Findings and discussion
Precipitation during the crop cycle was 284 mm, being significantly lower (-40%) than the historical records (series 1931-2015) (https://inta.gob.ar/documentos/informacion-meteorologica-mensual-de-la-eea-manfredi) of 465 mm, and the irrigation applied was 210 mm. Water consumption through the crop cycle showed two contrasting responses (Fig. 1a). The first, represented by the sowing period – 65 days after sowing (DAS) – was similar between treatments and determined a consumption of 250 mm. The second response, represented from 65 DAS on, showed a dissimilar consumption between treatments. The decrease in consumption under non-irrigated treatment coincided with the stages of fixation and growth of the pods and grains. The percentage of usable water at the beginning of that period was 23%, while towards the end of that period (122 DAS) it was close to the permanent wilting point (8% of the total usable water). This edaphic scenario, together with the unsatisfied environmental demand, intensified the water deficit, which was reflected in increases in canopy temperature, leaf folding and leaf fall. The total water consumption was 507 mm for the irrigation treatment and 364 mm for the non-irrigated treatment,. a 28% reduction of the latter compared to the former.
In the early stages of the cycle, where water consumption and environmental demand are lower, the arrangement of the leaves with respect to the main stem of the plant was similar under both treatments and resulted in similar levels of intercepted radiation (66% at 45 DAS) (Fig. 1b). Later on, and due to the low rainfall, the water deficit was accentuated under the non-irrigated treatment and triggered a marked decrease in intercepted radiation (Fig. 1b) as a consequence of leaf folding, and an increase in leaf temperature (Fig. 1c) due to stomatal closure. These responses on non-irrigated land started to reverse at 110 DAS due to continuous rain events that remained until harvest (108 mm for the 110 DAS-harvest period), reflected in increases in intercepted radiation (Fig. 1b) and biomass production (Fig. 1d), and sustained similar leaf temperature gap between treatments (Fig. 1c). Under the irrigated condition, the plants showed contrasting growth with respect to those on non-irrigated land (Fig. 1d) and the thermal stress index was significantly lower from post-flowering onwards due to sustaining perspiration at environmental demand levels (Fig. 1c). Intercepted radiation reached maximum levels from early stages of the cycle (66 DAS) (Fig. 1b) and biomass production grew steadily over time (Fig. 1d).
Certain aspects of biomass production in contrast to water scenarios should be highlighted. The growth rate of the crop under the irrigated condition was 10.7 g/m2/day between days 23 and 108 of the cycle, while under the non-irrigated condition it varied according to the environmental scenario. In this sense, due to low environmental demand and sufficient provision of soil water, a non-irrigated crop growth rate of 10.3 g/m2/day was determined between 23 and 63 DAS, while, due to a severe water deficit between 63 and 108 DAS, which resulted in pronounced loss of leaves, the growth rate of the crop was only 4.6 g/m2/day. These responses converged in productions of total biomass at harvest of 975.3 g/m2 (9753 kg/ha) and 573.9 g/m2 (5739 kg/ha) for irrigated and non-irrigated land, respectively (Fig. 1d). Pod biomass represented the contrasting water condition between treatments with pod growth rates of 5 g/m2/day on irrigated land and 2.4 g/m2/day on non-irrigated land.
The efficiency in the use of total biomass water was 3.1 g/mm and 2 g/mm for irrigated and non-irrigated land, respectively; while for grain production it was 0.59 g/mm and 0.42 g/mm for irrigated and non-irrigated land, respectively. These values are considerably lower than those determined by Haro et al. (2010), raising the suspicion that low levels of radiation and temperature during fixation and growth of implicit grains at a late sowing date, such as the one in this study, triggered such responses.
At harvest, the pod biomass was 432.9 g/m2 (4329 kg/ha) and 217.6 g/m2 (2176 kg/ha) for irrigated and non-irrigated land, respectively (Fig. 1d). Grain yield and number of grains were significantly different between treatments, with values of 301.0 g/m2 (3010 kg/ha) and 154,2 g/m2 (1542 kg/ha) for irrigated and non-irrigated land, respectively, and 564 grains/m2 and 316 grains/m2 for irrigated and non-irrigated land, respectively. Grain weight was 0.31 g and 0.27 g for irrigated and non-irrigated land, respectively, and did not show significant differences between treatments.
Figure 1. Responses associated with water and carbon economy in peanut cultivation growing under contrasting water conditions.
The depth of the irrigation laterals was 35 cm, significantly distant from the growth and harvest zone of the pods (0-15 cm) (Fig. 2a). The depth of the laterals coincides with the region of greater density and radical architecture, which guarantees the fluid access of water and nutrients supplied by this technology. Meanwhile, the subsurface provision of water minimises the evaporation of water from the soil, thereby increasing the availability of water to the crop and exacerbating the differences in growth between water environments (Fig. 2b).
Figure 2. (a) Subsurface supply of irrigation to peanut crops. (b) Peanut crop growing in contrasting water conditions.
