Take home message

• Time of sowing impacts mungbean yield due to environmental conditions during the reproductive phase (flowering to grain-fill)
• High vapor pressure deficit (VPD) during reproductive periods decreased seed yield. High VPD increases crop water requirements, decreases water-use efficiency and elevates the risk of terminal crop stress. These conditions are more commonly experienced in spring mungbean plantings and are magnified in rainfed crops. Lower starting soil water reduces both yield potential and yield stability
• Planting mungbean in summer, combined with adequate to high starting soil water, typically produces higher and more consistent yield in most regions compared with spring planting.

Background

Over the past decade, there has been extensive investment across multiple projects to determine the optimum sowing times for mungbean crops, ideal conditions for high yield and the agronomic levers that can be manipulated to mitigate the risk of crop failure. The cash flow and break crop benefits of growing mungbean as a summer rotational crop following a winter fallow or in a double-crop situation have seen the area of production expand in the northern region.

While mungbeans are a valuable addition to farming systems, yield stability and management of the crop to achieve profitable yields has been a challenging task, and often leaves seasoned growers scratching their heads. Pasley et al., 2024 eloquently summarised the most common management decisions growers face as: “When should I sow?”; “Is there enough soil water to sow?”; and “What is the risk of crop failure should I sow outside the optimal window?”.

In this paper, we will look at the mechanisms that contribute to mungbean yield from a physiological level, what drives our yield potential and what this means for time of sowing and potential management strategies to achieve profitable yield. This paper will report results specifically from the trial site at the University of Queensland, Gatton Campus and will draw upon the findings of other recent projects to support the results of these experiments.

Methods

From December 2018 to February 2021, twenty-two treatments were established at the University of Queensland, Gatton Campus, utilising different times of sowing and water availability to capture the diverse environments mungbeans are grown in. To elucidate the role of water availability in yield response to environment, two treatments were used for this experiment: ‘wet’ and ‘dry’. The wet treatments received an average of 25–50 mm irrigation (overhead sprinkler) weekly to avoid moisture stress. The dry treatments only received irrigation within the first two weeks of crop establishment and no further irrigation for the duration of the crop. The dry treatment was intended to impose an increasing water deficit starting from seedling emergence. Starting soil moisture was measured to a depth of 90 cm. Average monthly water applied to dry treatments was 90 ± 5 mm (primarily rainfall) and 145 ± 8 mm in wet plots (rainfall + irrigation) across all three seasons. Water supply dynamics (water in vs. water out) was calculated using water available (starting soil water plus in-crop rainfall/irrigation) against crop evapotranspiration potential. The water supply demand dynamic across the three seasons showed the dry plots experienced mild to moderate water deficits pre-anthesis (0.8) and severe water deficits post-anthesis (0.36) compared with the wet treatment at 1.09 pre-anthesis and 0.86 post-anthesis.

The varieties planted were Jade-AU, Crystal and Opal-AU. Not all varieties were planted in each season. Each trial was planted at a seeding rate of 30 seeds/m² and inoculated using a Group I peat-based inoculant.

Measurements:

• Environmental conditions (minimum/maximum temperature, relative humidity, vapor pressure deficit, solar radiation)
• In-crop rainfall/irrigation
• Vegetative, flowering, mid pod-fill and physiological maturity biomass
• Yield (kg/ha) and yield parameters such as pod and seed number.

Results

The mechanisms – what makes them yield?

Water availability is one of the most important components of mungbean yield. Mungbean yields are largely influenced by starting soil water levels coupled with timing and amount of in-crop water (rainfall and irrigation). The typical rule of thumb has been that starting soil moisture <75 mm is thought to significantly reduce mungbean yield, especially with little follow up rain. Most crops in both the wet and dry treatments had less than optimal starting soil moisture prior to planting and required early irrigation to achieve acceptable crop establishment (Figure 1). Wet and dry treatments received similar water pre flowering for all TOS, with minimal water stress in the vegetative stages. In contrast, in the reproductive stage post-flowering,  wet treatments received significantly more water (95 – 262mm) than the dry treatments (30 – 120mm). Total amount of water applied post-flowering in the wet treatments was driven by the season conditions. Total in-crop water plus starting soil water was higher for the wet treatments, with all TOS receiving an average of 330 – 530mm of water throughout the season. In contrast, in the dry treatments this ranged from 210mm – 300mm of in-crop water throughout the season.

Figure 1. Starting soil water and in-crop moisture for the wet and dry treatments for September to February for each time of sowing (TOS). Starting soil moisture was measured pre-planting, pre-flowering water was measured prior to flowering and post-flowering water was measured after flowering had commenced.

Water availability influenced seed yield in both the wet and dry treatments with higher water availability in the wet treatments significantly increasing final yields whilst decreasing water-use efficiency as measured by kg ha^-1 seed yield per mm of water available (Figure 2).  For example, in crops yielding 1500 kg ha^-1 the WUE of wet crops was approximately 3 kg.ha.mm in contrast to dry crops at 6 kg.ha.mm at the same yield point. Increasing water availability decreased WUE whilst overall increasing yield, likely due to poor harvest index / excess biomass in conditions with high water availability (e.g. wet treatments).

Figure 2. Mungbean yield (kg ha^-1 per mm of available water) for dry and wet mungbean treatments.

While biomass is often correlated with yield, it is not the best indicator of yield for mungbean and the relationship with yield gets more complicated as the crop progresses into the reproductive phases. Under rainfed conditions, where crops are often subject to low water availability (e.g. dry treatments), biomass and yield are more strongly correlated at both flowering (R²=0.68) and pod-filling (R²=0.49) than crops with higher water availability such as the wet treatments. This tighter relationship between biomass and yield illustrates development becomes more determinate under dry conditions, with the switch from vegetative development to reproductive development occurring more definitively. In contrast, the wet treatments had higher biomass and seed yield but a much weaker relationship due to luxuriant water (and other resource) availability. This results in an uncoupling of the biomass and yield relationship, particularly during the later stages of crop growth where biomass exceeds 3000 kg ha^-1 (Figure 3b). While this relationship can be messy, it is clear adequate biomass development is still a key element for the crop to set a good number of pods and subsequently, produce higher seed numbers. Seed number is still one of the main drivers of seed yield (Figure 4). Increasing crop biomass can potentially result in seed yield increases, however where mungbean are grown in non-limiting conditions (water, nutrients, solar radiation, low pest and disease pressure), mungbean can produce excess biomass, more than what is required to develop the number of pods/seeds set by the crop.

Figure 3. The relationship between shoot dry weight (SDW) and yield at flowering (a) and pod-filling (b) for wet and dry treatments. Shoot dry weight is measured without pods or grain.

Figure 4. Seed number is the major factor determining total yield of mungbean in both rainfed/dryland and irrigated/high rainfall crops.

The drivers – what influences their yield?

Crop response to abiotic stress changes as it moves through the developmental stages and is co-dependent on water availability. Early stress impacts biomass development, mid-season and late stress impact on yield components such as pod and seed number, seeds per pod and pod retention. To better understand this relationship, yield sensitivity in the wet and dry treatments to environmental stressors were examined across the different TOS. As yield in mungbeans is strongly associated with seed number (Table 1) the interactions between environment and crop development were investigated using seed number as the response variable. Sensitivity of seed number to environmental factors associated with crop stress such as maximum temperature, minimum temperature, radiation, calculated crop water requirement also known as potential crop evapotranspiration (daily ETc) and maximum daily VPD (max VPD) were assessed across crop development. Many of these factors directly or in-directly impact photosynthesis an essential plant process. Changes to the environment can have major consequences for growth and production, such as a reduction in biomass accumulation and limiting carbohydrate supply to developing flowers and grains, all potentially resulting in reduced yields. Sensitivity to these factors changes with crop phenology or developmental stages. Crop development was split into three phases based on phenology: (1) vegetative - germination to first flower (FF); early reproductive stage (2) pod-set - 10 days from FF to pod-set and later reproductive stages (3) seed-fill (> 10 days after FF to 90% physiological maturity).

Table 1. Correlation matrix for seed number and pod number, maximum daily temperature (°C), minimum daily temperature (°C), solar radiation, daily crop water requirements (calculated crop evapotranspiration; ETc) and maximum VPD (kPa). Development phases were: vegetative (emergence to first flower), pod-set (first flower to pod-set; 10 days) and seed fill (> 10 days after first flower) to physiological maturity (90% black pod). Pearson’s correlation coefficients are listed and significant (P < 0.05) correlation relationships are single underlined (negative) and double underlined (positive).

The correlation coefficient analysis consistently found seed number and yield had a strong relationship in both dry (r 0.99) and wet (r 0.93) treatments (Table 1). Seed number and yield in mungbean was not strongly affected by environmental conditions during the vegetative phase under all treatments. Water availability played an important role in mitigation of environment on crop yield. As the crop progressed through the reproductive phases, all environmental factors were negatively correlated with seed number in the dry treatments only. There were no significant correlations between environment and seed number in the wet treatments during the pod-set and seed fill periods.

In the rainfed water-limited dry treatments, conditions leading to high crop water requirement (crop evapotranspiration; ETc) such as high vapour pressure deficit (VPD), maximum temperatures and radiation were all negatively correlated with seed number during both pod-set and seed filling phases. The rate of transpiration or water-loss from the canopy (via stomata opening and closing) is driven by changes in VPD which is a combined function of temperature and humidity. High VPD conditions lead to high transpiration rates per unit of carbon fixed by photosynthesis, increasing crop water requirements and often leading to drought stress.

During the early reproductive stages from first flower to pod-set (10-day period) factors such as high VPD and solar radiation that directly affect stomatal opening, crop transpiration (water-loss rate), photosynthesis and photoassimilate (sugar) production were the most negatively correlated with seed number. Moreover, there is also a very strong negative correlation (r 0.79) between seed number and crop evapotranspiration, a calculation of crop water requirements based on weather conditions, during this period of early pod-set when pod number and grain number are largely determined. This data illustrates that environmental conditions that increase crop water-requirements and directly impact the crop’s ability to supply photoassimilate (carbon) to flowers and developing pods in early have large impact on yield potential.

High VPD creates a thirsty environment or atmosphere when it is consistently high over multiple weeks and is often associated with declining stomatal conductance and photosynthetic rates. While the range for optimum VPD has not been fully characterised for mungbeans, it is thought to be between 0.4 and 1.6 kPa for other legumes.  Based on this, and the correlation matrix in Table 1, it helps to explain the variation in yield across different times of sowing. In Figure 5, the average daily VPD was calculated for the TOS planted from October through to February. The VPD during the reproductive phase for the September, October, November and December TOS exceeds the optimum range for other legumes, indicating increased VPD during the reproductive period is likely responsible for yield decline, particularly in the dry treatments, for those TOS.

Figure 5. Vapour pressure deficit (kPa) over the reproductive period (from 50% flowering) of mungbean planted in September, October, November, December, January and February based on the average weather conditions in spring and summer of the 2018/2019 season. The green (shaded) box indicates the optimum VPD range reported for other legumes.

What does that mean for time of sowing?

Yield varied significantly between wet and dry treatments, from September through to February TOS for the three varieties Crystal, Jade-AU and Opal-AU (Figure 6). Yield data for both the wet and dry treatments show yield is lower when mungbeans are planted in spring versus summer, except for the wet treatment planted in October (data only available for Jade-AU and Crystal). The January plantings were the most successful summer TOS for all three varieties with >1500 kg/ha yield achieved in the wet treatments for all three varieties. When looking at the combination of VPD (Figure 5) and yield (Figure 6), VPD during the reproductive period was likely to be within the optimum range for the summer plantings (December, January, February), contributing to higher yields in both wet and dry treatments. Pasley et al., 2024 found during time of sowing simulations over multiple sites/years, that while yield was somewhat influenced by time of sowing, yield stability was far more likely to be impacted by time of sowing (Figure 7), contributing to the yield fluctuations we commonly see with mungbeans. Figure 7 (Pasley et al., 2024) shows the simulations conducted for sowing times from September through to February for different areas and starting soil moisture (40%, 60%, 80% and 100% of plant available water for those soils). Based on these simulations, mungbean crops at Gatton are likely to have higher and more stable yield planted in November or December compared with other TOS, particularly when starting plant available water is higher. Locations such as Goondiwindi or Emerald are likely to achieve higher and more stable yield with later summer planting (January to February), with higher initial water availability (100% PAWC) but may also achieve adequate yield with earlier summer planting (December to January) where starting soil moisture availability is lower.

Figure 6. Yield (kg/ha) of Crystal, Jade-AU and Opal-AU planted from September through to February.
Blue bars represent wet treatments, red bars represent dry treatments.
Yield data has been combined from 2018/2019, 2019/2020 and 2020/2021. Error bars indicate standard error.

Figure 7. Source: Pasley et al., 2024. Simulated achievable yields for mungbeans planted from September through to February for different growing regions.The orange markers indicate yield (kg/ha) and the green markers represent yield stability (acv %). The lower the acv, the more stable mungbean yields are. Starting soil plant available water levels are 40% (0.4, top row) increasing to 100% (1, bottow row).