The effects of root confinement on the growth, assimilate partitioning, mineral nutrition, nitrogen assimilation, and water relations of tomato plants (Lycopersicon esculentum Mill.)

Abstract

A large number of experimental investigations have established that the root growth in many species is very sensitive to mechanical impedance or to confinement in small volumes, but little is known about the consequent effects on growth of the whole plant and the mechanism involved. The results reported in this thesis suggest a mechanism by which root confinement affected the growth and assimilate partitioning, mineral nutrition, uptake and assimilation of nitrogen, and water relations of tomato plants (Lycopersicon esculentum Mill.) grown in solution culture. The experiments were conducted using either continuously circulating-nutrient solution or static-nutrient solution culture in a temperature controlled glasshouse.

Tomato plants were grown in either 4500 ml (nonconfined) or 75 ml (confined) containers filled with aerated nutrient solutions. Plants were allowed to grow with lateral shoots (Experiment I) or as single stems (Experiments II, III, and IV). Root, leaf, stem, and fruit dry weights, leaf area, root length and number, mineral uptake, nitrogen assimilation and composition of nitrogenous compounds were measured. In addition, transpiration, leaf-air temperature differences, and leaf diffusive resistances were measured frequently. Root and shoot hydraulic resistances were estimated from measurements of leaf water potential and transpiration rate, using a steady-state technique.

Confining root growth to small containers substantially reduced the average rate of increase in dry weight production of shoots and roots of tomato plants. Because root growth rate was affected most, confined plants had greater shoot:root and top:root ratios. Root confinement reduced the average rate of increase in total root length, root number, leaf number, stem length, and stem diameter. There was a much higher dry weight per unit length of roots and stem, and per unit area of leaves of confined plants.

Root confinement also generally reduced the total amount and concentration of K, Ca, Mg, P, and N and reduced the rate of translocation of these minerals to the aerial parts of the plants. The concentration of Ca and P were much higher in the roots and lower in all parts of the shoot of confined plants. Although root confinement reduced the rate of uptake of N (NO₃ and NH₄) and water, the rate of uptake of N was affected more than water resulting in a lower ratio of N to water uptake. The results in this work revealed that at lower levels of solution N (52ppm or 104ppm), root confinement affected the rate uptake of NO₃ and NH₄ nitrogen more than their assimilation, but at higher level of solution N (208ppm), the assimilation of NO₃ and NH₄ may become a limiting factor affecting the growth of these plants.

The results in this work suggest that the mechanism by which root confinement caused these effects was due to drought stress. The sharp reduction in the growth rate of confined roots triggered a secondary series of physiological responses which led to a larger increase in the hydraulic resistances to water flow in the root and shoot systems (by a factor of 7 to 8). The larger hydraulic resistance within the confined root system (2.04 x 1010 s m⁻²) than that in the shoot system (1.61 x 1010 s m⁻²) led to a more negative leaf water potential, increased leaf diffusive resistance, reduced accumulative transpiration (by a factor of 2.5) and reduced net assimilation rate (by a factor 2.5). Because of the physical limitation imposed on root expansion and because of the large hydraulic resistance within the plant which limited the capacity of leaves to act as a sink, much more of the available photosynthates in the confined plants were either diverted in the fruits or stored in the stem. Thus a combination of root confinement and the consequent drought stress progressively and simultaneously reduced the total assimilating capacity of the plants. The rate of photosynthate translocation was altered by drought stress through reduction in the strength activity of the individual sinks and through affects on source capacity to supply assimilate. The drought stress experienced by the confined plants affected the utilization of photosynthate by altering the efficiency with which photosynthate is converted to new growth. The drought stress in confined plants would have reduced the uptake and rate translocation of mineral nutrients through an effect on transpiration and through an effect on the active transport mechanism by suppressing plant growth thereby reducing plant demand. The relatively lower growth rate of the confined plants with a smaller root system resulted in a smaller leaf transpiration surface and a disproportionate decrease in the N demand.