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SUMMARY: The yield of maize (Zea mays L.) crops depends on their ability to intercept sunlight throughout the growing cycle, transform this energy into biomass and allocate it to the kernels. Abiotic stresses affect these eco‐physiological determinants, reducing crop grain yield below the potential of each environment. Here we analyse the impact of combined abiotic stresses, such as water restriction and nitrogen deficiency or water restriction and elevated temperatures. Crop yield depends on the product of kernel yield per plant and the number of plants per unit soil area, but increasing plant population density imposes a crowding stress that reduces yield per plant, even within the range that maximises crop yield per unit soil area. Therefore, we also analyse the impact of abiotic stresses under different plant densities. We show that the magnitude of the detrimental effects of two combined stresses on field‐grown plants can be lower, similar or higher than the sum of the individual stresses. These patterns depend on the timing and intensity of each one of the combined stresses and on the effects of one of the stresses on the status of the resource whose limitation causes the other. The analysis of the eco‐physiological determinants of crop yield is useful to guide and prioritise the rapidly progressing studies aimed at understanding the molecular mechanisms underlying plant responses to combined stresses. Significance Statements: Drought, nitrogen deficiency, warming temperatures and neighbour crowding reduce the grain yield of plants. Here we analyse how these stresses impact on maize crops in the field when combined. [ABSTRACT FROM AUTHOR]

Cotton is often exposed to high temperatures during the reproductive stage, which can negatively affect its productivity. The objectives were to: i) test whether heat stress impacts during the reproductive stage on photosynthesis are due to instant temperature effects or to acclimation produced during the heat stress period, ii) evaluate the role of stomatal and mesophyll conductance on net photosynthetic rate and iii) identify possible interactions between heat stress and different source/sink ratios during the reproductive period. Two field experiments were carried out in 2016 (Exp. 1) and 2017 (Exp. 2). Two heating treatment periods were imposed as follows: pre‐flowering between 15 days before flower bud and flowering (H1) and post‐flowering between flowering and 15 days later (H2). Each treatment had a control group (C1 and C2, respectively). In Exp. 1, two genotypes with contrasting crop cycles were compared. In Exp. 2, 50% defoliated plants (D‐) were compared with intact plants (D+) under the same temperature treatments using one genotype. Average daily maximum temperature of heated treatments for both experiments was 37.9 ± 0.79ºC, 5.8ºC higher than the controls. Independently of the period, thermal stress had a negative impact on photosynthesis in both genotypes through an acclimation response, reducing it up to 35% compared with controls when heath‐stressed and control plants were measured at the same temperature. Instant responses to temperature were not observed. This decrease was mainly determined by mesophyll conductance, and no recovery was observed 15 days after the end of treatments. Photosynthesis depletion was conditioned by the source/sink ratio, showing a complete recovery only in defoliated plants. It is concluded that thermal stress had a negative acclimation impact on photosynthesis, without responses to changes in instant temperature, and this acclimation is modulated mainly by mesophyll conductance. [ABSTRACT FROM AUTHOR]

Identifying the physiological traits indirectly selected during the search for high-yielding maize hybrids is useful for guiding further improvements. To investigate such traits, in this study we focused on the critical period of kernel formation because kernel number is the main yield component affected by breeding. Our results show that breeding has increased the number of florets per ear and ear growth rate but not the vegetative shoot growth rate, suggesting localised effects around the ear. Consistent with this possibility, breeding has increased the net CO2 exchange of the ear leaf in field-grown crops grown at high population densities. This response is largely accounted for by increased light interception (which increases photosynthesis) and by reduced rates of respiration of the ear leaf in modern hybrids compared to older ones. Modern hybrids show increased ear-leaf area per unit leaf dry matter (specific leaf area), which accounts for the reduced respiratory load per unit leaf area. These observations are consistent with a model where the improved ear leaf CO2 exchange helps the additional florets produced by modern hybrids to survive the critical period of high susceptibility to stress and hence to produce kernels. [ABSTRACT FROM AUTHOR]