Heat and frost impact on lentil and how remote sensing can benefit growers

Take home messages

• Knowledge on the impact of frost on yield and quality of lentil, depending on its timing during the flowering and grain filling phase, allows growers to limit production risk through sowing time and cultivar choice.
• For lentil crops surveyed within the Victorian Mallee in 2018, varieties with group B tolerance (e.g. PBA HurricaneXT) were not linked with increased frost susceptibility.
• Linking remote sensing tools to frost damage in pulse crops provides new opportunities for spatially zoning paddocks for informing management decisions, such as maximising grain quality.

Background

Pulse production in southern Australia is limited by the occurrence of heat waves and frost during the growing season. The major pulse crops in the southern region are lentil, chickpea, faba bean and field pea where the suitability of these crops varies across agroecological zones, for example significant production of lentil in the Wimmera, and faba bean in the high rainfall zone. Typically, for lentil, temperatures that exceed 30°C during the flowering and pod filling phase cause yield losses, where the effects are amplified under dry (low rainfall and stored soil water) and windy conditions. During a frost event, temperatures of 0°C or below at the crop canopy also translate to yield loss and grain quality penalties in pulse crops. For both high temperature and frost, the growth stage of the crop, duration and intensity of exposure are critical in defining the extent of damage, where cumulative time (load) above or below a critical threshold are used to classify expected yield loss.

The principles applied for lentil in this paper broadly apply to other pulse crops in terms of risk management and susceptibility to extreme events.

Definition: cold and heat load

To account for the varying severity in frost (temperature × duration) imposed on lentil, we calculated the cold load as the sum of degrees Celsius (°C) below 0°C, with time (°C.hr) for temperature measured at the canopy. The same approach is used for heat when temperature exceeds 30°C.

Key Question 1 - What affect does high temperature and frost have on lentil?

Pulse crops, including lentil, are most sensitive to frost and high temperature during the reproductive phase which extends from first flowering to filled pod.

Crop damage of lentil caused by frost and heat at different growth stages, looks similar:

• Vegetative – premature death of leaves and tendrils.
• Flowering – dropping of flowers and buds.
• Flat pod – dropping of pods, generally translates to yield losses (reduced grain number) and formation of underdeveloped dark grain.
• Filled pod – deterioration in grain quality, generally shrivelling of grain and some darkening of seed coat.

If a frost (<0°C at canopy) or heat wave (>30°C) is forecast, knowing the growth stage will provide insight into the damage you can expect and importantly inform, if your crop may recover. The indeterminant nature of pulse crops provides a partial recovery mechanism to maintain yield potential, by continuing to set pods following extreme events, although the extent of recovery will be influenced by timing (the earlier in the season the greater recovery) and water availability.

Lentil response to heat

As a rule of thumb, for every degree-hour (>30°C) during the podding stage, there is a 0.13% reduction in yield. Further research is required to determine how this rate varies for different growth stages.

Lentil response to frost

A relationship between lentil yield response and frost (cold load) was defined using field data during 2017 at Horsham, Victoria, where no natural frost was recorded during the reproductive window. At flowering, damage occurs when a threshold of 31°C.hr (<0°C) is reached, here after yield decline was 3.8% per °C.hr (<0°C) (Figure 1). At pod filling, for every degree below zero there was a 2% reduction in grain yield indicating that this is the most sensitive phase for lentil to frost. The difference in response to frost at flowering and pod filling indicates that timing, intensity and duration has an effect the extent to which lentil will recover from frost.

Figure 1. Relationship between lentil (cv. Jumbo 2) grain yield and cold load (°C.hr <0°C) associated with frost treatments applied at five different growth stages; late vegetative, flowering, early pod, flat pod and filling pod in 2017. CC, chamber control; OC, open control data set (black) is for data < 70 °C.hr <0°C). Segmented regression model is for CC and flowering data and linear models are for CC, OC, and pod filling stages (black solid line) (Delahunty et al. 2019a).

Key question 2 - Does the lentil variety influence the response of lentil to heat/frost?

High temperature

Genetic variation to high temperatures exists for lentil across commercial cultivars and landraces. This variation in plant response was demonstrated through a field trial which screened a combination of landraces and commercial varieties for yield stability under high temperature in the Wimmera, Victoria, 2014. For the commercial varieties tested, cv. PBA BoltA was the most stable variety, but had lower absolute yield potential, whereas cv. NipperA, a small-seeded variety, had both moderate yield stability and high yielding potential (Figure 2). Importantly this research indicates that further opportunity to increase the tolerance of lentil to high temperature exists through utilising high temperature tolerant landraces within current pulse breeding programs (Figure 2).

Figure 2. Screening of lentil varieties and landraces for high temperature tolerance. The relationship between the absolute yield of ‘control’ plants (protected from sun/ high temperature) and yield stability index (yield of plants grown under high temperature compared to the control). Commercial and breeding lines are highlighted in orange (Delahunty et al. 2020, unpublished).

Frost

For current commercial lentil varieties, there is little difference in capacity to tolerate frost. It is likely that genetic variation does exist across landraces, where a range in tolerance has been observed in other pulses including field peas (Davies & Pham, 2017).

Following the release of PBA HurricaneXT, there was concern that varieties with Group B herbicide tolerance were more susceptible to frost compared with conventional lentil varieties. In 2018, a field experiment at Ouyen, Victoria assessed lentil yield for natural and artificial frost effects which occurred during the late vegetative and late podding stage, across four imi-tolerant lentil varieties (PBA Herald, PBA HurricaneXT, PBA HallmarkXT, CIPAL1721) and two conventional varieties (PBA Jumbo 2 and PBA Flash) (Delahunty et al. 2019b).

Figure 3. Comparison of grain yield of conventional (con) to imi-tolerant (imi) varieties tested in 2018 field trial at Ouyen. Frost treatments are natural frost, applied at vegetative and natural frost, applied at reproductive and natural frost.

For the natural frost conditions which occurred in 2018, all varieties were equally susceptible to frost, indicating that imi-tolerance was not linked to increased frost sensitivity under these conditions (Figure 3). Furthermore, under applied frost conditions (severe), conventional and imi-tolerant lines were equally affected by frost during the late vegetative and reproductive period (Figure 3). This result suggests that the increased visual symptoms of frost damage in imi-tolerant lines (e.g. PBA HurricaneXT), did not translate to greater yield loss due to frost, in this study. Future work is required to verify this pattern of response to frost under wetter growing conditions where yield potential is greater than in 2018, and for alternative frost stress patterns.

Key question 3 - When does heat/frost have greatest impact on lentil yield and quality and how does this influence growers’ decisions?

Pulse crops, including lentil, are most sensitive to extreme temperature (heat waves and frost) during the reproductive period. This period spans from flowering to when grain is formed in the pods. Current management strategies to mitigate frost and high temperature damage are largely through avoidance; manipulating variables such as sowing date, crop and cultivar selections. However, these strategies possess the dilemma of weighing up avoidance of either the frost or heat wave window (depending on what risk you are trying to manage) (Figure 4). This challenge is significant and limiting the impact of frost and high temperature effects will be informed by understanding the region-specific historic occurrence of frost and high temperature and their probability of occurrence (Figure 4a versus Figure 4b). Pulse crops also range in their susceptibility to frost damage, where faba bean are more tolerant and field pea the most sensitive. These differences are primarily due to plant architecture (e.g. faba bean thick pod wall and field pea thin pod wall) and flowering time of each crop (e.g. chickpea and lentil flower later and avoid some frost).

Figure 4. Comparison of the risk profile of frost and high temperature for a) Bordertown and b) Horsham. Measure of occurrence is percentage of days in month for either frost (< 2°C at 1.8m) or high temperature (> 30°C) which has been calculated from the past 100 years of weather data (Source: Bureau of Meteorology).

When sowing time is used to manipulate flowering time and related frost and high temperature risk windows, the impact of time of sowing (TOS) on yield potential must also be considered. Generally, sowing at the optimal time based on region and cultivar choice translates to highest yield potential (Table 1). For example, for lentil crops at Curyo in 2014, delaying sowing by three weeks caused yield reductions of between 53 and 63% across a range of varieties (Table 1). These reductions in yield were due to the combined effects of mis-matching cultivar phenology with season, and high temperature and water stress associated with late maturing crops in this region.

Table 1. Percentage change in yield (negative is reduction) due to delayed sowing compared to recommended sowing time for lentil varieties in the Wimmera (Rupanyup, Pimpinio and Kalkee) and Southern Mallee (Curyo).

NB: Recommended sowing for the Wimmera is around the 10th of May and the last week of April/ early May at Curyo, delayed sowing is approximately three weeks. (Source: Jason Brand, Southern Pulse Agronomy).

In some years, delayed sowing can be beneficial due to the occurrence of other abiotic and biotic constraints, such as high incidence of disease (correlated to wet conditions). An example of this was in 2016 when there was a high infection rate of Botrytis Grey Mould (BGM) for crops sown on the recommended sowing date at both Curyo and Rupanyup which caused yield losses for susceptible varieties (PBA Bolt, PBA HurricaneXT and PBA Jumbo), where PBA Jumbo 2 which is rated resistant – moderately resistant (RMR) was not affected by the disease. This response highlights the value of selecting the variety best suited to your region, e.g. wetter areas should consider disease rating.

Key Question 4 - Remote sensing for frost damage in lentil – can it be done and how does it assist growers’ decisions

While not a solution, rapid estimation of damage, using remote and proximal sensing, would allow for tactical decision making for limiting financial losses through cutting for hay, precision harvesting and quality segregation opportunities.

To determine the utility of remote sensing for ultimately informing management decisions, the impact of frost to lentil was measured using proximal sensing following the application of artificial frost treatments. Several reflectance indices were strongly correlated with cold load, including Normalised Difference Vegetative Index (NDVI) and Photochemical Response Index (PRI). NDVI and PRI are surrogates for chlorophyll content/ greenness, biomass and photosynthetic efficiency/ plant stress, respectively.

There was a strong negative correlation between cold load and NDVI (Figure 5a), where PRI was more sensitive to frost damage compared to NDVI (Figure 5b). The good agreement of remote sensing indices; NDVI and PRI to frost affected crop, supports the potential to utilise non-destructive measurements taken from vehicle, airborne or satellite platforms for making in-season management decisions on-farm which limit losses due to abiotic constraints.

Figure 5. Reflectance indices for lentil cv. Jumbo 2A exposed to frost at varying intensities expressed as cold loads. Reflectance indices a) NDVI and b) PRI were measured using a portable spectrometer 10cm above the canopy at 6 days after frost (DAFr)and 13 DAFr (following flowering application) and 3 DAFr (following podding treatment). Segmented regression models are for CC (protected from frost events), OC (exposed to natural frost events), flowering and early podding data. 0 DAFr is the day following the night of frost application (Delahunty et al. 2019b).

Conclusion

Frost and high temperature during the reproductive phase of pulse crops, such as lentil, pose a significant challenge for growers to manage. Generally, the best management option to maximise yield is to sow crops in the optimal window recommended for the cultivar and district, where some reduction in yield and quality may occur due to frost and heat. In this case, selecting cultivars that flower in the period where region-specificchances of frost and heat severity are least will provide maximum chance for avoidance. Genetic variation to high temperature exists in lentil and provides future opportunity to increase lentil adaptation through breeding. Under severe frost conditions there is no difference between imi tolerant and conventional varieties. Finally, this work indicates that there is potential to spatially manage frost damage at the paddock scale using remote sensing technologies. Ongoing work is required to validate the use of remote sensing for detection of abiotic stresses and extend the use of such diagnostics to other pulse crops.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. This work was delivered through the ‘Improving practices and adoption through strengthening D&E capability and delivery in the southern region’, Regional Research Agronomist program (DAV00143) and funded by Agriculture Victoria and the GRDC. The authors also wish to acknowledge the assistance of Agriculture Victoria staff Ashley Purdue, Kate Finger, Mitchell Fromm and Alexander Clancy and Frontier Farming Systems.

Useful resources

Managing frost risk 

Climate guides

Rapid detection of frost damage in wheat using remote sensing. 2020 GRDC Grains Research Update paper

Plant growth physiology - lentils. GRDC GrowNote^TM Western Region

References

Davies P, Pham T (2017) A method to select for reproductive tolerance in field pea (Pisum sativum L.), Journal of Agronomy and Crop Science, 203(4), 332-337

Delahunty A, Perry E, Wallace A, Nuttall J (2019a) Frost response in lentil. Part 1. Measuring the impact on yield and quality, 19thAustralian Agronomy Conference, Wagga Wagga, Australia

Delahunty A, Perry E, Wallace A, Nuttall J (2019b) Frost response in lentils, GRDC Grains Research Update, Bendigo, Australia

Contact details

Audrey Delahunty
Cnr Eleventh St & Koorlong Ave, Irymple, Victoria, 3498
0427 580 131
audrey.j.delahunty@agriculture.vic.gov.au
@AudreyDelahunty