Principal component analysis
A PCA, using MAP and leaf-level structural and anatomical traits
associated with gm and Kleaf, was
performed to complement the trait-to-trait comparisons. The first two
major axes (PC1 and PC2) along with the average position of 18
C4 grasses in PC1-PC2 space are shown in Fig. 6. The,
first three axes with eigenvalues and scores are shown in Table S4. PC1
explained about 50.56 % of the total variation in the
C4 grasses. PC1 scaled positively with
gm, SDada, Smes, SR,
Narea, IVD, leaf thickness and average VED but
negatively with total VLA, BSias and MAP. Thus, PC1
delineated the C4 grasses into those which occupy
relatively drier habitats and show traits associated with greater
gm and hence photosynthetic C-gain (higher score on
PC1)) from those which occupy relatively wetter habitats and show traits
associated with greater Kleaf and water-loss (lower
score on PC1). PC2 explained about 16 % of the total variation and
scaled positively with MAP and BS area ratio but negatively with total
VLA and BSCW. PC3 explained 10.25% of total variation
and scaled positively only with MAT. Thus, MAT was unrelated to any of
the leaf-level structural and anatomical traits measured in the current
study. Together, the first three axes explained about 77% of the total
variation. To test if in addition to greater gm species
adapted to drier habitats also exhibit greater Anet and
leaf-level WUE, we looked at the relationship of the PC1 with
Anet (Fig. 7a) and two measures of leaf-level WUE, that
is, instantaneous water-use efficiency (Anet/E, Fig. 7b)
and intrinsic of water-use efficiency
(Anet/gsw, Fig. 7c). A higher score on
PC1 indicates presence of a ‘suite’ of traits associated with greater
gm and photosynthetic C-gain and lower
Kleaf and transpirational water-loss. There was a
significant positive relationship between Anet and PC1
(R 2 = 0.56, P = 0.002, Fig.7a).
Furthermore, Anet/E showed a significant positive
relationship with PC1 (R 2 = 0.25, P =
0.035, Fig.7b). However, Anet/gs did not
relate with PC1. In summary, C4 grasses with greater
score on PC1 also showed greater Anet and
Anet/E.