Plants plastically regulate resource allocation patterns and show various physiological and morphological traits in response to multiple environmental factors, such as availabilities of light, soil nitrogen, and water. In this thesis, I investigated actual resources and biomass allocation patterns for plants in response to these factors and criteria which determine the allocation patterns theoretically and experimentally. Further, a mechanism which regulated these allocation patterns was discussed.

In Chapter 1, it was investigated that whether changes in morphological and physiological traits of plants such as leaf to root ratio (L/R), leaf nitrogen content per area (N(area)), and leaf mass per area (LMA), in response to light and nitrogen availabilities were optimal biomass allocation which maximizes whole-plant relative growth rate (RGR). Here, I developed a biomass allocation model based on previous studies to predict optimal L/R and N(area). In the model, net assimilation rate (NAR) was determined by light-photosynthesis curve, light availability measured during experiments, and leaf temperature affecting the photosynthesis and leaf dark respiration rate in high and low-light environments. Two pioneer trees, Mulberry (Morus bombycis) and trident maple (Acer buergerianum), were grown in various light and nitrogen availabilities in an experimental garden and used for parameterizing and testing the model predictions. They were grouped into four treatment groups (relative photosynthetic photon flux density, RPPFD 100% or 10% × nitrogen-rich or nitrogen-poor conditions) and grown in an experimental garden for 60 to 100 days. The model predicted that optimal L/R is higher and N(area) is lower in low-light than high-light environments when compared in the same soil nitrogen availability. Observed L/R and N(area) of the two pioneer trees were close to the predicted optimums. From the model predictions and pot experiments, I conclude that the pioneer trees, Mulberry and trident maple, regulated L/R and N(area) to maximize RGR in response to nitrogen and light availability.

In Chapter 2, I tested whether saplings of devil maple (Acer diabolicum) produce fine root, which absorb water, more than enough to meet water demand for photosynthesis in leaves. If that's the case, stomatal conductance (Gs) and photosynthetic rate (A) would be affected little even if fine root biomass was decreased to some extent. Therefore, effects of decreasing fine root biomass on Gs and A were evaluated by simulation models and laboratory experiments using saplings of devil maple grown in high- and low-light environments. A-Gs relationships, hydraulic conductance of each organ were determined and used for the simulations. In the laboratory experiments, Gs of each sapling was measured with stepwise decrease in fine root biomass in a growth chamber where light intensity, temperature, and relative humidity were regulated at constant. The model predicted that Gs and consequently A decreased little even if fine root biomass was decreased to some extent. Furthermore, results of laboratory experiments were almost consistent with the model simulations. In conclusion, it was suggested that fine root biomass to leaf biomass was more than enough to meet the water demand for maintaining maximum photosynthetic rate for saplings of devil maple in high- and low-light environments.

Chapter 3 aimed to clarify allocation patterns of nitrogen and carbohydrate for trees under heterogeneous light environments using saplings of devil maple which had Y-shaped two branches. Saplings of Y-shaped devil maple were transplanted to pots in the experimental field and grown in following light treatment from 2009 to 2011. To create simplest heterogeneous light environments within the individual saplings, one branch was in 100% RPPFD (HL-branch) and the other branch was in 10%RPPFD (HS-branch) for HLS saplings. For comparison, both branches were in 100%RPPFD (L-branch) and 10%RPPFD (S-branch) for WL and WS saplings, respectively. Leaf nitrogen content per area (N(area)) and stem volume of each branch were measured throughout growth period. In addition, net assimilated glucose (NAG) was estimated by a photosynthesis model, where relationships between photosynthesis parameters and N(area) and actual meteorological datasets were considered, for each growth period. Throughout the growth period, concentrative allocation of nitrogen to HL-branch and suppressive allocation of nitrogen to HS-branch were observed for HLS saplings when compared with L-branch for WL and S-branch for WS saplings. It was also shown that there were highly-correlated relationships between branch stem growth and NAG. As a result, growth of HL-branch with highest N(area) was largest and that of HS-branch with lowest N(area) was smallest. It was also shown that allocation patterns of current photosynthate between branch growth and root growth was determined by local light environments, and not affected by light environments of the other branch. As a whole, whole-plant growth was increased by these resource allocation patterns for HLS saplings in heterogeneous light environments. These results suggest that tree canopy would be developed through such resource allocation patterns also in natural heterogeneous light environments.

Finally, in the section of general discussion, a mechanism regulating biomass allocation in response to changes in external and internal environmental factors was discussed. I particularly focused on allocation between shoot and root, and consequent L/R and N(area), and a probable model was designed by theoretical approach. It was suggested that plants need to recognize light availability and net assimilation rate in leaves, specific nitrogen absorption rate in roots, and current amount of leaves and roots or L/R. It was simulated that L/R could be optimized in response to changes in external and internal environments if these information were recognized and integrated per unit leaf area basis and transmitted to shoot apical meristem.

本論文は3章からなり、植物体の資源分配パターンが複合的な環境要因に応じてどのような規範に応じて決定されているのかという問題に対し、数理的モデル的による理論的な解析と、栽培した植物を用いた理論の検証を行なっている。

第1章では、光環境および土壌窒素条件に応じて植物体の葉/根比が大きく変化する現象について解析した研究が述べられている。本章では、植物体の葉/根比および葉の窒素濃度は、植物個体レベルの相対成長速度を最大化させるような資源分配パターンであることを示し、特に弱光環境下で葉/根比が高くなる理由を初めて理論と実証から明らかにすることができた。

第2章では、根の水吸収機能と、葉の光合成速度と蒸散速度の関係に着目し、葉に対する根の量は水吸収のためには十二分に作られていることを、数理モデルおよび植物体の根の切除実験によって明らかにされている。多くの植物において、水供給の不足に対して根の量を極端に増加させることはないことが知られていたが、これは最初から根の量を多めに作っているという植物側の戦略であることが示唆された。第1章、第2章を通して、植物体レベルの葉と根への資源分配パターンは、これまでに知られていた土壌からの窒素供給という評価軸に加え、光環境で決定される葉の窒素需要という評価軸を用いることで、統一的に説明できることを明らかにした点で、評価することができる。

第3章では、不均一な光環境下の樹木において、どのような資源分配パターンを通じて樹木全体が成長していくのかという問題に対し、Y字型に2分枝した植物体を用いて取り組んだ研究が述べられている。本章では、均一な強光下、弱光下の樹木と比べた場合、不均一な光環境下の樹木では、強光下の枝に集中的に窒素資源が分配され、弱光下の枝への窒素分配が抑制されることで、成長が強光下の枝に集約されていく現象が明らかにされている。また、光合成産物の分配パターンも、局所的な枝の光環境で決定され、近隣の枝の光環境には影響されないことも明らかにされた。これらの研究結果によって、これまで未解明であった樹木個体内の資源分配パターンの一端が解明され、複雑な樹冠構造を持つ樹木の成長の全容解明の端緒となった点を評価することができる。

なお、本論文第1章は、舘野正樹氏との共同研究であるが、論文提出者が主体となって分析おおび検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士 (理学) の学位を授与できると認める。