Phloem chemistry: Effects of genotype and environment and implications for nutritional ecology

The contents of mineral elements in phloem vary among trees, and this variation is often associated with host choice by diverse parasites and herbivores. The relative importance of genotype and environment was analyzed in order to determine the broad-sense heritability (H-2) of accumulation of 15 elements (Al, B, Ca, Cu, Fe, K, Mg, Mn, Mo, N, Na, P, Si, Ti, Zn) in phloem of ponderosa pine (Pinus ponderosa var. ponderosa Laws.). The phloem was collected in 1997 from trees planted between 1979 and 1983 at two locations in the Sierra Nevada of California. Each plantation consists of multiple copies, called ramets, of specific genotypes grafted onto root stocks of very heterogeneous genotypes. Within each plantation, we sampled five ramets of each of 17 different genotypes. The same 17 genotypes were analyzed at two locations. There are many differences in heritabilities and other features of elemental contents between the two plantations. The H-2 values range from 0.0 to 0.65 and vary between the plantations, with 13 of the 15 elements having a H-2 of at least 0.3 in at least one of two locations. There are also significant differences in elemental contents among genotypes at both locations. Finally, the correlation between locations in element contents of the 17 genotypes is significant for some elements (Ca, Fe, K, N, and Si), whereas for the other elements, there was no correlation between locations, even when H-2 estimates were high at both locations. These patterns provide strong evidence for genotype-by-environment interactions. The results are relevant to tree physiology and ecology as they indicate that mechanisms of element transport and storage are influenced by shoots somewhat independently of root systems. The results also underscore the potential evolutionary significance of intergenotype variation in biochemical profile of nutritive tissues in the context of host-plant choice.