4.4. Bud fertility and berry number per bunch are influenced by low water and nitrogen stress
Relationships between bud fertility and berry number per bunch in season 2 and LNC and predawn at flowering in season 1 are strong and significant. But an unexpected feature of these relationships is the low levels of both water and nitrogen stresses that could reduce bud fertility and berry number per bunch. Bud fertility in season 2might be reduced by 40% when predawn at flowering in season 1decreased from −0.1 to −0.3 MPa, which is generally considered as almost no water stress. The same conclusion holds for nitrogen: bud fertility in season 2 reacted to somewhat low levels of nitrogen stress at flowering in season 1, which were indicated by high values of LNC (Christensen, 1984). This highlights the great sensitivity of the early steps of yield formation to water deficit and nutrient deficiency. However, the two studied cultivars displayed different behavior: Aranel’s bud fertility was more sensitive to nitrogen stress than that of Shiraz, whereas their berry number per bunch seemed to react to water deficit in the same way (Fig. 3) in the field conditions. Whether these differences are attributable to different intrinsic sensitivities of the two cultivars or to differences in the combination of soil and weather stresses in experiments 1 and 2remains to be assessed.
4.5. Inflorescence formation in the latent buds during season 1 is a vegetative process very susceptible to water stress
We showed that bud fertility and berry number per bunch at harvest in season 2 were affected by very low levels of water deficit. To compare the response of these stages of yield formation with the responses of some vegetative processes of grapevine to water stress, we plotted relative bud fertility against FTSW, taking a bud fertility of 3 bunches per shoot as a maximum, and added responses of the other processes on the same plot (Fig. 6). Response curves of net assimilation, leaf emergence rate on main stem (LERI) and leaf emergence rate on branches (LERII) are from Lebon et al. (2006).Although the explored range of FTSW in our experiments was not wide enough to draw a regression line, this plot shows that bud fertility decreases much faster with FTSW than do other vegetative processes (Fig. 6).