There has been little written on the penetration of light through soil. In this review, we attempt to collate most of the work that has been published on this topic in order to stimulate further research and to clarify the often-confused literature.

Light penetration can be measured directly with, for example, a spectroradiometer, or indirectly by using germination of light-sensitive seeds or the presence of growing algae as bioindicators. Although the penetration of light through soil is greatly affected by factors such as soil moisture content and particle size and colour, it generally appears that physiologically and ecologically significant amounts of light rarely penetrate more than 4–5 mm through the soil, and may often penetrate much less than this. Any penetration beyond 10 mm would generally not be significant, especially as most soils are covered with litter, algae or lichens, or are shaded from direct sunlight. However, for some light-stimulated geo-tropic responses of roots, which can be sensitive to very low fluences, the penetration of light to greater depths could well be significant. The role of light in soil in directing root growth is also discussed.

The penetration of light through soil

The effects of clearfelling a tropical rainforest and establishing pasture on soil microbial biomass and nitrogen transformations were assayed monthly over 1 yr in three adjacent systems in the central Amazon region: (1) virgin rainforest; (2) slashed-and-burnt forest; and (3) recently established pasture. The amounts of soil organic matter (SOM) and soil microbial biomass-carbon (biomass-C) were substantial in all systems. Total soil-C ranged between 1.9 and 5.2% depending on management and soil layer, whereas biomass-C ranged between 3.5 and 5.3% of total soil-C. The soil biomass-C decreased upon slashing-and-burning to 64% of its original value (1287 μg g −1 ) in the forest (0–5 cm soil layer) and increased after establishment of pasture to 1290 μg g −1 , but remained unchanged in the deeper 5–20 cm soil layer. No significant seasonal variation was measured in any system or soil layer. Soil respiration responded to management like microbial biomass-C but varied significantly over the season with the smallest respiration found in the driest month (October) and the largest respiration at end of the rains in May. Pools of mineral N varied considerably in all systems and soil layers and displayed identical seasonal variations. The forest topsoil contained the highest amounts (on average 47 μg N g −1 ) and the pasture soil the smallest amounts (on average 24 μg N g −1 ). The transition of the forest ecosystem to a pasture resulted in increased NO 3 − concentrations. Net N-mineralization and net NO 3 − production monitored during short-term laboratory incubations were used as indices of N mineralization and nitrification. No significant differences in N-mineralization indices were measured between systems, but substantial within season variations were recorded in all systems and soil layers. The variations were synchronized in time with extreme net N-mineralization in September and net N-mineralization in October. Significant nitrification indices were measured in all systems. They were identical in the systems, except for small indices found in topsoil of the slashed and burnt area, where, on the other hand, certain localized areas with extreme nitrification rates were detected.

Seasonal variation of soil microbial biomass—The effects of clearfelling a tropical rainforest and establishment of pasture in the central Amazon

Low concentrations of ethylene, comparable with those known to occur in anaerobic soil, inhibited extension of pea roots to a similar extent to that previously reported for barley. Thus pea appeared to be less sensitive to ethylene than some non-leguminous dicotyledons. Perfusion of 10ppm of ethylene through the soil around the roots of pea and white clover resulted in reduced shoot dry weight. Nodulation, and the nitrogenase activity of those nodules which did form, was also greatly reduced. The ecological consequences of the sensitivity of nodulation and nitrogen fixation to ethylene, and the possible significance for quantitative studies involving the acetylene-reduction assay, are discussed.

Effects of ethylene on root extension and nodulation of pea ( Pisum sativum L.) and white clover ( Trifolium repens L.)

A symbiotic nitrogen-fixing relationship has been established in culture between soybean cells and the bacterium Rhizobium.

Establishment of symbiosis between Rhizobium and plant cells in vitro.

Practical solutions are described to problems incurred in growing cultivars of cowpea, soyabean, lima bean and pigeon pea under simulated tropical conditions in controlled environments, namely (a) ‘Saxcil’ growth cabinets in which daylength, day and night temperatures, light intensity and quality, relative humidity and CO 2 , concentration are precisely controlled and can be varied over time; (b) compartmentalized glasshouses and adjoining dark compartments where daylength, and day and night temperatures are automatically controlled and night-break illumination can be provided, and (c) heated plastic film houses in which daylength-insensitive tropical legumes can be grown to maturity during the UK summer months since the environment of the wet tropics can be simulated in all respects other than daylength.

Plant Husbandry and Management Techniques for Growing Grain Legumes Under Simulated Tropical Conditions in Controlled Environments

Nodulation of pea and broad bean plants grown in the light was found to be reduced when the roots were exposed to far-red light for 5–15 minutes daily during 5 consecutive days following inoculation with nodule bacteria. Similar results were obtained following a single exposure to far-red light during a period of 15 minutes at the 3rd or 4th day after inoculation. When the roots were exposed to far-red light either before inoculation or during the first two days afterwards there were either no effects or only slight effects on nodulation The inhibitory effect of far-red light on nodulation was partly reduced by subsequent exposure to red light, provided that the same part of the plant was exposed to both red and far-red light,viz either the root or the shoot. When different parts of the plant were exposed to red and far-red light respectively, there was no interaction between the two kinds of light on nodulation. Plants whose roots were exposed to far-red light did not subsequently show stem elongation. Nodules were found to develop on the roots of pea plants grown in the dark, provided that the plants were kept at or below 22°C. At 25°C nodulation was almost absent. Nodulation was decreased by addition of kinetin and IAA. In contrast to plants grown in the light pea plants grown in the dark, inoculated with either an effective or ineffective strain of Rhizobium, developed equal numbers of nodules. Exposure to red light slightly increased the percentage of nodulated plants but decreased the number of nodules per plant. Exposure to far-red light slightly decreased both the percentage of nodulated plants and the number of nodules per plant. The effect of far-red light was counteracted by red light andvice versa.

T. A. Lie

Papers

Non-photosynthetic effects of red and far-red light on root-nodule formation by leguminous plants