent observations were made of the appearance and elongation of pegs in all treatments. Peg initiation was sensitive even to mild water deficit, but elongation of pegs halted by water deficit could continue after rewatering. This may be an important attribute particularly where intermittent drought occurs. In both water-deficit treatments, peg initiation and elongation in all cultivars halted after about 80% of the extractable soil water had been exhausted. The yield advantage of Robut-33 was mainly in producing a large number of pods prior to water deficit, and in partioning a greater amount of biomass to pods after rewatering. The Virginia type cultivars were also apparently better able to tolerate the effects of severe water deficit. While only three cultivars were examined in this glasshouse experiment, the performance of Robut-33 points to the usefulness of highly synchronous development (generally a Spanish type characteristic) and high HI as well as an ability to tolerate drought in groundnut cultivars exposed to water deficits during early reproductive development.