Abstract

<jats:p> Stomatal conductance (g) and leaf photosynthetic rate (P) of many species in the field are often less sensitive to water deficits than when grown in small pots under controlled conditions. This may result from stomatal adjustment in field-grown plants in response to water deficits that develop slowly, whereas adjustment does not occur under the rapid drying experienced by plants in small pots. To test this hypothesis we studied the response to water potential (Ψl) of g and P in three tropical C4 grasses, green panic (Panicum maximum var. trichoglume), buffel grass (Cenchrus ciliaris) and spear grass (Heteropogon contortus) and a tropical legume, Siratro (Macroptilium atropurpureum), grown under controlled conditions and in the field.
Field experiments clearly showed that stomatal adjustment occurred so that g and P were progressively less sensitive to the decline in Ψl as water deficits increased during a long soil drying cycle. For example in one experiment, the Ψl at which P approached zero fell from - 1.9, -2.0 and -2.4 MPa to -4.0, -4.0 and -3.3 MPa for green panic, spear grass and buffel grass, respectively. This stomatal adjustment was reversed within 10 days after rewatering to the well watered condition. Little stomatal adjustment occurred in plants grown under controlled conditions in small pots in which both soil water and Ψl fell rapidly. However, if plants were grown in similar conditions but in large pots of soil so that soil water and Ψl decreased slowly, stomatal adjustment comparable with field-grown plants was observed.
Siratro showed much less stomatal adjustment than the grasses and Ψl at which P approached zero only fell from - 1.2 to - 1-5 MPa. </jats:p>