1. Introduction
In the area of the Mediterranean Basin, climate change is characterized by high temperature with frequent heat waves, erratic precipitation, and extreme meteorological events (both floods and droughts). In particular, a trend of increasing drought severity is expected to show up in the future, as reported by [
1]. The Palmer Drought Severity Index (PDSI) is a standardized index of drought that uses precipitation and temperature data to measure the cumulative deficit (relative to local mean conditions) in surface land moisture.
Figure 1 shows the projected change of PDSI in the 2021–2050 period compared to the thirty-year period from 1961–1990 in Europe. This scenario of increasing heat stress occurrence and longer dry spells poses a serious threat to summer crop yield, as well as to the quality of crop production, also making a proper crop choice for farmers difficult [
2].
Proso millet (
Panicum miliaceum L.) could have great potential to be adopted as a promising crop resilient to climate change in the Mediterranean Basin. Millet used to be cultivated in ancient times in the Po Plain area. In particular, proso millet was the main species cultivated in the north of Italy after the Bronze age [
3], and it was abandoned in favor of maize later on. It is now necessary to redefine agricultural practices and management and describe its phenology. It is an annual herbaceous plant of the Gramineae family. It has optimal nutrition parameters in terms of protein, minerals, and micronutrient content [
4]. Millet is the sixth world’s most important cereal, sustaining food security in arid regions and marginal lands [
4]. In developed countries, as a low-demanding additional source of income, it can be grown as a secondary crop with winter cereals or as a catch crop in case of major crop failure [
5,
6]. Proso millet can be successfully grown under drought and intense heat conditions in arid non-irrigated lands with just 200–500 mm of average annual precipitation [
7,
8]. In fact, as reported by [
9], proso millet has a shallow root system that is generally limited to the first 90 cm of soil and is really efficient at removing water from the topsoil and converting it into grain. Thus, it requires very little water compared to other cereals and can survive on topsoil moisture and summer precipitation, without the need for irrigation, restoring subsoil moisture for subsequent crops with a deeper root system. It also avoids drought sensitivity by having a really short cycle, carried out in 60–90 days [
7]. In addition, proso millet is a C4 crop whose advantages in terms of drought stress resistance are well known [
10,
11]. Moreover, it can successfully adapt to various pedological conditions, such as saline, low fertility, and slightly acid soils [
4]. As a warm season grass, proso millet needs to be planted in spring (soil temperature requested for a good germination must be at least 12 °C, as reported in [
7]) in a moist and firm seedbed. The first two weeks after planting are considered the most critical time in proso millet cultivation. This is due to the fact that final yield highly depends on soil water content at planting, so light precipitation is very helpful, but heavy rain can have a negative effect [
9]. Generally and in particular after a fallow year, proso millet does not need any fertilization. The effects of drought stress on yield and water use efficiency (WUE) clearly depend on the phenological stage in which stress appears and on its severity. Water stress at the ear emergence stage causes the greatest grain yield loss (around 40%) due to the reduction of both seed number per ear (as a result of stress on pollination and floret abortion) and seed weight (as a result of cytokinin reduction, which causes less endosperm production) [
12,
13,
14]. As reported in [
15], the seed filling stage appears to be less susceptible to drought stress. The proso millet panicle shows a staggered ripening that starts from the top and gradually continues to the bottom. Generally, as the bottom part of the panicle reaches full-ripening, the distal part of it starts grain loosening. Grain shattering causes yield loss if harvest is delayed [
9,
16]. At maturity, grains generally present about 20% or less moisture.
This case study of proso millet cultivation in the Mediterranean Basin is in the frame of a LIFE-CCA EU project, called Growing REsilience AgriculTure (GREAT LIFE). The goal of GREAT LIFE is to face the effects of climate change on agricultural activities in Italy and on the European side of the Mediterranean Basin in general. Through the experimentation of rational rotation schemes and sustainable agronomic practices, the project aims to experiment with stress-resistant low-demanding crops as potential alternatives to maize in crop rotation at the Italian and European levels to improve the resilience of agroecosystems and reduce water consumption. In fact, in addition to being an extremely water demanding crop (thus reducing its sustainability), corn productivity is becoming less reliable and profitable for farmers in Southern Europe as a consequence of climate change effects on corn yield [
17]. From 1974 to 2008, the corn yield losses caused by climate change in Western and Southern Europe regions have been estimated by [
17] to be around 6.3%. In addition, climate change impacts are not just related to quantitative crop performances but also to quality aspects, favoring the increase of mycotoxins and corn pathogens. In fact, a recent study focusing on aflatoxin contamination in maize within the next 100 years, under a +2 °C and +5 °C climate change scenario, demonstrated that the increasing temperatures clearly are closely related to aflatoxin contamination risk [
18]. The exact definition of proso millet phenology encoded in the BBCH (Biologische Bundesanstalt, Bundessortenamt and CHemical industry) scale is an important part of the GREAT LIFE project since it allows progress in research and gives researchers comprehensive indications for future agronomic surveys on the crop. Additionally, the calculation of temperature-driven heat-unit accumulation (cumulative growing degree days, CGDD) is important, and knowing the relationship between CGDD, days after sowing (DAS), and phenological BBCH stages can help to successfully cultivate proso millet, allowing us to identify the best sowing time and the most susceptible phases to abiotic stress during the life cycle. For this reason, in the first year of experimentation,
P. miliaceum L. phenological development was encoded in the BBCH scale, also indicating the thresholds in terms of CGDD and DAS necessary to achieve each phase (for more details, refer to [
19]). The aim of this study was to assess proso millet’s ability to maintain a high vegetative vigor and a good water status (comparable to that of an irrigated corn) without any irrigation, as well as to evaluate its resilience to climate change conditions in the Mediterranean climate.
3. Results
3.1. Meteorological Data
Figure 2a–c show the meteorological data for the VM site in 2019, 2020, and 2021, when proso millet was grown in an open field. Data from 2019, 2020, and 2021 were compared with the 30-year mean air temperature in Cadriano (Lat 44°33′03″, Lon 11°24′36″), where the main DISTAL agrometeorological station is located, for which a solid historical dataset is available. We also calculated the 30-year mean precipitation for an average crop cycle. The chosen period was 15 April to 15 August (cfr with sowing/harvesting data in
Table 1). Climatological precipitation during the crop cycle was 230.8 mm, while for 2019 it amounted to 310.6 mm, quite a large quantity, and the two following years showed low rainfall quantities, namely 159.8 mm (in 2020) and 119.6 mm (in 2021). As it was more than the total quantity during the cycle, its distribution in single years is interesting.
The 2019 season was characterized by a cold and rainy May (T mean 14.7 °C, most of the time under the climatic average, 183.6 mm of precipitation) during the BBCH stage 00–16 (from sowing to six fully expanded leaves), followed by a hot summer, with heat waves in June and July. In 2020, spring temperatures were mostly close to the climate, but precipitation was extremely restrained, with just 18.8 mm during the entire month of May, during BBCH stage 00–16. In 2021, only one precipitation event (11 mm) was recorded in the period, including the entire month of June and the first ten days of July. Two heat waves occurred in the same period, with temperature peaks over 35 °C (37.4 °C on July 7th), between the end of leaf development (BBCH 16) and flowering (BBCH 60-69). Although very different, 2019, 2020, and 2021 were three difficult seasons from an agronomical point of view, in the full manifestation of the uncertainties related to climate change.
3.2. Phenological Development
Phenological data collected at the VM and AG sites in 2019 and 2020 have been compared to test the consistency of CGDD thresholds necessary to reach each stage.
Figure 3 reports phenological data. The difference in the thermal thresholds between 2019 and 2020, expressed as the average between the 2 experimental sites, is mostly reduced, suggesting good reliability.
3.3. Satellite Indexes
Table 3 reports NDVI and NDWI calculated for corn at the AC site and millet at the VM site during the 2019, 2020, and 2021 seasons. For both crops, the BBCH principal growth stage is reported at each date. As an example,
Figure 4 shows vegetation indexes calculated with ESA SNAP software on 5 July 2019.
Table 3.
Corn (in AC site) and millet (in the VM site) NDVI and NDWI in 2019, 2020, and 2021 seasons.
Table 3.
Corn (in AC site) and millet (in the VM site) NDVI and NDWI in 2019, 2020, and 2021 seasons.
2019 | 18 June | 05 July | 25 July |
---|
Millet NDWI | 0.35 | 0.38 | 0.23 |
(BBCH 6) | (BBCH 7) | (BBCH 8) |
Corn NDWI | 0.28 | 0.35 | 0.34 |
(BBCH 3) | (BBCH 6) | (BBCH 7) |
Millet NDVI | 0.80 | 0.80 | 0.65 |
(BBCH 6) | (BBCH 7) | (BBCH 8) |
Corn NDVI | 0.78 | 0.80 | 0.78 |
(BBCH 3) | (BBCH 6) | (BBCH 7) |
2020 | 22 June | 22 July | 11 August |
Millet NDWI | 0.18 | 0.37 | 0.23 |
(BBCH 6) | (BBCH 8) | (BBCH 9) |
Corn NDWI | 0.40 | 0.36 | 0.30 |
(BBCH 3) | (BBCH 6) | (BBCH 8) |
Millet NDVI | 0.62 | 0.77 | 0.63 |
(BBCH 6) | (BBCH 8) | (BBCH 9) |
Corn NDVI | 0.86 | 0.75 | 0.76 |
(BBCH 3) | (BBCH 6) | (BBCH 8) |
2021 | 12 June | 12 July | 6 August |
Millet NDWI | 0.17 | 0.34 | 0.26 |
(BBCH 3) | (BBCH 6) | (BBCH 8) |
Corn NDWI | 0.06 | 0.20 | 0.22 |
(BBCH 3) | (BBCH 6) | (BBCH 7) |
Millet NDVI | 0.58 | 0.77 | 0.74 |
(BBCH 3) | (BBCH 6) | (BBCH 8) |
Corn NDVI | 0.42 | 0.62 | 0.68 |
(BBCH 3) | (BBCH 6) | (BBCH 7) |
In 2019, NDVI and NDWI values for millet were extremely close to those of an irrigated corn when comparing data at the same phenological stage; in particular, values at BBCH principal stage 6 (flowering) were perfectly overlapping (0.35 for NDWI and 0.8 for NDVI), but differences remained really restrained during BBCH principal stage 7 (milky ripe) (and a bit in favor of millet, which shows slightly higher values). Millet index values started lowering, as predicted, with the reaching of maturity, when leaves physiologically dry out. In 2020, lower NDVI and NDWI values were recorded for millet at the end of spring compared to the previous year, but the reduction was not so heavy as to show clear water stress. The indexes then settled at high values during summer (once again higher than corn at the same date and at the same BBCH stage), and then fell again at the end of the cycle, in line with what was observed in the previous year. In 2021, millet index values were quite low in June but then greatly improved in summer, surpassing those of irrigated corn. The particularly low values of corn indexes in June are due to the fact that the crop had not yet completely closed the row, and the presence of bare soil significantly reduced the NDVI and NDWI values.
3.4. Spad
Figure 5 reports a box-and-whisker plot of SPAD measurements on proso millet during the 2020 life cycle. The “box” is bounded by the first and third quartiles, divided inside by the median. The segments (“whiskers”) are delimited by the minimum and maximum of the values.
SPAD values drop throughout the life cycle, from 38.9 during BBCH principal stage 2 (tillering, contemporary in millet to stem elongation-BBCH principal stage 3, see Ventura et al., 2020) to 17.6 at BBCH 89 (full maturity), in accordance with crop progressive senescence as the life cycle is completed.
Figure 6 shows the good accordance of 2019, 2020, and 2021 SPAD values for the main phenological stages. Therefore, in all experimentation years, millet SPAD values stayed high until flowering, indicating optimal crop vigor, without any clue of drought stress, and then dropped in the last phenological stages, in line with a normal drop in physiological parameters associated with crop senescence [
30].
3.5. Agronomic Performances
Millet had a good agronomic performance at the VM site open field in the three years of experimentation (
Figure 7). The crop homogeneously and abundantly covered the soil, showing excellent competition with weeds. The mean results of the agronomic survey, carried out before harvesting on 3 separate sampling areas of 1 m
2, are presented in
Table 4.
For panicle length transformed via logarithmic transformation, ANOVA highlighted significant differences among the 3 years (p < 0.05). Panicle length in 2020 was significantly decreased with respect to 2019, and in 2021, it was significantly shortened with respect to 2020, as highlighted by Tukey’s test (p < 0.05). Height data were normally distributed, but they were not homoscedastic. Therefore, the non-parametric Welch test was performed, and the Duncan–Waller test was used to separate the means. Plant height at panicle insertion showed significant differences among the three years, as revealed by the Welch test (p < 0.05). Specifically, it was significantly greater in 2020 compared to 2019 and 2021. In 2019 and 2021, differences in plant height were not significant. Regarding the yield/panicle, the Kruskal–Wallis test did not show any significant difference (p > 0.05), so there was no difference in yield/panicle in the 3 years of experimentation. Therefore, panicle length significantly decreased, passing from 2019 to 2020 and then to 2021, while plant height appeared significantly lower in 2020 compared to the other two years of experimentation. However, these differences did not produce any difference in yield per panicle, which was unchanged over the 3 years.
3.6. Water Balance and WUE
Proso millet and corn grain yield and water balance for 2019, 2020, and 2021 seasons are reported in
Table 5. The table also reports the calculation of crop WUE, expressed as the ratio between grain production (kg dry grain/m
2) and water consumption (m
3/m
2), as the mean of all separated sampling areas. Regarding the period from sowing to BBCH 16 (six leaves fully expanded), calculations confirm that the net water loss from the soil system compared to the water gained from precipitation was zero (2019) or negligible (2020 and 2021). So, the decision to compute millet water balance from BBCH 16 was solid.
4. Discussion
The aim of this study was to assess the contribution offered by proso millet to enrich agricultural sustainability and resilience to climate change in Italy and on the European side of the Mediterranean Basin, to reduce water consumption for cereal production, and to offer a new resource to farmers, for whom corn production becomes less and less reliable. During experimentation, millet faced three difficult seasons from an agronomic point of view, in the full manifestation of the uncertainties caused by climate change. Crop season 2019 was characterized by a quite cold spring, followed by heat waves in June and July. The 2020 season, on the other hand, was characterized by an extremely dry spring. In 2021, drought and heat waves manifested simultaneously in June and July. The extremely different meteorological trend in the period following sowing could have influenced the first stages of development, explaining at least partially the difference in the 107 degrees days necessary to reach BBCH 16 in 2019 and 2020. For the rest of the life cycle, the thermal thresholds identified in 2019 for the achievement of subsequent phenological stages have been largely confirmed, suggesting that these thresholds are consistent.
NDVI and NDWI vegetative indexes calculated on dry-grown millet and on traditional irrigated corn have allowed us to make some interesting considerations. In 2019, although no irrigation was used, millet NDVI was never less than 0.8 until the ripening phase (BBCH principal stage 8) and then settled at 0.65 (in any case a high value, considering natural crop senescence in the final stages of the life cycle). The same consideration can be made, in qualitative terms, for NDWI; index values were 0.35 and 0.38 at the flowering and grain development stages, respectively, showing no water stress for the crop, despite summer heat waves. The index drop to 0.23 appears to be in line with senescence given by the achievement of the final phases of the life cycle. We can therefore state that millet showed high vigor and no signs of water stress during the 2019 growing season. The ability of this crop to effectively resist hot summers without any need for irrigation emerges even better if we compare NDVI and NDWI calculated on millet with those calculated on conventional irrigated maize at the same phenological stages. The indexes of the two crops are in fact widely matching (and slightly in favor of millet during the grain development stage), despite the fact that corn could take advantage of 180 mm of irrigation during the crop cycle.
In 2020, the extreme drought in spring resulted in lower NDVI and NDWI indexes (although not severely deficient) until BBCH principal stage 6 (flowering) compared to the previous year. However, millet proved to be able to effectively take advantage of summer rainfall, showing July NDVI and NDWI values of 0.77 and 0.37, respectively, perfectly overlapping those of irrigated corn on the same date. The subsequent decline in the indexes at the end of the season appears to be in line with the physiological senescence of the crop and perfectly comparable to the decline observed in the survey of the previous year. Therefore, despite the drought in spring, proso millet was able to recover successfully, taking advantage of summer precipitation and showing good resilience. Maize, in the same season, was supported by 190 mm irrigation, without which it would have presumably shown lower indexes and yield.
In 2021, as in 2020, spring water scarcity resulted in quite low NDVI and NDWI up to anthesis, compared to 2019. Then, the crop greatly improved its vigor and water status, confirming its resilience to adverse meteorological conditions, reaching NDVI and NDWI values of 0.77 and 0.34, respectively, in the month of July, showing higher values than those of irrigated corn.
With regard to SPAD values, we found that chlorophyll content decreases over the life cycle as full ripening is reached. This is in accordance with the normal crop physiology, without indicating that the crop was in a water stress condition. In fact, [
30] also observed a progressive reduction in proso millet chlorophyll content starting from the grain filling stage. Moreover, the mean SPAD value we observed in the first part of the life cycle (close to 40), when millet was still green and in full photosynthetic activity, was quite close to SPAD values observed by [
31] on proso millet (44.4 ± 0.5) cropped in a 100% ETm (maximum crop EvapoTranspiration) water restitution regime between 34 and 48 DAS (days after sowing). The proximal SPAD measurements confirm what was observed through the vegetative indexes obtained from SENTINEL-2; proso millet, despite three growing seasons characterized by summer heat waves, extremely dry spring, and concomitant drought and heat waves, was able to sustain satisfactory photosynthetic activity, without suffering from water stress. The progressive reduction of SPAD values, which can be easily overlapped in the three years of experimentation, seems to be in accordance with normal crop physiology, without indicating a stressful condition.
The only indication of a possible partial effect of water stress on millet in 2020 and 2021 is given by the significantly shorter panicle length compared to 2019. In fact, as reported by [
32], panicle length in cereals is one of the traits that can be affected by water stress. However, the reduction of this trait generally appears contained, and the effect of this reduction on yield is also contained. In fact, in cereals such as millet, when water stress causes a strong reduction in grain productivity, this is mainly due to a drastic drop in terms of grain number per panicle and grain weight, instead of panicle length [
13,
33]. The effect on yield of panicle length reduction appears to be very limited [
32]. This is confirmed by our measurements, as yield per panicle showed no significant difference among 2019, 2020, and 2021. In particular, the 2021 season was characterized by a very low level of precipitation in the period from mid-leaf development (BBCH 13) to the end of flowering (BBCH 69) (only 21.6 mm of total precipitation), combined with 3 heat waves, with temperature peaks over 35 °C in the period from stem elongation (BBCH 32-33) to the end of heading (BBCH 59). Thus, proso millet was subjected in this season to drought and thermal stress, higher than in the previous two years. The effects of these stressors may be found in a significant reduction in panicle length.
Other sources in the literature show a reduction of proso millet WUE in conditions of drought stress [
13,
14]. However, considering that yield per panicle in 2021 is completely comparable to the two previous years and considering that reduction in panicle length is known to cause a negligible yield loss in millet, it is conceivable that the reduction in WUE observed in 2021, due to a decrease in kg m
−2 dry matter grain production, is not the sign of a stressed crop but could be mainly caused by a higher weed presence and a lower ground cover rate compared to 2019 and 2020. In fact, there are no evident signs of stress in SPAD values and satellite indexes, which were highly satisfactory during the summer.
Moreover, it is interesting to note that, although it is well known that drought stress causes a reduction in plant height in cereals [
34], no significant difference was observed for this trait between 2019 and 2021. On the contrary, plant height appears significantly higher in 2020, despite a notably drier spring than in 2019. It is therefore conceivable that the effect of water stress on plant height in millet is variable, depending on the intensity and the moment in which the stress occurs.
Considering the average proso millet grain yield in Southern Europe (1.8 t/ha-FAOSTAT), we can state that proso millet showed a good performance during this three-year experimentation, despite three differently adverse agricultural years, especially if we consider that not only was the crop never irrigated, but no other productive inputs or cultivation treatments were used. Regarding WUE, this remained satisfactory, as well as the yield, showing values around 2 kg m
−3 in 2019 and 2020 and slightly above 1.5 kg m
−3 in 2021, corresponding to 20 kg ha
−1 mm
−1 and 15 kg ha
−1 mm
−1, respectively. In the FAO Thematic Report “Status of water use efficiency of main crops” [
35], values of WUE up to 12 kg ha
−1 mm
−1 are reported for dry land millet in Sahel (semi-arid African region), and in a long-term experiment in the semi-arid Central Great Plains of the United States (ideally suited for dry land millet production) [
36], values ranging from 8.37 to 33.62 kg ha
−1 mm
−1 were recorded. Therefore, the results obtained during the experiment were satisfactory, if compared with the data available in the literature, despite adverse and different meteorological trends.
Regarding maize, it was cropped following the best agronomic techniques available and irrigated with efficient and calibrated water use, thanks to the indications produced by the IRRIFRAME platform. This careful agronomic management allowed us to obtain very satisfactory yields in the three-year experimentation and WUE values in line with what is reported by the FAO Thematic Report. In 2021, summer NDVI and NDWI values were lower than the previous two years, as 2021 was the experimentation year with the lowest rainfall amount.
The higher WUE values in corn were due not only to much greater agronomic inputs on the crop, but to the fact that modern corn hybrids are the result of decades of selection and breeding, unlike millet, whose biodiversity potential offered to researchers and farmers is still to be explored.