Leaf growth, stomatal diffusion resistances and photosynthesis during droughting of Lolium perenne populations selected for contrasting stomatal length and frequency

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Abstract
SUMMARY: Lolium perenne selection lines with high calculated stomatal resistances to diffusion (rs) as a result of either few or short stomata, maintained leaf extension rates and photosynthetic rates longer than selection lines with low resistances when deprived of water. There were no significant differences between high and low rs plants in light saturated CO2 uptake of turgid attached leaves.When grown in soil drying to 21% moisture, plants with low calculated rs exhibited lower minimum leaf resistances (rl) than those with high, measured with a diffusion porometer, on all except the last day. The daily maximum rl (1.5 h after the start of the light period) became greater among low than high rs plants as the difference in rate of leaf extension between the two groups of plants increased with drying soil. Rate of leaf extension was negatively correlated with daily maximum rl and started to decline when relative leaf water content (RLWC), at 5 h after the start of the light period, fell below about 88%. Transpiration rate of plants grown in different soil moisture regimes was correlated (r=+ 0.83, P < 0.01) with mean maximum adaxial leaf conductance (reciprocal of resistance). There was a highly significant correlation (r=+ 0.62, P < 0.01) between calculated adaxial rs and mean minimum measured rl among plants growing at high or intermediate soil moisture, but not at low. Therefore, some random variation in minimum rl, even with adequate moisture, seemed to be unaccounted for by variation in stomatal numbers or size. Selection for increased numbers of adaxial stomata also resulted in more on the abaxial surface, but mean adaxial/abaxial ratio in the ‘frequent’ stomata plants was still only about 9:1.