玉米初生根向水性诱导优化试验研究

为了研究湿度梯度对根系向水性反应的影响,采用Takahashi and Scott于1993年创建的方法,设置以下3个试验:1)向水性诱导物不同倾斜角试验;2)根系距向水性诱导物不同距离试验;3)根尖距底部饱和K2CO3溶液不同距离试验。同时,还研究了根长和根系延伸速率对根系向水性弯曲的影响。结果表明,用饱和K2CO3溶液控制湿度时根系的向水性弯曲度明显大于纯水。随着诱导物倾斜角的增大,向水性弯曲增强。与距诱导物3 mm和6 mm相比,根系直接接触诱导物时表现出最大的向水性反应。与根尖距底部盐溶液6 cm相比,相距4 cm时向水性弯曲度增大,这些与根尖周围的湿度梯度增大有关。当根长为1.0、1.5、2.0、2.5、3.0 cm时,短根比长根表现出更大的向水性反应,这可能与其较慢的延伸速率为根系对湿度梯度的反应提供了更充足的时间有关。为了验证这个假说,用相同长度的根系、通过控制不同温度进行试验,结果表明根系的向水性弯曲随温度升高而降低。可见,玉米初生根的向水性反应受环境和根系发育阶段两方面影响。当根系相距诱导物较近、根系周围的湿度梯度较大时,根系向水性反应更强。而且,具有较小延伸速率根系的向水性反应更大。考虑到干旱条件下根系伸长慢、且土壤中湿度梯度大,因而可以认为干旱条件下根系的向水性生长在玉米吸收水分中有重要作用。同时,对根系向水性诱导方法的优化有助于其生理机制的进一步研究。

Optimizing hydrotropic response in the maize primary roots

Roots can sense and then grow toward a water source. This extraordinary capability of roots - hydrotropism has not been well studied. The objective of this study was to examine the environmental and physiological factors that might affect hydrotropic response in the maize primary roots. A system that was first established by Takahashi and Scott in 1993 was adopted in this study. Maize seedlings with uniform root length were fixed on a vertically placed wet pad with a slanted angle at the bottom. The root tips were aligned with the upper edge of the slanted pad bottom. The pad with seedlings was then vertically placed in a small chamber. The humidity of the chamber was controlled by placing a reservoir of either saturated K₂CO₃ solution or pure water at the bottom of the chamber. The root continued to elongate downward in the darkened, closed chamber. Due to the higher humidity of the air near the slanted bottom of the wet pad (compared to the relatively dry air on the other side of the root), the root curved and grew toward the wet pad and grew away from the drier air. The curvature away from the vertical (gravity) axis was measured to quantify the strength of the hydrotropic response. To test the effect of the humidity gradient on the hydrotropic response, three experiments were conducted: 1) roots were grown on the wet pads with different slanted angels; 2) the roots were placed a few mm away from the wet pad; and 3) the roots were positioned at different distances from the saturated K₂CO₃ solution. The effect of root length and of the elongation rate on hydrotropic curvature was also tested. Because the results showed that hydrotropic curvature was greater, when the saturated K₂CO₃ solution was used to control the chamber humidity compared to pure water, all the other experiments in this study were conducted by using the saturated K₂CO₃ solution. When pads with different slanted degrees were compared (40°, 50°, or 60°), the root curvature was greater as the slanted angle increased (Note: The greater the slanted angle is, the closer the root tip is to the hydrostimulant - the wet pad.) Also, the roots showed the best hydrotropic curvature when the roots directly touched the hydrostimulant compared to when the roots were placed 3 mm or 6 mm away from the hydrostimulant. Finally, root curvature was greater when the distance between the root tip and the salt solution was 4 cm compared to 6 cm. When roots of different lengths (1.0, 1.5, 2.0, 2.5, or 3.0 cm) were compared, the shorter roots showed much better hydrotropic response than the longer roots did. Since maize primary roots elongate faster as the roots get longer during germination (before reaching a relatively steady rate), it is possible that a better hydrotropic response in the shorter roots might be explained by a slower elongation, providing more time for the roots to respond to the humidity gradient. To test this hypothesis, roots of the same length, but with different elongation rates, were tested for their hydrotropic response. The root elongation rate was controlled by varying growth temperatures, and the time was adjusted to allow the roots at different temperatures to reach the same final length before root curvature was determined. The results showed that root curvature decreased as the growth temperature increased. In summary, this study demonstrated that hydrotropic response in the maize primary roots is affected by environmental factors as well as the developmental stages of the roots. Maize roots showed a stronger hydrotropic response when the roots are closer to the hydrostimulant and when the humidity gradient around the roots becomes steeper. Moreover, roots with slower elongation showed greater hydrotropic response. Since roots elongate slower under drought, and the humidity gradient is steeper in dry soil, it can be hypothesized that hydrotropism may play an important role in water acquisition in maize plants under drought. The findings from this study also allow us to optimize an experimental system that can be used to study the mechanisms of hydrotropism in the future.