Interaction of Temperature and Light in the Development of Freezing Tolerance in Plants

Abstract Freezing tolerance is the result of a wide range of physical and biochemical processes, such as the induction of antifreeze proteins, changes in membrane composition, the accumulation of osmoprotectants, and changes in the redox status, which allow plants to function at low temperatures. Even in frost-tolerant species, a certain period of growth at low but nonfreezing temperatures, known as frost or cold hardening, is required for the development of a high level of frost hardiness. It has long been known that frost hardening at low temperature under low light intensity is much less effective than under normal light conditions; it has also been shown that elevated light intensity at normal temperatures may partly replace the cold-hardening period. Earlier results indicated that cold acclimation reflects a response to a chloroplastic redox signal while the effects of excitation pressure extend beyond photosynthetic acclimation, influencing plant morphology and the expression of certain nuclear genes involved in cold acclimation. Recent results have shown that not only are parameters closely linked to the photosynthetic electron transport processes affected by light during hardening at low temperature, but light may also have an influence on the expression level of several other cold-related genes; several cold-acclimation processes can function efficiently only in the presence of light. The present review provides an overview of mechanisms that may explain how light improves the freezing tolerance of plants during the cold-hardening period

TIMING OF MACROELEMENT SPRAYS FOR OPTIMUM ABSORPTION BY CITRUS

Six percent solutions of 12-6-6 liquid fertilizer were applied in the morning, at noon, and in the evening to 3-yearold 'Hamlin' and 'Valencia' orange, Citrus sinensis (L.) Osbeck, trees on sour orange, C. aurantium L., and Volkamer lemon, C. limon Burm. f., rootstock. The trees were in pots in the open. Leaf samples were taken immediately before spraying and 7 days after spraying and analyzed for N, P and K. The experiment was repeated 3 times. Rain within 12 hours of application sharply reduced uptake of the applied material. The best application time under the semi-tropical conditions of the experiment (36°C day temperature, 23°C night temperature, relative humidity 30 to 100%) was evening. ----- Soluciones de seis por ciento de abono liquido 12-6-6 fueron applicados por la manana, al mediodta y al anochecer a arboles de naranja (Citrus sinensis (L.) Osbeck) 'Hamlin' y 'Valencia' injertados en patrones de naranja agria (C. aurantium L.) y limon Volkamer (C. limon Burm. f.), Se mantuvo los arboles in cestos al aire libre. Se tom6 muestras de hojas imediatemente antes y siete dras despues de la aplicaci6n foliar para analisis de nitrogeno, f6sforo y potasio. EI experimento fue repetido tres veces. Uuvia dentro de doce horas de aplicaci6n redujo la absorci6n de los nutrientes aplicados. Bajo las condiciones semi-tropicales de experimento (36°C durante el dia, 23°C de noche) el mejor tiempo para aplicar la materia era al anochecer