Leaf age-structure effects on plant water use and photosynthesis of two wheat cultivars.

The wheat cultivars 'Katya Al' and 'Mexipak 65', grown in pots under field conditions in a Mediterranean climate, differed only slightly in growth and water use on a whole-plant basis, although Mexipak had a smaller leaf area. When expressed on a leaf area basis, plant biomass increase and transpiration were significantly greater in Mexipak. However, previous gas exchange measurements on the youngest fully expanded leaf showed only minor differences between the two cultivars. The hypothesis that different patterns of leaf ageing contributed to the differences in plant growth and water use was tested by combining leaf demography and gas exchange measurements. Plants were subjected to two moisture treatments: 'well-watered' (frequent, watering to initial soil water content) and 'intermittent drought' (five drying-rewatering cycles). Mexipak had, on average, older leaves than Katya in both treatments because of fast early growth, which in turn was clue to a larger area of individual leaves rather than a faster leaf appearance rate. Stomatal conductance and net photosynthetic rate were nearly the same for the two cultivars, but were considerably reduced by drought and did not fully recover after rewatering. Stomatal conductance and photosynthesis tended to decrease with ape, but effects of age or treatment or their interactions were not significant and could not explain the discrepancy between whole plant and leaf observations. One possible explanation is that age effects-do exist in leaves older than those investigated. Another explanation may be that changes in microclimatological conditions caused photosynthesis and transpiration to differ from rates measured under standard conditions.