Abstract Studies focusing on terminal drought combined with heat impacts on plants of agronomic value remain scarce, and even less under field conditions. The objective of this study was to investigate leaf structural and ultrastructural changes induced by heat stress (HS) and drought stress (DS) during seed filling and their relationship with physiological variables and yield determination. Two soybean cultivars were grown in field conditions. During seed filling four treatments were applied, including a control (without manipulation, at ambient temperature and field capacity), HS (episodes exceeding 32°C for 6 h d-1) during 21-d, DS (20% of field capacity soil water content) during 35-d, and HS×DS. Drought principally reduced leaf area, whereas heat decreased leaf thickness, possible as acclimation strategies, but also irreversible reducing CO2 assimilation sites. Both stresses damaged the outer and inner membranes of chloroplasts, causing swollen chloroplasts and accumulation of plastoglobules, loss of chlorophyll content, and negatively affecting chlorophyll fluorescence. Thus, the performance and integrity of the photosynthetic machinery were reduced. Through a morpho-functional perspective and a holistic multiscale approach, our results provide evidence of photosynthesis impairment and yield drops under stressful conditions which were associated with structural and ultrastructural (particularly at the level of chloroplasts) modifications of leaves.

The mixture of strobilurin and triazole (S+T) was applied to 2 contrasting cultivars in susceptibility to frogeye leaf spot (FLS) (DM3700 y DM3810) at growth stages R3 or R3+R5; untreated control and healthy control were also included. Incidence and severity of FLS, yield, grain number (NG), grain weight (GW), grain oil and protein were evaluated. Different levels of severity between DM3700 (susceptible) treatments, and absence of FLS in DM3810 (resistant) treatments were observed. Negative correlation was found between severity and yield. In DM3700 higher values of severity corresponded to lower yields (2117 kg/ha). DM3810 exhibited the higher yields without statistical differences between treatments (3478 kg/ha). Although NG and PG decreased in the untreated control DM3700, NG reductions were more pronounced. No increments of yield, NG and/or PG were attributed to S+T in DM3810. The maximum severity achieved by the more susceptible cultivar was not sufficient to significantly decrease oil and protein. FLS could be controlled on the susceptible cultivar with a single application of S+T at R3. The use of the resistant cv to FLS was effective in controlling the disease, and showed no additional benefits in yield due to fungicide application. It was concluded that FLS did not affect the chemical quality of the grain.

La mezcla de fungicidas estrobilurina y triazol (E+T) se aplicó a 2 cultivares contrastantes en susceptibilidad a mancha ojo de rana (MOR) (DM3700 y DM3810) en los estadíos R3 o R3+R5; se incluyeron además 2 controles: enfermo y sano. Se avaluó incidencia y severidad de MOR, rendimiento, número de granos (NG), peso de granos (PG), aceite y proteína del grano. Se observaron distintos niveles de severidad entre tratamientos de DM3700 y ausencia de MOR en los tratamientos de DM3810. Se encontró correlación negativa entre severidad y rendimiento. En DM3700 los valores más elevados de severidad se correspondieron con rendimientos más bajos (2117 kg/ha). DM3810 exhibió rendimientos elevados y sin diferencias estadísticas entre tratamientos (3478 kg/ha). Aunque NG y PG disminuyeron en el testigo enfermo DM3700, las reducciones del NG fueron más pronunciadas. No hubo incrementos de rendimiento, NG y/o PG atribuidos a E+T en DM3810. La severidad máxima en el cultivar más susceptible no fue suficiente para disminuir significativamente aceite y proteína. MOR pudo ser controlada en el cultivar susceptible con una única aplicación de E+T en R3. El uso del cultivar resistente a MOR resultó efectivo para controlar la enfermedad y no manifestó beneficios adicionales en rendimiento debido a la aplicación de fungicidas. En este trabajo se concluyó que MOR no afectó la calidad industrial del grano.