Plant Root Responses to Shade and Temperature

From April 1998 to March 2001 I worked as a post-doc at The University of York. The project was to follow up results from a previous TIGER project which indicated that root growth and respiration may be determined by received radiation flux rather than soil temperature as commonly thought. The aim of my work was initially to provide further evidence of this by using a combined controlled environment and field study approach. Subsequently, a possible role of nitrogen was to be examined and a model parameterised. The following is adapted from the final report to the funding body. You can skip to the conclusions here.

Three controlled environment experiments lasting 7, 20 and 40 days examined the ability of Holcus lanatus roots to acclimate to a temperature change of around 10 C and shading up to 70%, both as a community and as individuals. In the two longer experiments, root dry weight (DWt) and length were not affected by transfer to low temperatures (Fig 1), whereas shoot DWt was 25-50% lower. Leaf area and specific leaf area (SLA) were also lower in transfered plants. These CE experiments show that root growth of H. lanatus can acclimate to a large temperature change over days to weeks, where other factors are not limiting, despite a large reduction in shoot growth.

Fig. 1 - Root DWt of Holcus lanatus grown as single plants (circles, right axis) or in a community (triangles, left axis) at 20 C (closed symbols) or grown at 20 C then transferred to 8 C (communities) or 10 C (single plants) (open symbols).

Shade treatments in the CE experiments mostly reduced shoot DWt but always increased SLA. Shading also generally lowered root DWt, but increased specific root length (SRL), so that root length was maintained. However, shaded plants transferred to low temperature had the lowest root biomass and even with an increase in SRL had shorter root length than unshaded plants.

In the 40-d experiment, plants received CO₂ with a high δ¹³C for 12 h at the start and end of the experiment. The δ¹³C of plant tissue showed that shading did not alter the root:shoot ratio of C allocation, either when first exposed to shading or after 30 d of shade, despite a difference in root fraction. This ratio was 1.3 in warm-grown and 1.5 in cold-treated plants: instantaneous allocation of C was therefore greater to roots than shoots in all treatments. Shade reduced total carbon uptake at LT, but not in plants kept at 20 C.

For root growth to acclimate to temperature, root respiration might also have at least partially acclimated, as growth accounts for 15-45% of root respiration. Acclimation of respiration occurred in all three CE experiments: there was no difference in respiration between temperature treatments when measured at growth temperature (Fig. 2a). In each experiment root respiration was measured at two temperatures. The expected response (an increase in respiration as measurement temperature increases) was seen (Fig. 2b), and there was no difference in Q₁₀ between control plants and those grown at low temperature. Q₁₀ ranged from 1.2 to 3.0 with 1.8 being a typical value. As the Q₁₀ of respiration was unaffected by treatment, respiratory flux was probably maintained despite the transfer, either because the respiratory capacity in warm-grown plants was strongly regulated by ATP turnover, or because the low temperature plants increased the activity/quantity of respiratory enzymes. Respiration is correlated with N concentration, [N], as seen even in the short term experiment (r² = 0.1, p<0.001; cf. Fig. 2b & c). Expressing root respiration per unit N showed that the difference between the measurements of acclimated and non-acclimated plants could be accounted for by variation in root [N] (Fig.2d).

Fig. 2 - Root respiration and N concentration of H. lanatus grown at 20 C (closed symbols) or 20 C and transferred to either 10 C (open circles) or 8 C (open triangles): a) root respiration at growth temperature (2 experiments), b) root respiration at 20 C, c) root %N, and d) root respiration at 20 C on a N basis.

We quantified the effect of [N] on acclimation in a CE experiment with 2 N supply rates (zero and low N), and shade and temperature treatments. As previously, shoot DWt was reduced by transfer to 10 C, but root DWt did not acclimate in either N limiting treatment. Shade responses were as before, but there were interactions with N. Shoot DWt was reduced by N starvation in unshaded plants, but shaded plants showed no response to N, even though they had similarly low [N]. Equally, root fraction increased significantly under zero N at both temperatures in the unshaded plants, but not in shaded plants. This difference in apparent allocation differs from the results of the C analyses (above), because the latter measures instantaneous C allocation, while biomass ratio reflects structural and stored C.

The controlled environment studies were complemented by field studies using a soil warming system built by D. Benham CEH Merlewood, which tracked ambient soil temperature, combined with shade frames set up in an experimental grassland (Fig. 3). Soil heating was raised by 3C above ambient at 2 cm depth with consequent heating to 20 cm depth. A weather station together with a Delta-T datalogger was used to measure environmental variables. The design was a randomised block split-plot: each plot (shaded/unshaded) had heated and unheated sub-plots. In shaded plots, the ambient sub-plot was warmed to the temperature of ambient unshaded plots. After seeding (with 4 grasses and 2 dicots.), the site became dominated by H. lanatus and Plantago lanceolata. Two 8-week experiments examined the effects of shade and heating on root responses in spring and autumn when the greatest heating impact was expected. Between experiments, treatments were removed.

Fig. 3 - Walled Garden experimental setup.

Root demography was different in spring and autumn: root turnover and root accumulation (the net increase in number of roots) at a depth of 2 cm was increased by heating in spring, but root deaths were increased with heating in autumn. Over these 8-week periods, shading did not alter root demography, although it did reduce root DWt and length in spring.

The third field experiment, with two shade treatments (60% and 85%), ran from January 2000 to January 2001. There were no carry-over effects from previous experimental runs. Root numbers increased rapidly in all plots from mid February. Then stabilised in late March in shaded plots, but increased for several weeks longer in unshaded plots (Fig 4a & b). Soil warming slightly delayed the onset of root growth in spring, but had a greater effect in autumn when root accumulation declined much further in warmed plots, to below the initial root number. This effect was most pronounced in the shaded plots. Plots receiving shading and heating lost roots throughout the winter and root numbers never stabilised.

Fig. 4 - Root accumulation of Holcus and Plantago dominated grassland at ambient soil temperature (a) and with soil warming giving 3 C above ambient (b). Shading treatments were ambient light (open symbols), 65% shade (grey symbols) and 85% shade (black symbols).

Root DWt in shaded plots matched root number and was lower than in unshaded plots. However, despite the lack of a heating effect on root number for most of the year, root DWt was consistently lower in heated plots. The difference between demography and biomass probably arises because root number was measured at a depth of 2 cm, whereas DWt was averaged over the top 10 cm.

Photosynthesis was measured at saturating light (A_sat) throughout the experiment on both H. lanatus and P. lanceolata. CO₂ and light response curves for H. lanatus were also measured during the summer. A_sat, stomatal conductance and transpiration were all unaffected by soil warming. Some acclimation to shade occurred in V_Cmax and J_max (Table 1). Even so, carbon acquisition must have been reduced by shading, with a reduction in C supply to the root system.

Reduced root growth in heated plots is better explained in terms of increased root respiration rather than C acquisition, because warming did not affect photosynthesis. However, respiration was not significantly affected by any treatment.

Significant relationships in stepwise regressions of root demographic variables on environmental variables, such PAR, air temperature and soil temperature were primarily with received PAR (Table 2). Soil temperature was not a significant explanatory variable for either root growth or respiration.

In these studies there was a strong positive relationship between root growth and PAR, which was always dominant over any temperature effect on root growth, despite a reduction in canopy biomass at low temperature. Where LT and shade were applied together in the absence of other stresses, root growth was typically less than when the treatments were applied separately, even where no temperature response was seen in the roots of an unshaded canopy. Root respiration rapidly acclimated to temperature change irrespective of acclimation of root growth. In optimal conditions, this acclimation could occur in 24 hours. Shading did not affect root respiration; respiration of the whole root system would then be determined by root growth, which in turn is governed by PAR. This supports the EUROFLUX finding that soil respiration is determined by primary productivity rather than temperature.

In controlled environment experiments, respiratory acclimation depended on root [N]. When N was limiting, root growth acclimation to temperature was reduced. Further, where N-stressed plants were shaded, root fraction was unable to respond to the N limitation, whereas root fraction of unshaded plants increased. There are complex interactions between N supply, PAR, temperature and root growth.

The root system clearly can respond to and acclimate to temperature fluctuation. However, the ability to acclimate depends on both N concentration and C supply. Where C supply is inadequate, root growth cannot be maintained and becomes subject to constraints on shoot growth and photosynthesis.

These findings have large implications. The IPCC predicts a global mean temperature rise of 1.4 to 5.8 C this century. Soil temperatures will reflect rises in air temperature, and are assumed in models to alter rates of many biotic and edaphic processes, including root growth, root respiration, nutrient uptake and N mineralisation. Understanding the mechanism and extent of acclimation is essential to accurate prediction of the impacts of changing climate.

© 1996-2006 Everard Edwards
This page was last updated Tue Sep 3 13:40:03 EST 2002.