Departamento de Botánica, Ecología y Fisiología Vegetal, Area de Fisiología Vegetal, Facultad de Ciencias, Universidad de Córdoba, Campus de Rabanales, edificio Celestino Mutis (C4), Córdoba, Spain.
Physiol Plant. 2010 Mar;138(3):256-67. doi: 10.1111/j.1399-3054.2009.01336.x. Epub 2009 Nov 23.
Different parameters which vary during the leaf development in sunflower plants grown with nitrate (2 or 20 mM) for a 42-day period have been determined. The plants grown with 20 mM nitrate (N+) showed greater leaf area and specific leaf mass than the plants grown with 2 mM nitrate (N-). The total chlorophyll content decreased with leaf senescence, like the photosynthetic rate. This decline of photosynthetic activity was greater in plants grown with low nitrogen level (N-), showing more pronounced senescence symptoms than with high nitrogen (N+). In both treatments, soluble sugars increased with aging, while starch content decreased. A significant increase of hexose to sucrose ratio was observed at the beginning of senescence, and this raise was higher in N- plants than in N+ plants. These results show that sugar senescence regulation is dependent on nitrogen, supporting the hypothesis that leaf senescence is regulated by the C/N balance. In N+ and N- plants, ammonium and free amino acid concentrations were high in young leaves and decreased progressively in the senescent leaves. In both treatments, asparagine, and in a lower extent glutamine, increased after senescence start. The drop in the (Glu+Asp)/(Gln+Asn) ratio associated with the leaf development level suggests a greater nitrogen mobilization. Besides, the decline in this ratio occurred earlier and more rapidly in N- plants than in N+ plants, suggesting that the N- remobilization rate correlates with leaf senescence severity. In both N+ and N- plants, an important oxidative stress was generated in vivo during sunflower leaf senescence, as revealed by lipid peroxidation and hydrogen peroxide accumulation. In senescent leaves, the increase in hydrogen peroxide levels occurred in parallel with a decline in the activity of antioxidant enzymes. In N+ plants, the activities of catalase and ascorbate peroxidase (APX) increased to reach their highest values at 28 days, and later decreased during senescence, whereas in N- plants these activities started to decrease earlier, APX after 16 days and catalase after 22 days, suggesting that senescence is accelerated in N-leaves. It is probable that systemic signals, such as a deficit in amino acids or other metabolites associated with the nitrogen metabolism produced in plants grown with low nitrogen, lead to an early senescence and a higher oxidation state of the cells of these plant leaves.
已确定在以硝酸盐（2 或 20mM）培养 42 天的向日葵植株的叶片发育过程中变化的不同参数。用 20mM 硝酸盐（N+）培养的植物的叶面积和比叶重比用 2mM 硝酸盐（N-）培养的植物大。总叶绿素含量随叶片衰老而降低，与光合速率一样。在低氮水平（N-）下生长的植物中，这种光合作用活性的下降更大，表现出比高氮（N+）更明显的衰老症状。在两种处理中，可溶性糖随老化而增加，而淀粉含量减少。在衰老开始时观察到己糖向蔗糖的比例显著增加，而在 N-植物中比在 N+植物中更高。这些结果表明，糖衰老的调节依赖于氮，支持叶片衰老受 C/N 平衡调节的假说。在 N+和 N-植物中，年轻叶片中的铵和游离氨基酸浓度较高，并在衰老叶片中逐渐降低。在两种处理中，天冬酰胺，以及较低程度的谷氨酰胺，在衰老开始后增加。与叶片发育水平相关的（Glu+Asp）/（Gln+Asn）比值的下降表明氮的动员更大。此外，该比值在 N-植物中的下降早于在 N+植物中，并且下降速度更快，这表明 N-再动员率与叶片衰老的严重程度相关。在 N+和 N-植物中，体内光合作用叶片衰老过程中产生了严重的氧化应激，如脂质过氧化和过氧化氢积累所揭示的。在衰老叶片中，过氧化氢水平的增加与抗氧化酶活性的下降平行发生。在 N+植物中，过氧化氢酶和抗坏血酸过氧化物酶（APX）的活性增加，在 28 天时达到最高值，随后在衰老过程中下降，而在 N-植物中，这些活性更早开始下降，APX 在 16 天后，过氧化氢酶在 22 天后，这表明氮叶的衰老加速。可能是系统信号，例如与低氮植物中产生的氮代谢相关的氨基酸或其他代谢物的缺乏，导致这些植物叶片的细胞早期衰老和更高的氧化状态。
