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Abstract
Se determinaron los efectos del estrés hídrico, alta temperatura y su combinación sobre la acumulación de biomasa (BA), rendimiento de grano (RG) y sus componentes directos [número de granos por mazorca (NG) y peso individual de grano (PIG)], así como el índice de cosecha (IC) en cuatro poblaciones de maíz tropical. Los resultados indican que el estrés hídrico no tuvo efectos significativos sobre las diferentes variables estudiadas, debido a la ausencia de un estrés hídrico severo aplicado durante la floración, fundamentado por la presencia de precipitaciones pluviales. El factor de mayor importancia en el estudio fue la alta temperatura, reduciendo de manera significativa todas las variables estudiadas excepto PIG. El estrés por alta temperatura redujo la acumulación de BA en un 45%, siendo la mazorca el órgano más afectado (66%); además se encontró una reducción del RG (74%) debido principalmente a una pérdida del NG del mismo nivel. Como resultado de la disminución de la BA y el RG, el IC también fue reducido (52%). Todas las poblaciones de maíz estudiadas mostraron susceptibilidad al estrés por alta temperatura durante la floración, encontrando grandes pérdidas del RG y la acumulación de BA. Esta respuesta indica que la selección para mejorar la tolerancia a la sequía en estas poblaciones no implica necesariamente obtener también una mayor resistencia a las altas temperaturas.
Our objective was to determine the sensitivity of components of the photosynthetic apparatus of maize (Zea mays), a C4 plant, to high temperature stress. Net photosynthesis (Pn) was inhibited at leaf temperatures above 38 degrees C, and the inhibition was much more severe when the temperature was increased rapidly rather than gradually. Transpiration rate increased progressively with leaf temperature, indicating that inhibition was not associated with stomatal closure. Nonphotochemical fluorescence quenching (qN) increased at leaf temperatures above 30 degrees C, indicating increased thylakoid energization even at temperatures that did not inhibit Pn. Compared with CO(2) assimilation, the maximum quantum yield of photosystem II (F(v)/F(m)) was relatively insensitive to leaf temperatures up to 45 degrees C. The activation state of phosphoenolpyruvate carboxylase decreased marginally at leaf temperatures above 40 degrees C, and the activity of pyruvate phosphate dikinase was insensitive to temperature up to 45 degrees C. The activation state of Rubisco decreased at temperatures exceeding 32.5 degrees C, with nearly complete inactivation at 45 degrees C. Levels of 3-phosphoglyceric acid and ribulose-1,5-bisphosphate decreased and increased, respectively, as leaf temperature increased, consistent with the decrease in Rubisco activation. When leaf temperature was increased gradually, Rubisco activation acclimated in a similar manner as Pn, and acclimation was associated with the expression of a new activase polypeptide. Rates of Pn calculated solely from the kinetics of Rubisco were remarkably similar to measured rates if the calculation included adjustment for temperature effects on Rubisco activation. We conclude that inactivation of Rubisco was the primary constraint on the rate of Pn of maize leaves as leaf temperature increased above 30 degrees C.
In order to determine the importance of timing of short periods of high temperature and drought on grain weight and grain quality, a glasshouse experiment was carried out in which Schooner barley was exposed to short periods of heat stress (40˚C for 6 h/day for 5 consecutive days) or drought at early grain filling (10–15 days after anthesis, DAA), mid grain filling (20–25 DAA), or late grain filling (30–35 DAA). Individual grain weight was most sensitive to heat stress and drought treatments imposed early in grain filling and was less sensitive to later treatments. The reduction in grain weight was greater under heat stress (average 13%) than under drought in this study (average 6%). Starch was reduced in amount and quality, especially with early stresses during grain filling, but grain nitrogen percentage was similar between treatments.