Effects of water stress on gas exchange of field grown Zea mays L. in Southern Italy: an analysis at canopy and leaf level.

Zea mays is cultivated in the Mediterranean regions where summer drought may lead to photoinhibition when irrigation is not available. In this work the response of maize to water stress was evaluated by gas exchange measurements at the canopy and leaf level. Leaf gas exchange was assessed before, during and after water stress, while canopy turbulent fluxes of mass and energy were performed on a continuous basis. In the early growth period, a linear increment of net ecosystem photosynthetic rate (PNE) to incoming of photosynthetic photon flux density (PPFD) was found and net leaf photosynthetic rate (PNL) showed the tendency to saturate under high irradiance. During water stress, the relationship between PNE and PPFD became curvilinear and both PNE and PNL saturated in a range between 1,000 and 1,500 mu mol (photons) m(-2) s(-1). Leaf water potential (psi(1)) dropped from -1.50 to -1.88 MPa during water stress, indicating that leaf and canopy gas exchanges were limited by stomatal conductance. With the restoration of irrigation, P-NE, P-NL and psi(1) showed a recovery, and P-NE and P-NL reached the highest values of whole study period. Leaf area index (LAI) reached a value of 3.0 m(2)m(-2). The relationship between P-NE and PPFD remained curvilinear and PNE values were lower than those of a typical well-irrigated maize crop. The recovery in PNE and P-NL after stress, and psi(1) values during stress indicate that the photosynthetic apparatus was not damaged while soil moisture stress after-effects resulted in a sub-optimal LAI values, which in turn depressed P-NE.